Understanding magnetic flux leakage testing reading 1

Understanding Magnetic Flux Leakage Reading 1

My ASNT Level III Pre-Exam Study Note 30th August 2015

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Permafrost Zone Pipeline MFLT

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Offshore Pipeline MFLT

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Cross Country Pipeline MFLT

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Offshore Pipeline MFLT

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Cross Country Pipeline MFLT

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Cross Country Pipeline MFLT

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Offshore Pipeline MFLT

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Tank Bottom MFLT

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Tank Bottom MFLT

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Tank Bottom MFLT

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Wire Rope MFLT

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Drilling String MFLT

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Reading I Content  Reading One: E1571 (Revisiting)  Reading Two: Magnetic Flux and SLOFEC Inspection of Thick Walled Components (Revisited)  Reading Three:  Reading Four:

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Principle of MFL Testing MFL testing is a magnetic based NDT method. The method is used to detect corrosion and cracks in ferromagnetic materials, such as pipelines, storage tanks, ropes and cables [10,27–29]. The basic principle of MFL testing is that the flux lines pass through the steel wires when a magnetic field is applied to the cable. At areas where corrosion or missing metal exists, the magneticfield leaks from the wires.

In an MFL tool, magnetic sensors are placed between the poles of the magnet to detect the leakage field. The signal of the leakage field is analyzed to identify the damaged areas and estimate the amount of metal loss. Thus, the transducer includes magnetizers and magnetic sensors.

The magnetic-field can be produced by a permanent magnet yoke, or a solenoid with the direct current. The magnetic-field density needs to meet near saturation under the sensor. Figure 3 shows the principle of MFL testing.

The permanent magnet yoke is used to produce the magnetic-field, and the coil is used to induce (detect?) the leakage field.

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Figure 3. Principle of MFL testing. (a) Undamaged cable; (b) Cable with metal loss.

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Reading 1 E1571 Standard Practice for Electromagnetic Examination of Ferromagnetic Steel Wire Rope

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1. Scope 1.1 This practice covers the application and standardization of instruments that use the electromagnetic, the magnetic flux, and the magnetic flux leakage examination method to detect flaws and changes in metallic crossectional areas in ferromagnetic wire rope products.

1.1.1 This practice includes rope diameters up to 2.5 in. (63.5 mm). Larger diameters may be included, subject to agreement by the users of this practice.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

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2. Referenced Documents 2.1 ASTM Standards: E 543 Practice for Agencies Performing Nondestructive Testing E 1316 Terminology for Nondestructive Examinations2

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3. Terminology 3.1 Definitions—See Terminology E 1316 for general terminology applicable to this practice.

3.2 Definitions of Terms Specific to This Standard: 3.2.1 dual- unction instrument—a wire rope NDT instrument designed to detect and display changes of metallic cross-sectional area on one channel and local flaws on another channel of a dual-channel strip chart recorder or another appropriate device.

3.2.2 local flaw (LF)—a discontinuity in a rope, such as a broken or damaged wire, a corrosion pit on a wire, a groove worn into a wire, or any other physical condition that degrades the integrity of the rope in a localized manner.

3.2.3 loss of metallic cross-sectional area (LMA)—a relative measure of the amount of material (mass) missing from a location along the wire rope and is measured by comparing a point with a reference point on the rope that represents maximum metallic cross-sectional area, as measured with an instrument.

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3.2.4 single-function instrument—a wire rope NDT instrument designed to detect and display either changes in metallic cross-sectional area or local flaws, but not both, on a strip chart recorder or another appropriate device.

Keywords: changes in metallic cross-sectional area local flaws

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4. Summary of Practice 4.1 The principle of operation of a wire rope nondestructive examination instrument is as follows: 4.1.1 AC Electromagnetic Instrument—An electromagnetic wire rope examination instrument works on the transformer principle with primary and secondary coils wound around the rope (Fig. 1). The rope acts as the transformer core. The primary (exciter) coil is energized with a low frequency alternating current (ac), typically in the 10 to 30 Hz range. The secondary (search) coil measures the magnetic characteristics of the rope. Any significant change in the magnetic characteristics in the core (wire rope) will be reflected as voltage changes (amplitude and phase) in the secondary coil.

Electromagnetic instruments operate at relatively low magnetic field strengths; therefore, it is necessary to completely demagnetize the rope before the start of an examination. This type of instrument is designed to detect changes in metallic crosssectional area.

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Keywords:  AC- Alternating Current System  Electromagnetic instruments operate at relatively low magnetic field strengths;  it is necessary to completely demagnetize the rope before the start of an examination.  This type of instrument is designed to detect changes in metallic crosssectional area.

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Alternating Field MFL method The Alternating Field MFL probe rotates at high speed around the longitudinally moved test material and scans its surface helically. The rotating probe scans „punctiform“ only a small area of the material surface at any moment, i.e. when testing, it focuses on a very small part of the overall surface. Thus, even an extremely small material flaw represents a major disturbance with respect to this relatively small material surface area. One other advantage of the rotating probe method: Long drawn-out material flaws are indicated over their full length.

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MAGNETIC FLUX LEAKAGE TESTING WITH CIRCOFLUX®

Alternating Field MFL method

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MAGNETIC FLUX LEAKAGE TESTING WITH CIRCOFLUX速

Alternating Field MFL method

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MAGNETIC FLUX LEAKAGE TESTING WITH CIRCOFLUX速

FIG. 1 Schematic Representation of an Electromagnetic Instrument SensorHead

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4.1.2 Direct Current and Permanent Magnet (Magnetic Flux) InstrumentsDirect current (dc) and permanent magnet instruments (Figs. 2 and 3) supply a constant flux that magnetizes a length of rope as it passes through the sensor head (magnetizing circuit). The total axial magnetic flux in the rope can be measured either by Hall effect sensors, an encircling (sense) coil, or by any other appropriate device that can measure absolute magnetic fields or variations in a steady magnetic field. The signal from the sensors is electronically processed, and the output voltage is proportional to the volume of steel or the change in metallic cross-sectional area, within the region of influence of the magnetizing circuit. This type of instrument measures changes in metallic cross-sectional area.

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FIG. 2 Schematic Representation of a Permanent Magnet Equipped SensorHead Using a Sense Coil to Measure the Loss of Metallic Cross- ectional Area

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FIG. 2 Schematic Representation of a Permanent Magnet Equipped SensorHead Using a Sense Coil to Measure the Loss of Metallic Cross- ectional Area Sensor Head

8.1.3 The sensor head, containing the energizing and detecting units, and other components, should be designed to accommodate different rope diameters. The rope should be approximately centered in the sensor head.

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FIG. 3 Schematic Representation of a Permanent Magnet Equipped Sensorhead Using Hall Devices to Measure the Loss of Metallic Cross-Sectional Area Sensor Head

Hall Devices Hall Devices

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4.1.3 Magnetic Flux Leakage Instrument- A direct current (DC) or permanent magnet instrument (Fig. 4) is used to supply a constant flux that magnetizes a length of rope as it passes through the sensor head (magnetizing circuit). The magnetic flux leakage created by a discontinuity in the rope, such as a broken wire, can be detected with a differential sensor, such as a Hall effect sensor, sensor coils, or by any appropriate device. The signal from the sensor is electronically processed and recorded. This type of instrument measures LFs. While the information is not quantitative as to the exact nature and magnitude of the causal flaws, valuable conclusions can be drawn as to the presence of broken wires, internal corrosion, and fretting of wires in the rope.�

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4.2 The examination is conducted using one or more techniques discussed in 4.1. Loss of metallic cross-sectional area can be determined by using an instrument operating according to the principle discussed in 4.1.1 and 4.1.2.

Broken wires and internal (or external) corrosion can be detected by using a magnetic flux leakage instrument as described in 4.1.3. The examination procedure must conform to Section 9. One instrument may incorporate both magnetic flux and magnetic flux leakage principles.

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5. Significance and Use 5.1 This practice outlines a procedure to standardize an instrument and to use the instrument to examine ferromagnetic wire rope products in which the electromagnetic, magnetic flux, magnetic flux leakage, or any combination of these methods is used. If properly applied, the electromagnetic and the magnetic flux methods are capable of detecting the presence, location, and magnitude of metal loss from wear and corrosion, and the magnetic flux leakage method is capable of detecting the presence and location of flaws such as broken wires and corrosion pits.

5.2 The instrument’s response to the rope’s fabrication, installation, and inservice-induced flaws can be significantly different from the instrument’s response to artificial flaws such as wire gaps or added wires. For this reason, it is preferable to detect and mark (using set-up standards that represent) real in-service-induced flaws whose characteristics will adversely affect the serviceability of the wire rope.

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6. Basis of Application 6.1 The following items require agreement by the users of this practice and should be included in the rope examination contract: 6.1.1 Acceptance criteria.

6.1.2 Determination of LMA, or the display of LFs, or both.

6.1.3 Extent of rope examination (that is, full length that may require several setups or partial length with one setup).

6.1.4 Standardization method to be used: wire rope reference standard, rod reference standards, or a combination thereof.

6.1.5 Maximum time interval between equipment standardizations.

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6.2 Wire Rope Reference Standard (Fig. 5): 6.2.1 Type, dimension, location, and number of artificial anomalies to be placed on a wire rope reference standard.

6.2.2 Methods of verifying dimensions of artificial anomalies placed on a wire rope reference standard and allowable tolerances.

6.2.3 Diameter and construction of wire rope(s) used for a wire rope reference standard.

6.3 Rod Reference Standards (Fig. 6): 6.3.1 Rod reference standard use, whether in the laboratory or in the field, or both.

6.3.2 Quantity, lengths, and diameters of rod reference standards.

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FIG. 5 Example of a Wire Rope Reference Standard

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FIG. 6 Example of a Rod Reference Standard

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7. Limitations 7.1 General Limitations: 7.1.1 This practice is limited to the examination of ferromagnetic steel ropes.

7.1.2 It is difficult, if not impossible, to detect flaws at or near rope terminations and ferromagnetic steel connections.

7.1.3 Deterioration of a purely metallurgical nature (brittleness, fatigue, etc.) may not be easily distinguishable.

7.1.4 A given size sensor head accommodates a limited range of rope diameters, the combination (between rope outside diameter and sensor head inside diameter) of which provides an acceptable minimum air gap to assure a reliable examination.

air gap Charlie Chong/ Fion Zhang

7.2 Limitations Inherent in the Use of Electromagnetic and Magnetic Flux Methods (LMA) : 7.2.1 Instruments designed to measure changes in metallic cross- sectional area are capable of showing changes relative to that point on the rope where the instrument was standardized.

7.2.2 The sensitivity of these methods may decrease with the depth of the flaw from the surface of the rope and with decreasing gaps between the ends of the broken wires.

Factor affecting measured LMA

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7.3 Limitations Inherent in the Use of the Magnetic Flux Leakage Method: 7.3.1 It may be impossible to discern relatively smalldiameter broken wires, broken wires with small gaps, or individual broken wires within closely-spaced multiple breaks. It may be impossible to discern broken wires from wires with corrosion pits.

7.3.2 Because deterioration of a purely metallurgical nature may not be easily distinguishable, more frequent examinations may be necessary after broken wires are detected to determine when the rope should be retired, based on percent rate of increase of broken wires.

Keywords: â– Electromagnetic Method (AC-LMA) (electromagnet) â– Magnetic Flux Method (DC-LMA) (electromagnet or permanent magnet) â– Magnetic Flux Leakage Method (DC-LF) (electromagnet or permanent magnet)

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8. Apparatus 8.1 The equipment used shall be specifically designed to examine ferromagnetic wire rope products.

8.1.1 The energizing unit within the sensor head shall consist of (1) permanent or (2) electromagnets, or (2a) AC or (2b) DC solenoid coils configured to allow application to the rope at the location of service.

8.1.2 The energizing unit, excluding the ac solenoid coil, shall be capable of magnetically saturating (except for electromagnetic AC method?) the range (size and construction) of ropes for which it was designed.

8.1.3 The sensor head, containing the energizing and detecting units, and other components, should be designed to accommodate different rope diameters. The rope should be approximately centered in the sensor head.

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8.1.4 The instrument should have connectors, or other means, for transmitting output signals to strip chart recorders, data recorders, or a multifunction computer interface. The instrument may also contain meters, bar indicators, or other display devices, necessary for instrument setup, standardization, and examination.

8.1.5 The instrument should have an (1) examination distance and (2) rope speed output indicating the current examination distance traveled and rope speed or, whenever applicable, have a proportional drive chart control that synchronizes the chart speed with the rope speed.

8.2 Auxiliary Equipment The examination results shall be recorded on a permanent basis by either 8.2.1 a strip chart recorder 8.2.2 and/or by an other type of data recorder 8.2.3 and/or by a multifunctional computer interface.

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9. Examination Procedure 9.1 The electronic system shall have a pre-examination standardization procedure.

9.2 The wire rope shall be examined for LFs or LMA, or both, as specified in the agreement by the users of this practice. The users may select the instrument that best suits the intended purpose of the examination. The examination should be conducted as follows: 9.2.1 The rope must be demagnetized before examination (ALL- AC electromagnetic, DC/PM Magnetic flux and DC/PM Magnetic Flux Leakage methods) by an electromagnetic instrument. If a magnetic flux or a magnetic flux leakage instrument is used, it may be necessary to repeat the examination to homogenize the magnetization of the rope.

9.2.2 The sensor head must be approximately centered around the wire rope.

9.2.3 The instrument must be adjusted in accordance with a procedure. The sensitivity setting should be verified prior to starting the examination by inserting a ferromagnetic steel rod or wire of known cross-sectional area. This standardization signal should be permanently recorded for future reference.

DC/PM = DC electromagnet of Permanent Magnet Charlie Chong/ Fion Zhang

9.2.4 The wire rope must be examined by moving the head, or the rope, at a relatively uniform speed. Relevant signal(s) must be recorded on suitable media, such as on a strip chart recorder, on a tape recorder, or on computer file(s), for the purpose of both present and future replay/analysis.

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9.2.5 The following information shall be recorded as examination data for analysis: 9.2.5.1 Date of examination, 9.2.5.2 Examination number, 9.2.5.3 Customer identification, 9.2.5.4 Rope identification (use, location, reel and rope number, etc.), 9.2.5.5 Rope diameter and construction, 9.2.5.6 Instrument serial number, 9.2.5.7 Instrument standardization settings, 9.2.5.8 Strip chart recorder settings, 9.2.5.9 Strip chart speed, 9.2.5.10 Location of sensor head with respect to a welldefined reference point along the rope, both at the beginning of the examination and when commencing a second set-up run, 9.2.5.11 Direction of rope or sensor head travel, 9.2.5.12 Total length of rope examined, and 9.2.5.13 examination speed.

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9.2.6 To assure repeatability of the examination results, two or more operational passes are required.

9.2.7 When more than one setup is required to examine the full working length of the rope, the sensor head should be positioned to maintain the same magnetic polarity (?) with respect to the rope for all setups. For strip chart alignment purposes, a temporary marker should be placed on the rope at a point common to the two adjacent runs. (A ferromagnetic marker shows an indication on a recording device.) The same instrument detection signals should be achieved for the same standard when future examinations are conducted on the same rope.

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9.2.8 When determining percent LMA, it must be understood that comparisons are made with respect to a reference point on the rope representing maximum metallic cross sectional area. The reference point may have deteriorated such that it does not represent the original (new) rope. The reference point must be inspected visually to evaluate its condition.

When determining percent LMA, it must be understood that comparisons are made with respect to a reference point on the rope that represents the rope’s maximum metallic crosssectional area. The reference point’s condition may have deteriorated during the rope’s operational use such that it no longer represents the original (new) rope values. The reference point must be examined visually, and possibly by other means, to evaluate its current condition.

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9.2.9 If the NDT indicates existence of significant rope deterioration at any rope location, an additional NDT of this location(s) should be conducted to check for indication repeatability. Rope locations at which the NDT indicates significant deterioration must be examined visually in addition to the NDT. 9.3 Local flaw baseline data for LF and LMA/LF instruments may be established during the initial examination of a (new) rope. Whenever applicable, gain settings for future examination of the same rope should be adjusted to produce the same amplitude for a known flaw, such as a rod or wire attached to the rope.

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10. Reference Standard 10.1 General: 10.1.1 The instrument should be standardized with respect to the acceptance criteria established by the users of this practice.

10.1.2 Standardization should be done the first time the instrument is used, during periodic checks, or in the event of a suspected malfunction.

10.1.3 The instrument should be standardized using one or more of the following: ■ wire rope reference standard with artificial flaws (see Fig. 5), or ■ rod reference standards (see Fig. 6).

For clarification, the following sections – 10.2 and 10.3 – are useful for laboratory purposes to more fully understand instrument limitations.

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10.2 Wire Rope Reference Standard: 10.2.1 The wire ropes selected for reference standards should be first examined to ascertain and account for the existence of interfering, preexisting flaws (if they exist) prior to the introduction of artificial flaws. The reference standard shall be that rope appropriate for the instrument and sensor head being used and for the wire rope to be examined unless rod reference standards are used. The reference standard shall be of sufficient length to permit the required spacing of artificial flaws and to provide sufficient space to avoid rope end effects. The selected configuration for the reference standard rope shall be as established by the users of this practice.

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10.2.2 Artificial flaws placed in the wire rope reference standard shall include gaps produced by removing, or by adding, lengths of outer wire. The gaps shall have typical lengths of 1/16 , 1/8 , 1/4 , 1/2 , 1, 2, 4, 8, 16, and 32 in. (1.6, 3.2, 6.4, 12.7, 25.4, 50.8, 101.6, 203.2, 406.4, and 812.8 mm, respectively).

The gaps shall typically be spaced 30 in. (762 mm) apart. There shall be a minimum of 48 in. (1219 mm) between gaps and the ends of the wire rope.

Some of the gap lengths may not be required. All wire ends shall be square and perpendicular to the wire. 10.2.3 Stricter requirements than those stated above for local flaws and changes in metallic cross-sectional area may be established by the users if proven feasible for a given NDT instrument, subject to agreement by the users.

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10.3 Rod Reference Standard: 10.3.1 Steel rods are assembled in a manner such that the total crossectional area will be equal to the cross-sectional area of the wire rope to be examined. The rod bundle is to be placed in the sensor head in a manner simulating the conditions that arise when a rope is placed along the axis of the examination head. Individual rods are to be removed to simulate loss of metallic area caused by wear, corrosion, or missing wires in a rope. This procedure gives highly accurate control of changes in instrument response and can be used to adjust and standardize the instrument.

10.3.2 The rods for laboratory standardization procedures should be a minimum of 3 ft (Approx. 1 m) in length to minimize end-effects from the rod ends, or as recommended by the instrument manufacturer.

10.3.3 Shorter rods or wires may be used for a preexamination check in the field.

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10.4 Adjustment and Standardization of Apparatus Sensitivity: 10.4.1 The procedure for setting up and checking the sensitivity of the apparatus is as follows: 10.4.1.1 The reference standard shall be fabricated as specified in the agreement by the users.

10.4.1.2 The sensor head shall be adjusted for the size of material to be examined.

10.4.1.3 The sensor head shall be installed around the reference standard.

10.4.1.4 The reference standard shall be scanned, and, whenever applicable, gain and zero potentiometers, chart recording scale, or other apparatus controls shall be adjusted for required performance.

10.4.1.5 If standardization is a static procedure, as with an electromagnetic instrument (see 4.1.1), the standard reference rope shall be passed through the detector assembly at field examination speed to demonstrate adequate dynamic performance of the examination instrument. The instrument settings that provide required standardization shall be recorded.

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11. Test Agency Qualification 11.1 Nondestructive Testing Agency Qualification—Use of an NDT agency (in accordance with Practice E 543) to perform the examination may be agreed upon by the using parties. If a systematic assessment of the capability of the agency is specified, a documented procedure such as Practice E 543 shall be used as the basis for the assessment.

12. Keywords 12.1 electromagnetic examination; flux leakage; local flaws (LF); Magnetic flux; magnetic flux leakage; percent loss of metallic cross-sectional area (LMA); rod reference standards; sensor head; wire rope; wire rope reference standard

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End Of Reading 1

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Reading 2 Magnetic Flux and SLOFEC Inspection of Thick Walled Components

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http://www.ndt.net/article/wcndt00/papers/idn352/idn352.htm

Summary: Magnetic Flux Leakage (MFL) inspection of low-alloy carbon steel components is attractive while, contrary to ultrasonic inspection, no acoustic coupling is needed between the sensor system and the object. Furthermore MFL is a fast and reliable method to detect local corrosion. The well-known and widely used traditional MFL method however is, despite efforts to improve, limited to a thickness of up to no more than 15 mm. This paper describes an improved highly sensitive MFL method with an upper thickness limit of at least 30 mm. The extended thickness capability of the new MFL tool makes the method suitable for a much wider range of applications, not only for inspection of thick components but also for thinner walls covered with thick non-metallic protection layers such as glass fibre reinforced epoxy coatings on floors of (oil)storage tanks. Moreover this improved MFL method is able to differentiate surface from back wall defects, which is a unique and very useful feature. The new MFL method, known as "SLOFEC" in the meantime has successfully been applied in the field on a variety of components. Background and applications of this new intriguing MFL tool for the NDT industry are described in this paper.

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1. Introduction NDT is an essential activity to establish the integrity of (petro)chemical) plants as part of regular maintenance[1]. Because of stringent maintenance cost reduction programs, application of NDT is ever more rationalised.

Conventional inspection programs are often not taken for granted any more when viewed from new and better understanding of safety and risk. So called Risk Based Inspection (RBI) philosophies gradually influence or dictate what is done by NDT, what method, qualitative or quantitative, to what extent but always at the lowest possible cost. As a consequence one can observe that some constructions are inspected over their full surface with NDT screening tools, [1], because all places are considered of equal risk, e.g. the floor of an (oil) storage tank.

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On other components, e.g. on a pressure vessel, NDT can be limited to certain critical areas. The increasing knowledge of risk, failure and fracture mechanics has influenced the need to improve or adapt the capabilities of some NDT methods. The MFL method to inspect steel components fits very well in a full surface coverage and low cost inspection approach. As such it has been the prevailing method to inspect long distance pipelines for decades.

Over the past decade, despite its limited quantitative capabilities, MFL became the most common method to inspect tank floors [2][3]. Unfortunately this "traditional“ MFL method is limited to a thickness of 10 or at best 15 mm under favourable field conditions. The demand for MFL tools with a larger thickness range is known for decades, but considerable efforts to increase the range of traditional MFL tools so far were hardly successful. Only marginal improvements could be achieved at a high cost and weight penalty.

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2. Progress in NDT capabilities These days capabilities of common NDT methods are stretched to the limit, one does hardly observe "quantum leaps" in performance any more, most progress was achieved in the past. Of course the implementation of computer technology and signal analysis in NDT systems, all accomplished in the last decade have resulted in sometimes large technical steps forward. A good illustration of this impressive progress is Computer Tomography in combination with ultrasonic or radiographic inspection. Such large improvements are often at high cost and only suitable and affordable for laboratory type use. Moreover due to the complexity of such systems they are not suitable for industrial bulk work and reduce the use of these CT systems to niche applications. From this historical viewpoint it is remarkable that only a few years ago a "quantum leap" was achieved with the relatively simple MFL technique.

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All of a sudden the thickness range could be increased to at least 30 mm in combination with several other unique inspection features. Besides, the improved MFL technique is very suitable for prevailing field conditions. After a period of proof of principle and verification the new method is now gradually becoming known in industry. The now maturing improved MFL method, offers economically affordable NDT solutions until recently not available to industry, it fulfils a demand and fits very well in the current inspection approaches.

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3. Traditional MFL Inspection systems The need for a fast and simple NDT technique which does not require acoustic coupling liquid as required in traditional ultrasonic inspection , was a major incentive to develop tools based on the MFL principle. Moreover an MFL system is rather tolerant to surface condition, removal of loose and excessive debris prior to inspection is sufficient. Because of this and other merits MFL has become the premier method to inspect long distance pipelines from the inside. This is done on stream with so called "intelligent pigs". The full pipe surface is inspected to reveal local metal loss either inside or outside. Over a number of decades these tools have been optimised and reached capabilities near perfection [4]. In the eighties the method was selected to inspect floors of (oil)storage tanks. Such tools now are a commodity. Figure 1 shows a system in use to inspect a tank floor. In more recent years some derivative MFL tools to inspect pipes from the outside were introduced.

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Fig 1: MFL inspection of a tank floor

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Fig 2: Adjustable MFL pipe scanner

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Silverwing UK - Floormap VS2i - MFL tank inspection, Corrosion Mapping and Detection Floors Scanner

■ https://www.youtube.com/watch?v=8JtVJJp3mc8 ■ https://www.youtube.com/watch?v=c22z9Mo0PVs Charlie Chong/ Fion Zhang

For inspection of bare or painted pipe from the outside, instead of single diameter scanners sometimes adjustable yoke and scanner constructions are used to make them suitable for a range of diameters. With such adjustable scanners, of which one is shown in Figure 2, inspection cost per meter of pipe can be reduced, a factor of paramount importance in the maintenance inspection world.

The MFL method is very suitable to detect local corrosion and is qualitative rather than quantitative.

Gradual thinning can not be detected.

Once one needs quantitative data complementary methods e.g. ultrasonic inspection has to be applied. Another considerable drawback of the MFL technique is that rather heavy and bulky scanners are needed, adapted to the geometry of the component. This limits the application to large constructions of uniform geometry such as storage tanks, pipe lines and long lengths of plant piping. Despite some of the described limitations MFL has obtained a good reputation in industry also due to its high reliability of defect detection.

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4. Principle of "traditional" MFL The MFL method can only be applied on low alloy carbon steels which have a high magnetic permeability. The well known principle is illustrated in Figure 3.

Fig 3: Principle of MFL to detect metal loss

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MFL method

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http://idea-ndt.en.alibaba.com/product/578144516-212166760/Magnetic_flux_leakage_testing_instrument.html

A magnet within a yoke construction is used to establish a uniform magnetic flux in the material to be inspected. The magnetisation should be up to a high level close to magnetic saturation. Usually strong permanent magnets are used to generate the magnetic field, but sometimes electromagnets are used if sufficient power is available, even combinations of both to achieve superimposed magnetisation. In a defect free plate the magnetic flux is uniform. In contrast a metal loss type defect, such as local corrosion or erosion, not only distorts the uniformity of the flux but a small portion of the magnetic flux is forced to "leak" out of the plate.

Sensors placed between the poles of the magnet or yoke construction can detect this small local "leakage".

The amount of distortion and leakage is dependent on depth, orientation, type and position (topside/back wall) of the defect. Defects are often of erratic form.

Various combinations of volume loss can result in the same flux leakage level although not having the same depth.

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This causes that the method is and remains rather qualitative and not quantitative, despite efforts to apply signal analysis and adaptive learning software programs to improve depth sizing. Very often this mainly qualitative character is acceptable for industry in return for its high speed , full surface coverage and in particular its high probability of defect detection.

Most of the MFL inspection tools make use of "passive" Hall effect sensors to detect flux leakage as indication of metal loss. The systems using Hall elements we call "traditional" MFL tools. Due to physical limits of the size of magnets and total weight of the necessary scanner there is an optimum in performance of traditional MFL tools. As a consequence thickness range is limited to 10 or at best 15 mm under favourable circumstances. Sensitivity drops dramatically with increasing thickness. Thus the challenge to design a tool for a much greater wall thickness remained.

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5. Principle of "improved" MFL (SLOFEC) Trying to increase the range of MFL tools, another sensor type in combination with a few other essential equipment modifications ultimately solved the problem. Instead of the "passive" Hall sensors, as illustrated in figure 3, "active" eddy current sensors are used to detect flux leakage, even better, these sensors can detect changes in flux density inside the plate. The sensing is virtually "in the plate" and this explains its higher sensitivity for variations of the magnetic flux than a passive sensor "at the plate surface".

The principle has been known and systems existed already for a considerable time [5]. It is applied for steel (boiler) tube inspection not exceeding say 5 mm wall thickness, thus not for extreme thicknesses up to 30 mm being the subject of this paper. Eddy currents in steel have a small penetration depth due to the high relative magnetic permeability, say 500 or more. This limits penetration of the eddy currents to the outer surface. δ= √ (2/ωσμ) = 1/ √(πfσμ) = (πfσμ)-½

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This so called "skin effect" is strongly reduced by magnetic saturation of the wall, causing a low relative permeability, say close to 1. This allows the eddy currents to penetrate much deeper, up to the full wall thickness.

Magnetic saturation not only creates a low permeability and uniform flux, it also suppresses the usual local permeability variations in the material. This eliminates an enormous source of noise, which can hardly be filtered out, and otherwise would prohibit proper functioning of flux sensing systems.

Keywords:  Magnetic saturation  Magnetic permeability μ=1  Uniform flux  Deeper penetration  Skin effect  Local permeability variations  Hall sensor (passive!)  Eddy current sensor (active)

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Figure 4: shows the relative sensitivity curves for traditional and improved MFL.

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These curves are typical for inspection results achieved on plates. This extreme high sensitivity is achieved with the eddy current sensors in combination with special electronics and fast on-line signal processing. In eddy current testing, phase information is provided and from that it can be established whether the defect is at the top or back wall of the component.

Using phase information the type of defect can automatically be sorted out including a reasonable level of defect severity. In addition, although there is still room for improvement, the new system provides some information on general wall thickness reduction. Despite all these merits it can not replace ultrasonic inspection in terms of absolute accuracy. Experiments in the laboratory and field trials proved that with the improved MFL system a thickness of up to 30 mm and probably more can be inspected with a much higher overall sensitivity than with traditional MFL, this applies certainly for thickness range beyond 5 mm.

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The technical thickness limit of this new system is determined by the combination of sufficient magnetic saturation ( bias field) of the full wall thickness of the component and a low enough eddy current frequency to penetrate the full wall without sacrificing on inspection speed.

Most probably the weight of the magnetic yoke and scanner dictate the real physical upper limit. The limits have not fully been explored yet. The now existing system, suitable for approximately 30 mm wall thickness, seems to be an optimum. The "new" MFL technology is called "SLOFEC". The acronym SLOFEC stands for Saturation LOw Frequency Eddy Current.

Keywords: â– phase information is provided and from that it can be established whether the defect is at the top or back wall of the component. â– low enough eddy current frequency to penetrate the full wall without sacrificing on inspection speed

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6. Development of tools - market demand At first SLOFEC systems were built for customer specific "one-off "solutions.

In fact a specific inspection problem which could not economically be solved with regular NDT provided the challenge. The customer needed a system to inspect thick large diameter buried bullet tanks from the inside to detect corrosion under the external tar coating. SLOFEC offered an affordable solution to inspect the tanks which otherwise had to be lifted, cleaned and inspected ; a not attractive expensive procedure. Figure 5 shows the partly buried tanks with a diameter of 5 metres. Figure 6 shows the typical "oneoff“ scanner, with adjustable diameter, built for this job …………….

For more read: http://www.ndt.net/article/wcndt00/papers/idn352/idn352.htm

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End Of Reading 2

Charlie Chong/ Fion Zhang

Reading 3: A Comparison of the Magnetic Flux Leakage and Ultrasonic Methods in the detection and measurement of corrosion pitting in ferrous plate and pipe

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn701/idn701.htm

INTRODUCTION Magnetic Flux Leakage (MFL) and manual Ultrasonics (UT) have been used extensively for the detection and sizing of corrosion pits in ferrous plates and pipes. Users and providers of these inspection services may have different perceptions and expectations of the sensitivity and accuracy of the methods.

This paper discusses the underlying principles of the methods and their effect on Probability of Detection (POD) and accuracy.

CORROSION PITTING There are many types and mechanisms of corrosion but in this instance we deal exclusively with corrosion that is typical between the pad and the underside of tank bottoms or from water contamination inside the tank. The ultrasonic means of detecting erosion in pipework was so successful during the 1960's that it has given a false impression of the accuracy that will be obtained with pitting type corrosion. To help appreciate the difference we will illustrate erosion and some typical pit shapes. Figure 1 shows erosion whereas Figures 2 to 4 sketch corrosion shapes that have been given the terms "Lake Type", "Cone Type" and "Pipe Type".

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Figures 5 to 8 are photographs of erosion and typical corrosion of the lake and cone type. It is interesting to note the steps or 'terraces' formed as the corrosion progressed.

Lake and pipe (cone?) types of corrosion are most commonly found in storage tank floors.

They are usually the result of moisture ingress between the floor and the pad (underside) or water in the product (topside).

Pipe type pitting is relatively uncommon (?)and is usually associated with water droplet erosion or Sulphur Reducing Bacteria (SRB).

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

METHOD PRINCIPLES MFL: The principles of both the MFL method and the UT method have been described in detail elsewhere. For the purposes of this paper these are briefly summarised here. Figure 9 illustrates the basic principle of the MFL method.

A magnet mounted on a carriage induces a strong magnetic field in the plate or pipe wall. In the presence of a corrosion pit, a magnetic flux leakage field forms outside the plate or pipe wall. An array of sensors is positioned between the magnet poles to detect this flux leakage. The sensors are usually (1) Hall Effect devices or (2) (Eddy current?) coils; there are advantages and limitations with either type of sensor.

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UT: Figure 10 illustrates a simple UT set-up using the pulse-echo principle and a twin crystal probe. In this configuration one crystal acts as transmitter and the other as the receiver. The transmitter is isolated from the receiving circuits so that the A-scan display is freed from the presence of a transmission signal. As a result the transmission pulse does not obscure the first back wall echo when testing relatively thin areas of plate or pipe. We shall see that simple digital thickness meters without an A-scan facility are not suitable for either detection or measurement of pitting.

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PROBABILITY OF DETECTION - MFL. The MFL method uses an array of sensors such that each sensing field overlaps with its neighbour. The probability of detection of any flux leakage signal depends on the amplitude of that leakage field in relation to any noise signals. In other words, the signal to noise ratio is the primary factor governing detection. Some of the parameters affecting the signal to noise ratio are related to the equipment design and performance, and some are related to the floor condition including the geometry of any pitting.

Equipment parameters

Floor parameters

Magnet design Sensor type and layout Speed control Vibration damping Signal processing Detection notification

Floor material Scanning surface condition Scanning surface coating Cleanliness Pit depth Pit volume Pit contour

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Equipment Parameters ■Magnet design The magnet must be strong enough to achieve a flux density in the material being tested that is close to saturation. The carriage design must be such that the magnet system can ride any undulations in the scanning surface without too much variation in the gap between the magnet poles and the test surface (lift off). Clearly, one advantage of using Electro-magnets is that the magnetising force can be adjusted to compensate for different material thicknesses and lift off changes. A practical advantage is also that the magnetic field can be switched off to aid removal of the scanning head from the test surface. The major disadvantages are size and weight. For this reason many scanners resort to permanent magnets using Neodymium – iron - boron in the magnet design. The result is a compact scanning head suitable for wall thicknesses up to 12.5 mm, or, at reduced sensitivity, up to 20 mm.

Greater thicknesses could be achieved provided that a suitable and safe system to place and remove the carriage from the test surface is devised.

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â– Sensor type and layout Two types of sensor are in common use, coils and Hall effect devices.

In either case the spacing between adjacent elements in the array must be small enough to ensure that there are no gaps in detection across the array. If sensors are arranged in differential pairs for noise cancelling purposes, the layout should take into account the fact that the leakage field may extend 3 or 4 times the diameter of the pit across the array but only about the diameter of the pit in the scanning direction. (?) The voltage signal generated by a given leakage field in a coil sensor is a function of the rate of cutting lines of force. This will be a function of the number of turns in the coil and the forward speed of the scanner. Thus the coil type of sensor is speed sensitive and this should be taken into account in the equipment design. Coils are also more sensitive to lift off variation than some configurations of Hall effect devices.

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One distinct advantage of the coil sensor is that it appears to be less affected than Hall effect devices by the strong eddy current signal that is generated during the acceleration and deceleration phases of the scanner.

Hall effect devices are in principle less sensitive to speed variation, however when filtration is used during signal processing to remove low and high frequency spurious signals, the resulting band pass window imposes some restriction on speed variation. When these devices are arranged to detect the Horizontal component of the leakage field, they are relatively insensitive to the eddy current signal mentioned above, but, like the coil, relatively sensitive to lift off variations. When arranged to detect the Vertical component, they are less sensitive to lift off variations but very sensitive to the eddy current signals.

One advantage of this arrangement, however, is that a larger gap between the sensor housing and the test surface can be accommodated which reduces housing wear and allows the housing to clear some of the surface imperfections such as weld spatter.

Keywords: acceleration and deceleration phases of the scanner Charlie Chong/ Fion Zhang

â– Speed control Some degree of speed control is necessary with all types of sensor but there is less latitude when coils are used. â– Vibration damping One source of background noise and false indications is due to surface roughness of the scanning surface. This is very common in the case of storage tank floors and above ground pipelines that have not been coated.

The resulting corrosion on those surfaces causes the scanning carriage to vibrate the magnet and sensor system. The resulting noise can be reduced in three ways: by fitting broader wheels, by incorporating shock absorbers and by signal processing since the vibration frequency is likely to be higher than that from pit signals.

Keypoints: by signal processing since the vibration frequency is likely to be higher than that from pit signals. (unlike crack and weld defect!)

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â– Signal processing The signals from leakage fields are relatively small and need amplification.

They also need to be discriminated from unwanted noise. Band pass filters are used to remove the low frequency (eddy current) (?) (lift off eddy current variation?) and high frequency (vibration) noise. Any residual noise can be countered by the use of thresholds set on the defect detection circuit or, in the case of dynamic detection notification displays, by the operator assessing the general noise level.

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â– Defect notification There are three ways in current use in which a defect may be drawn to the attention of the operator: Autostop. The scanner automatically stops when a defect is encountered and a visual display indicates which sensors in the array have detected the pit.

The scanner cannot be restarted until the operator has cancelled the indication. The operator marks the floor so that pit depth measurement can be performed.

Dynamic display. The operator views a dynamic display indicating the current status of signals across the array. A signal above the general noise level indicates the presence of a pit. In these systems the operator may be assisted by an audible or visual alarm which triggers above a pre-set threshold. The operator marks the floor so that pit depth measurement can be performed.

Computer data acquisition. Some systems use a computer to store data from the inspection for subsequent analysis and reporting. This may include software to allow mapping of the tank floor with colour coded indications of material loss. The operator can access the data at the end of each scan in order to mark the floor so that some cross checking of results can be performed.

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Floor Parameters â– Material Clearly a ferrous material is necessary for MFL, but the magnetic permeability of the ferrous material will affect the results. It follows that the calibration plate or pipe used to set up the equipment should be made of the same grade of steel as the material to be inspected. This is generally not a problem with storage tank floors since with very rare exceptions they are constructed using low carbon mild steels. Greater care is needed when selecting a calibration pipe to ensure that the correct grade of steel is selected. For a given magnetising field, material thickness will affect the degree of saturation achieved and this in turn will affect the flux leakage amplitude for a given pit

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â– Scanning surface condition The scanning surface should be clean and free from debris (particularly from corrosion products that may have fallen from the tank roof). Surface roughness may cause vibration noise requiring a relatively high threshold to be set (reduced pit sensitivity). In some cases laying a thin sheet (circa 1mm) of plastic over the scanning surface can alleviate this. Other anomalies such as weld spatter or weld repairs that have been ground flush will give large false indications.

It must also be remembered that the MFL method does not discriminate between pitting on the scanning surface and that on the remote surface, however, for pits penetrating 50% or more through the material, the MFL method is more sensitive to remote surface pitting. (comment: some vendor provide eddy current probe with phase discrimination for depth analysis?)

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These curves are typical for inspection results achieved on plates. This extreme high sensitivity is achieved with the eddy current sensors in combination with special electronics and fast on-line signal processing. In eddy current testing, phase information is provided and from that it can be established whether the defect is at the top or back wall of the component.

Using phase information the type of defect can automatically be sorted out including a reasonable level of defect severity. In addition, although there is still room for improvement, the new system provides some information on general wall thickness reduction. Despite all these merits it can not replace ultrasonic inspection in terms of absolute accuracy. Experiments in the laboratory and field trials proved that with the improved MFL system a thickness of up to 30 mm and probably more can be inspected with a much higher overall sensitivity than with traditional MFL, this applies certainly for thickness range beyond 5 mm.

Charlie Chong/ Fion Zhang

■ Scanning surface coating One major advantage of the MFL method is that it is able to function with relatively thick surface coating and maintain reasonable sensitivity. Fibreglass coatings up to 6mm thick on 6.32mm thick floors have been inspected and 20% wall loss detected.

■ Cleanliness MFL is less sensitive to floor surface condition that ultrasonics but heavily ribbed scale can cause false indications and corrosion products can build up on the magnet poles and then give false indications as they break away and pass under the sensor head. Generally removal of product and subsequent water jetting of the surface is sufficient.

■ Pit depth Pit depth is one of the main factors affecting flux leakage amplitude at a particular distance above the test surface. Volume and contour also affect this amplitude and these are discussed below. However within prescribed limitations the amplitude of the flux leakage field can be used to assess the percentage wall loss and thus reduce the amount of cross checking needed.

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â– Pit Volume It has been claimed elsewhere that the volume of the pit is the most significant factor affecting signal amplitude and for this reason it is claimed that no quantitative information about the pit can be deduced from the MFL results. Since the claim mostly appears as a bald statement we decided to carry out a study of the effects of volume and depth using modelling techniques and some empirical trials on real corrosion. A series of models of pits of given depth and varying volumes were produced. The results for depths of 40%, 50% and 60% pits in 6.35mm plate are shown at Figure 11.

These show that as the volume increases its affect on signal amplitude decreases. This suggests that for typical tank floor corrosion of the cone and lake type it should be possible to "band" corrosion severity with reasonable accuracy using MFL alone. Pipe - like pitting such as that encountered with Sulphur Reducing Bacteria attack, however, are likely to give inaccurate results because the volumes will correspond to the region where the curves in Figure 11 converge.

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60%, 50%, 40%,

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â– Pit contour Very often people producing test plates with machined pitting choose simple shapes such as flat-bottomed holes (borrowed from ultrasonics) or simple conical impressions using drill bits. It has been shown that the contour of the pit will affect the leakage field. Since corrosion pitting usually progresses in such a way as to produce "terracing" in its profile, we have used artificial pits for calibration purposes that mimic the terracing as shown in Figure 12. These have been used to calibrate the MFL system used in the empirical results shown below.

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■Human factors As with other NDT methods, human factors must be considered in assessing probability of detection. Especially in the case of storage tanks, the environment is not friendly! The interior of the tank is dark, dirty and has the lingering smell of the product. It can at times be extremely hot (+50°C) or extremely cold (-20°C) depending on location and season. It is therefore essential that the demands made on the operator are as light as possible.

However, the operator must also ensure that the equipment is maintained in the best possible condition and that the calibration routine is carried out with precision.

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POD Summary for MFL The probability of detection of pitting using the MFL is high within certain limits. With well-maintained equipment, trained and conscientious operators working on clean unpitted scanning surfaces on material thicknesses up to 10mm thick losses of 20% (sometimes as low as 10%) can be reliably detected. On less clean surfaces and on thicknesses up to 13mm 40% losses can be detected. Within these limits MFL is able to scan at speeds around 0.5m/sec with scan widths from 150mm to 450mm wide. The method is less influenced by surface condition than ultrasonics and for most MFL systems less operator dependant.

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PROBABILITY OF DETECTION - ULTRASONICS The probability of detection of corrosion pitting using the ultrasonic method is also dependent on many factors. Because the method is rather slower than MFL, it was common practice until recently to use spot checks on a grid pattern in the same way that was used for erosion detection on pipe bends.

Clearly the probability of detecting isolated pitting using this technique is negligible. Area scanning is now preferred and can be applied manually using contact scanning or using automated scanning with water irrigated probes.

The reflecting surface that is offered by typical corrosion pitting is often poor for ultrasonic purposes and the operator needs to be able to see the character of the signal to avoid errors. For this reason simple digital thickness meters are not suitable for corrosion detection. Equipment with an A-Scan presentation is preferred and this can be complimented by B-Scan and CScan facilities. As with the MFL, the factors affecting POD with Ultrasonics include those that relate to the equipment and technique and those that relate to the floor and any pitting that may be present.

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Equipment Parameters Flaw Detector Probe Type Couplant method and type Scanning technique Calibration Training and experience

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Floor Parameters Floor Thickness Scanning surface condition Floor coating Pit characteristics

Flaw detector As a minimum it should have an A-Scan display but the use of data storage techniques with facilities for producing both C-Scan and BScan images greatly enhances the probability of detection. In particular, these facilities demonstrate that continuous coupling has been achieved during the inspection.

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Probe Type In many cases the thickness of material being examined is less than 10mm and the scanning surfaces are not completely smooth. This means that the initial pulse of single crystal transducers will occupy a significant portion of the nominal thickness so these transducers are not suitable. Twin crystal (Dual) transducers overcome this problem but it must be remembered that the optimum distance at which the maximum amount of transmitted energy is able to be captured by the receiver is a function of the probe design. Figure 13 illustrates this and shows clearly why reflectors below this distance will give reduced amplitude signals even when the reflecting surface in question is flat and parallel to the scanning surface. The operator should be aware of this possibility especially as corrosion pits are not ideal reflectors and should be prepared to vary the gain when backwall echoes are 'lost'. The rough surfaces encountered will rapidly wear Perspex shoes and change the beam angle so it is necessary to fit a wear ring to the probe. The crystal size should be between 10 and 15 mm diameter.

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Couplant method and type Two methods of coupling ultrasound to the material are in current use. For manual scanning the contact method is used, whilst for automated and semiautomated scanning, water irrigation is preferred. In either case it is essential that the couplant is able to 'wet' the surface. Suitable gels are available for manual scanning and for water irrigation it may be necessary to add a wetting agent (soap).

Scanning technique It should be obvious that taking spot readings on a grid pattern is only suitable for detecting areas of general corrosion and is useless in detecting isolated pits. Therefore it is necessary to use an area scan technique with a suitable overlap to ensure coverage by the effective area of the probe. With manual scanning it is better to use a fairly rapid probe movement with suitable calibration than to use a slow painstaking approach to the detection phase.

This is because the human eye naturally responds to a sudden change (movement) in signal pattern. Once the pit has been detected, a more careful investigation of pit depth can be carried out.

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Calibration For the detection phase of the inspection when using manual scanning, it is better to calibrate the flaw detector on the actual test material by selecting an area on the floor where the thickness is known to be at the nominal plate thickness. The timebase is then set to display 3 backwall echoes positioned at 3, 6 and 9. The gain should be set so that the third backwall echo is at 80% full screen height. With this arrangement, using the fast scanning movement described above, loss of couplant will show as a vertical drop in all three echoes. The presence of a pit will show as a progressive loss (3rd then 2nd and then 1st echo) coupled with a general movement of the signals towards zero. With practice the eye becomes well adapted to recognise these patterns.

Training and experience The detection of corrosion pits is more difficult than simple thickness measurement or the detection of laminations or erosion. The slow scanning technique, with a timebase calibrated to display only one backwall echo, used by some operators is prone to miss pits that have poor reflectivity such as the conical types. Operators often say that they 'lost' the signal due to poor scanning surface when they have just encountered a pit. Specific training and experience is required for corrosion detection.

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Floor Parameters Thickness Thinner wall thicknesses present the main difficulty when using the ultrasonic method. Below 6 mm the signal from a good reflector is reduced as described above and shown in Figure 13. The operator must be aware that more gain will be required. For thicker sections (above 12 mm) the ultrasonic method is far less restricted than MFL, however the POD limitations with respect to shape and reflectivity of pits still apply.

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Scanning surface condition The ultrasonic method is much more sensitive to the condition of the scanning surface than is the MFL method. This applies to both contact scanning and irrigated 'gap' scanning. Reflections in the couplant layer create 'noise' that obscures part of the timebase as shown in Figure 14. Since the velocity of sound in the couplant is about one quarter of the velocity in the material, top surface pits may give clear echoes that appear to show a reduced wall thickness. Figure 15 illustrates a lake type pit 1 mm deep. The echo from the bottom of the pit appears at a steel thickness of 4 mm. If unnoticed the operator may report a 6mm deep underfloor pit in a 10mm plate (60% loss).

The same pit is likely to be misinterpreted with automated and semiautomated systems whether or not they use interface triggering and/or echoto-echo monitoring.

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Floor Coatings Painted and epoxy coated floors in which the coating is in good condition and has been applied from new present few problems to ultrasonic inspection and pit detection. The accuracy of measurement of remaining wall thickness is improved if the echo-to-echo method is used to eliminate paint thickness errors. Thicker, fibreglass coatings present more of a problem. Although in theory it may be possible to inspect through such a coating if the adhesion to the metal surface is good, it is seldom suitable for inspection.

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Pit characteristics The easiest pits to detect are the lake type because in the deepest region they are relatively parallel to the scanning surface and can be expected to give reasonable reflectivity. On the other hand the conical pits tend to reflect sound away from the receiver and the centre of the pit is often too small in area to give a strong signal (Figure 16). These are the pits that are most likely to be missed by the ultrasonic operator. Often one of the 'terrace' facets is the strongest reflector and the pit is detected but its depth is underestimated.

Pipe like pits such as those typical of SRB attack present very small targets to the ultrasonic beam and may also be as difficult to detect. Where the reflectivity of the pit is favourable, the ultrasonic method is capable of detecting smaller changes of thickness than the MFL method but, since the corrosion allowance is often as much as 50%, this advantage is not always significant.

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

POD Summary for Ultrasonics On good scanning surfaces the probability of detecting Lake Type pits is high.

For poor scanning surfaces and for Cone Type pitting, the probability of detection is less satisfactory. To some extent the POD can be improved using the automated techniques with data storage and at least a C-scan presentation using colour coding to 'band' thickness.

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SOME PRACTICAL RESULTS Some sections of floor were cut from storage tank bottoms after MFL inspection. Sections were taken from areas where underfloor corrosion was reported and also from areas where there was said to be no corrosion that was deeper than 20%. Some of the sections had been inspected using the Silver Wing 'Floormap' system that produced a map of the floor with colour coded indications of corrosion, each colour representing a 'band' of percentage wall loss. The corroded sections were subjected to mechanical pit depth measurement and the results compared with the MFL report. The pitting included both lake and cone examples. The approximate locations of the pits were marked on the opposite side of the plates (scanning surface) and two teams of UT operators were asked to locate the pits and measure their depth.

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Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Figures 17 to 21 are photographs of some of the corrosion detected. Figures 22 and 23 are graphs showing actual pit depth against reported depths for the two UT teams. Figure 24 show the same for the MFL results. It can be seen that on average the MFL system overestimates the depth of pitting by about 10% whereas the ultrasonic method has underestimated by about 10%.

However one UT team missed two of the pits even though the approximate location had been marked.

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Charlie Chong/ Fion Zhang

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CONCLUSIONS Both methods have limitations in the thickness range that can be reliably inspected and the smallest pit that can be detected. Within the limitations described for MFL, the probability of detection of isolated pitting is better than ultrasonics and the method is also quicker than ultrasonics so more economic.

In terms of accuracy of depth measurement, both methods have the same percentage error though in opposite senses. Since there is a remote chance that the floor material may not be mild steel and thus may have a permeability that differs from the calibration plate, it is always necessary to carry out at least limited cross checking of MFL results with UT before relying on MFL depth assessment.

Charlie Chong/ Fion Zhang

End Of Reading 3

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Reading 4: The Truth About Magnetic Flux Leakage As Applied To Tank Floor Inspections

Charlie Chong/ Fion Zhang

The Truth About Magnetic Flux Leakage As Applied To Tank Floor Inspections Magnetic Flux Leakage Inspection techniques have been widely used in the Oil field Inspection Industry for over a quarter of a century for the examination of pipe, tubing and casing both new and used. It is only in the last fifteen years that this inspection technique has been applied to above ground storage tank floors in an attempt to provide a reliable indication of the overall floor condition within an economical time frame. In most cases these inspections are being carried out by Industrial Inspection NDT Companies who do not have the depth of experience in the technique that most of the Oil field Tubular Inspection Companies have.

At the same time this relatively new application of Magnetic Flux Leakage brings with it some additional problems not evident in the inspection of tubulars where certain parameters can be quite closely controlled. Probably the greatest of these is that tank floors are never flat, whereas tubulars are generally always round.

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The ability to obtain any reasonably consistent quantitative information is seriously impacted by this general unevenness of most tank floors. The application of rigid accept/reject criteria based on signal amplitude thresholds has proved to be absolutely unreliable as regards truly quantitative information. A more realistic approach is required in the application of this inspection technique and in the design of the MFL inspection equipment to ensure that there are fewer incidences of significant defects being missed.

The following information outlines some of the major considerations that need to be addressed in order to achieve reliable, fast and economical inspections of above ground storage tank floors.

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MAGNETIC FLUX LEAKAGE (MFL) In order to understand some of the problems associated with this particular application of Magnetic Flux Leakage (MFL), it is necessary to understand the basic principles of the technique. Most people are familiar with a magnet’s ability to “stick” to a carbon steel plate. This happens because the magnetic lines of force (flux) prefer to travel in the carbon steel plate rather than in the surrounding air. In fact, this flux is very reluctant to travel in air unless it is forced to do so by the lack of another suitable medium. For the purposes of this particular application, a magnetic bridge is used to introduce as near a saturation of flux as is possible in the inspection material between the poles of the bridge. Any significant reduction in the thickness of the plate will result in some of the magnetic flux being forced into the air around the area of reduction. Sensors which can detect these flux leakages are placed between the poles of the bridge. Figure 1 graphically illustrates this phenomenon.

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FIGURE “1”

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THE MFL INSPECTION ENVIRONMENT In order to optimize the effectiveness of the MFL inspection, it is necessary to consider the environment and address the physical restrictions imposed by the actual conditions found when examining the majority of tank floors. â– CLIMATIC CONDITIONS Invariably, the range of temperature and humidity conditions will vary enormously worldwide. The effect on both operator and equipment must be taken into consideration. Human beings do not function well in extremes of temperature. Use of the MFL equipment should not place too great a burden on them from either a physical or mental point of view. In other words, the simpler, more reliable and easy to use the MFL inspection equipment is made, the more reliable the inspection results.

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â– TANK FLOOR CLEANLINESS By their very nature, the majority of above ground storage tanks are dirty and sometimes dusty places to work. The conditions in this regard vary widely and are dependent upon how much effort the tank owner/operator is willing to expend in cleaning the floors in preparation for Magnetic Flux Leakage Scanning. As an absolute minimum, a good water blast is necessary and all loose debris and scale should be removed from the inspection surface. The surface does not necessarily have to be dry but puddles of standing water need to be removed. The cleaner the floor, the better the inspection.

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â– STORAGE TANK SURFACE CONDITION Significant top surface corrosion and/or buckling of the tank floor plates represents a serious limitation to both the achievable coverage in the areas concerned and also the achievable sensitivity. While it is understood that very little can be done to improve this situation prior to inspection, it must be considered in the design of the MFL inspection equipment and its effect on the sensitivity of the inspection appreciated by both the owner/operator of the tank as well as the person conducting the examination. Any physical disturbance of the MFL scanning system as it traverses the tank floor will result in the generation of noise. The rougher the surface, the greater the noise and, therefore, a reduction in achievable sensitivity.

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MFL EQUIPMENT DESIGN CONSIDERATIONS It is vital that Magnetic Flux Leakage NDT equipment used for storage tank floor inspection is designed to handle the environmental and practical field conditions that are consistently present. A piece of MFL equipment designed in a laboratory and tested in ideal conditions will invariably have significant short comings in real world applications.

■ ELECTROMAGNETS/PERMANENT MAGNETS Powerful rare earth magnets are ideally suited for this application. They are more than capable of introducing the required flux levels into the material under test. Electromagnets by comparison are bulky and heavy. They do have an advantage in that the magnetic flux levels can be easily adjusted and “turned off” if necessary for cleaning purposes. Permanent magnet heights can be adjusted to alter flux levels but the bridge requires regular cleaning to remove ferritic debris. The buildup of debris can have a significant impact on system sensitivity.

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â– SENSOR TYPES MFL tools typically use one of two types of sensors: Coils and Hall Effect Sensors. They are both capable of detecting the magnetic flux leakage fields caused by corrosion on tank floors. There is a fundamental difference, however, in the way that they respond to leakage fields.

COILS Coils are passive devices and follow Faraday’s Law in the presence of a magnetic field. As a coil is passed through a magnetic field, a voltage is generated in the coil and the level of this voltage is dependent on the number of turns in the coil and the rate of change of the flux leakage. From this, it is clear that speed will have some influence on the signals obtained from this type of sensor.

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HALL EFFECT SENSORS Hall Effect Sensors are solid state devices which form part of an electrical circuit and, when passed through a magnetic field, the value of the voltage in the circuit varies dependent on the absolute value of the flux density. It is necessary to carry out some cross referencing and canceling with this type of sensor in order to separate true signals from other causes of large variations in voltage levels generated by the MFL inspection process.

There is disagreement within the industry as to which is the best type of sensor to use for this application. Hall Effect Sensors are undeniably more sensitive than coils. However, in this application, coils are more than adequately sensitive and are more stable and reliable. Hall Effect sensors prove to be too sensitive when surface conditions are less than perfect which results in an unreliable inspection and the generation of significant false calls.

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MFL TECHNIQUE APPLICATION CONSIDERATIONS COVERAGE LIMITATIONS It is virtually impossible to achieve 100% coverage using this technique due to the physical access limitations. The MFL inspection equipment should be designed so that it can scan as close as possible to the lap joint and shell.

There are obviously compromises to be made as the wheel base of the scanner is an important consideration on tank floors that are not perfectly flat.

Smaller scanning heads can be used in confined spaces to increase coverage.

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■ TOPSIDE/BOTTOM SIDE DIFFERENTIATION Magnetic Flux Leakage cannot differentiate between the response from topside and bottom side indications. Some attempt has been made to use the eddy current signals from topside defects for the purposes of differentiation based on frequency discrimination. This is unreliable on real tank floors due to the uneven nature and lack of cleanliness of the inspection surface. In most cases, visual techniques are perfectly adequate for this purpose.

Contrary to what is expected, the Magnetic Flux Leakage response from a topside indication is significantly lower in amplitude than that from an equivalent bottom side indication. This means that, to some degree, the influence of the top side indications can be “tuned out” to allow a reliable assessment of the under floor condition. (?)

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â– QUANTITATIVE ASSESSMENT OF INDICATIONS Magnetic Flux Leakage is a qualitative, not quantitative inspection tool and is a reliable detector of corrosion on tank floors. Due to the environmental and physical restrictions encountered during real inspections, no reliable quantification of indications are possible. Amplitude alone is an unreliable indication of remaining wall thickness as it is more dependent on actual volume loss. Defects exhibiting various combinations of volume loss and through wall dimension can give the same amplitude signal. Couple to this the continually changing spatial relationship of magnets, sensor and inspection surface and it is absolutely clear that an accurate assessment of remaining wall thickness is virtually impossible. Truly quantitative results can only be obtained using a combination of Ultrasonic testing and Magnetic Flux Leakage.

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60%, 50%, 40%,

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â– THE SINGLE LEVEL THRESHOLD Commercial expediency has brought about the implementation of accept/reject criteria using a single level threshold approach. MFE Enterprises, as a manufacturer of Magnetic Flux Leakage equipment, does not support this approach. As previously stated, the amplitude of signals alone is not a reliable indicator of remaining wall thickness. Significant indications can be completely missed especially in cases where the equipment does not incorporate some form of real time on line display. In order to carry out a reliable MFL inspection, the operator must have as much information as possible available to him in the form of an easy-to-interpret real time display. The use of a blind single threshold is absolutely indefensible in this application.

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MFL OPERATOR TRAINING AND QUALIFICATION REQUIREMENTS Currently, there is limited training available to users of the MFL equipment in regard to this application. MFE Enterprises Inc. recognizes this fact and offers initial basic training in magnetic flux leakage and the use of MFL inspection equipment on delivery of the scanner. This is obviously geared to our equipment and is quite specific. The ultrasonic prove up necessary must be carried out by personnel who are adequately trained and qualified. This is not just a “thickness measurement,� but rather a corrosion evaluation and the technician must have a full understanding of the technique that should be applied.

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End Of Reading 4

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Reading 5: Magnetic Flux Leakage Testing for Back-side Defects Using a Tunnel Magnetoresistive Device

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Abstract— Magnetic non-destructive testing is limited to surface inspection, however demand for the detection of deep defects is increasing. Therefore, we developed a magnetic flux leakage (MFL) system using a tunnel magnetoresistive (TMR) device that has high sensitivity and wide frequency range in order to detect deep defects. Using the developed system, back-side pits of steel plates having different depth and diameter were measured and 2D images were created. Moreover, we analyzed the detected vector signal with optimized phase data. As a result, the developed MFL system can detect a defect that has a wall thinning rate of more than 56 % of 8.6 mm thick steel plates. Furthermore, the defect’s diameter size was estimated by spatial signal change.

Keywords-MFL; magnetic imaging; TMR device; Low-Freaquency field; backside pit.

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Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a few nanometers), electrons can tunnel from one ferromagnet into the other. Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly quantum mechanical phenomenon.

Magnetic tunnel junctions are manufactured in thin film technology. On an industrial scale the film deposition is done by magnetron sputter deposition; on a laboratory scale molecular beam epitaxy, pulsed laser deposition and electron beam physical vapor deposition are also utilized. The junctions are prepared by photolithography.

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https://en.wikipedia.org/wiki/Tunnel_magnetoresistance

History The effect was originally discovered in 1975 by M. Jullière (University of Rennes, France) in Fe/Ge-O/Co-junctions at 4.2 K. The relative change of resistance was around 14%, and did not attract much attention.[1] In 1991 Terunobu Miyazaki (Tohoku University, Japan) found an effect of 2.7% at room temperature. Later, in 1994, Miyazaki found 18% in junctions of iron separated by an amorphous aluminum oxide insulator [2] and Jagadeesh Moodera found 11.8% in junctions with electrodes of CoFe and Co.[3] The highest effects observed to date with aluminum oxide insulators are around 70% at room temperature.

Since the year 2000, tunnel barriers of crystalline magnesium oxide (MgO) have been under development. In 2001 Butler and Mathon independently made the theoretical prediction that using iron as the ferromagnet and MgO as the insulator, the tunnel magnetoresistance can reach several thousand percent.[4][5]

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https://en.wikipedia.org/wiki/Tunnel_magnetoresistance

The same year, Bowen et al. were the first to report experiments showing a significant TMR in a MgO based magnetic tunnel junction [Fe/MgO/FeCo(001)].[6] In 2004, Parkin and Yuasa were able to make Fe/MgO/Fe junctions that reach over 200% TMR at room temperature.[7][8] In 2008, effects of up to 600% at room temperature and more than 1100% at 4.2 K were observed in junctions of CoFeB/MgO/CoFeB.[9] Applications The read-heads of modern hard disk drives work on the basis of magnetic tunnel junctions. TMR, or more specifically the magnetic tunnel junction, is also the basis of MRAM, a new type of non-volatile memory. The 1st generation technologies relied on creating cross-point magnetic fields on each bit to write the data on it, although this approach has a scaling limit at around 90–130 nm.[10] There are two 2nd generation techniques currently being developed: Thermal Assisted Switching (TAS)[10] and Spin Torque Transfer (STT). Magnetic tunnel junctions are also used for sensing applications. For example, a TMR-Sensor can measure angles in modern high precision wind vanes, used in the wind power industry.

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https://en.wikipedia.org/wiki/Tunnel_magnetoresistance

I. INTRODUCTION Accidents due to defects in steel structures such as power plants or pipe line cause serious injuries to humans and harm to the natural environment.

Therefore, it is important to use non-destructive testing for detecting defects at an early stage. In many cases, it is difficult to find defects in the interior or on the back side, and thus a detection method for deep defects is desired.

There are many non-destructive testing methods such as radiographic testing, ultrasonic testing, magnetic flux leakage (MFL) testing, and eddy current testing. Among them, MFL is commonly used for ferromagnetic material such as steel and it is a method for detecting flux with bypass defects due to differences in permeability and leakage from the sample’s surface when an external field is applied to the sample.

MFL for deep defects needs to be operated at low frequency because the penetration of the applied external field becomes deeper with decreasing frequency. However, the conventional MFL method, which uses a detection coil as a magnetic sensor, cannot be operated at low frequency because it has low sensitivity at low frequency due to Faraday’s law of induction.

Therefore, it can detect only surface defects near the detection coil.

(I =dΦ/dt?)

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Moreover, the detection of deep defects also requires a high magnetic resolution because the change of flux generated by the deep defect is very small. The other problem of MFL is that the magnetic field intensity of MFL needs to be operated at the saturation region of the B-H curve in order to obtain measurable large magnetic flux leakage. However, a measurement system that gives such large magnetic field intensity is costly because a high power current source is necessary.

One way to solve these problems is to use a high sensitivity magnetic sensor that can detect a low magnetic intensity field at low frequency such as a magnetoresistive (MR) sensor. If such a sensor were installed, we could operate MFL at extra low frequency, which would give deep skin depth and detect small magnetic flux leakage caused by a low power source.

We reported a MFL system using an anisotropic magnetoresistive (AMR) sensor [11]. Recently, the tunnel magnetoresistive (TMR) sensor has progressed because it has a larger MR ratio than other MR devices with a wide frequency range.

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In this study, we developed the MFL system using a TMR device having high sensitivity at extreme-low frequency in order to enable us to detect defects deeper and more clearly than the AMR sensor and other magnetic sensors.

Moreover, we investigated the performance of the developed system using samples having various back-side pits. δ= √(2/ωσμ) = (πfσμ)-½

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II. TMR DEVICE A TMR device is a kind of MR device and is usually applied in the magnetic head of a hard disk. It has a larger MR ratio than other MR devices. A common TMR device shows a step response to magnetic fields and has hysteresis. The TMR device used in this study was designed for sensor application [12]-[14]. It was annealed at different temperatures and directions two times in order to make easy directions of the pin layer and the free layer orthogonal. In this structure, the output is linear with respect to the magnetic field. In addition, it has a large MR ratio because of magnetic coupling of the free layer and the soft magnetic material layer. Figure 1 shows the TMR resistance as a function of an applied field. The range from -400ÎźT to 400ÎźT, which is treated in MFL, can be applicable to the sensor application.

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Figure 1. Resistance of the TMR device to an applied field.

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III. EXPERIMENTAL The developed MFL system (Figure 2) consists of a sensor probe with a TMR device, a lock-in amplifier, a current source, an oscillator, two excitation coils, a half shaped ferrite yoke, a sample stage, and a PC. Two excitation coils with 30 turns were connected to both ends of the yoke and an AC field was induced in the sample between both ends. The sensor probe was installed between the ends of the yoke and they were 1 mm away from the sample’s surface. The TMR device measured magnetic flux leakage bypassing defects.

In this study, the TMR device had sensitivity to the direction parallel to both ends of the yoke in order to obtain a larger output [11]. The excitation coils were operated by a sine wave of 1.2 App and 5 Hz or 10 Hz from the current source controlled by the oscillator. The effect of the eddy current can be ignored in such an extreme-low frequency field. The output signal from the TMR device was detected by the lock-in amplifier, which is synchronized with the current source in order to obtain a high signal-to-noise ratio.

Comments: How eddy current density Jo of J= Joe-x/δ calculated?

How does the frequency affect Jo?

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Faraday Law E(emf) = - N ∆(BA)/∆t = -N (dФ/dt) ∆Ф = B┴ ∆A = E ∆A = A ∆E

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Figure 2. Schematic diagram of the developed MFL system.

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The signal from the lock-in amplifier contains the signal intensity R and the phase θ. In this measurement system, magnetic flux leakage is very small so that it is strongly affected by the phase shift of the entire measurement system. Therefore, we calculated the imaginary part of the signal intensity with the common phase φ [11].

R’ = R sin(θ+φ)

Here, φ is a common phase adjusting the phase shift of the entire measurement system.

The samples used in this study were two steel plates (SPHC) with four backside pits as shown in Figure 3. Both samples were 8.6 mm thick. The pits of Sample (a) are of the same diameter (6 mm) and different wall thinning rates (23, 57, 70, 93 %). Sample (b) has the same wall thinning rate (70 %) and different diameters (4, 6, 8. 10 mm). Multipoint measurement was carried out in the range of 20 mm × 20 mm around a pit from front surface with an interval of 1 mm for 21 × 21 steps as shown in Figure4.

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Figure 3. Schematic diagram of the test plates with pits.

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Figure 4. Measuring points for back-side pits.

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We investigated the common phase φ in this measurement system. The measurement was carried out around a pit that has a wall thinning rate of 70% and a diameter of 4 mm. The excitation coils were operated by sine wave of 1.2 App and 10 Hz or 5 Hz from the current source. The measurement results show as contour maps of calculated intensity (mV) with different common phases.

Figure 5 shows magnetic images with a frequency of 10 Hz and different common phases and Figure 6 shows that with 5 Hz and different common phases. Magnetic images with a common phase φ of 130 ° show the emphasis of the intensity change due to the pit in the center of the scanning range. The magnetic image with a frequency of 5 Hz shows the presence of the back-side pit more clearly than that of 10 Hz because the skin depth becomes deeper with decreasing frequency. Therefore, the frequency was 5 Hz and the optimized common phase φ was 130 ° for the measurement system.

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Figure 7 shows the power spectrum of the developed system when the magnetic field was not applied and the sine field was applied at 100μT and 5 Hz in the unshielded environment. The sensitivity at 5 Hz of the developed system is 2.44 mV/μT. We estimated the magnetic noise without an applied field that corresponds to the minimum magnetic field resolution at 5 Hz. As a result, the magnetic field resolution was 1.08 nT. To evaluate the performance of the developed MFL system, we analyzed the magnetic image change of a steel plate having different pit wall thinning rates and diameters under optimum conditions. The excitation coils were operated by a sine wave of 5 Hz and 1.2 App from the current source. We calculated the signal vector with the optimized phase φ = 130°. The aforementioned Sample (a) and Sample (b) were measured and we made contour maps of the calculated signal vector.

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Figure 5. Magnetic images with 10 Hz and different phase.

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Figure 5. Magnetic images with 10 Hz and different phase.

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Figure 6. Magnetic images with 5 Hz and different phase.

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Figure 6. Magnetic images with 5 Hz and different phase.

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Figure 7. Power spectrum of the developed system.

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IV. RESULTS AND DISCUSSION First, we used Sample (a) and investigated the change of magnetic images of the steel plates with different wall thinning rates. The map showed the existence of the pit and it becomes clear with increasing the actual pit’s wall thinning rate (Figure 8). However, the magnetic image of a pit that has a wall thinning rate of 23 % is unclear. This was caused by the weak magnetic flux leakage from the small thinning rate of the wall. The detection limit was a thinning rate 57 % corresponding to a wall thickness of 4.6 mm. Next, we used Sample (b) and investigated the changes of the magnetic images by changing the diameter (Figure 9). Apparent differences were observed in each figure. The contour map change became large according to the increment of the diameter.

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Moreover, we quantitatively evaluated the magnetic field intensity change and examined the relationship of the defect’s characteristics and the calculated intensity. The center line of the contour of the magnetic image was extracted as shown in Figure 10 and ΔB was defined as the value obtained by subtracting the minimum value from the maximum value as shown in Figure 10. Figure 11 shows the relationship of ΔB and the wall thinning rate and Figure 12 shows that of ΔB and the diameter. ΔB was increased with the increment of the wall thinning rate and the diameter. Therefore, we can estimate the defect’s characteristics using the magnetic image and ΔB.

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Figure8. Magnetic images of pits with different wall thinning rate.

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Figure 9. Magnetic images of pits with different diameter.

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Figure 10. Example of the extracted line and the definition of ΔB.

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Figure 11. Relationship of the defect’s wall thinning rate and ΔB.

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Figure 12. Relationship of the defect’s diameter and ΔB.

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V. CONCLUSIONS We developed a magnetic flux leakage (MFL) testing system using TMR for back-side defects. Analysis using the signal vector with optimized phase was effective for magnetic imaging of the back-side pits. The magnetic images reflected the actual defect’s characteristics and were able to detect more than the wall thinning rate of 57%. The developed MFL system does not require a high power current source so that this measurement system is expected to be applicable to field testing.

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End Of Reading 5

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Reading 6 Chapter Nine: Magnetic Flux Leakage Testing

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9.1

PART 1. Introduction to Magnetic Flux Leakage Testing MFLT

9.1.0

Introduction

Magnetic flux leakage testing is part of the widely used family of electromagnetic nondestructive techniques. Magnetic particle testing is a variation of flux leakage testing that uses particles to show indications. When used with other methods, magnetic tests can provide a quick and relatively inexpensive assessment of the integrity of ferromagnetic materials. The theory and practice of electromagnetic techniques are discussed elsewhere in this volume. The origins of magnetic particle testing are described in the literature1 and information that the practicing magnetic test engineer might require is available from a variety of manuals and journal articles. The magnetic circuit and the means for producing the magnetizing force that causes magnetic flux leakage are described below. Theories developed for surface and subsurface discontinuities are outlined along with some results that can be expected.

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9.1.1 Industrial Uses Magnetic flux leakage testing is used in many industries to find a wide variety of discontinuities. Much of the world’s production of ferromagnetic steel is tested by magnetic or electromagnetic techniques. Steel is tested many times before it is used and some steel products are tested during use for safety and reliability and to maximize their length of service.

9.1.1.1 Production Testing Typical applications of magnetic flux leakage testing are by the steel producer, where blooms, billets, rods, bars, tubes and ropes are tested to establish the integrity of the final product. In many instances, the end user will not accept delivery of steel product without testing by the mill and independent agencies.

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9.1.1.2 Receiving Testing The end user often uses magnetic flux leakage tests before fabrication. This test ensures the manufacturer’s claim that the product is within agreed specifications. Such tests are frequently performed by independent testing companies or the end user’s quality assurance department. Oil field tubular goods are often tested at this stage.

9.1.1.3 In-service Testing Good examples of in-service applications are the testing of used wire rope, installed tubing, or retrieved oil field tubular goods by independent facilities.

Many laboratories also use magnetic techniques (along with metallurgical sectioning and other techniques) for the assessment of steel products and prediction of failure modes.

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9.1.2

Discontinuities

Discontinuities can be divided into two general categories: those caused during manufacture in new materials and those caused after manufacture in used materials. Discontinuities caused during manufacture include cracks, seams, forging laps, laminations and inclusions.

1. Cracking occurs when quenched steel cools too rapidly.

2. Seams occur in several ways, depending on when they originate during fabrication.

3. Discontinuities such as piping or inclusions within a bloom or billet can be elongated until they emerge as long tight seams or gouges during initial forming processes. They may later be closed with additional forming.

4. Their metallurgical structures are often different but the origin of manufactured discontinuities is not usually taken into account when rejecting a part.

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5. Forging laps occur when gouges or fins created in one metal working process are rolled over at an angle to the surface in subsequent processes.

6. Inclusions are pieces of nonmagnetic or nonmetallic materials embedded inside the metal during cooling. Inclusions are not necessarily detrimental to the use of the material.

7. The pouring and cooling processes can also result in lack of fusion within the steel. Such regions may be worked into internal laminations.

Discontinuities in used materials include fatigue cracks, pitting corrosion, erosion and abrasive wear.

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Much steel is acceptable to the producer’s quality assurance department if no discontinuities are found or if discontinuities are considered to be of a depth or size less than some prescribed maximum. Specifications exist for the acceptance or rejection of such materials and such specifications sometimes lead to debate between the producer and the end user. Discontinuities can either remain benign or can grow and cause premature failure of the part.

Abrasive wear can turn benign subsurface discontinuities into detrimental surface breaking discontinuities.

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For used materials, fatigue cracking commonly occurs as the material is cyclically stressed. Fatigue cracks grow rapidly under stress or in the presence of corrosive materials such as hydrogen sulfide, chlorides, carbon dioxide and water. For example, drill pipe failure from fatigue often initiates at the bases of pits, at tong marks or in regions where the tube has been worn by abrasion. Pitting is caused by corrosion and erosion between the steel and a surrounding or containing fluid. Abrasive wear occurs in many steel structures.

Good examples are: (1) the wear on drill pipe caused by hard formations when drilling crooked holes or (2) the wear on both the sucker rod and the producing tubing in rod pumping oil wells.

Specifications exist for the maximum permitted wear under these and other circumstances. In many instances, such induced damage is first found by automated magnetic techniques.

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9.1.3 Steps in Magnetic Flux Leakage Testing There are four steps in magnetic flux leakage testing: (1) (2) (3) (4)

magnetize the test object so that discontinuities perturb the flux, scan the surface of the test object with a magnetic flux sensitive detector, process the raw data from these detectors in a manner that best accentuates discontinuity signals and present the test results clearly for interpretation.

The next section discussion deals with the first step, producing the magnetizing force.

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Steel Mill – Expert at Works

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Steel Mill – Expert at Works

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Steel Mill

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Steel Mill

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http://jyhengrun.en.made-in-china.com/productimage/LXxJqIKWhmhC2f1j00bScaFVoCEUql/China-Rolling-Ring-Forging-Stainless-Steel-Flange.html

Seams & Laps

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http://azterlan.blogspot.com/2013/09/sensitivity-in-magnetic-particle.html

9.2

PART 2. Magnetization Techniques

9.2.0

Introduction

Successful testing requires the test object to be magnetized properly. The magnetization can be accomplished using one of several approaches: (1) permanent magnets, (2) electromagnets and (3) electric currents used to induce the required magnetic field.

Excitation systems that use permanent magnets offer the least flexibility.

Such systems use high energy product permanent magnet materials such as neodymium iron boron, samarium cobalt and aluminum nickel. The major disadvantage with such systems lies in the fact that the excitation cannot be switched off. Because the magnetization is always turned on, it is difficult to insert and remove the test object from the test rig. Although the magnetization level can be adjusted using appropriate magnetic shunts, it is awkward to do so. Consequently, permanent magnets are very rarely used for magnetization.

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Keywords:  Excitation systems.  Neodymium iron boron, samarium cobalt and aluminum nickel.  Appropriate magnetic shunts.

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http://en.wikipedia.org/wiki/Neodymium_magnet

Electromagnets, as well as electric currents, are used extensively to magnetize the test object. Figure 1 shows an excitation system where the test object is part of a magnetic circuit energized by current passing through an excitation coil. The magnetic circuit passes through a yoke made of a soft magnetic material and through a test object placed between the poles of the yoke. When the coil wound on the yoke carries current, the resulting magnetomotive force drives magnetic flux through the yoke and the test object. The total magnetic flux Ф (phi) ( in weber) is given by: (1)

Ф = N I / S = ampere/(ampere/weber)

where I is the current (ampere) in the coil, N is the number of turns in the coil and S is the reluctance (ampere per weber) of the magnetic circuit.

Keywords: Reluctance (ampere per weber)

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FIGURE 1. Electromagnetic yoke for magnetizing of test object.

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Yangtze River China - 水落石出

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http://en.wikipedia.org/wiki/Magnetic_field

Reluctance.

Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance.

The unit for magnetic reluctance is inverse henry, H-1.

In a DC field, the reluctance is the ratio of the "magnetomotive force” (MMF) in a magnetic circuit to the magnetic flux in this circuit. In a pulsating DC or AC field, the reluctance is the ratio of the amplitude of the "magnetomotive force” (MMF) in a magnetic circuit to the amplitude of the magnetic flux in this circuit. (see phasors)

S = N I / Ф, F =NI where S is the reluctance in ampere-turns per weber (a unit that is equivalent to turns per henry).

F is the magnetomotive force (MMF) in ampere-turns Ф is the magnetic flux in webers.

"Turns" refers to the winding number of an electrical conductor comprising an inductor 1/Henry = ampere/ weber, Henry= Weber/ampere?

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http://en.wikipedia.org/wiki/Magnetic_reluctance

 Henry, unit of either self-inductance or mutual inductance, abbreviated h, and named for the American physicist Joseph Henry. One henry is the value of self-inductance in a closed circuit or coil in which one volt is produced by a variation of the inducing current of one ampere per second.

One henry is also the value of the mutual inductance of two coils arranged such that an electromotive force of one volt is induced in one if the current in the other is changing at a rate of one ampere per second.  Weber, unit of magnetic flux in the International System of Units (SI), defined as the amount of flux that, linking an electrical circuit of one turn (one loop of wire, N=1), produces in it an electromotive force (E) of one volt as the flux is reduced to zero at a uniform rate in one second.  Tesla, a flux density of one Wb/m2 (one weber per square metre) is one tesla.

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http://global.britannica.com/EBchecked/topic/261372/henry

L, Henry – H, The inductance of an electric circuit is one henry when an electric current that is changing at one ampere per second results in an electromotive force across the inductor of one volt. Ф, Weber – Wb, magnetic flux B, Tesla – T, magnetic flux density (1 weber / m2) = 10000 Gauss S, Reluctance – Ampere Turn/ weber NI/Ф F, Magnetomotive force – Ampere Turn H, Magnetic field intensity – Amperes per meter (symbol: A·m-1 or A/m) μ, permeability – B/H, henries per meter (H·m-1), or newtons per ampere squared (N·A-2).

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http://global.britannica.com/EBchecked/topic/261372/henry

A magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where ■ H is measured in units of amperes per meter (symbol: A·m-1 or A/m) in the SI. ■ B is measured in teslas (symbol:T) and newtons per meter per ampere [symbol: N·m-1·A-1 or N/(m·A)] in the SI. (1 teslas = 10000 Gauss) B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges. Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.

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https://en.wikipedia.org/wiki/Magnetic_field

Weber (Magnetic Flux Ф)

In physics, specifically electromagnetism, the magnetic flux (often denoted Φ or ΦB) through a surface is the surface integral of the normal component of the magnetic field B passing through that surface.

■ The SI unit of magnetic flux is the weber (Wb) (in derived units: voltseconds), and the CGS unit is the maxwell.

Magnetic flux is usually measured with a fluxmeter, which contains measuring coils and electronics, that evaluates the change of voltage in the measuring coils to calculate the magnetic flux.

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https://en.wikipedia.org/wiki/Magnetic_flux

Gauss (Magnetic Flux Density) gauss, unit of magnetic induction in the centimetre-gram-second CGS system of physical units. One gauss corresponds to the magnetic flux density that will induce an electromotive force of one abvolt (10-8 volt) in each linear centimetre of a wire moving laterally at one centimetre per second at right angles to a magnetic flux. One gauss corresponds to 10-4 tesla (T), the International System Unit. The gauss is equal to 1 maxwell per square centimetre, or 10-4 weber per square metre. Magnets are rated in gauss. The gauss was named for the German scientist Carl Friedrich Gauss.

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http://global.britannica.com/science/gauss

Electrical Inductance - Henry The henry (symbol H) is the unit of electrical inductance in the International System of Units. The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

The magnetic permeability of a vacuum μo is 4π×10-7 H m-1 (henries per metre).

The National Institute of Standards and Technology provides guidance for American users of SI to write the plural as "henries". The inductance of an electric circuit is one henry when an electric current that is changing at one ampere per second results in an electromotive force across the inductor of one volt:

v(t) = L di/dt where v(t) denotes the resulting voltage across the circuit, i(t) is the current through the circuit, and L is the inductance of the circuit.

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https://en.wikipedia.org/wiki/Henry_(unit)

Magnetic Permeability (B/H) In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Hence, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter μ. The term was coined in September 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.

In SI units, permeability is measured in henries per meter (H·m-1), or newtons per ampere squared (N·A-2). The permeability constant (μ0), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined) value µ0 = 4π×10-7 H·m-1≈ 1.2566370614…×10−6 H·m-1 or N·A-2).

A closely related property of materials is magnetic susceptibility, which is a dimensionless proportionality factor that indicates the degree of magnetization of a material in response to an applied magnetic field.

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https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

More Reading on: Magnetic field A magnetic field is the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, Where: H: (magnetic field intensity) is measured in units of amperes per meter (symbol: A路m-1 or A/m) in the SI. B: (magnetic flux density) is measured in teslas (symbol: T) and newtons per meter per ampere (symbol: N路m-1路A-1 or Newtons per Ampere meter N/(m路A)) or weber/m2 in the SI. B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges.

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http://en.wikipedia.org/wiki/Magnetic_field

 Henry, unit of either self-inductance or mutual inductance, abbreviated h, and named for the American physicist Joseph Henry. One henry is the value of self-inductance in a closed circuit or coil in which one volt is produced by a variation of the inducing current of one ampere per second.

One henry is also the value of the mutual inductance of two coils arranged such that an electromotive force of one volt is induced in one if the current in the other is changing at a rate of one ampere per second.  Weber, unit of magnetic flux in the International System of Units (SI), defined as the amount of flux that, linking an electrical circuit of one turn (one loop of wire, N=1), produces in it an electromotive force (E) of one volt as the flux is reduced to zero at a uniform rate in one second.  Tesla, (B: magnetic flux density) a flux density of one Wb/m2 (one weber per square metre) is one tesla.  H: (magnetic field intensity) is measured in units of amperes per meter (symbol: A·m-1 or A/m) in the SI.

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http://global.britannica.com/EBchecked/topic/261372/henry

The weber The weber may be defined in terms of Faraday's law, which relates a changing magnetic flux through a loop to the electric field around the loop. A change in flux of one weber per second will induce an electromotive force of one volt (produce an electric potential difference of one volt across two opencircuited terminals).

Officially, Weber (unit of magnetic flux) - The weber is the magnetic flux which, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second

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https://en.wikipedia.org/wiki/Weber_(unit)

Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.(?)

In everyday life, magnetic fields are most often encountered as a force created by permanent magnets, which pull on ferromagnetic materials such as iron, cobalt, or nickel and attract or repel other magnets. Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. The Earth produces its own magnetic field, which is important in navigation, and it guards Earth's atmosphere from solar wind. Rotating magnetic fields are used in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits.

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http://en.wikipedia.org/wiki/Magnetic_field

History: Although magnets and magnetism were known much earlier, the study of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles Noting that the resulting field lines crossed at two points he named those points 'poles' in analogy to Earth's poles. He also clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them.

Almost three centuries later, William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet Published in 1600, Gilbert's work, De Magnete, helped to establish magnetism as a science.

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http://en.wikipedia.org/wiki/Magnetic_field

In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law. Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated (dipoles) . Building on this force between poles, Siméon Denis Poisson (1781–1840) created the first successful model of the magnetic field, which he presented in 1824. In this model, a magnetic H-field is produced by 'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles.

Comment: H-field – magnetic field intensity? Relates to A∙m-1

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http://en.wikipedia.org/wiki/Magnetic_field

Three discoveries challenged this foundation of magnetism, though.

First, in 1819, Hans Christian Oersted discovered that an electric current generates a magnetic field encircling it. Then in1820, André-Marie Ampère showed that parallel wires having currents in the same direction attract one another. Finally, Jean-Baptiste Biot and Félix Savart discovered the BiotSavart law in 1820, which correctly predicts the magnetic field around any current- carrying wire.

Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model. This has the additional benefit of explaining why magnetic charge can not be isolated. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, which, like the Biot–Savart law, correctly described the magnetic field generated by a steady current. Also in this work, Ampère introduced the term electrodynamics to describe the relationship between electricity and magnetism.

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http://en.wikipedia.org/wiki/Magnetic_field

In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field. He described this phenomenon in what is known as Faraday's law of induction.

Later, Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law. In the process he introduced the magnetic vector potential, which was later shown to be equivalent to the underlying mechanism proposed by Faraday.

In 1850, Lord Kelvin, then known as William Thomson, distinguished between two magnetic fields now denoted H and B. The former applied to Poisson's model and the latter to Ampère's model and induction. Further, he derived how H and B relate to each other.

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http://en.wikipedia.org/wiki/Magnetic_field

Between 1861 and 1865, James Clerk Maxwell developed and published Maxwell's equations, which explained and united all of classical electricity and magnetism. The first set of these equations was published in a paper entitled On Physical Lines of Force in 1861. These equations were valid although incomplete. Maxwell completed his set of equations in his later 1865 paper A Dynamical Theory of the Electromagnetic Field and demonstrated the fact that light is an electromagnetic wave. Heinrich Hertz experimentally confirmed this fact in 1887.

The twentieth century extended electrodynamics to include relativity and quantum mechanics. Albert Einstein, in his paper of 1905 that established relativity, showed that both the electric and magnetic fields are part of the same phenomena viewed from different reference frames. (See moving magnet and conductor problem for details about the thought experiment that eventually helped Albert Einstein to develop special relativity.) Finally, the emergent field of quantum mechanics was merged with electrodynamics to form quantum electrodynamics (QED).

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http://en.wikipedia.org/wiki/Magnetic_field

The B-field: The magnetic field can be defined in several equivalent ways based on the effects it has on its environment.

Often the magnetic field is defined by the force it exerts on a moving charged particle. It is known from experiments in electrostatics that a particle of charge q in an electric field E experiences a force

F = q·E. However, in other situations, such as when a charged particle moves in the vicinity of a current-carrying wire, the force also depends on the velocity of that particle. Fortunately, the velocity dependent portion can be separated out such that the force on the particle satisfies the Lorentz force law,

F = q·(E + v × B) Here v is the particle's velocity and × denotes the cross product. The vector B is termed the magnetic field, and it is defined as the vector field necessary to make the Lorentz force law correctly describe the motion of a charged particle.

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http://en.wikipedia.org/wiki/Magnetic_field

Lorentz Force Law Both the electric field and magnetic field can be defined from the Lorentz force law: The electric force is straightforward, being in the direction of the electric field if the charge q is positive, but the direction of the magnetic part of the force is given by the right hand rule.

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http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html

Electric Force

F = q·E The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge:

The implications of this expression include: 1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B. 2. The magnitude of the force is F = q∙v∙Bsinϴ where ϴ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero.

3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product.

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http://en.wikipedia.org/wiki/Magnetic_field

From the force relationship above it can be deduced that the units of magnetic are Newton seconds /(Coulomb meter) or Newtons per Ampere meter. This unit is named the Tesla. It is a large unit, and the smaller unit Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is on the order of half a Gauss.

Keywords: Fmagnetic = q∙v∙B sinϴ The magnitude of the force is F = q∙v∙B sinϴ where ϴ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero.

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http://en.wikipedia.org/wiki/Magnetic_field

The H-field In addition to B, there is a quantity H, which is also sometimes called the magnetic field. In a vacuum, B and H are proportional to each other, with the multiplicative constant depending on the physical units.

Inside a material they are different (see H and B inside and outside of magnetic materials). The term "magnetic field" is historically reserved for H while using other terms for B. Informally, though, and formally for some recent textbooks mostly in physics, the term 'magnetic field' is used to describe B as well as or in place of H. There are many alternative names for both Comments: H: (magnetic field intensity) is measured in units of amperes per meter (symbol: A路m-1 or A/m) in the SI. B: (magnetic flux density) is measured in teslas (symbol: T) and newtons per meter per ampere (symbol: N路m-1路A-1 or Newtons per Ampere meter N/(m路A)) or weber/m2 in the SI.

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http://en.wikipedia.org/wiki/Magnetic_field

Alternative names for B   

Magnetic flux density Magnetic induction Magnetic field

Alternative names for H    

Magnetic field intensity Magnetic field strength Magnetic field Magnetizing field

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http://en.wikipedia.org/wiki/Magnetic_field

Fleming Right Hand Rule

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http://en.wikipedia.org/wiki/Magnetic_field

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http://www.encyclopedia-magnetica.com/doku.php/magnetism

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http://www.electronenergy.com/magnetic-design/magnetic-design.htm

Reluctance S is the sum of the reluctance Sg of air gaps (between the test object and the yoke), test object reluctance Ss and yoke reluctance Sy. The reluctance values of the air gaps, test object and yoke are given by Eq. 2 to 4: (2)

(4)

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(3)

(2)

(3)

(4) Where: • ax is the cross sectional area (square meter) of the air gaps, test object or yoke; • Lx is the length (meter) of the air gaps, test object or yoke; μ0 is the permeability of free space (μ0 = 4π.10–7 H·m–1); μr is relative permeability; and subscripts g, s and y denote the air gaps, test object and yoke, respectively.

Note that the magnetic circuit consists of two air gaps, one at each end of the test object. Both air gaps need to be taken into account in calculating the total reluctance of the magnetic circuit.

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Reluctance S

Reluctance, S = Length L / (cross sectional area a â&#x2C6;&#x2122; permeability Îź)

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To obtain maximum sensitivity, it is necessary to ensure that the magnetic flux is perpendicular to the discontinuity. This direction is in contrast to the orientation in techniques that use an electric current for inspection of a test object, where it may be more advantageous to orient the direction of current so that a discontinuity would impede the current as much as possible.

Because the orientation of the discontinuity is unknown, it is necessary to test twice with the yoke, in two directions perpendicular to each other. A grid is usually drawn on the test object to facilitate the tests.

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9.2.1

Magnetizing Coil

A commonly used encircling coil is shown in Fig. 2. The field direction follows the right hand rule. (The right hand rule states that, if someone grips a rod, holds it out and imagines an electric current flowing down the thumb, the induced circular field in the rod would flow in the direction that the fingers point.) With no test object present, the field lines form closed loops that encircle the current carrying conductors. The value of the field at any point has been established for a great many coil configurations. The value depends on the current in the coils, the number of turns N and a geometrical factor.

Calculation of the field from first principles is generally unnecessary for nondestructive testing; a hall element tesla meter will measure this field.

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FIGURE 2. Encircling coil using direct current to produce magnetizing force.

Legend I = electric current P, Q = points of discontinuities in example R = point at which magnetic field intensity H is measured S = point at which magnetic flux density B is measured

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Introduction of the test object into the field of the coil changes the field. The metal becomes part of the magnetic circuit, with the result that, close to the surface of the test object, magnetic field intensity H is lower than it would be if the test object were removed. Again, a hall element tesla meter will show the field intensity at the test object. This reduces the need for semi-empirical formulas. With the test object inserted, the flux density changes and the flux lines get concentrated within the test object. Thus, the fields inside and outside the test object are not the same. However, two boundary conditions allow assessment of the magnetic state of the test object. The fact that the tangential field is continuous across the air-to-metal interface allows measurement of H at the point R to yield the value of the tangential field at the test surface. In addition, because the normal component of magnetic flux density B is continuous, a tesla meter at point S will yield B inside the test object at that point. Two totally different situations, common in magnetic flux leakage testing, are described below.

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9.2.1.1 Testing in Active Field In this technique, the test object is scanned by probes near position R in Fig. 2, in the presence of an active field. Air fields of 16 to 24 kA¡mâ&#x20AC;&#x201C;1 (200 to 300 Oe) are commonly used. In this situation, application of small fields is sufficient to cause magnetic flux leakage from transversely oriented surface breaking discontinuities. For subsurface discontinuities or those on the inside surface of tubes, larger fields are required. The inspector must experiment to optimize the applied field for the particular discontinuity.

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9.2.1.2 Testing in Residual Field Test objects are first passed through the coil field and then tested in the resulting residual field. Elongating the coil and placing the test object next to the inside surface of the coil will expose the test object to the largest field that the coil can produce. This technique is often used in magnetic particle testing.

The main problem to avoid is the induction of so much magnetic flux in the test object that the magnetic particles stand out like fur along the field lines that enter and leave the test object, especially close to its ends. Optimum conditions require that the test object be somewhat less than saturated. The inspector should experiment to optimize the coil field requirements for the test object because this field depends on test object geometry.

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9.2.2

Applied Direct Current

If an electric current is used to magnetize the test object, it may be more advantageous to orient the direction of current in a manner where the presence of a discontinuity impedes the current flow as much as possible.

Bars, billets and tubes are often magnetized by application of a direct current I to their ends (Fig. 3). Figure 4 shows a system where the current I is passed directly through a tubular test object to magnetize the test object circularly.

Figure 5 shows a central conductor energized by a current source I, again, to establish a circular magnetic field intensity H (ampere per square meter) in a tubular test object: (5) where a is area (square meter).

Question: a or r?

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FIGURE 3. Circumferential magnetization by application of direct current: (a) rectilinear bar; (b) round bar; (c) tube.

Legend H = magnetic field intensity I = electric current

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FIGURE 4. Current carrying clamp electrodes used for testing ferromagnetic tubular objects with small diameters.

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FIGURE 5. Simple technique for circumferential magnetization of ferromagnetic tube.

Legend H = magnetic field intensity I = electric current r = tube radius Charlie Chong/ Fion Zhang

9.2.2.1 Capacitor Discharge Devices For the circular magnetization of tubes or the longitudinal magnetization of the ends of elongated test objects, a capacitor discharge device is sometimes used. The capacitor discharge unit represents a practical advance over battery packs and consists of a capacitor bank charged to a voltage V and then discharged through a rod, a cable and a silicon controlled rectifier of total resistance R. The full system, considered mathematically, also contains a variable amount of inductance, so that if the current Ic were allowed to oscillate, it would do so according to the theory of LCR circuits (that is, circuits described by inductance L, capacitance C and resistance R). The theory is complicated by the time required to magnetize the material and to induce an eddy current in the test object.

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Typical configurations shown in Fig. 6 illustrate the complexity of the situation.

In the case of the magnetization of a tube, the current Ic first rises rapidly, inducing magnetic flux in the tube. This time varying flux changes rapidly and induces an electromotive force in the tube, as dictated by Faradayâ&#x20AC;&#x2122;s law, the result being that an eddy current Ie flows around the tube as shown in Fig. 6a, where the dashed line is the inner surface eddy current and the solid line is the outer surface current.

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FIGURE 6. Capacitor discharge configurations causing magnetization perpendicular to current direction: (a) conductor internal to test object creates circular field; (b) flexible cable around test object creates longitudinal field.

Legend C = capacitor Ic = capacitor discharge current Ie = eddy current SCR = silicon controlled rectifier

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FIGURE 6. Capacitor discharge configurations causing magnetization perpendicular to current direction: (a) conductor internal to test object creates circular field; (b) flexible cable around test object creates longitudinal field.

Legend C = capacitor Ic = capacitor discharge current Ie = eddy current SCR = silicon controlled rectifier

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The net result is a lack of penetration of the field caused by the capacitor discharge current Ic. For a centered rod, in effect, the magnetic field intensity in the test object at radius r is given not by H = Ic·(2πr)–1 but rather by Eq. 6: (6) Here Ie is the amount of eddy current (ampere) contained within the cylinder of radius r (meter). Investigation of the effect of the eddy current is theoretically quite complicated because of its effect on the inductance, which in turn affects Ic. In practice, however, measurement of the magnetic flux density B in the material will yield the final degree of magnetization of that material.

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A good rule is that, if H(r) in Eq. 6 can be maintained at about 3.2 kA·m–1 (40 Oe), the material will be magnetized almost to saturation and can be tested for both surface and subsurface discontinuities.

Several other practical conclusions can be drawn from the above discussion.

• • • •

Pulse duration plays a greater role than pulse amplitude Ic(max) in determining the amount of flux induced in a test object. This is intuitively seen in direct current tests.

It is not possible to give simple rules that relate Ic(max) to magnetization requirements. This relationship can be shown with a magnetic flux meter.

The eddy currents induced during pulse magnetization play an important role in the result. They can shield midwall regions from magnetization.

Larger capacitances at lower voltages provide better magnetization than smaller capacitances at higher voltages because larger capacitances at lower voltages lead to longer duration pulses and therefore to lower eddy currents. The lower voltage is an essential safety feature for outdoor use.

A maximum of 50 V is recommended.

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9.2.3

Magnitudes of Magnetic Flux Leakage Fields

The magnitude of the magnetic flux leakage field under active direct current excitation naturally depends on the applied field. An applied field of 3.2 to 4.0 kA¡mâ&#x20AC;&#x201C;1 (40 to 50 Oe) inside the material can cause leakage fields with peak values of tens of millitesla (hundreds of gauss). However, in the case of residual induction, the magnetic flux leakage fields may be only a few hundred microtesla (a few gauss). Furthermore, with residual field excitation, an interesting field reversal may occur, depending on the value of the initial active field excitation and the dimensions of the discontinuity.

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9.2.4 Optimal Operating Point Consider raising the magnetization level in a block of steel containing a discontinuity (Fig. 7). At low flux density levels, the field lines tend to crowd together in the steel around the discontinuity rather than go through the nonmagnetic region of the discontinuity. The field lines are therefore more crowded above and below the discontinuity than they are on the left or right.

The material can hold more flux as the permeability rises, so there is no significant leakage flux at the surfaces (Fig. 7a). However, an increase in the number of lines causes ΔB·(ΔH)–1 to fall — the material is becoming less permeable. At about this point, magnetic flux leakage is first noticed at the surfaces. Although the lines are now closer together, representing a higher magnetic flux density, they do not have the ability to crowd closer together around the discontinuity where the permeability is low.

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FIGURE 7. Effects of induction on magnetic flux lines at discontinuity: (a) no surface flux leakage occurs where magnetic flux lines are compressed at low levels of induction around discontinuity; (b) lack of compression at high magnetization results in surface magnetic flux leakage.

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At higher and higher values of applied field, the permeability falls. It is, however, still large compared to the permeability of air, so the reluctance of the path through the discontinuity is still larger than through the metal. As a result, magnetic flux leakage at the outside surface helps provide a sufficiently high flux density in the material for the leakage of magnetic flux from discontinuities (Fig. 7b) while partially suppressing long range surface noise. For residual field testing, it is best to ensure that the material is saturated. The magnetic field starts to decay as soon as the energizing current is removed.

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The Great Rationalizer

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http://www.heitu5.com/kehuan/mojingxianzong/player-0-0.html

9.3

PART 3. Magnetic Flux Leakage Test Results

9.3.0

Introduction

Magnetic flux leakage testing continues to be one of the most popular nondestructive test techniques in industry. A number of factors, including low cost and simplicity of the data interpretation process, contribute to this popularity. The underlying principles and modeling techniques are described elsewhere in this volume. The discussion below focuses on probes and excitation schemes to detect and measure magnetic leakage fields.

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9.3.1

Magnetic Flux Leakage Probes

The purpose of probes for magnetic testing is to detect and possibly quantify the magnetic flux leakage field generated by heterogeneities in the test object.

The leakage fields tend to be local and concentrated near the discontinuities.

The leakage field can be divided into three orthogonal components: normal (vertical), tangential (horizontal) and axial directions. Probes are usually either designed or oriented to measure one of these components. Typical plots of these components near discontinuities are shown in this volumeâ&#x20AC;&#x2122;s chapter on probes. A variety of probes (or transducers) are used in industry for detecting and measuring leakage fields.

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Magnetic Flux Leakage fields.

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Magnetic Flux Leakage fields.

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BS EN 10246-5:2000 MFLT Set-up

1: Transducer 2: Tube 3: Rotating Magnet & Transducer

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BS EN 10246-5:2000

Magnetic Flux Leakage fields.

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http://www.railwaystrategies.co.uk/article-page.php?contentid=9524&issueid=303

MFLT- Expert at Work

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MFLT- Expert at Works

MFLT- Expert at Works

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http://www.puretechltd.com/articles/newsletter/2012/03/California_MFL.shtml

MFLT- Expert at Works

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http://www.puretechltd.com/articles/newsletter/2012/03/California_MFL.shtml

The most commonly used in-service inspection tools utilize the Magnetic Flux Leakage (MFL) technique in order to detect internal or external corrosion. The MFL inspection pig uses a circumferential array of MFL detectors embodying strong permanent magnets to magnetize the pipe wall to near saturation flux density. Abnormalities in the pipe wall, such as corrosion pits, result in magnetic flux leakage near the pipe's surface. These leakage fluxes are detected by Hall probes or induction coils moving with the MFL detector. The demands now being placed on magnetic inspection tools are shifting from the mere detection, location and classification of pipeline defects, to the accurate measurements of defect size and geometry. Modern, high-resolution MFL inspection tools are capable of giving very detailed signals. However, converting these signals to accurate estimates of size requires considerable expertise, as well as a detailed understanding of the effects of inspection conditions and the magnetic behaviour of the type of pipeline steel used.

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http://www.physics.queensu.ca/~amg/expertise/inline.html

Magnetic Flux Leakage fields.

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http://www.physics.queensu.ca/~amg/expertise/inline.html

Magnetic Flux Leakage fields.

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http://www.physics.queensu.ca/~amg/expertise/inline.html

Intelligence Pigging with MFLT

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http://www.physics.queensu.ca/~amg/expertise/inline.html

Intelligence Piggy

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http://www.physics.queensu.ca/~amg/expertise/inline.html

9.3.1.1 Pickup Coils One of the simplest and most popular means for detecting leakage fields is to use a pickup coil.6 Pickup coils consist of very small coils that are either air cored or use a small ferrite core. The voltage induced in the coil is given by the rate of change of flux linkages associated with the pickup coil.

(7) Where: N is the number of turns in the coil, V is the voltage induced in the coil and Ф is the magnetic flux (weber) linking the coil.

It must be mentioned that only the component of the flux parallel to the axis of the coil (or alternately perpendicular to the plane of the coil) is instrumental in inducing the voltage.

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This induction direction makes it possible to orient the pickup coil so as to measure any of the three leakage field components selectively. Thus, a coil A whose axis is perpendicular to the surface of the test object (Fig. 8a), is sensitive only to the normal component. In contrast, the coil in Fig. 8b is sensitive only to the tangential component. Consider the case where the pickup coil is moving over the test object in the X direction. Making use of the fact that 肖 = B路A, where B is the magnetic flux density (tesla) and A is the cross sectional area (square meter) of the pickup coil, Eq. 7 can be rewritten:

(8)

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FIGURE 8. Effect of pickup coil orientation on sensitivity to components of magnetic flux density: (a) coil sensitive to normal component; (b) coil sensitive to tangential component.

(a)

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(b)

It must be mentioned that only the component of the flux parallel to the axis of the coil (or alternately perpendicular to the plane of the coil) is instrumental in inducing the voltage. (a)

plane of the coil axis of the coil

plane of the coil

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(b)

axis of the coil

It must be mentioned that only the component of the flux parallel to the axis of the coil (or alternately perpendicular to the plane of the coil) is instrumental in inducing the voltage. (b) flux (a)

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flux

This equation indicates that the output of the pickup coil is proportional to the spatial gradient of the flux along the direction of the coil movement as well as the velocity of the coil. Two issues arise as a result.

1. It is essential that the probe scan velocity (relative to the test object) should be constant to avoid introducing artifacts into the signal through probe velocity variations.

2. The output is proportional to the spatial gradient of the flux in the direction of the coil. The output of the pickup coil can be integrated for measurement of the leakage flux density rather than of its gradient.

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Figure 9 shows the output of a pickup coil and the signal obtained after integrating the output. The coil is used to measure, in units of tesla (or gauss), the magnetic flux density B leaking from a rectangular slot. The sensitivity of the pickup coil can be improved by using a ferrite core. Tools for designing pickup coils, as well as predicting their performance, are described elsewhere in this volume.

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FIGURE 9. Pickup coil and signal integrator (magnetic flux leakage) output for rectangular discontinuity.

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Signal and Magnetic Disturbances

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Magnetic Flux Leakage & Signals

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Magnetic Flux Leakage & Signals

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Magnetic Flux Leakage & Signals

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9.3.1.2 Magnetodiodes The magnetodiode is suitable for sensing leakage fields from discontinuities because of its small size and its high sensitivity. Because the coil probe is usually larger than the magnetodiode, it is less sensitive to longitudinally angled discontinuities than the magnetodiode is. However, the coil probe is better than the magnetodiode for large discontinuities, such as cavities.

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Magnetodiodes

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http://www.craft-3.com/Semiconductor/SONY_Transistor/sony_diode.html

9.3.1.3 Hall Effect Detectors Hall effect detector probes are used extensively in industry for measuring magnetic flux leakage fields in units of tesla (or gauss). Hall effect detector probes are described in this volumeâ&#x20AC;&#x2122;s chapter on probes for electromagnetic testing.

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Hall Effect Detectors

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Hall Effect Detectors

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http://movableparts.org/rear-wheel-tachometer/

9.3.1.4 Giant Magnetoresistive Probes Magnetic field sensitive devices called giant magnetoresistive probes, at the most basic level, consist of a nonmagnetic layer sandwiched between two magnetic layers. The apparent resistivity of the structure varies depending on whether the direction of the electron spin is parallel or antiparallel to the moments of the magnetic layers. When the moments associated with the magnetic layers are aligned antiparallel, the electrons with spin in one direction (up) that are not scattered in one layer will be scattered in the other layer. This increases the resistance of the device.

This is in contrast to the situation when the magnetic moments associated with the layers are parallel where the electrons that are not scattered in one layer are not scattered in the other layer, either. Giant magnetoresistive probes use a biasing current to push the magnetic layers into an antiparallel moment state and the external field is used to overcome the effect of the bias.

The resistance of the device, therefore, decreases with increasing field intensity values.

Figure 10 shows a typical response of a giant magnetoresistive probe.

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Keywords: The resistance of the device, therefore, decreases with increasing field intensity values.

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FIGURE 10. Resistance versus applied field for 2 Îźm (8 . 10â&#x20AC;&#x201C;5 in.) wide strip of anti-ferromagnetically coupled, multilayer test object composed of 14 percent giant magnetoresistive material.

Charlie Chong/ Fion Zhang

More Reading on: Giant Magnetoresistive Probes There are better alternatives to detect pneumatic cylinder end of stroke position than reed switches or proximity switches. By better, I mean they are faster and easier to implement into your control system. In addition, you can realize other benefits such as commonality of spare sensors and lower longterm costs. So what are the better solutions? Three types of sensor technologies lead the way to better alternatives. First, there is the Hall Effect magnetic field sensor, see figure 1.

The benefit of Hall Effect sensors is speed; they are electronic so there are no moving parts and nothing to wear out.

They are not affected by shock and vibration unlike the reed switch.

figure 1 Charlie Chong/ Fion Zhang

https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

However, there are some disadvantages of Hall Effects such as they typically require fairly high magnetic gauss strength and they require a radially magnetized magnet. Typically, a Hall Effect will not work as a replacement of a reed switch or if it does operate, it may produce double switch points. A Hall Effect sensor is looking for a single magnetic pole, so if it is used with an axially magnetized magnet, it will switch when it sees the north pole and then again with the south pole, thus causing the double switch points.

The second and newer technology is the magnetoresistive sensor shown in figure 2 or sometimes referred to as AMR (Anisotropic magnetoresistance).

Unlike the Hall Effect sensor that uses a change in voltage the AMR is based off a change in resistance. This change in resistance is more sensitive thus; a lower strength magnet can be utilized. The best advantage of the AMR sensor is that it will work with the axially magnetized magnet and in most cases the radially magnetized magnet. Like the Hall Effect, the AMR has no moving parts and nothing to wear out and is fast therefore it is a good solution for high-speed applications. The magnetoresistive sensor does not suffer from double switch points and has a much better noise immunity than Hall Effects.

Charlie Chong/ Fion Zhang

Figure 2:

Charlie Chong/ Fion Zhang

https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

Giant Magnetoresistive or GMR sensors shown in figure 3 are technologically the newer of the magnetic field sensors. They operate on a change in resistance, as does the AMR, however; the magnetic field causes a larger or giant change in resistance. Although you would think the GMR sensors are physically larger than the AMR, they are actually smaller. Major advantages of the GMR sensor are they are more sensitive, are more precise and have a better hysteresis than the AMR.

Charlie Chong/ Fion Zhang

https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

Giant Magnetoresistive Probes

Charlie Chong/ Fion Zhang

https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

Okay so the AMR and GMR sensors seem to be the better or even the best solution. Are there other advantages to them? Higher quality sensor manufacturers offer better output circuitry that includes reverse polarity protection, overload protection and short circuit protection. Couple that with lifetime warranty offered on some manufacturerâ&#x20AC;&#x2122;s sensors and you end up with a better alternative to the pneumatic cylinder end of stroke sensor.

I know what you are thinking there must be some negatives. The initial cost of the AMR or GMR sensor may be slightly more than the reed sensor however this cost is becoming less and less and it is even less once you figure the cost of down time after your reed switch fails or the proximity flag is moved. In addition, the AMR and GMR sensors are 3-wire devices unlike the 2-wire reed switch. However, in the end the AMR and GMR sensors are still the better solution.

Charlie Chong/ Fion Zhang

https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

9.3.1.5 Magnetic Tape For the testing of flat surfaces, magnetic tape can be used. The tape is pressed to the surface of the magnetized billet and then scanned by small probes before being erased. This technique is sometimes called magnetography. In automated systems, magnetic tape can be fed from a spool. The signals can be read and the tape can be erased and reused.

Unfortunately, the tangential leakage field intensity at the surface of the material is not constant. To optimize the response, the amplification of the signals can be varied. Scabs or slivers projecting from the test surface can easily tear the tape

Charlie Chong/ Fion Zhang

9.3.2

Magnetic Particles

Magnetic particles are one of the most popular means used in industry for detecting magnetic fields. Indeed, magnetic particle testing is so popular that an entire volume of the Nondestructive Testing Handbook is devoted to the subject. The descriptions below are therefore cursory ç˛&#x2014;ç&#x2022;Ľç&#x161;&#x201E;. Magnetic particle testing involves the application of magnetic particles to the test object after it is magnetized by using an appropriate technique. The ferromagnetic particles preferentially adhere to the surface of the test object in areas where the flux is diverted, or leaks out. The magnetic flux leakage near discontinuities causes the magnetic particles to accumulate in the region and in some cases form an outline of the discontinuity. Heterogeneities can therefore be detected by looking for indications of magnetic particle accumulations on the surface of the test object either with the naked eye or through a camera. The indications are easier to see if the particles are bright and reflective. Alternately, particles that fluoresce under ultraviolet or visible radiation may be used. The test object has to be viewed under appropriate levels of illumination with radiation of appropriate wavelength (visible, ultraviolet or other).

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9.3.2.1 Application Techniques Magnetic particles are applied to the surface by two different techniques in industry.

(A) Dry Testing.

Dry techniques use particles applied in the form of a fine stream or an aerosol.

They consist of high permeability ferromagnetic particles coated with either reflective or fluorescent pigments. The particle size is chosen according to the dimensions of the discontinuity sought. Particle diameters range from ≤50μm to 180 μm (≤0.002 to 0.007 in.). Finer particles are used for detecting smaller discontinuities where the leakage intensity is low. Dry techniques are used extensively for testing welds and castings where heterogeneities of interest are relatively large.

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(B) Wet Testing.

Wet techniques are used for detecting relatively fine cracks. The magnetic particles are suspended in a liquid (usually oil or water) usually sprayed on the test object. Particle sizes are significantly smaller than those used with dry techniques and vary in size within a normal distribution, with most particles measuring from 5 to 20μm (2·10–4 to 8·10–4 in.). As in the case of dry powders, the ferromagnetic particles are coated with either reflective or fluorescent pigments. More information on this subject is available elsewhere.

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MPI

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9.3.2.2 Imaging of Magnetic Particle Indications The magnetic particle distribution can be examined visually after illuminating the surface or the surface can be scanned with a flying spot system or imaged with a charge coupled device camera.

(A) Flying Spot Scanners.

To illuminate the test object (Fig. 11), flying spot scanners use a narrow beam of radiation - visible light for non-fluorescent particles and ultraviolet radiation for fluorescent ones. The source of the beam is usually a laser. The wavelength of the beam is chosen carefully to excite the pigment of the magnetic particles. The incidence of the radiation beam on the test object can be varied by moving the scanning mirror. The photocell does not sense any light when the test object is scanned by the narrow radiation beam until the beam is directly incident on the magnetic particles adhering to the test object near a discontinuity. When this occurs, a large amount of light is emitted, called fluorescence if excited by ultraviolet radiation. The fluorescence is detected by a single phototube equipped with a filter that renders the system blind to the radiation from the irradiating source. The output of the photocell is suitably amplified, digitized and processed by a computer.

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FIGURE 11. Flying spot scanner for automated magnetic particle testing.

Charlie Chong/ Fion Zhang

FIGURE 11. Flying spot scanner for automated magnetic particle testing.

filter that renders the system blind to the radiation from the irradiating source Charlie Chong/ Fion Zhang

(B) Charge Coupled Devices CCD. An alternative approach is to flood the test object with radiation whose wavelength is carefully chosen to excite the pigment of the magnetic particles.

Charge coupled device cameras, equipped with optical filters that render the camera blind to radiation from the source but are transparent to light emitted by the magnetic particles, can be used to image the surface very rapidly. In very simple terms, charge coupled devices each consist of a two dimensional array of tiny pixels that each accumulates a charge corresponding to the number of photons incident on it. When a readout pulse is applied to the device, the accumulated charge is transferred from the pixel to a holding or charge transfer cell. The charge transfer cells are connected in a manner that allows them to function as a bucket brigade or shift register. The charges can, therefore, be serially clocked out through a charge-to-voltage amplifier that produces a video signal.

Charlie Chong/ Fion Zhang

In practice, charge coupled device cameras can be interfaced to a personal computer through frame grabbers, which are commercially available. Vendors of frame grabbers usually provide software that can be executed on the personal computer to process the image. Image processing software can be used for example to improve contrast, highlight the edges of discontinuity or to minimize noise in the image.

Charlie Chong/ Fion Zhang

CCD

Charlie Chong/ Fion Zhang

http://oneslidephotography.com/ccd-vs-cmos-dslr-camera-wich-one-is-better/

CCD

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CCD

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http://www.smartinfoblog.com/cmos-vs-ccd-sensor/

CCD

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http://www.rocketroberts.com/astro/ccd_fundamentals.htm

9.3.3

Test Calculations

In determining the magnetic flux leakage from a discontinuity, certain conditions must be known: 1. the discontinuityâ&#x20AC;&#x2122;s location with respect to the surfaces from which measurements are made, 2. the relative permeability of the material containing the discontinuity and 3. the levels of magnetic field intensity H and magnetic flux density B in the vicinity of the discontinuity.

Even with this knowledge, the solution of the applicable field equations (derived from Maxwellâ&#x20AC;&#x2122;s equations of electromagnetism) is difficult and is generally impossible in closed algebraic form. Under certain circumstances, such as those of discontinuity shapes that are easy to handle mathematically, relatively simple equations can be derived for the magnetic flux leakage if simplifying assumptions are made. This simplification does not apply to subsurface inclusions.

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9.3.3.1 Finite Element Techniques An advance in magnetic theory since 1980 has been the introduction of finite element computer codes to the solution of magnetostatic problems. Such codes came originally from a desire to minimize electrical losses from electromagnetic machinery but soon found application in magnetic flux leakage theory. The advantage of such codes is that, once set up, discontinuity leakage fields can be calculated by computer for any size and shape of discontinuity, under any magnetization condition, so long as the B,H curve for the material is known. In the models of magnetic flux leakage discussed so far, the implicit assumptions are (1) that the field within a discontinuity is uniform and (2) that the nonlinear magnetization characteristic (B,H curve) of the tested material can be ignored. Much of the early pioneering work in magnetic flux leakage modeling used these assumptions to obtain closed form solutions for leakage fields.

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Nonlinear magnetization characteristic (B,H curve) of the tested material

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http://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html

The solutions of classical problems in electrostatics have been well known to physicists for almost a century and their magnetostatic analogs were used to approximate discontinuity leakage fields. Such techniques work reasonably well when the permeability around a discontinuity is constant or when nonlinear permeability effects can be ignored. The major problem that remains is how to deal with real discontinuity shapes often impossible to handle by classical techniques. Such deficiencies are overcome by the use of computer programs written to allow for nonlinear permeability effects around oddly shaped discontinuities. Specifically, computerized finite element techniques, originally developed for studying magnetic flux distributions in electromagnetic machinery, have also been developed for nondestructive testing. Both active and residual excitation are discussed above. The extension of the technique to include eddy currents is detailed elsewhere in this volume.

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Further Reading: Understanding Magnetic Flux Leakage Signals from Mechanical Damage in Pipelines In-line inspection using the Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall stresses. Therefore, MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, the combined influence of stress and geometry make MFL signals from dents and gouges difficult to interpret.

Accurate magnetic models that can incorporate both stress and geometry effects are essential to improve the current understanding of MFL signals from mechanical damage. MFL signals from dents include a geometry component in addition to a component due to residual stresses. If gouging is present, then there may also be an additional magnetic contribution from the heavily worked material at the gouge surface. The relative contribution of each of these components to the MFL signal depends on the size and shape of the dent in addition to other effects such as metal loss, wall thinning, corrosion, etc.

Charlie Chong/ Fion Zhang

http://prci.org/index.php/site/projects_single/understanding_magnetic_flux_leakage_signals_from_mechanical_damage_in_pipel/

FEA Model

Charlie Chong/ Fion Zhang

http://prci.org/index.php/site/projects_single/understanding_magnetic_flux_leakage_signals_from_mechanical_damage_in_pipel/

Key Results Magnetic Finite Element Analysis (FEA) can be applied to model MFL signals from mechanical damage defects having various sizes, shapes, and configurations. These models included geometry effects, contributions due to elastic strain (either residual strain or strain due to in-service loading), and also magnetic behavior changes due to severe deformation. The modeled results were then compared with experimental MFL signal measurements on dents and gouges produced in the laboratory as well under â&#x20AC;&#x153;fieldâ&#x20AC;? conditions. Magnetic FEA models were produced of circular dents as well as dents elongated in the pipe axial and pipe hoop directions. Residual stress patterns were predicted in and around the dent using stress FEA modeling. The magnetic effects of these predicted residual stresses were incorporated into the magnetic FEA model by modifying the magnetic permeability in stressed regions in and around the dent. The modeled stress and geometry contributions to the MFL signal were examined separately, and also combined for comparison with experimental MFL results. Agreement between modeled and measured MFL signals was generally good, and the measured MFL signals were used to validate and refine the models.

Charlie Chong/ Fion Zhang

http://prci.org/index.php/site/projects_single/understanding_magnetic_flux_leakage_signals_from_mechanical_damage_in_pipel/

Other Reading: Leakage signals due the two defects. Field shown in (a) corresponds to the deeper defect and field shown in (b) to the shallow one.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn269/idn269.htm

9.4 PART 4. Applications of Magnetic Flux Leakage Testing 9.4.0

Introduction

Magnetic flux leakage testing is a commonly used technique. Signals from probes are processed electronically and presented in a manner that indicates the presence of discontinuities. Although some techniques of magnetic flux leakage testing may not be as sophisticated as others, it is probable that more ferromagnetic material is tested with magnetic flux leakage than with any other technique. Magnetizing techniques have evolved to suit the geometry of the test objects. The techniques include yokes, coils, the application of current to the test object and conductors that carry current through hollow test objects. Many situations exist in which current cannot be applied directly to the test object because of the possibility of arc burns.

Charlie Chong/ Fion Zhang

Design considerations for magnetization of test objects often require minimizing the reluctance of the magnetic circuit, consisting of (1) the test object, (2) the magnetizing system and (3) any air gaps that might be present.

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Reluctance S

Reluctance, S = Length L / (cross sectional area a â&#x2C6;&#x2122; permeability Îź)

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9.4.1

Test Object Configurations

9.4.1.1 Short Asymmetrical Objects A short test object with little or no symmetry may be magnetized to saturation by passing current through it or by placing it in an encircling coil. If hollow, a conductor can be passed through the test object and magnetization achieved by any of the standard techniques (these include half-wave and full-wave rectified alternating current, pure direct current from battery packs or pulses from capacitor discharge systems). For irregularly shaped test objects, testing by wet or dry magnetic particles is often performed, especially if specifications require that only surface breaking discontinuities be found.

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9.4.1.2 Elongated Objects The cylindrical symmetry of elongated test objects such as wire rope permits the use of a relatively simple flux loop to magnetize a relatively short section of the rope. Encircling probes are placed at some distance from the rope to permit the passage of splices. Such systems are also suited for pumping well sucker rods and other elongated oil field test objects. After a well is drilled, the sides of the well are lined with a relatively thin steel casing material, which is then cemented in. This casing can be tested only from the inside surface. The cylindrical geometry of the casing permits the flux loop to be easily calculated so that magnetic saturation of the well casing is achieved. As with in-service well casing, buried pipelines are accessible only from the inside surface. The magnetic flux loop is the same as for the well casing test system. In this case, a drive mechanism must be provided to propel the test system through the pipeline.

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Elongated Objects- Pump Jack

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9.4.1.3 Threaded Regions of Pipe An area that requires special attention during the inservice testing of drill pipe is the threaded region of the pin and box connections. Common problems that occur in these regions include fatigue cracking at the roots of the threads and stretching of the thread metal. Automated systems that use both active and residual magnetic flux techniques can be used for detecting such discontinuities.

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Threaded Regions of Sucker Rod

Charlie Chong/ Fion Zhang

9.4.1.4 Ball Bearings and Races Systems have been built for the magnetization of both steel ball bearings and their races. One such system uses specially fabricated hall elements as detectors.

9.4.1.5 Relatively Flat Surfaces The testing of welded regions between flat or curved plates is often performed using a magnetizing yoke. Probe systems include coils, hall effect detectors, magnetic particles and magnetic tape.

Charlie Chong/ Fion Zhang

9.4.1.4 Ball Bearings and Races Systems have been built for the magnetization of both steel ball bearings and their races. One such system uses specially fabricated hall elements as detectors.

9.4.1.5 Relatively Flat Surfaces The testing of welded regions between flat or curved plates is often performed using a magnetizing yoke. Probe systems include coils, hall effect detectors, magnetic particles and magnetic tape.

Charlie Chong/ Fion Zhang

Relatively Flat Surfaces

Charlie Chong/ Fion Zhang

Relatively Flat Surfaces

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9.4.2

Discontinuity Mechanisms

In the metal forming industry, discontinuities commonly found by magnetic flux leakage techniques include overlaps, seams, quench cracks, gouges, rolled-in slugs and subsurface inclusions. In the case of tubular goods, internal mandrel marks (plug scores) can also be identified when they result in remaining wall thicknesses below some specified minimum. Small marks of the same type can also act as stress raisers and cracking can originate from them during quench and temper procedures. Depending on the use to which the material is put, subsurface discontinuities such as porosity and laminations may also be considered detrimental. These types of discontinuities may be acceptable in welds where there are no cyclic stresses but may cause injurious cracking when such stresses are present.

Charlie Chong/ Fion Zhang

In the metal processing industries, grinding especially can lead to surface cracking and to some changes in surface metallurgy. Such discontinuities as cracking have traditionally been found by magnetic flux leakage techniques, especially wet magnetic particle testing. Service induced discontinuities include cracks, corrosion pitting, stress induced metallurgy changes and erosion from turbulent fluid flow or metal-to-metal contact. In those materials placed in tension and under torque, fatigue cracking is likely to occur. A discontinuity that arises from metal-to-metal wear is sucker rod wear in tubing from producing oil wells. Here, the pumping rod can rub against the inner surface of the tube and both the rod and tube wear thin. In wire rope, the outer strands will break after wearing thin and inner strands sometimes break at discontinuities present when the rope was made. Railroad rails are subject to cyclic stresses that can cause cracking to originate from otherwise benign internal discontinuities.

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Loss of metal caused by a conducting fluid near two slightly dissimilar metals is a very common form of corrosion. The dissimilarity can be quite small, as for example, at the heat treated end of a rod or tube. The result is preferential corrosion by electrolytic processes, compounded by erosion from a contained flowing fluid. Such loss mechanisms are common in subterranean pipelines, installed petroleum well casing and in refinery and chemical plant tubing. The stretching and cracking of threads is a common problem. For example, when tubing, casing and drill pipe are overtorqued at the coupling, the threads exist in their plastic region. This causes metallurgical changes in the metal and can create regions where stress corrosion cracking takes place in highly stressed areas at a faster rate than in areas of less stress. Couplings between tubes are a good example of places where material may be highly stressed. Drill pipe threads are a good example of places where such stress causes plastic deformation and thread root cracking.

Charlie Chong/ Fion Zhang

9.4.3

Typical Magnetic Flux Leakage Techniques

9.4.3.1 Short Parts For many short test objects, the most convenient probe to use is the magnetic particle. The test object can be inspected for surface breaking discontinuities during or after it has been magnetized to saturation. For active field testing, the test object can be placed in a coil carrying alternating current and sprayed with magnetic particles. Or it can be magnetized to saturation by a direct current coil and the resulting residual induction can be shown with magnetic particles. In the latter case, the induction in the test object can be measured with a flux meter. Wet particles perform better than dry ones because there is less tendency for the wet particles to fur (that is, to stand up like short hairs) along the field lines that leave the test object. These techniques will detect transversely oriented, tight discontinuities.

Charlie Chong/ Fion Zhang

The magnetic flux leakage field intensity from a tight crack is roughly proportional to the magnetic field intensity Hg across the crack, multiplied by crack width Lg. If the test is performed in residual induction, the value of Hg (which depends on the local value of the demagnetization field in the test object) will vary along the test object. Thus, the sensitivity of the technique to discontinuities of the same geometry varies along the length of the test object.

For longitudinally oriented discontinuities, the test object must be magnetized circumferentially. If the test object is solid, then current can be passed through the test object, the surface field intensity being given by Ampereâ&#x20AC;&#x2122;s law: (9) Where: dl is an element of length (meter), H is the magnetic field intensity (ampere per meter) and I is the current (ampere) in the test object.

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Ampere's Law The magnetic field in space around an electric current is proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source.

Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.

In the electric case, the relation of field to source is quantified in Gauss's Law which is a very powerful tool for calculating electric fields

Charlie Chong/ Fion Zhang

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html

If the test object is a cylindrical bar, the symmetry of the situation allows H to be constant around the circumference, so the closed integral reduces: (10)

(11) Where: R is the radius (meter) of the cylindrical test object. A surface field intensity that creates an acceptable magnetic flux leakage field from the minimum sized discontinuity must be used. Such fields are often created by specifying the amperage per meter of the test objectâ&#x20AC;&#x2122;s outside diameter.

Charlie Chong/ Fion Zhang

9.4.3.2 Transverse Discontinuities Because of the demagnetizing effect at the end of a tube, automated magnetic flux leakage test systems do not generally perform well when scanning for transverse discontinuities at the ends of tubes. The normal component Hy of the field outside the tube is large and can obscure discontinuity signals. Test specifications for such regions often include the requirement of additional longitudinal magnetization at the tube ends and subsequent magnetic particle tests during residual induction. This situation is equivalent to the magnetization and testing of short test objects as outlined above.

Charlie Chong/ Fion Zhang

The flux lines must be continuous and must therefore have a relatively short path in the metal. Large values of the magnetizing force at the center of the coil are usually specified. Such values depend on the weight per unit length of the test object because this quantity affects the ratio of length L to diameter D. Where the test object is a tube, the L·D–1 ratio is given by the length between the poles divided by twice the wall thickness of the tube. (The distance L from pole to pole can be longer or shorter than the actual length of the test object and must be estimated by the operator.)

As a rough example, with L = 460 mm (18 in.) and D = 19 (0.75 in.), the L·D–1 ratio is 24.

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The effective permeability of the metal under test is small because of the large demagnetization field created in the test object by the physical end of the test object. An empirical formula is often used to calculate approximately the effective permeability Îź: (12) so effective permeability Îź = 139 in the above example. For wet magnetic particle testing, the surface tension of the fluids that carry the particles is large enough to confine the particles to the surface of the test object. This is not the case with dry particles, which have the tendency to stand up like fur along lines of magnetizing force. In many instances, it may be better to use some other test technique for transverse discontinuities, such as ultrasonic or eddy current techniques.

Charlie Chong/ Fion Zhang

9.4.3.3 Alternating Current versus Direct Current Magnetization Alternating current magnetization is more suitable for detection of outer surface discontinuities because it concentrates the magnetic flux at the surface. For equal magnetizing forces, an alternating current field is better for detecting outside surface imperfections but a direct current field is better for detecting imperfections below the surface. In practice, the ends of tubes are tested for transverse discontinuities by the following magnetic flux leakage techniques.

Charlie Chong/ Fion Zhang

1. Where there is a direct current active field from an encircling coil, magnetic particles are thrown at the tested material while it is maintained at a high level of magnetic induction by a direct current field in the coil.

This technique is particularly effective for internal cracks. Fatigue cracks in drill pipe are often found by this technique.

2. Where there is an alternating current active field from an encircling coil, magnetic particles are thrown at the tested material while it lies inside a coil carrying alternating current. Using 50 or 60 Hz alternating current, the penetration of the magnetic field into the material is small and the technique is good only for the detection of outside surface discontinuities.

When tests for both outer surface and inner surface discontinuities are necessary, it may be best to test first for outer surface discontinuities with an alternating current field, then for inner surface discontinuities with a direct current field.

Charlie Chong/ Fion Zhang

9.4.3.4 Liftoff Control of Scanning Head To obtain a stable detection of discontinuities, liftoff between the probe and the surface of the material must be kept constant. Usually liftoff is kept constant by contact of the probe with the surface but the probe tends to wear with this technique. A magnetic floating technique has been used for noncontact scanning. In this technique, liftoff is measured by a gap probe and the probe holder is moved by a voice coil motor, controlled by the gap signal.

This system and related technology are described in this volumeâ&#x20AC;&#x2122;s chapter on primary metals applications.

Charlie Chong/ Fion Zhang

9.4.4

Particular Applications

9.4.4.1 Wire Ropes An interesting example of an elongated steel product inspected by magnetic flux leakage testing is wire rope. Such ropes are used in the construction, marine and oil production industries, in mining applications and elevators for personnel and raw material transportation. Testing is performed to determine cross sectional loss caused by corrosion and wear and to detect internal and external broken wires. The type of flux loop used (electromagnet or permanent magnet) can depend on the accessibility of the rope. Permanent magnets might be used where taking power to an electromagnet might cause logistic or safety problems. By making suitable estimates of the parameters involved, a reasonably good estimate of the flux in the rope can be made.

Because discontinuities can occur deep inside the rope material, it is essential to maintain the rope at a high value of magnetic flux density, 1.6 to 1.8 T (16 to 18 kG). Under these conditions, breaks in the inner regions of the rope will produce magnetic flux leakage at the surface of the rope.

Charlie Chong/ Fion Zhang

The problem of detecting magnetic flux leakage from inner discontinuities is compounded by the need to maintain the magnetic probes far enough from the rope for splices in the rope to pass through the test head. Common probes include hall effect detectors and encircling coils. The cross sectional area of the rope can be measured by sensing changes in the magnetic flux loop that occur when the rope gets thinner. The air gap becomes larger and so the value of the field intensity falls. This change can easily be sensed by placing hall effect probes anywhere within the magnetic circuit.

Charlie Chong/ Fion Zhang

9.4.4.2 Internal Casing or Pipelines The testing of in-service well casing or buried pipelines is often performed by magnetic flux leakage techniques. Various types of wall loss mechanisms occur, including internal and external pitting, erosion and corrosion caused by the proximity of dissimilar metals. From the point of view of magnetizing the pipe metal in the longitudinal direction, the two applications are identical. The internal diameters and metal masses involved in the magnetic flux loop indicate that some form of active field excitation must be used. Internal diameters of typical production or transportation tubes range from about 100 mm (4 in.) to about 1.2 m (4 ft). If the material is generally horizontal, some form of drive mechanism is required. Because the test device (a robotic crawler) may move at differing speeds, the magnetic flux leakage probe should have a signal response independent of velocity.

Charlie Chong/ Fion Zhang

For devices that operate vertically, such as petroleum well casing test systems, coil probes can be used if the tool is pulled from the bottom of the well at a constant speed. In both types of instrument, the probes are mounted in pads pressed against the inner wall of the pipe. Because both line pipe and casing are manufactured to outside diameter size, there is a range of inside diameters for each pipe size. Such ranges may be found in specifications. To make the air gap as small as possible, soft iron attachments can be screwed to the pole pieces. For the pipeline crawler, a recorder package is added and the signals from discontinuities are tape recorded. When the tapes are retrieved and played back, the areas of damage are located. Pipe welds provide convenient magnetic markers. With the downhole tool, the magnetic flux leakage signals are sent up the wire line and processed in the logging truck at the wellhead. A common problem with this and other magnetic flux leakage equipment is the need to determine whether the signals originate from discontinuities on the inside or the outside surface of the pipe.

Charlie Chong/ Fion Zhang

Production and transmission companies require this information because it lets them determine which form of corrosion control to use. The test shoes sometimes contain a high frequency eddy current probe system that responds only to inside surface discontinuities. Thus, the occurrence of both magnetic flux leakage and eddy current signals indicates an inside surface discontinuity whereas the occurrence of a magnetic flux leakage signal indicates only an outside surface discontinuity. Problems with this form of testing include the following: 1. The magnetic flux leakage system cannot measure elongated changes in wall thickness, such as might occur with general erosion.

2. If there is a second string around the tested string, the additional metal contributes to the flux loop, especially in areas where the two strings touch.

3. A relatively large current must be sent down the wire line to raise the pipe wall to saturation. Temperatures in deep wells can exceed 200째C (325 째F).

4. The tool may stick downhole or underground if external pressures cause the pipe to buckle.

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9.4.4.3 Cannon Tubes In elongated tubing, the presence of rifling affects the ability to perform a good test, especially for discontinuities that occur in the roots of the rifling.

Despite the presence of extraneous signals from internal rifling, however, rifling causes a regular magnetic flux leakage signal that can be distinguished from discontinuity signals. As a simulated discontinuity is made narrower and shallower, the signal will eventually be indistinguishable from the rifle bore noise. In magnetic flux leakage testing, cannon tubes can be magnetized to saturation and scanned with hall elements to measure residual induction.

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Cannon Tubes

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9.4.4.5 Round Bars and Tubes In some test systems, round bars and tubes have been magnetized by an alternating current magnet and rotated under the magnet poles. Because the leakage flux from surface discontinuities is very weak and confined to a small area, the probes must be very sensitive and extremely small. The system uses a differential pair of magnetodiodes to sense leakage flux from the discontinuity. The differential output of these twin probes is amplified to separate the leakage flux from the background flux. In this system, pipes are fed spirally under the scanning station, which has an alternating current magnet and an array of probe pairs. The system usually has three scanning stations to increase the test rate. In one similar system, round billets are rotated by a set of rollers while the billet surface is scanned by a transducer array moving straight along the billet axis. Seamless pipes and tubes are made from the round billets. In another tube test system, the transducers rotate around the pipe as the pipe is conveyed longitudinally. Overlapping elliptical printed circuit coils are used instead of magnetodiodes and are coupled to electronic circuits by slip rings. The system can separate seams into categories according to crack depth.

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9.4.4.6 Billets A relatively common problem with square billets is elongated surface breaking cracks. By magnetizing the billet circumferentially, magnetic flux leakage can be induced in the resulting residual magnetic field. Magnetic flux leakage systems for testing tubes exhibit the same general ability to classify seam depth. It is generally accepted that even with the lack of correlation between some of the instrument readings and the actual discontinuity depths, the automatic readout of these two systems still represents an improvement over visual or magnetic particle testing. One technique, often called magnetography, for the detection of discontinuities uses a belt of flux sensitive material, magnetic tape, to record indications. Discontinuity fields magnetize the tape, which is then scanned with an array of microprobes or hall effect detectors. Finally, the tape passes through an erase head before contacting the billet again. Because the field intensity at the corners is less than at the center of the flat billet face, a compensation circuit is required for equal sensitivity across the entire surface.

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9.4.5

Damage Assessment

In most forms of magnetic flux leakage testing, discontinuity dimensions cannot be accurately measured by using the signals they produce. The final signal results from more than one dimension and perhaps from changes in the magnetic properties of the metal surrounding the discontinuity. Signal shapes differ widely, depending on location, dimensions and magnetization level. It is therefore impossible to accurately assess the damage in the test object with existing equipment. Under special circumstances (for example, when surface breaking cracks can be assumed to share the same width and run normal to the material surface), it may be possible to correlate magnetic flux leakage signals and discontinuity depths. This correlation is normally impossible. Commercially available equipment does not reconstruct all the desired discontinuity parameters from magnetic flux leakage signals. For example, the signal shape caused by a surface breaking forging lap is different from that caused by a perpendicular crack but no automated equipment uses this difference to distinguish between these discontinuities.

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As with many forms of nondestructive testing, the detection of a discontinuity and subsequent follow up by either nondestructive or destructive methods pose no serious problems for the inspector.

Ultrasonic techniques, especially a combination of shear wave and compression wave techniques, work well for discontinuity assessment after magnetic flux leakage has detected them. In some cases, however, the discontinuity is forever hidden. Such is very often the case for corrosion in downhole and subterranean pipes.

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9.5 PART 5. Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects Vijay Babbar and Lynann Clapham

9.5.0

Preface

Simulated defects of different shapes and sizes were created in a section of API X70 steel line pipe and were investigated using a residual magnetic flux leakage (MFL) technique. The MFL patterns reflected the actual shape and size of the defects, although there was a slight shift in their position. The defect features were apparent even at high stresses of 220 MPa when the samples were magnetized at those particular stresses. However, unlike the active flux technique, the residual MFL needs a sensitive flux detector to detect the comparatively weaker flux signals.

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Journal of Nondestructive Evaluation, Vol. 22, No. 4, December 2003 (Š 2004)

9.5.1

Introduction

The magnetic flux leakage (MFL) technique is frequently used for in-service monitoring of oil and gas steel pipelines, which may develop defects such as corrosion pits as they age in service. Under the effect of typical operating pressures, these defects act as “stress raisers” where the stress concentrations may exceed the yield strength of the pipe wall. The main objective of MFL inspection is thus to determine the exact location, size, and shape of the defects and to use this information to determine the optimum operating pressure and estimate the life of a pipeline. Most MFL tools rely on active magnetization in which the pipe wall is magnetized to near saturation by using a strong permanent magnet, and the flux leaking out around a defect is measured at the surface of the pipeline.

Keywords: ■ Near saturation ■ Active magnetization ■ Flux leaking out

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The magnitude of the leakage flux density depends on the strength of the magnet, the width and depth of the defect, the magnetic properties of the pipeline material. and running conditions such as velocity and stress. A typical peak-to-peak value of leakage flux density from a surface defect may be around 30 G. Another way of employing the MFL technique for studying the pipeline defects is through residual magnetization. After a magnet is passed over a portion of the steel pipe, some residual magnetization remains.

A study of the residual magnetization MFL signal can provide useful information about the size and shape of the defect. However, little published work exists about residual MFL, probably because of the comparatively weak leakage flux signals, which require sensitive detectors.

Keywords: â&#x2013; 30 G (Gauss)

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An earlier study of samples magnetized by strong electric currents revealed that the residual flux patterns are basically similar to the active flux patterns, with exceptions that they are very weak and may have opposite magnetic polarity in comparison to the latter. The opposite polarity occurs only when the excitation current is low, whereas for high excitation current level, there is no reversal of polarity. A finite element modeling technique has been proposed by Satish to predict the reversal of the residual leakage field.

Keywords: â&#x2013; Residual flux patterns are basically similar to the active flux patterns. â&#x2013; Very weak and may have opposite magnetic polarity â&#x2013; For high excitation current level, there is no reversal of polarity

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The present work investigates the residual flux patterns of defects after the passing of a permanent magnet (similar to the situation in pipeline inspection).

The residual flux patterns of three different blind defects, that is, circular, elongated pit (henceforth named racetrack), and irregular gouge, are investigated. The effect of pipe wall stresses on the active and residual leakage flux signals from some of the defects is also reported.

Note: â&#x20AC;&#x153;Blindâ&#x20AC;? indicates a hole that is not completely through-wall.

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9.5.2

EXPERIMENTAL

Three simulated defects were used in the present study: a circular blind hole, a blind racetrack-shaped defect, and a gouge. The first two defects were produced on the surface of a hydraulic pressure vessel (HPV) constructed for a previous study and were nearly 50% of the wall thickness. These are illustrated in Figure 1. The circular defect has a 15-mm diameter and 5-mm depth; the racetrack has about a 53-mm length, 15-mm width and 4.4-mm depth. An electrochemical-milling process, which prevents the introduction of additional stresses around the defects, was used for creating the first two defects in the HPV. The gouge of about 125-mm length, 26-mm width, and a graded maximum depression of about 14 mm was created on another section of similar steel pipe by using a single backhoe tooth. It is shown in Figure 2.

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Fig. 1. Geometric details of blind hole (a) and blind racetrack (b) defects.

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Fig. 2. Camera picture of a gouge on a steel line pipe section. The main groove is nearly rectangular, having dimensions of 53 mm 15 mm and depth varying from zero to 4.4 mm maximum. An extended depression as indicated by a closed contour is present around the gouge.

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The HPV used in the present study is shown in Figure 3 and is briefly described here; the details can be found elsewhere. It consists of an outer section of API X70 steel pipeline of 635-mm length, 610-mm diameter, and 9m wall thickness separated from an inner steel spool by a hydraulic chamber that contains hydraulic oil. On pressurizing the chamber, circumferential (hoop) stresses can be created in the outer wall of the pipeline and hence the in-service pressure stresses can be simulated. Axial stresses are minimized because they are carried by free end caps sealed with O-rings to prevent leakage.

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Fig. 3. Outline of pipeline sample (high-pressure vessel), magnet, the Hall probe, and scanning system assembly.

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The pipe wall was magnetized by using an assembly of strong permanent magnets. High-strength NdFeB permanent magnet blocks, approximately 55 x 55 x 6 mm3, were connected in parallel and held in place by aluminum cover plates at each pole piece. Steel brushes, having the same curvature as the pipe, were used to couple the flux into the pipe wall. A back-iron mounting plate was connected to the pole pieces, thus completing the magnetic circuit from the NdFeB magnets through to the pipe wall and back again. To magnetize the defect, the magnet was pulled along the axis and across the surface of the HPV over the defect from left to right with south pole ahead.

This is consistent with typical inspection procedures, although in this case the detector is on the outer wall of the pipe while inspection is internal. The magnet was pushed from the pipe end to a cylindrical aluminum platform, where it was lifted off, turned in a direction perpendicular to the axis, and returned to the left of the pressure vessel. This procedure was repeated three times for each magnetization process.

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After the three magnetization cycles the magnet remained on the HPV producing a flux density of 1.4 T (tesla). The gouge was similarly magnetized.

All the measurements were repeated three times with time intervals of several days to verify the reproducibility of results, keeping the direction of magnetization always the same. The scanning system used in the present investigation can be seen in Figure 3. More details are available in a previous paper. It consisted of an SS94A1 Micro-Switch Hall probe that was controlled by a computer software and moved smoothly over the surface of defects in a two- imensional grid with increments of 1 x 1 mm2. It was connected to a Roland DXY-1100 XY digital plotter, which was controlled by a Tecmar A/D board operated by a compiled Microsoft Visual BASIC 4.0 program called Aquis. Finally, a three-dimensional plotting package called Surfer 7.0 from Golden Software was used for obtaining surface and contour maps.

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9.5.3

RESULTS AND DISCUSSION

9.5.3.1 Active and Residual MFL Results in an Unstressed Pipe Wall (A) Active MFL The contour map of the active radial MFL scan from the circular blind-hole defect is shown in Figure 4. The magnetic field lies along the axial direction, whereas the stress is circumferential. A corresponding axial line scan through the center of the blind hole is shown in Figure 5, where the solid line is only a guide to the eye. The scan is approximately symmetric along the axis of the pipe; a region of high positive flux is present on one side of the defect and a high negative flux on the other. The peak-to-peak value of the radial leakage flux (MFLpp) is about 27.0 Gauss. The shape of the flux pattern is well understood and has been reported by many workers. Although the size and shape of the circular defect are not obvious from this contour map, some useful information can be obtained. For example, this type of circular defect is typically located between high positive and high negative flux regions, with its center almost on the zero flux line. Also, the MFLpp is used to determine the defect depth. However, for irregular defect shapes, such contour maps may not reveal very useful information about the defect geometry.

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Fig. 4. Contour map of radial active magnetic leakage flux density (B) from circular blind-hole defect. Solid circle represents the actual location of the defect. The applied magnetic field and stress are along the axial and circumferential directions, respectively.

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Fig. 5. Radial active MFL axial line scan through the center of the circular defect showing the variation of the radial active magnetic leakage flux density (B) along the axial direction.

MFLpp

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Keywords: ■ Contour map scan ■ MFL axial line scan

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(B1) Vertical Lift-Off- Residual MFL The residual radial MFL scan and the corresponding axial line scan through the center of the defect are shown in Figures 6 and 7, respectively. These were obtained after lifting the magnet perpendicularly upward from the defect.

The residual flux pattern shows magnetic polarity exactly opposite to that of active flux pattern of Figure 4. This is consistent with reports by Heath for comparatively low excitation levels. The residual peak-to-peak flux density in the present case is about 4.3 Gauss. It may also be noted from Figures 6 and 7 that, as for active flux patterns, the regions of positive and negative flux in the residual pattern appear to exhibit axial symmetry around the center of the defect. A small change in orientation of the flux pattern with respect to the axial direction is believed to be due to the rotation of the magnet after lifting it off the pipe. To summarize, as for active MFL patterns, the residual patterns with perpendicular liftoff can reveal information about the size and shape of the defect only on the basis of positions of high positive and negative flux regions. However, as with active MFL patterns, the shape of the defect is not directly obvious from the signal.

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Fig. 6. Contour map of radial residual magnetic leakage flux density (B) after perpendicular lift-off of the magnet from the circular defect. Solid circle represents the actual location of the defect.

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Fig. 7. Radial residual MFL axial line scan through the center of the circular defect after perpendicular lift-off of the magnet. B represents the radial residual magnetic leakage flux density.

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Compare the magnetic flux density of active & residual MFLT

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(B2) Sliding the magnet along the axial direction- Residual MFL During actual service conditions the magnets always slide along the pipe axis; therefore subsequent residual scans were made after sliding the magnet along the axial direction on the outer surface of the pipe wall with south pole leading. The contour map and the line scan obtained with this end lift-off method are shown in Figures 8 and 9 and are markedly different from those shown for perpendicular lift-off. There is now a marked asymmetry between the regions of positive and negative flux; the center of positive and negative regions no longer coincide with the edges of the defect, and the region of positive flux is more spread out over the defect.

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Fig. 8. Contour map of radial residual magnetic leakage flux density (B) after end lift-off of the magnet. Solid and dotted circles represent the actual and apparent locations of the defect, respectively.

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Fig. 9. Radial residual MFL axial line scan through the center of the circular defect after end lift-off of the magnet. B represents the radial residual magnetic leakage flux density.

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Active and residual flux distributions The possible active and residual flux distributions for the above cases are depicted in Figure 10. In the active case when magnet is on the defect, the flux and hence the domains are parallel to the top horizontal surface of the pipe, while those near the sides are oriented almost vertically. The path of flux lines near the edges of the defect is shown in Figure 10(a). When the magnet is lifted perpendicularly, the domains on either side of the defect tend to remain in the vertical orientation. A localized symmetric flux distribution is thus established around the defect, with flux being directed downward on the left, upward on the right, and from right to left over the defect. The flux path is shown in Figure 10(b) and is similar to that reported by Heath. There appear to be induced south and north polarities near the edges of the defect along the axial direction.

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In the third case of end lift-off with north pole leaving the pipe at the end, the asymmetric flux distribution shown in Figure 10(c) appears to account for the asymmetric MFL pattern of Figure 9. This is due apparently to the slight displacement of the S-N dipole developed on the axial diameter of the defect toward the left, owing to the repulsion from the north pole of the magnet before end lift-off. However, there is a need to verify these results by other methods. Unfortunately, finite element model simulations cannot be used for this purpose unless the domain level phenomena are incorporated into the model.

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Fig. 10. Probable flux distributions around the circular defect: (a) active, (b) residual with perpendicular lift-off, and (c) residual with end lift-off.

Active magnetization

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Fig. 10. Probable flux distributions around the circular defect: (a) active, (b) residual with perpendicular lift-off, and (c) residual with end lift-off.

Vertical lift-off

S

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N

End lift-off

One of the interesting features of the asymmetric contour map of residual MFL scan with end lift-off of the magnet is that the defect shape is reflected in the radial MFL signal. It is also easy to estimate the size and position of the defect. A close look at Figure 8 indicates an almost circular defect centered on a point of high positive flux marked by the dotted circle. The true location is marked by the solid circle and is slightly toward the negative flux region. The magnitude of the shift in the position of the defect apparently depends on the strength of the magnet and the magnetic properties of the pipeline and can be determined experimentally. It is about 3 mm for the present system. It is also possible to estimate the size of the defect from the axial line scan shown in Figure 9. The diameter of the apparent defect is approximately the length of the horizontal projection of the positive peak.

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Racetrack defect The active and residual radial MFL contour maps of the racetrack defect are shown in Figure 11. The solid racetrack boundary in Figure 11(a) indicates the true location of the defect, and the broken boundary in Figure 11(b) indicates its apparent location according to the residual signal. In the active scan, the ends of the defect are located slightly outside the positive and negative peak positions of the flux density but the shape and size of the defect cannot be seen clearly. Conversely, the residual scan gives a clear view of the size and shape of the defect, except with an axial shift of about 3 mm as observed in case of circular defect. The nature of flux pattern of this residual scan, however, differs from that of circular defect. In the residual racetrack pattern, the region of high negative flux is not concentrated at the end of the defect, but on the axial side of it, while the region of high positive flux is present almost everywhere over the defect as observed for circular defect.

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Fig. 11. Active (a) and residual (b) MFL radial contour maps of racetrack defect in the absence of stress. The actual and apparent locations of defect are indicated by the solid and broken racetrack boundaries, respectively.

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This 90-degree rotation of the magnetic flux pattern from the expected axial direction is probably due to the large length of the defect, which does not permit the flux to make long axial loops. Instead, short circumferential flux loops around the defect are energetically more favorable wherein most of the flux lines emerge out of the defect, make loops around one of the long axial sides, and reenter the pipe slightly outside the region of defect. The domains are apparently aligned horizontally along the circumferential direction beneath the defect, but vertically along the axial wall of the defect. This is in spite of the fact that, even in the absence of applied stress, there exists a macroscopic easy axis that is parallel to the axis of the steel pipe section.

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Irregular gouge The active and residual MFL scans of the third defect, an irregular gouge, are shown in Figure 12. Although the actual length, width, and maximum depression of the gouge are about 125 mm, 26 mm, and 14 mm, respectively, the overall depression is not limited to an area of just 125 mm 26 mm because of depression of the surrounding region during the gouge formation.

The defect is spread over a non-uniform area of about 155 mm 65 mm as indicated in Figure 12 by the elongated closed contour. The active flux pattern of the gouge, as shown in Figure 12(a), does not exhibit longitudinal symmetry, which is expected owing to the non-uniformity in depression as well as width. The only resemblance this pattern has to the racetrack flux pattern of Figure 11(a) is that the upper half pattern shows a region of positive active flux and the lower half shows a region of relatively weak negative flux.

The shape of the gouge is not apparent from this pattern.

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The extreme axial regions of high positive and negative flux are not due to the defect itself, but to the closer approach of the Hall probe detector to the magnetic brushes, where the induced magnetic poles produce spurious flux leakage signals. The residual flux pattern of Figure 12(b), on the other hand, shows a region of positive flux spread over the defect, which helps to estimate the size of the defect more conveniently. Thus, instead of active scans, the residual scans look more promising to reveal the size and shape of this type of irregular defect.

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Fig. 12. Active (a) and residual (b) radial contour maps of the gouge in the absence of stress. The approximate location of the defect is shown in both.

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9.5.3.2 Active and Residual MFL Results as a Function of Pipe Wall Stress In-service oil and gas pipelines are subjected to high stresses (up to 70% of the yield strength); thus the variations in the active MFL patterns brought about by the increased level of stress have been the subject of study. When the pipe is axially magnetized, the higher circumferential stresses are known to affect the active MFL signals and patterns from circular blind-hole defects in two ways: (1) they rotate the macroscopic magnetic easy axis of the pipe from the axial direction toward the circumferential direction, which causes the change in MFLpp, and (2) they modify the MFL pattern by producing localized flux variations as a result of stress concentrations around defects.

To study such changes in the residual MFL patterns, measurements were made on circular and racetrack defects at different stress levels.

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The main interest was to determine if, as at zero stress, the residual patterns could reveal the shape and size of the defects at high stress levels. Figure 13 depicts the residual MFL patterns of both circular and racetrack defects, which were magnetized at a stress level of 0 MPa but then studied at 220 MPa. The corresponding 0 MPa patterns are shown in Figure 8 and 11(b). A comparison of these patterns indicates that a flux rotation of 180 degrees occurs at stress values of 220 MPa, with positive and negative flux regions interchanging their locations. In the case of a circular defect, the negative flux region has two localized regions of comparatively higher flux along the circumferential or stress direction where the stress concentration is higher.

Two similar localized positive flux regions, though not clearly seen in Figure 13(a), are developed on the positive side of the flux at higher stresses. The positions of such localized flux regions may be linked to the localized stress concentrations around the defect. The residual pattern of the racetrack defect in Figure 13(b) also shows two pockets of positive and negative flux regions near the four corners of the racetrack.

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The residual patterns of Figure 13 do not depict the shape of the defects as clearly as seen from patterns of Figures 8 and 11(b), which indicates that the application of stress reorients the magnetic domains along the stress direction, thus disturbing the original pattern. However, if the stress is applied before magnetization, as is done during in-service operation, the residual patterns can still be employed to get useful information about the shape and size of the defect. This is obvious from the residual patterns shown in Figure 14, where the defects were magnetized and also scanned at 220 MPa.

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Fig. 13. Residual MFL scans of circular (a) and racetrack (b) defects taken at a stress of 220 MPa after magnetizing at 0 MPa. The actual defect locations are shown.

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Fig. 14. Residual MFL scans of circular (a) and racetrack (b) defects taken at a stress of 220 MPa after magnetizing at the same stress. The actual defect locations are shown.

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9.5.4

CONCLUSIONS

The residual MFL technique with end lift-off of the magnet appears to be very promising to provide useful information about defect geometry. Although the flux leakage signals weaken at high pressures, the technique still can be used to obtain reasonably good information provided the samples are magnetized at the same high pressure. However, the technique involves the use of sensitive probes to detect the flux leakage signals, which have about one tenth of the strength of the active flux leakage commonly used.

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End Of Reading 6

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