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TechnicalHandbook Where and how to implement fixed ultrasonic gas leak detection


WHY ULTRASONIC GAS LEAK DETECTION (UGLD)? �Field trials have indicated that gas plumes in well ventilated areas may not be detected by fixed point or open path detectors. Analysis of hydrocarbon release incidents shows that conventional gas detectors have 65% detection efficiency. The potential therefore exists for gas from undetected leaks to disperse to adjacent areas, accumulate in ventilation dead spots in the modules, or for the release to continue undetected for some time...� Health and Safety Executive in the UK, April 2010 @

Ultrasonic gas leak detector

Conventional gas detector

Figure 1: Ultrasonic gas leak detectors do not need physical contact with the gas. They are unaffected by wind, gas dilution, and the direction of the gas plume.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Where to Install UGLD . . . . . . . . . . . . . . . . . . . . 37 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

1 How does UGLD Work? . . . . . . . . . . . . . . . . . . . . . 9

Applications: Gas Processing Plant . . . . . . . . . . . . . 41

Gas Release Event Tree . . . . . . . . . . . . . . . . . . . . . 11

Applications: Separation Area . . . . . . . . . . . . . . . . . 43

Total Speed of Response . . . . . . . . . . . . . . . . . . . . 12

Applications: Wellheads . . . . . . . . . . . . . . . . . . . . . 44

Detection Coverage . . . . . . . . . . . . . . . . . . . . . . . . 15

Applications: Compressor Area . . . . . . . . . . . . . . . . 45

LEL vs Leak Rate . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Gas Pressure, Leak Size and Detection Coverage . . 20

4 How to Implement and Maintain UGLDs . . . . . . . 47

Frequency and Amplitude . . . . . . . . . . . . . . . . . . . . 22

Implementation Practice . . . . . . . . . . . . . . . . . . . . . 48

UGLD Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Phase One: Where in the Project should UGLD be Implemented? . . . . . . . . . . . . . . . . . . . . . 48

2 Which Gases Can UGLD Detect? . . . . . . . . . . . . 27

Which Areas are Suited for UGLD? . . . . . . . . . . . . . 48

Example of Gases

Phase Two: Where should Individual Detectors

Hydrocarbon Gases (CxHx) . . . . . . . . . . . . . . . . . . . 30

be Installed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Hydrogen (H2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Offshore Installation of UGLD . . . . . . . . . . . . . . . . . 51

Carbon Dioxide (CO2) . . . . . . . . . . . . . . . . . . . . . . . 30

Onshore Installation of UGLD . . . . . . . . . . . . . . . . . 56

Hydrogen Sulphide (H2S) . . . . . . . . . . . . . . . . . . . . 31

Phase Three: Detailed Engineering Considerations . 58

Condensate, 2-phase, and Liquid Leaks . . . . . . . . . 31

Choosing the Correct Height . . . . . . . . . . . . . . . . . . 58

Gases and Coverage . . . . . . . . . . . . . . . . . . . . . . . 32

Shadowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 How to Obtain Optimal Detection Coverage . . . . . . . 60

Setting the Right Trigger Level . . . . . . . . . . . . . . . . 61

Communication Buses and Protocols . . . . . . . . . . . 71

Ultrasonic Noise Sources

Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . 71

and Plant Noise Levels . . . . . . . . . . . . . . . . . . . . . . 62

Product Comparison Table . . . . . . . . . . . . . . . . . . . 72

How to Avoid Influence from Other Noise Sources and False Alarms . . . . . . . . . . . . . . . 63

6 How to Interface UGLD

Services – Maintenance . . . . . . . . . . . . . . . . . . . . . 64

and Mix with Other Technologies . . . . . . . . . . . . . . 75

Services – Onsite Mapping Survey . . . . . . . . . . . . . 65

How to Interface Detectors with the Control System . . . . . . . . . . . . . . . . . . . . . . . . 76

Phase Four: How are the UGLDs

Configuring Trigger Level

Commissioned and Tested? . . . . . . . . . . . . . . . . . . 67

and Alarm Delay Time . . . . . . . . . . . . . . . . . . . . . . 77

Services – Onsite Commissioning

Executive Action and Voting Configuration . . . . . . . 78

and Leak Simulation . . . . . . . . . . . . . . . . . . . . . . . 67

UGLD as an Independent Protection Layer

5 Which UGLD Products are Available? . . . . . . . . . 69

UGLD – SIL Rating, SIS and Risk . . . . . . . . . . . . . . . 81

in Safety Instrumented Systems . . . . . . . . . . . . . . . 79 Considerations when Specifying UGLDs Explosion Proof and Intrinsically Safe . . . . . . . . . . . 70

A Final Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Sensor Element . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Explanation and Expressions . . . . . . . . . . . . . . . . . 84

Acoustic Self-Test (Self-Diagnostics) . . . . . . . . . . . . 70 Hazardous Area Approvals and Safety Integrity Levels . . . . . . . . . . . . . . . . . . . 71

Introduction Ultrasonic gas leak detection (UGLD) is a comparatively

and limitations are also discussed. Illustrations show

recent detection technique and has emerged as an

how UGLD can be deployed to provide wide area coverage

effective means of establishing the presence of gas leaks.

per device, even in noisy areas. Last, the handbook

It works especially well in open, ventilated areas where

gives guidelines on sensor placement, installation, and

other methods of gas detection may not be independent of

commissioning, as well as procedures for maintenance

ventilation. Because UGLDs respond to the source of the

of the equipment.

leak, rather than the gas itself, they complement sensors that measure gas concentration. Such application of the

All data found in this handbook is based on knowledge

technology as an additional and complementary means


of detecting gas leaks is supported by the Health and

Gassonic ultrasonic gas leak detectors since 1996.

Safety Executive (HSE) in the UK. In a recent report,

Since recommendations are based on the acoustic

the organization’s Offshore Division recommends the

characteristic of Gassonic UGLDs, Gassonic cannot be

use of a combination of sensors, with ultrasonic gas

held responsible for implementation of other types of

leak detectors providing early warning and IR detectors

UGLDs according to information found in the handbook.

identifying the gas accumulation (HSE 2004).






We hope readers will gain useful insights on ultrasonic gas

This handbook details the operation and use of UGLDs.

leak detectors and greater confidence on the application

The principles of ultrasonic detection and its strengths

of these instruments to plant safety.

Page 6


This handbook is divided into six main sections:

The document is meant to be an active document, which will be updated frequently as more knowledge is gained.

Chapter 1

Links to web-based pages are included wherever relevant

How does UGLD Work?

to provide further documentation and better visualization

Chapter 2

of the different topics. Use the link below to subscribe to

Which Gases can UGLD Detect?

updates on material in this handbook as well as ultrasonic gas leak detection in general.

Chapter 3

Where to Install UGLD Note that ultrasonic gas leak detection is sometimes

Chapter 4

How to Implement and Maintain UGLD

referred to as acoustic leak detection (ALD) in certain countries.

Chapter 5

Which UGLD Products are Available? Chapter 6

How to Interface UGLD and Mix with Other Technologies

Updates See also

Reference HSE. 2004. Fire and Explosion Strategy, Issue 1. Hazardous Installations Directorate, Offshore Division.


Page 7

1 How does UGLD work? Fixed gas detection in open ventilated areas like offshore or onshore oil and gas facilities is generally considered problematic because the gas easily dilutes and drifts away from conventional gas sensors. Ultrasonic gas leak detectors solve this problem by detecting the airborne acoustic ultrasound generated when pressurized gas escapes from a leak. When a gas leak occurs, the ultrasound generated by the leak travels at the speed of sound, through the air, from the source to the detector. Ultrasonic gas leak detectors are non-concentration based detectors. They send a signal to the control system indicating the onset of a leak.


Immediate Ignition

Vapor Cloud Forms and Ignites

Liquid Rainout and Ignition

Explosion Occurs

Toxic Chemical


Outcome Jet Fire


Vapor Cloud Explosion

Yes No

Flash Fire

No Yes No

Pool Fire Yes





Toxic Exposure No Consequences But Environmental Impact

Figure 2. The gas release event tree illustrates the sequence of events that can take place in the event of a gas release. The figure shows that UGLD responds at gas leak initiation whereas conventional detectors only respond when the gas has accumulated and formed a vapour cloud.

Page 10

How does UGLD Work?

Gas Release Event Tree The importance of speed of detection can be visualized

go undetected for hours or days before the concentration

using an event tree for gas releases. As illustrated,

becomes sufficiently high to raise an alarm.

an ultrasonic gas leak detector alarms as soon as a pressurized gas leak occurs. After the gas leak begins,

An UGLD provides the fastest response, preventing a

the gas can either be ignited or accumulate. If a gas

dangerous situation from escalating. For this reason

cloud builds up, conventional gas sensors can detect the

UGLD should be considered a first layer of protection

gas and produce an alarm. Similarly, a well placed flame

in pressurized gas installations and used together with

detector can respond to leaking gas in the event it ignites

conventional gas detection methods to secure optimal

and creates flames.

protection in outdoor or well ventilated areas. The total response time for UGLD and conventional gas detectors

The timeframe for the evolution of different scenarios

is further described on the next pages.

varies according to the location of the installation (offshore or onshore), ambient condition (wind direction and speed), gas and leak properties (leak rate and gas type), and other factors. If the gas leak takes place inside a building, the gas can quickly accumulate and prompt a point or open path detector to alarm. However, if the gas leak is outdoors or where the air current is strong, it may

How does UGLD Work?

Page 11

TOTAL Speed of Response Response time for conventional gas detectors is often

speed of response, comprising the time for diffusion to

measured in seconds. Nevertheless, this response is based

the sensor and gas accumulation:

on gas coming directly in contact with the sensor element. This can be difficult in open, well ventilated areas where

Total speed of response for conventional gas detectors

dilution and the direction of the plume can carry the gas

can be calculated as:

away from the sensor.

Ttotal = Tdetector + Tgas

Conventional Gas Detectors When it comes to the response time of a conventional gas detection system, it is important to consider the total

Tdetector is also referred to as T90 (and T50) and it simply tells how long it takes for the gas detector to reach 90%

90% response

Tdetector for UGLD Tgas for conventional detector

50% response

Figure 3

Tdetector for conventional detector

Ttotal for UGLD

T50 T90


Ttotal for conventional detector Page 12

How does UGLD Work?

(or 50%) of the correct reading when 100% of full-scale

The total speed of response for an UGLD can be cal-

gas concentration is injected directly into the sensor-

culated as:

head of the detector. Tdetector is normally 15-30 seconds. Tgas tells how long it takes for a certain gas concentration

Ttotal = Tdetector + Tultrasound

to travel from the leak to the sensor. This parameter

Tdetector for an UGLD is the alarm delay time implemented

is often taken for granted simply because it is difficult

on the instrument, commonly 10-30 seconds.

to predict mainly due to changing wind directions and dilution of the gas cloud. In practice, Tgas can range from minutes to hours!

Tultrasound represents the time it takes ultrasonic noise to travel from the leak source to the detector. This is typically measured in milliseconds.

In a safety system with gas detectors, it is inadequate to use just Tdetector. One must consider the total speed of response, Ttotal, which is the only parameter that provides a true picture of the actual response time of the gas detection system. Ultrasonic gas leak detectors

The response of the UGLD is not dependent on the gas to travel to the detector, which means that it reacts much faster to the dangerous gas leak. Figure 3 on the opposite page illustrates the superior Ttotal for an ultrasonic gas leak detector.

The main advantage of an UGLD compared to a conventional gas detector is that it does not need to wait for a gas concentration to accumulate and form a potentially explosive cloud before it can detect the leak.

How does UGLD Work?

Page 13


High noise areas Low noise areas Very low noise areas

1 2




15 Meter

Figure 4. The graphic shows the detection coverage characteristics for UGLD. The distances are based on the detection of methane based gas leaks using a leak rate of 0.1 kg/s as the performance standard. Page 14

How does UGLD Work?

Detection Coverage Since the sound pressure level decreases over distance at

From the illustration it could be implied that the detector

a predictable rate, operators and engineers can establish

detects gas leaks below ground, but this is rarely the

detection coverage before ultrasonic gas leak detectors

case. The only instance in which a detector responds to

are installed. The location and number of detectors can

gas leaks below ground is when the device is installed on

be planned based on plant drawings when the facility is

a grid floor, which allows ultrasound to travel through the

in the design stage. UGLDs are used to cover both large

cells in the grid with minimum impairment. An UGLD may,

outdoor facilities and single installations. UGLD detection

for example, be installed on an upper platform deck while

coverage depends on the ultrasonic background noise

providing coverage to lower decks as well.

level of the area and on the minimum gas leak rate to be detected. For the purposes of sensor allocation, plant

As shown also, the shape of the detection coverage is

environments can be divided into three types: high noise,

the same for the three plant areas, but the maximum

low noise, and very low noise, as represented in the

detection range varies according to ultrasonic background

graphic to the left.

noise. A more detailed description of specific noise levels and sensor implementation follows in Chapter 4.

The image shows a detector installed on a mounting pole 2 meters (6 feet) above ground as seen from the front. Because the sensor points down when installed, the detection coverage is greater below and to the sides of the sensor than above. Notice that when not obstructed by a floor, the detection coverage is “apple shaped�.

How does UGLD Work?

Page 15

Figure 5

Detection coverage for high, low, and very low noise levels is illustrated in the figures below. Coverage is based on detection of methane leaks using a leak rate of 0.1 kg/s as the performance standard. • High noise areas (eg compressor area) Audible noise: 90-100 dBa Ultrasonic background noise < 78 dB Alarm trigger level = 84 dB Detection coverage = 5-8 meter (16-26 ft)

• Low noise areas (eg normal process area) Audible noise: 60-90 dBa Ultrasonic background noise < 68 dB Alarm trigger level = 74 dB Detection coverage = 9-12 meter (30-39 ft)

• Very low noise areas (eg remote onshore wellhead) Audible noise: 40-55 dBa Ultrasonic background noise < 58 dB Alarm trigger level = 64 dB Detection coverage = 13-20 meter (43-66 ft) Note that very low noise areas settings are generally only used in onshore installations. How does UGLD Work?

Page 17

LEL Vs Leak rate Whereas conventional gas detectors measure gas concentrations as a percentage of the lower explosive limit (LEL) or in parts per million (ppm), the performance of ultrasonic gas leak detectors is based on the leak rate, usually measured in kilograms per second. 20% LEL

100% LEL

Figure 6: The conventional gas detector above measures gas concentration in the lower explosive limit (LEL). The LEL level measured by the sensor depends on the leak rate (mass flow rate), leak directionality, and where the sensor is positioned relative to the leak.

Page 18

How does UGLD Work?

LEL For conventional gas detection, gas concentration is

The categories developed by the body HSE are used

measured in either LEL or ppm. The term LEL is used

to define the guidelines for UGLD. For methane based

for combustible gases and is measured as a percentage.

leaks then UGLD must respond to small leaks of minimum

When the concentration of combustible gas in air reaches

0.1 kg/s.

100% LEL, an ignition of the gas causes an explosion. Guidelines for other gases are found in Chapter 2.

Leak rate The term leak rate describes the amount of gas escaping

Notice an UGLD does not measure the leak rate. The leak

from a leak per unit time. A leak can be considered large,

rate is used to set the performance criteria, and in effect

for instance, if a large quantity of gas escapes every

define, which leaks the UGLD must pick up. The UGLD

hour or every second. Conversely, a leak can be said

provides a measure of the ultrasonic sound measured

to be small if a small amount of gas jets out from the

in decibels (dB). When there is a gas leak with a leak

pressurized system over a given period.

rate of 0.1 kg/s inside the detector’s coverage area, the sound level will exceed the trigger level of the UGLD and

The leak rate, which defines how fast a potential

cause an alarm. As a result, in order to prevent injury or

dangerous gas cloud accumulates, can be divided into

loss of life, UGLDs must detect methane leaks of at least

three categories according to hazard severity:

0.1 kg/s.

· Minor gas leak < 0.1 kg/s

* Reference: HSE website, April 2010: otopdf/2001/oto01055.pdf; page 10

· Significant gas leak 0.1 - 1.0 kg/s · Major gas leak > 1.0 kg/s

How does UGLD Work?

Page 19

Gas pressure, leak size and detection coverage Gas pressure and influence on UGLD The detection coverage illustrated in the previous section is based on a gas pressure of at least 10 bar (145 psi). There is no upper limit. Nonetheless, ultrasonic gas leak detectors can detect gas leaks from pressurized systems kept at much lower pressures. For methane, for instance, a minimum pressure of 2 bar (30 psi) is required to generate ultrasound. Use of the technology in such cases, however, results in reduced detection coverage. For allocation of UGLDs in low pressure systems the manufacturer should be consulted. Figure 7

< 2 Bar (30 psi)

Page 20

2-10 Bar (30-145 psi)

>10 Bar (145 psi)

How does UGLD Work?

Leak size and influence on UGLD

for a leak with a small hole size like 0.5 mm (0.02 in),

The leak size influences the performance of the UGLD in

the systemâ&#x20AC;&#x2122;s pressure must be almost 3,000 bar (or

the following way: the greater the leak size, the bigger the

around 43,500 psi). Since tiny pinhole leaks are found in

leak rate and thus the greater the detectorâ&#x20AC;&#x2122;s coverage

fittings especially on offshore facilities, UGLDs are neither

(assuming the gas pressure is kept constant). Some of

designed for pinhole leaks nor for big pipe ruptures.

the most frequently asked questions pertain to the leak

Pinhole leaks increase in size over time and become

size and whether the opening can be too small or too

easier to detect while pipe ruptures can be identified by the

large to create adequate levels of ultrasound.

pressure drop. Instead of considering specific hole sizes or pressures, UGLD should be related to the leak rate.

The most important thing to understand is that the leak rate can derive from an infinite number of combinations of leak size and gas pressure (gas properties also have some influence). As the hole becomes larger, the leak rate increases. However, with extremely large leaks it becomes more and more difficult to sustain the systemâ&#x20AC;&#x2122;s pressure. When the system pressure starts dropping it causes a reduction of the leak rate and thereby decrease the ultrasonic sound level. In theory, there is no limitation to the rule when the leak becomes small. However, to achieve the commonly used leak rate for methane of 0.1 kg/s

Figure 8. When the pressure is kept constant a small leak has a smaller leak rate and makes less ultrasound compared to a bigger leak.

How does UGLD Work?

Page 21

Frequency and AMPLITUDE Ultrasonic gas leak detection differs from conventional gas detection mainly because it responds to the airborne acoustic sound from the gas leak, and not by sensing the gas molecules. Two new parameters are fundamental to understand ultrasonic technology â&#x20AC;&#x201C; amplitude and frequency, where amplitude is measured in decibels [dB] and frequency is measured in Hertz [Hz].


Frequency (Hz)

The term amplitude is the parameter that describes the

The term frequency is the parameter that describes the

sound level or volume of the acoustic sound. Imagine that

high and low pitches in acoustic sound. To illustrate this,

you sit in front of the radio and turn up the volume, the

low frequencies can be heard from the bass drums in

sound level increases and in the world of acoustics, we

music, whereas high frequencies can be heard from for

say the dB level increases.

example cymbals. This means there are low frequencies and high frequencies.

Amplitude (dB) Figure 9

104 dB

34 dB

Frequency (Hz)

0 Hz

25 kHz

70 kHz

Human hearing range Ultrasonic range detected by UGLD Page 22

Above detection range

The human ear can hear both high and low frequencies,

for example, in spaces with turbines, compressors,

but only within a certain frequency range, typically from

and other high speed rotating machines. In other areas

20 Hz to 20000 Hz (20 kHz). This frequency range is also

there is a simple mix of sound frequencies at low decibel

called the audible frequency range. Frequencies above 20

levels. This is the case in process areas with no rotating

kHz up to 100 kHz are called ultrasonic frequencies. The

equipment or in remote installations in outdoor locations.

human ear cannot hear acoustic sound in this frequency range. The UGLD is designed to ignore audible and

In very noisy plant locations where the audible noise level

lower ultrasonic frequencies and only sense ultrasonic

may be around 95 dB (very loud), the ultrasonic sound

frequencies in the range 25 kHz to 70 kHz.

level will, as a rule of thumb, be 20-30 dB lower (6575 dB) simply because the machine made noise does

An example of the relation between amplitude (dB) and

not generate a lot of ultrasonic frequencies - only a lot

frequency (Hz) is shown in Figure 9. An interactive version

of audible sound frequencies. For actual examples see

can be found on Gassonicâ&#x20AC;&#x2122;s website: www.gassonic.

Table 1 on page 62.

com/simulator/frequency_and_amplitude/ For this reason UGLDs can be installed in very noisy

Frequencies in plant ENVIRONMENTS

locations without interference from the normal audible

In normal industrial plant environments there can be a

background noise.

wide variety of acoustic sound frequencies present or there may be only a limited number. Basically it depends on the process equipment installed in various parts of the plant. In some areas there is a complex mixture of sound frequencies at high amplitude (high dB level);

How does UGLD Work?

Page 23


Are there any installations where UGLD cannot be used? Ultrasonic gas leak detection is only applicable when the gas is under pressure because it is the drop to atmospheric pressure that makes the leak generate ultrasound. In addition, UGLDs cannot be used to detect liquid leaks or in locations with extreme levels of ultrasonic background noise (>95 dB).


• Instant detection of pressurized gas leaks – improved Total Speed of Response

Ultrasonic gas leak detection is used for pressurized gas

• Immunity to gas dilution, leak direction and windy conditions

leak detection. Its detection principle is different from that of concentration-based detectors, and consequently, shares few of the conventional devices’ vulnerabilities. Making UGLD part of the plant fire and gas detection system adds an alternative or complementary layer of protection, which may increase detection efficiency while reducing the need for a high point sensor count.

• Immunity to audible noise and lower ultrasonic noise frequencies • Wide detection coverage (area coverage up to 20 m or 65 ft radius) • Lower implementation and maintenance costs improve total cost of ownership

As the UGLD technology is based on sound propagation instead of transport of gas molecules, detectors respond to hazards at a significantly faster rate than concentrationbased sensors. The detectors are unaffected by environmental conditions like wind, leak dilution, and the direction of the leak, which indicate that they have high detection reliability. The box to the right lists the most important benefits of applying UGLD.

How does UGLD Work?

• Failsafe operation due to integrated acoustic self-test technology • Robust microphone sensor and no consumable components – ensures long lifetime (MTBF > 11 years Naval Sheltered) • Easy to operate and field test (using traceable portable test and calibration unit) • Hazardous area certification: ATEX, C-UL, IECEx, FM, CSA – and SIL certified

Page 25

2 WHICH gases can UGLD DETECT? Ultrasonic gas leak detectors can be used for any application that contains pressurized gas since UGLDs detect the leak and not the gas plume.

Figure 10: Lighter gas molecules typically create higher levels of ultrasound at the same leak rate.

Page 28

How does UGLD Work?

Which gases can UGLD detect? Ultrasonic gas leak detectors can be used for any application that contains pressurized gas since the UGLDs respond to the leak noise and not the gas itself. Ultrasonic gas leak detectors respond to leaks when gas

However, gas properties vary, which influence UGLD

is pressurized and in a gaseous, not liquid, state when

coverage and performance. For example, gas properties

it leaks.

such as molecular weight of the gas and boiling point are important factors. Tests carried out by General

When gas molecules move from a high pressure pipe or

Monitors/Gassonic indicate that gas with a low

vessel to lower or atmospheric pressure, the pressure

molecular weight and a low boiling point generates the

drop generates ultrasound. The ultrasonic sound

highest sound pressure levels (SPL) when the leak rate

pressure waves are then detected by the ultrasonic gas

(mass flow rate) is kept constant.

leak detectors. In extremely cold environments special attention should For standard applications and standard coverage, the gas

be taken to ensure the boiling point of the gas is lower

should be under a minimum pressure of 10 bar (145 psi).

than the ambient temperature. The gas, which would leak

For any application that contains pressurized gas, UGLDs

as gas under normal temperatures, may escape as liquid

can be used since they respond to the leak noise and not

due to the cold environment. In consequence, the leak

the gas itself.

may not generate adequate levels of ultrasonic sound and the detection range of the instrument may be reduced.

Which Gases can UGLD Detect?

Page 29

Example of Gases Hydrocarbon Gases (CxHx)

sparks, or static discharge. Since hydrogen molecules

UGLDs have been used for hydrocarbon gas detection

have a low molecular weight and a low boiling point, they

since the mid-1990â&#x20AC;&#x2122;s and the technology has proven

produce high ultrasonic sound levels. UGLD is therefore

effective detecting hydrocarbon gases that leak in

ideal for identifying hydrogen leaks.

a gaseous state. As previously mentioned, the specific gas properties influence detection coverage. In general,


lighter hydrocarbons (methane, ethane, and ethylene) are

Because CO2 can only be a liquid or a supercritical fluid

detected at greater distances than heavier hydrocarbons

when pressurized, the material escapes as a gas when

(propane and propylene).

released into atmospheric pressure and temperatures above -78°C. This CO2 gas generates high levels of

Hydrogen (H2)

ultrasound that can be detected by UGLDs.

Due to its chemical properties, hydrogen poses unique challenges in the plant environment. Hydrogen gas is

Carbon dioxide detection is becoming increasingly relevant

colorless, odorless, and not detectable by human senses.

due to heavy investments in massive carbon capture and

It is lighter than air and hence difficult to detect where

storage (CCS) projects as well as projects where CO2

accumulations cannot occur. It also cannot be detected by

is used for reinjection to enhance reservoir pressure.

infrared gas sensing technology. Besides being difficult to

The investments in CCS may reduce CO2 emissions to

detect, hydrogen gas itself possesses severe safety risks:

the atmosphere. CO2 detection is essential to limit the

it is easily ignitable on contact with open flames, electrical

environmental impact of a release and production losses.

Page 30

Which Gases can UGLD Detect?

H2S may also be mixed into the CO2, which increases the

Condensate, 2-Phase, and Liquid Leaks

need for detection. UGLD responds well to CO2 leaks.

The detection of mixed or 2-phase (liquid/gas) leaks

Hydrogen Sulfide (H2S)

equilibrium with liquids are common in many process

by UGLDs is a subject of recent interest. Gases in

Because H2S is highly toxic, industry and governmental

streams. Preliminary work has shown UGLDs provide an

organizations often demand the earliest possible

adequate response to certain water/steam leaks. The

detection of H2S gas releases. As for other pressurized

sound pressure level generated by these leaks is low,

gas installations, UGLDs can be used as a first protective

however, restricting the area coverage of the device

layer in H2S with technologies specifically designed to

to a few meters, or areas with low ambient ultrasonic

detect H2S.

background noise. The results are consistent with those

Reservoirs that contain high concentrations of H2S are likely to hold a mixture of gases and liquids. When

published by the Health and Safety Executive (HSE) on the response of UGLDs to pressurized water leaks.*

H2S escapes before it is separated, it is usually mixed

To summarize, pure liquids do not generate adequate

with hydrocarbon gases. An UGLD does not distinguish

levels of ultrasound to make the use of UGLDs viable for

between the different gases but the leaking pressurized

these types of applications. It is assumed, however, that

hydrocarbons with H2S mixed into it will generate high

most 2-phase leaks containing small liquid fractions are

levels of ultrasonic noise, which are picked up by the

detectable by UGLD at a reduced coverage.

UGLDs. UGLDs should not be used to detect H2S after it has been separated from the hydrocarbon gas.


M. Royle et el. â&#x20AC;?Measurement of Acoustic

Spectra from Liquid Leaksâ&#x20AC;?, Research Report RR568. HSE Books, Colegate, England, 2007.

Which Gases can UGLD Detect?

Page 31

Gases and coverage As described on the previous pages it is understood that

the industry guideline for detection of methane based

different gases have different characteristics. This will

gas leaks and a useful guideline for carbon dioxide and

have some influence on how high ultrasonic noise the

ethylene leak detection.

gas will make. In effect this will influence on how far away an UGLD can pick up gas leaks.

The graphs represent the ultrasonic sound pressure levels [SPL], which the UGLD picks up (Y-axis). It shows

In the graphs on the following pages, the design engineer

that the SPL becomes lower when the distance between

can find the detection coverage for different gases at

the detector and the leak source increases (X-axis). The

different background noise levels. This makes it relatively

three solid horizontal lines represent the trigger level in

simple to estimate the number of ultrasonic detectors

the three standard plant noise areas. When the line that

required for the specific application. NOTE: The graph

represents the relevant noise area meets the graph the

should not be stretched to its limits as detection range

detection coverage for that gas can be found.

of an UGLD may decrease when the leak is pointed away from the detector. The implementation practice (chapter

As an example, consider an installation in a high-noise

4) provides further guidance. When using the graphs some

area by a compressor module: The ultrasonic background

overlapping coverage can be used to ensure full detection.

noise is typically between 65-75 dB. The trigger level is then set to 84 dB or a corresponding value in mA (see also


page 61 for further directives). The individual detectorâ&#x20AC;&#x2122;s

The graphs on the opposite page show UGLD coverage

coverage in radius can now be found in the graph. For

when detecting leaks at a leak rate of 0.1 kg/s. This is

methane the detection radius is 7 meters (23 feet).

Page 32

Which Gases can UGLD Detect?


Trigger level [dB (ultrasound)]

Reference test set-up for these curves, see Figure 12



High noise area



Low noise area

69 CH4 (methane)

Very low noise area

64 C2H4 (ethylene) C02 (carbon dioxide)






Example: Detection coverage in high noise area for CH4


12 CH4 (0,1 kg/s)



C2H4 (0,1 kg/s)



Detection distance [Meter]

C02 (gaseous) (0,1 kg/s)

Figure 11

Page 33


should not be used without taking other factors (such

The graphs on the opposite page show UGLD coverage

as blockage) into account. Full verification can always be

when detecting leaks at a leak rate of 0.01 kg/s. This

performed at commissioning (see phase four in chapter 4).

may be used for a lighter and more explosive gas such as hydrogen. It may also be used for detection of propane

The digital version of the Technical Handbook is updated

or propylene, which are typically found in processes with

on a frequent basis as more gases are tested and further

lower pressure. Note that the coverage for propane or

knowledge is obtained. To find the latest graphs and most

propylene is based on 0.018 kg/s leaks or 0.016 kg/s

recent updates and to sign up for automatic updates


you can go to For further input it is recommended to contact a Gassonic or General

Again, as an example on how to read the graph, consider

Monitors representative.

an installation in a high-noise area by the compressor Gassonic UGLD

module: It is found that the detection radius for hydrogen in this area is 7-8 meter (25 feet).

Test gas 3 meter (10 feet)

NOTE: The graphs on these pages are based on comprehensive live tests carried out at Gassonic, Denmark, and

Test nozzle

General Monitors, USA. The results are published with

Detection distance

a safety factor to enhance reliability and repeatability. It should be noted that the detection coverage is based on

1 meter (3 feet)

the assumption that the escaping material is a gas when it is released. It should also be noted that the information on these pages should be seen as guidelines only and

Page 34

Ground Figure 12: Reference test set-up for curves in Figures 11 and 13.

Which Gases can UGLD Detect?


Trigger level [dB (ultrasound)]

Reference test set-up for these curves, see Figure 12



High noise area



Low noise area H2/He (hydrogen/helium)


Very low noise area

64 C3H8 (propane) C3H6 (propylene)






Example: Detection coverage in high noise area for H2


12 H2/He (0,01 kg/s)




C3H8 (0,018 kg/s)


Detection distance [Meter]

C3H6 (0,016 kg/s)

Figure 13

Page 35

3 WHERE TO INSTALL UGLD As a rule of thumb, ultrasonic gas leak detectors offer distinct advantages in outdoor or ventilated areas over conventional gas detection methods which are susceptible to changing wind directions and gas dilution.


How can UGLD work in noisy plant environments? The detectors can be installed in noisy environments because the technology only responds to high frequency noise. Escaping pressurized gas generates so-called â&#x20AC;?white noiseâ&#x20AC;?, in both audible and ultrasonic frequencies. Normal background noise generated by humans and plant equipment, makes noise primarily in the lower frequencies, which are filtered out by the UGLD.

Applications Ultrasonic gas leak detectors can be used in a wide range

• Chemical Processing Plants

of application areas where there is pressurized gas. As a rule of thumb, UGLDs offer distinct advantages in outdoor or ventilated areas. Conventional gas detection

• Floating Production Storage and Offloading Vessels (FPSOs)

methods have some imminent limitations as gas must accumulate reach a sensor (point gas detection) or pass within a beam (open path detection) to be detected. In such areas, there is a risk that the gas will quickly dilute before accumulation can occur or the wind will blow the gas cloud away before reaching the sensor. Studies carried out on gas leaks in offshore facilities in the UK state that conventional detection methods only detect 65% of the registered gas leaks (see inside front cover). As UGLDs respond to the leak noise, which is

• Gas Compressor and Metering Stations • Gas Turbine Power Plants • LNG/GTL Trains • LNG Re-gasification Plants • Offshore Oil & Gas Platforms • Onshore Oil & Gas Terminals

omni-directional and indifferent to wind directions, they may be used to improve the gas detection efficiency.

• Refineries

A selection of typical installations where UGLDs are being

• Underground Gas Storage Facilities

used can be found in the box to the right. This chapter further details different applications for UGLD. Which Gases can UGLD Detect?

Page 39 Page 39

9 Pig launcher and receiver 8 Gas metering

6 Filter station 3 Power substation

7 Separator

5 Cooler 2 Gathering manifold 1 Wellheads

4 Gas compressor

Figure 14: A generic gas processing plant where the ultrasonic gas leak detectors are installed to protect potential leak spots in all areas with high-pressure gas. These are marked with a number. Page 40

Which Gases can UGLD Detect?

Applications: Gas Processing Plant A typical gas processing plant can be used to illustrate where UGLDs can be applied.

In this facility, gas is being extracted from the wellheads

containment. Other detection means like infrared point

(1) and piped through a central manifold (2) to the rest of

detectors can identify and measure the concentration of

the processing facility. The water knockout drums remove

the gas, giving users a better sense of the scale of the

the water before it reaches the gas compressor (4). The

hazard. Chapter 6 addresses the application of different

cooler (5) ensures optimal temperature before the filter

detection technologies for greater safety.

and separation processes (6 and 7). After separation, the gas is transferred to a gas metering skid (8) on to the gas outlet, where it is distributed through a grid for consumption, storage, or export. Ultrasonic gas leak detectors are applied in all outdoor locations with pressurized gas. Locations in this example include wellheads, compressor area, cooler, filter station, separator, gas metering skid, and receiver area. UGLDs should be installed as a first line of defense to provide the earliest possible warning to a loss of

Where to Install UGLD

Page 41


Figure 14a

Page 42

Where to Install UGLD

Applications: Separation Area In the separation process, moisture, hydrogen sulfide, and other contaminants are removed from the gas stream.

As shown in the opposite diagram, the separator unit includes the separator itself, a gas exchanger, and gas coolers that ensure the correct temperature for the process. Ultrasonic gas leak detectors are positioned so they cover potential leak sources like the flanges and valves on the separator module and on the coolers. Obstructions are taken into account when determining the number of detectors required for optimal coverage. Detector implementation and placement is discussed further in Chapter 4.

Gassonic Observer UGLD

Where to Install UGLD

Page 43

Applications: Wellheads Wellheads are used to extract and produce oil and gas. The oil and gas pressure is often high in the wellhead area are therefore suitable for ultrasonic gas leak detection.

The top of the wellhead usually consists of a collection

compressor stations, or oil export terminals. As depicted

of valves that makes up a production tree or â&#x20AC;&#x2122;Christmas

on the onshore wellhead below, one ultrasonic gas leak

treeâ&#x20AC;&#x2122;. These valves regulate pressures, control flows, and

detector adequately covers the production tree including

allow access to the wellbore in case further completion

valves and flanges on the topside of the wellhead and

work is needed. In offshore installations the wellheads

potential leak areas inside the wellhead pit if it is exposed.

are normally found on the lower platform decks (in the well bay area) whereas onshore they are found either


in close vicinity of a processing plant (Figure 14b) or in remote locations. A wellhead may also be situated on the top of an underground gas storage facility. From the outlet valve of the production tree, the flow can be connected to a distribution network of pipelines to supply the extracted substance to refineries, natural gas UGLD Figure 14b

Page 44

Where to Install UGLD

Applications: Compressor Area Gas compression takes place in the compressor area to increase the gas pressure for transmission

The natural gas compressor area is prone to leaks be-

in the front ensures coverage along the front side of the

cause the operating environment includes a combination

compressors whereas the second detector placed on the

of heat, high operational pressure, and vibration. Leaks

staircase covers the flanges on the top and the back of the

could be caused by the following failures:

compressors. Conventional gas detectors are used inside the housing and on the air intakes.

• Degraded pipes from exposure to corrosive gases · Broken or unseated seals or bearings within the compressor engines


• Rupture in pipe systems or machinery • Degraded flanges or valves Figure 14c exemplifies a typical compressor module with two compressors. The gear box and turbine are situated in the housing to the left. High levels of ultrasound may be generated by the turbine inside the enclosure but the housing attenuates the ultrasound so it does not interfere with the ultrasonic gas leak detectors. The UGLD installed

Where to Install UGLD

UGLD Figure 14c

Page 45

Gassonic service engineer simulating real gas leaks while performing onsite commissioning. Besides testing that the ultrasonic detectors are within accepted tolerances, the commissioning verifies the actual detection coverage and performance of the detectors.

4 How to implement and maintain UGLDâ&#x20AC;&#x2122;s Implementation and installation of ultrasonic gas leak detectors are different than those for conventional gas detectors because UGLD is based on detection of airborne acoustic sound waves. This means there are some particular guidelines that need to be followed when implementing UGLDs.

Phase One: Where in the project should UGLD be implemented? Implementation Practice

wellhead or if it is a large gas plant where hundreds of

The implementation of ultrasonic gas leak detectors can

detectors are needed.

be divided into the following four phases: • Where in the project should ultrasonic gas leak detection be implemented?

WHICH AREAS ARE SUITED FOR UGLD? The first question to be asked is if the project comprises process facilities with compressed gas with a pressure higher than 10 bar (145 psi). If the answer is yes, UGLD is

• Where should each individual detector be installed? • Detailed engineering considerations • After installation: How are the UGLDs commissioned and tested? These four phases are carried out for each new project where UGLD is implemented. This applies whether the project consists of one single detector on a high pressure

Page 48

well suited for the project (see page 20 for installation in systems with lower pressures). Next, is the pressurized gas installation indoors or outdoors? If it is an outdoor or ventilated gas installation, UGLD is the optimal detection technology to use as a first line of defense to ensure fast and reliable detection. UGLDs are well suited for installation both onshore and offshore and several thousand UGLDs are installed both onshore and offshore in areas with high pressure gas.

How to Implement and Maintain UGLD

Examples of process areas adequate for UGLD are listed in the box to the right. For installation near air intakes for machines and in accommodation facilities, conventional gas detectors are more suited than UGLDs due to lack of

Areas suited for UGLD: Fin fan gas coolers


Gas compressor and separation areas Gas metering skids and manifold areas Hydrocracking, hydrotreating, catalytic reforming Pig launchers and receivers Wellheads

How to Implement and Maintain UGLD

Page 49 Page 49

PHASE TWO: Where should individual detectors be installed? Figure 15

Once the project team has determined that UGLDs are suitable for a project, the next step is to determine detector placement. Step 1: Obtain the layout design of the plant.



Step 1

Step 2

Step 2: Mark the areas where there is pressurized gas and where there are potential leak sources. Step 3: Determine the detection coverage each ultrasonic detector should have based on the ultrasonic plant noise. Next, draw circles on the plant fire and gas layouts to determine how many and where the detectors should be installed. Gassonic (General Monitors), a local representative or a design consultant can provide input at this stage. By following this procedure, users can obtain an estimate of the number of detectors required to cover the entire area. MAC

Step 3 Page 50

How to Implement and Maintain UGLD

Figure 16

Offshore Installation of UGLD A platform used to exemplify an offshore installation is divided into different sections:

Top deck Main deck Lower deck Wellbay deck

How to Implement and Maintain UGLD

Page 51

Top deck

Figure 16a

On the top deck, which is sometimes referred to as the weather deck, the fin-fan coolers help lower the temperature when the gas is compressed and thus heated. The pressurized gas is typically found on the side of the cooler structure and not over the center of the finfans. Ultrasonic gas leak detectors are installed to cover all potential leak sources down the side of the coolers. Since the top deck is exposed to potential high levels of ultrasound the detection coverage is based on high-noise area settings. The detection radius is based on 5-8 meter coverage (16-28 feet). In this example nine detectors protect the area. UGLDs are unaffected by helicopter landings and take offs. The air intakes at the bottom of the drawing are covered with conventional gas detectors because accumulation of gas may occur there.


Page 52






How to Implement and Maintain UGLD

Main deck





The main deck houses the main processing facilities,

Figure 16b

which consist of the compressor skids, inlet gas separation, export gas metering, gas exchanger, and the fuel gas skid. The entire process area is protected with ultrasonic detectors because most of the facilities FP

operate at high pressure. Generally the detection



area settings, which gives at least 9-12 meters (30-39


coverage in the process area is based on low-noise feet) coverage in radius. However, near the compressors that produce relatively high levels of ultrasound along with the loud audible noise, the settings should be based on high-noise area settings. This means that the detection radius decreases to 5-8 meters (16-28 feet) in radius. When implementing detectors here it must be considered if there is any blockage from big vessels such as the compressor unit, which could make it necessary FP

to include additional detectors. A total of nine detectors cover the deck in this example. MaC FP

How to Implement and Maintain UGLD

Page 53

LOWER deck The lower deck of the platform contains pipe work, which includes potential leak sources such as flanges and valves. In the bottom of the drawing is the pigging section where the pig is launched or received as part of maintenance and inspection of the pipeline. The whole area is regarded as low-noise because there is no heavy or rotating machinery. The detection radius is therefore FP


set at a minimum of 9 meters (30 feet). Most of the area includes potential high-pressure leak sources. A total of six detectors are implemented to provide optimal coverage of the area.




Page 54






Figure 16c

How to Implement and Maintain UGLD

Wellbay deck The wellbay area primarily consists of the wellheads where the gas comes in at high pressure after it is extracted from underground. Often the wellbay could be located on one or more smaller dedicated platforms separated from the main processing platform. On the wellbay deck, manifolds and separation facilities (at the bottom) can also be found. Since the ultrasonic background noise in this area is low, coverage for each detector is set at 9-12 meters (30-39 feet). A total of eight detectors cover this area due to the physical shape of the wellheads. There is some overlapping coverage because the risk of shadowing needs to be taken into account, which could limit coverage in some angles.


Figure 16d

How to Implement and Maintain UGLD

Page 55

Onshore Installation of UGLD

Most of the facility can be considered a low-noise area

The onshore oil and gas plant is divided into different

providing 9-12 meters (30-39 feet) coverage in radius. In

sections or trains. Unlike the offshore facility, the onshore

the compressor area, which is considered a high-noise

plant covers a large area with long distances between

area, the coverage is 5-8 meters (16-28 feet) in radius.

units. However, the overall implementation practice for

In the plant drawing an average of 10 meters (33 feet)

UGLDs is the same. In the image to the right a typical

is used to provide a rough estimate of the number of

gas plant can be seen. The numbers indicate the areas

detectors required.

where there is pressurized gas: wellheads, manifold, separator, coolers, and compressor (see also Figure 14

In this facility, there is a total number of 23 UGLDs

on page 40). On Figure 17 the gas plant is shown as

identified to provide coverage of the entire facility.

a plant layout drawing. The steps required to allocate UGLDs are as follows: Step 1: Obtain plant drawings and identify where there is

9 8

pressurized gas making it suitable for UGLDs 6

Step 2: Determine the detection coverage for each



UGLD based on the ultrasonic plant noise in the area of installation.

2 1


Step 3: Draw circles on the plant fire and gas layouts to determine how many and where the detectors should be installed.

Page 56

How to Implement and Maintain UGLD















Figure 17: This plant layout shows the gas processing plant from the left hand side in top view. Each circle represents the coverage from an ultrasonic detector. How to Implement and Maintain UGLD

Page 57

PHASE THREE: Detailed engineering considerations Choosing the correct height

With UGLDs, the installation height is always the same. It

For optimal operation an ultrasonic gas leak detector

is not dependent on the gas because the ultrasonic sound

must be installed above or to the side of potential leak

generated by the gas travels in all directions around the

sources and where no solid structures obstruct the path

leak regardless of the weight of the specific gas.

between the detector and the potential leak source. The main rule of thumb is that the UGLD should be installed 1-2 meters (3-6 feet) above the potential leak source. This ensures optimal use of coverage and provides easy access for installation and maintenance, (see also coverage characteristics on Figure 4, Page 14). The installation height of conventional LEL-based gas detectors often depends on the target gas. Some gases

1-2 meter

are heavier and others are lighter than air. As a result, an LEL gas detector needs to be installed in a high position and sometimes in a low position depending on the properties of the gas.

Figure 18: To the left correct installation of an UGLD in 1-2 meters height above the potential leak source. To the right the detector installed too high. Page 58

How to Implement and Maintain UGLD

Figure 19

ShadowING or blockage Ultrasonic gas leak detectors do not operate optimally if there is a solid, physical blockage between the leak source and the detector element. If there are solid structures such as big vessels, tanks, compressors, or separator units that can block the path from the detector to the potential leak source, they could attenuate or block the ultrasound before reaching the detector. This is

Correct installation of UGLD with regards to blocking and shadows

known as shadowing or blockage. As shown on Figure 19 normal pipework does not block the sound. When implementing UGLDs in the design stage and upon physical installation of the detectors, shadowing/ blockage must be taken into consideration. In the example on the bottom right, the detector must be moved to the other side of the tank or a second detector installed to cover the valve on the right-hand side in order to ensure adequate coverage. The commissioning practice (see page 67) reveals spots at a plant where shadowing or

Incorrect installation of UGLD from the gas leak because the ultrasound is blocked by the vessel.

blockage could affect detection coverage.

How to Implement and Maintain UGLD

Page 59

How to obtain optimal detection coverage

These noise sources represent roughly 95% of worldwide industrial noise in plants. Even though plant noise can be very loud most of it is low frequency noise, which does not

Ultrasonic gas leak detectors detect the acoustic

influence UGLD. The ultrasonic detectors are equipped

ultrasonic noise generated from pressurized gas leaks.

with high pass filters, which effectively eliminate audible

Since the UGLD only “hears” the high frequency sound

and lower ultrasonic background noise.

(the ultrasound), it is immune to most acoustic noise present in plant environments. Normal background

If plant noise didn’t contain any ultrasound, the detector

noise in an industrial plant may come from the following

could always be in its most sensitive alarm level setting,


and thereby always obtain the same wide detection coverage from the detector. However, in plant areas

• Process fluids or gases running inside pipes and vessels • Rotating machines such as compressors and fin fan coolers • Plant personnel and vehicles

that contain compressors or other high-speed rotating equipment, some ultrasonic noise is generally present, which in turn can interfere with the UGLD. Therefore, it is important to have an overall understanding of the area each UGLD is going to be installed in to ensure the correct alarm trigger level set-point on each detector and that it is always above the base level of ultrasound in the area.

• Airplanes or helicopters

Page 60

How to Implement and Maintain UGLD

Setting the right trigger level Figure 20 shows the three typical noise scenarios in

Figure 20

84 dB

plant environments and the optimal detector trigger level (see also page 17 for a more detailed description of the

74 dB

different areas). The straight black line shows the trigger level set-point in dB and the red trend curve shows

64 dB

the actual ultrasonic noise at the detector. Notice that unlike concentration based detectors, which read out a

Trigger level in high noise area

percentage of LEL, the UGLD reads out the ultrasonic noise in dB, which has a corresponding value on the 4-20

84 dB

mA signal. As an example, 84 dB may correspond to 13 mA. An online demo enables the user to try different noise settings and set-points on Gassonicâ&#x20AC;&#x2122;s website,

74 dB 64 dB

noise/ Trigger level in low noise area 84 dB 74 dB 64 dB

Trigger level in very low noise area How to Implement and Maintain UGLD

Page 61

Ultrasonic Noise Sources AND Plant Noise Levels

important to understand that the ultrasonic detectors are

The table below exemplifies some of the normal noise

away. This means that audible noise measurements made

only affected by ultrasound â&#x20AC;&#x201C; the audible noise is filtered

sources found in plant environments and the normal noise

in plants must not be used in relation to UGLD. Such studies

levels they produce. It also incorporates the recommended

are done to ensure the proper work environment and to

trigger level and resulting detection coverage. The table

know when the audible noise becomes so loud that hearing

shows both audible and ultrasonic noise levels. It is

protection must be used by personnel.









84 dB

5-8 m

Helicopter deck



84 dB

5-8 m

Choke valves



74 dB

8-12 m




74 dB

8-12 m

Well bays



74 dB

8-12 m




74 dB

8-12 m

Metering skids



74 dB

8-12 m




74 dB

8-12 m

Coolers (fin-fans)



74 dB

8-12 m

Onshore wellheads



64 dB

13-20 m

Table 1: Note: the above readings are based on representative recordings taken during onsite surveys done by Gassonic. The noise levels may fluctuate from plant to plant. Page 62

How to Implement and Maintain UGLD

How to Avoid Influence from Other Noise Sources and False Alarms

High level acoustic noise generated by gas flowing inside

As explained before, UGLDs are immune to most normal

ultrasonic noise that could generate false alarms,

plant noise since it is low frequency noise, which the

but that is not the case. Although the acoustic sound

UGLD filters away. However, other noises can occur in

generated inside the piping, and especially inside a choke

the plant environment which could trigger the UGLD

valve, may contain high frequencies (ultrasound) most

(for example, air driven valves and actuators). This kind

of that ultrasound is blocked by the pipe wall. Compare

of noise is typically of short duration (1-10 seconds),

it with hearing music through a wall. The low frequency

whereas a real gas leak is continuous and lasts for a

bass sounds are heard whereas the high frequencies are

much longer period of time. To avoid false alarms, it is

blocked by the wall.

the pipes is often mentioned as a potential source of

recommended to always implement a short alarm time delay (â&#x2030;Ľ10 sec).

Gas leak

Gas leak Alarm

Short high dB spike Trigger level at 74 dB Background noise at 64 dB

Figure 21

How to Implement and Maintain UGLD

10 sec. Delay

Page 63

Services - maintenance

the detectorâ&#x20AC;&#x2122;s linearity and the loop to the control room

An ultrasonic gas leak detector is a low-maintenance

are verified. For the most advanced models, calibration

product since it does not have any consumable parts.

can be performed onsite.

Conventional gas sensor elements degrade over time due to exposure to gas or other substances. Routine

Some detector models have an integrated self-testing

maintenance service is therefore limited to once every

mechanism (such as Gassonicâ&#x20AC;&#x2122;s Senssonic self-test),

6-12 months for UGLDs.

which tests the detectorâ&#x20AC;&#x2122;s performance at short intervals. This ensures the operation and performance of the

Routine testing is carried out by means of a traceable

ultrasonic detector between inspections.

portable calibration device. During routine maintenance,

Gassonic 1701 test and calibration unit

Page 64

Test carried out with traceable portable test and calibration unit.

How to Implement and Maintain UGLD

Services - onsite mapping survey

onsite mapping survey should always be performed by

An onsite mapping survey can be performed in order

engineers trained and certified by the manufacturer or an

to optimize detector positioning or to verify detector

experienced consultant.

allocation based on plant layouts. When Gassonic trained engineers are carrying out the In existing (brownfield) installations it is possible to

mapping survey, a full documentation report is created.

determine the ultrasonic background noise prior to

The report includes specific suggestions for where to

installation by performing an onsite mapping survey. A

install each detector and how to set the trigger level in

survey may reveal sources of short timescale intermittent

each location for optimal coverage.

ultrasonic noise like pressure relief valves or sources that may produce some continuous ultrasonic noise levels like turbo compressors and turbines. Based on the survey findings, one can determine the exact coverage from each ultrasonic detector and therefore the total number of detectors required to cover the facility. An

Onsite mapping survey performed by Gassonic trained engineer.

How to Implement and Maintain UGLD

Page 65

Gassonic service engineer simulating real gas leaks while performing onsite commissioning. Besides testing that the ultrasonic detectors are within accepted tolerances, the commissioning verifies the actual detection coverage and performance of the detectors.

PHASE FOUR: How are the UGLDs commissioned and tested? Services - onsite commissioning and

the predefined safety standards â&#x20AC;&#x201C; and that it picks up

leak simulation

gas leaks. For optimal verification, onsite commissioning

Start-up commissioning of an ultrasonic gas leak

and leak simulation should always be carried out by

detection system consists of two steps. First, the detector

engineers trained and certified by the manufacturer or

must be tested to ensure detector linearity. This is done

by experienced maintenance engineers using the proper

by means of a customized portable test unit (Gassonic


1701). This exercise can be compared to the test that is carried out on a point sensor with a test gas or on a beam detector by means of a filter that absorbs infrared light. Second, simulated leaks are generated at critical leak spots in the perimeter of the coverage area to ensure full detection of the facility and verify that coverage is not influenced by shadowing. The leaks are carried out by means of a non-explosive replacement gas (nitrogen for methane and helium for hydrogen) at the performance standard leak rate of 0.1 kg/sec (methane). This verifies that the ultrasonic gas detection system complies with

How to Implement and Maintain UGLD

Equipment used to perform leak simulation onsite.

Page 67

5 Which UGLD products are available? The UGLD technology has been on the market since 1996. Today a number of different detector models are available. These pages describe the overall principles and features to consider when implementing UGLD.

Considerations when specifying ULTRASONIC GAS LEAK DETECTORS The UGLD technology has been on the market since 1996. Today a number of different detector models are available. These pages describe the overall principles and features to consider when implementing UGLD. You can find a table outlining the available main detector models here as well. Explosion proof AND intrinsically safe

addition, these sensors require little maintenance, about

There are various reasons for choosing explosion proof

1 or 2 times per year, and in rare occasions, calibration.

(Exd) or intrinsically safe (Exi) designs in hazardous

Calibration can be performed onsite with a portable test

location equipment. Choices may be based on a preferred


engineering practice or the characteristics of the facility (ex. voltage and power requirements).

Acoustic Self-Test (self-diagnostics) To ensure failsafe operation and avoid unrevealed

Sensor element

failures in between inspections, certain ultrasonic gas

The most common type of sensor for UGLD is stainless

leak detectors are equipped with an acoustic self-test like

steel microphone technology. Stainless steel microphone

Gassonicâ&#x20AC;&#x2122;s Senssonic. This self-test emits an ultrasonic

sensors are reliable, exhibit little drift, and have

signal every 15 minutes. If the signal is not detected by

an excellent record of operation in harsh industrial

the sensor element, the detector produces a fault.

environments in both arctic and desert conditions. In

Page 70

Which UGLD Products are Available?

Hazardous Area Approvals and

Power Consumption

Safety Integrity Levels

UGLDs consume little power, since the technology

Detector models are certified by several regulatory

requires no constant power source for operation. For

agencies and under several directives. Regulatory

installation in isolated locations like a remote solar

approvals include FM, CSA, C-UL, ATEX, IECEx, and GGTN

panel powered application, certain detector models can

K. UGLDs are also certified under FM Approvals to comply

operate on minimal power. Maximum consumption is

with IEC 61508 being as SIL 1 and 2 suitable.

found to be lower than 1 Watt (36 mA at 24 VDC) in these instruments.

Communication buses and protocols Standard 4-20 mA analog output and digital communication are available with UGLD. Information on serial connections and other communication topics are available on the Gassonic website.

Which UGLD Products are Available?

Page 71

Product Comparison Table

Sensor technology Protection method Response time Hazardous area certification

Gassonic Surveyor

Gassonic Observer

Stainless steel electret condenser

Stainless steel electret condenser



EEx ia

EEx d

< 1 sec

< 1 sec



SIL suitability



Ingress protection



Acoustic self test Interface 4 â&#x20AC;&#x201C; 20 mA

X (Integrated Senssonicâ&#x201E;˘ self-test) X

Digital communication


Interface: Alarm relay



Interface: Fault relay



5 dB

5 dB

0 to 480 sec in steps

0 to 600 sec in 10 sec steps

Alarm trigger level steps Internal alarm delay setting Page 72

Which UGLD Products are Available?

User interface Cable connection

Gassonic Surveyor

Gassonic Observer

LED indications

Full interactive display

conduit/cable gland

conduit/cable gland

Detector set up

Factory set-up/Field adjustable

Factory set-up/Field adjustable

Coverage radius @ 0.1 kg/sec

≤ 20 meters (background noise

≤ 20 meters (background noise



Field testing with calibrated device

Yes (with the Gassonic 1701)

Yes (with the Gassonic 1701)

Field calibration option

Yes (with the Gassonic 1701)

Yes (with the Gassonic 1701)

44-104 dB

58-104 dB

Stainless steel

Stainless steel

-40 to +75°C

-40 to +60°C

>10 years

> 10 years

Dynamic range Material Temperature range Sensor life Accuracy Input Power

+/- 3 dB

+/- 3 dB

13 - 28 VDC

15 - 30 VDC

36 mA @ 24 VDC

250 mA @ 24 VDC

Table 2 - In general, all Gassonic detectors can be used for any high-pressure gas installation. However, one feature that makes the Gassonic Observer stand out is the fact that it has integrated self-test capabilities. This means that the risk of unrevealed failures in-between inspections can be eliminated. In certain applications, such as unmanned or remote installations (on NUIs or remote wellheads etc.), where access and frequent routine testing is more problematic, this is particularly important. Although all Gassonic detectors consume relatively little power, in locations with particularly limited power resources, the Gassonic Surveyor can be the optimal choice as its maximum drag is less than 1 Watt (43 mA at 13 VDC).

Which UGLD Products are Available?

Page 73

6 How to interface UGLD and mix with other technologies Ultrasonic gas leak detectors can interface with control systems just like conventional detection methods. The main difference is that the output from UGLDs corresponds to leak noise (measured in dB), rather than gas concentration (measured in % LEL).

How to interface the detectors with the control system Ultrasonic gas leak detectors have various customer selected output methods, which both include analog and digital communication modes. Whether an analog or digital communication method is preferred depends on how the specific safety system is designed. Output methods make it easy to integrate the ultrasonic gas leak detectors into standard control systems like a DCS, PLC, local fire and gas panel as well as customized General Monitors control systems (MC600 and HazardWatch). An ultrasonic gas leak detector can interface with control systems just like traditional detection methods. The main difference is that the output from UGLDs correspond to leak noise (measured in dB), and not gas concentration (measured in %LEL). This difference in unit of measurement must be taken into consideration when the safety system is configured and the alarm levels are incorporated. Alternatively, a dedicated multi-channel controller or input card can be used to transform the signal from the UGLD into a dB value.

Page 76

How to Interface UGLD and Mix with Other Technologies

Configuring Trigger Level and Alarm Delay Time

alarm. When determining the right delay time, the surrounding machinery must be taken into consideration since valves and actuators could be sources of spurious

When installing ultrasonic gas leak detectors two

alarm. Typically, a time delay of 10-15 seconds should be

essential factors must be considered - the alarm trigger

implemented. For general guidelines regarding prevention

level and the time delay.

of spurious alarms (see page 17).

The alarm trigger level refers to a specific dB value. If

The trigger level and time delay must either be

this value is reached, the detector goes into alarm mode.

implemented in the control system or fire and gas panel.

When determining what the right trigger level is for the

Alternatively, it can be configured in the detector itself if

detector, the ultrasonic background noise in the area

relays are being used.

must be taken into consideration. It is recommended to set the trigger level at least 6 dB higher than the measured ultrasonic background noise or use the general guidelines for implementation in ‘high-noise’, ‘low-noise’, ‘very low-noise’ areas (Chapter 4, phase three). The dB range of an ultrasonic detector directly corresponds to a 4-20 mA value so that if the dB range goes from 58-104 dB, 4 mA equals 58 dB and 20 mA equals 104 dB. The introduction of a time delay is used to prevent spurious alarms. Without a time delay, any ultrasonic sound exceeding the trigger level would produce an

How to Interface UGLD and Mix with Other Technologies

Page 77

Executive Action and Voting Configuration Ultrasonic gas leak detectors can be used for alarm or executive action depending on the operatorâ&#x20AC;&#x2122;s needs.

follows examples of how voting configurations between different technologies can work together: Open path detectors and UGLDs

These detectors are commonly used as part of a voting

UGLDs are used for area coverage and gas leak detection in

mechanism, which would either vote the UGLD with

all high-pressure areas. The open path detectors are used

other UGLDs or with conventional detectors (open path

for fence line monitoring to measure the concentration of

or points). The voting configuration may be 2 out of any

the gas.

detector on a deck (2ooN), regardless of the number of sections or enclosed spaces on that deck, or 2 out of 3 detectors in an area (2oo3). In case two detectors identify a gas leak, the fire and gas system automatically issues an alarm and initiates a protective response according to the nature, location, and severity of the detected hazard. This may include starting up the deluge system, initiating beacons and horns, or by performing a full emergency shutdown. It is considered good practice to vote gas detectors to prevent spurious alarms and to ensure at least one detector is available if another one were to fail. Here

Page 78

UGLDs with UGLDs Voting of UGLDs can be done in all high-pressure zones. When using the UGLDs in voting configurations with other UGLDs, overlapping coverage from the detectors must be considered. Alarm delays must also be implemented. Points detectors and UGLDs UGLDs are used as a fist protection layer for early gas leak detection in areas with pressurized gas. The point detectors are implemented in areas where accumulation of gas and vaporization of depressurized liquids are likely to occur. They may also be installed strategically by major flanges and valves where the risk of gas leaks is high.

How to Interface UGLD and Mix with Other Technologies

UGLD as an independent protection layer in

action once the hazard can no longer be prevented. At

the safety instrumented system (SIS)

the heart of the IPL scheme is the assumption that no

The development of international standards like ISA-84

device is 100% reliable. Over the course of their life cycle,

and industry codes of practice that ensure the availability

instruments will fail to perform their functions; and as a

and effectiveness of fire and gas systems has advanced

result, the best defense is multiple independent layers

risk-based concepts in the process industry sector.

of protection.

Among the important assumptions from this regulatory framework is that accidents are rarely the result of a single








failure. Rather, a series of errors combined to expose

common safeguards. Properly defined, however, an IPL

a flank through which the initiating cause propagates.

is â&#x20AC;&#x153;any independent mechanism that reduces risk by

These errors begin with an initiating event and include the

control, prevention or mitigationâ&#x20AC;? (ISA-84 2004). While

enabling conditions which must be present in order for

preventative IPLs serve to stop a scenario from developing

the scenario to proceed. Invariably, as the hazard evolves

further, mitigation IPLs simply reduce the consequences

unchecked, its consequences become more severe and

of an incidence once it occurs. By their nature then, fire

the accident more difficult to control.

and gas detection systems are mitigation layers.

In order to reduce the likelihood and severity of

Just as protection layers need to be independent from

accidents, functional safety professionals recommend

one another, so too do the individual elements within an

the application of independent protection layers [IPLs].

IPL. This is particularly the case with fire and gas sensors,

Layered protection provides a formidable defense against

which are often the first line of defense against the risk of

accident sequences. IPLs help bring a basic process into a

several types of major incidents. In the IPL scheme, each

safe state or provide early warning and prompt appropriate

sensor serves as a protection layer against a common

How to Interface UGLD and Mix with Other Technologies

Page 79

risk like those of fires and gas releases (Figure 22). Since

and equipment. Their benefit toward hazard mitigation

all detection systems are vulnerable, it stands to reason

lies in their complementary use, so that the limitations

they should be implemented in a way that provides the

of one are offset by the strengths of the others. This

greatest degree of independence between them.

concept of safety in diversity is the premise behind our Total Solution Provider (TSP) philosophy and our belief

As a member of the General Monitors family, we recog-

that diverse safety systems, combined with a design that

nizethere is no absolute security with any one gas

prevents leaks and eliminates possible ignition sources,

detection system. For this reason, we offer a whole host

offers the best approach for reducing the chances of

of detection technologies that safeguard plant personnel

hazard propagation.

Fire Detection

A human sensory model (Table 3 on the next page) applied to gas and flame safety suggests detection

Gas Detection N Gas Detection 2 Gas Detection 1

technologies associated with the classical senses can be combined to provide superior protection. Devices that rely on ultrasound, optics, or mass transport to trace gas are independent, as their methods of operation are based on different physical principles. Sound propagation,

Leak Detection

emission or absorption of light, or an exothermic chemical reaction can inform the presence of a gas under the right conditions but share few common elements; and therefore, few common failures.

Figure 22. Independent Protection Layers Within Automatic Safety Instrumented System IPL. (For Illustration Only â&#x20AC;&#x201C; Number and Types of IPL May Vary.) Page 80

Reference ANSI/ISA-84.00.01-2004 â&#x20AC;&#x201C; Part 1 (IEC 61511-1 Mod), 2 September 2004. Research Triangle Park, NC: ISA.

How to Interface UGLD and Mix with Other Technologies

UGLD â&#x20AC;&#x201C; SIL rating, SIS and Risk

technology is having a known coverage area that is not

There is much industry debate on whether a conventional

influenced by the physical presence of a gas. Unlike

gas detection system can be considered a safety

conventional point and open path detectors, UGLD is

instrumented system. This is due to the fact that a fully

not impacted by wind directionality, wind speed or gas

functioning point or open path detector may never have the

dilution. To maximize a plantâ&#x20AC;&#x2122;s safety and decrease risk

ability to perform as intended simply because a gas leak

as much as possible, it is wise to employ a gas detection

did not reach the detector due to wind conditions or gas

system that includes UGLD and improves the probability of

dilution. Optimal sensor placement and scenario coverage

leak detection.

is absolutely vital when designing a gas detection SIS to ensure that if a leak occurs, a sensor will come in contact

Functional safety is a fascinating topic with many facets of discussion. Please refer to the General Monitors SIL

with the gas and initiate the appropriate response.

Resource Center for further information on SIL and how it Ultrasonic gas leak detection has now become an

relates to gas and flame detection and UGLD. This can be

important part of the functional safety solution for gas

found on:

detection systems. A remarkable benefit of ultrasonic


Detection Technologies


Gas cloud imaging, UV/IR enabled flame detection


Fixed point IR gas detection, open path detection, catalytic, metal oxide semiconductor, electrochemical cell


Ultrasonic gas leak detection Table 3. Detection Technologies Classified According to Human Sensory Model.

How to Interface UGLD and Mix with Other Technologies

Page 81



Uses the hearing sense to pick up the ultrasonic leak

UGLD is primarily installed in outdoor ventilated

noise from pressurized gas leaks.

locations where conventional detector types can be

This enables gas leak detection at the speed of sound

applications include among other offshore platforms,

and a significantly improved total speed of response.

onshore processing plants, compressor and metering

disadvantageous due to wind and leak properties. Typical

UGLD provides area coverage with each unit covering up to 40 meter (130 ft) in diameter. Gases Detects the leak noise generated by all gases under pressure â&#x20AC;&#x201C; from around 10 Bar (145 psi). Suited for detection of lighter hydrocarbons (methane, ethane/ethylene, propane/propylene), light gases such as hydrogen and helium and carbon dioxide. Furthermore, UGLD is excellent for early detection of hydrogen sulphide releases provided it is mixed into methane for example.

Page 82

stations along pipelines, LNG facilities, FPSOs, gas fired power plants and certain parts of chemical facilities. Implementation and operation Implementation of UGLD is relatively simple. Allocation of detectors can be done either based on F&G plant layouts or by performing an onsite mapping survey. UGLD is integrated directly into the F&G system (plant DCS or PLC). During the commissioning, onsite testing and calibration can be carried out as well as live leak simulation to verify the coverage at a distance from the detectors.

The UGLDs do not have any consumables and significant drift. This enables a long life time and minimal maintenance requirements.

Reasons for including UGLD from a loss and prevention point of view:

Integration with other F&G devices UGLD can be used on its own or together with other detector types. To optimize the plant safety level and increase detection reliability, it is always recommended to consider applying different independent layers of protection. It is further advised to incorporate UGLDs in the overall plant F&G voting architecture and thereby increase speed of response.

Speed of response â&#x20AC;&#x201C; the leak noise can be detected at the speed of sound Probability of failure of detection due to wind conditions and leak properties is low Coverage verification can be carried out during commissioning Self-test mechanism limits the risk of unidentified failures

Page 83 Page 83

Explanation and Expressions

Hertz (Hz): Measure for frequencies.

Amplitude: The degree of change in atmospheric pressure

High noise areas: In this handbook, area where ultrasonic

caused by sound waves. It describes the volume of the

background noise is less than 78 dB.

acoustic sound (measured in dB). Background noise: Ambient noise in the area where the UGLD is to be implemented. In this handbook it refers to ultrasonic ambient noise.

Human sensory model: A construct for developing fire and gas sensors based on the classical human senses. Independent Protection Layers (IPLs): An independent mechanism that reduces risk by control, prevention or

Conventional gas detectors: A detector that relies on detection methods that are well established in the

mitigation (ISA-84 2004).

fire and gas industry usually based on identifying and

Leak rate: Amount of gas escaping from a leak per unit

measuring gas concentrations - for example open-path

time, also known as mass flow rate.

and point detectors.

Leak size: the size (diameter) of a leak. Measured in

Decibel (dB): Unit for sound pressure level or volume.

millimeter (or inches).

Detection coverage: The area that a detector covers.

Leak sources: Refers to locations where gas leaks may occur, e.g. flanges or valves.

Frequency: Number of crests of sound wave that pass a given point per unit time. Describes the high and low

Lower Explosive Limit (LEL): Lowest concentration of gas

pitches in acoustic sound (measured in Hz).

in air capable of producing a flash of fire in the presence of an ignition source.

Page 84

Low noise areas: Area where the ultrasonic background

safety instrumented systems.

noise is less than 68 dB. SIL suitability: Capacity of a device to operate in a certain Mapping Survey: A service provided to determine the

SIL environment based on compliance with IEC 61508.

ultrasonic background noise level prior to installation. Sound Pressure Level (SPL): A logarithmic measure Parts per million (ppm): A unit of concentration corre-

of the effective sound pressure of a sound relative to

sponding to one part, either by weight or volume, for every

a reference value, measured in decibels above the

million parts.

standard reference level.

Safety Instrumented System (SIS): An instrumented

Total speed of response: The time elapsed from the onset

system used to implement one or more safety instrumented

of a leak until it is picked up by the detector (includes the

functions. It is composed of any combination of sensors,

time required for the gas to reach the sensor head of a

logic solvers, and final elements.

conventional gas detector).

Senssonicâ&#x201E;˘ self-test: Acoustic self-test that ensures the

UGLD: Abbreviation for ultrasonic gas leak detector or

failsafe operation of certain Gassonic gas leak detectors.

ultrasonic gas leak detection.

Shadowing or blockage: Refers to situations where there

Very low noise areas: Area where the ultrasonic

is a solid, physical obstruction between the leak source

background noise is less than 58 dB.

and the detector. Voting: Alarm configuration procedure in the control Safety Integrity Level (SIL): A discrete level (one out of

room. Voting of detectors prior to executive action is

four) for specifying the safety integrity requirements of

often performed to ensure automatic operations from the

the safety instrumented functions to be allocated to the

fire and gas detection system. Page 85

Gassonic Handbook: BP2009 - 1

Gassonic Denmark Energivej 42 A DK-2750 Ballerup Tel: +45-44-700-910 Fax: +45-44-700-911

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General Monitors - UK Heather Close Lyme Green Business Park Macclesfield, Cheshire United Kingdom, SK11 0LR Tel: +44-1625-619583 Fax: +44-1625-619-098

Handbook on Ultrasonic Gas Leak Detection