MEASUREMENT AND INSTRUMENTATION
should be consulted before an instrument is installed that has a combination of these or other conditions. Within types of instruments there is often a wide variance in price and standard features from different manufacturers that may influence applicability of a specific instrument. The legend letters for the instrument type are shown below the chart. The key is as follows: Y = normally applicable, N = not recommended for this application, NA = not applicable for the instrument type alone, O = may be used, but more information or a sample conditioning system is normally used. Relative cost is a factor when several techniques can work in the application and an instrument may work but be prohibitive in cost. This cost factor may cause an instrument type not to be recommended here.
METHODS OF MEASUREMENT IN LIQUIDS Liquid applications use some of the same sensing types that are used for gases, with the exception of sensors that would be damaged or influenced by the liquid phase of the process. The techniques described below will only highlight the unique liquid issues if the technique has been described above. Infrared Adsorption This technique, as described above for gases, works with liquids as well—as long as the process connections and internal sample piping can maintain a seal. The liquid must also be transparent so the light beam is not obstructed. If the sample has a tint or color it is best if the color is constant since changes in color may interfere with the accuracy. This method is typically used off line and for applications for which the concentration of moisture is low to trace levels.
This type of capacitance sensor is of a rather large scale compared with that of the sensors discussed in the gases section. This device uses the fluid itself as the dielectric for determining the moisture content. Since the water in the fluid has a dielectric of ≈ 80, the carrier or process stream should have a very low and fairly stable dielectric of <2. If the carrier dielectric changes, a new calibration should be performed. These devices are often used in crude oil measurements to monitor flows to determine internal distribution for initial treatment—all the way to measuring for custody transfer. These devices have an insulated rod in a pipe carrying the sample. The capacitance is measured between these two conductors, and the dielectric is a function of the water content. Simple as this may seem, the thermal coefficient of this measurement, along with the salt and other contamination and buildup issues, can cause serious inaccuracies (see Fig. 27). The common basic sand and water (BS&W) devices have been refined to reduce these effects. More sophisticated devices include dual sensors and mechanical and electronic refinements to achieve titration accuracies. These features do add cost to these systems, but when applied to processes that move thousands of barrels of oil per day this cost is amortized quickly. These units can measure from 0.05% to 30% water in oil and are made to handle the high pressures of the oil application.
Aluminum Oxide Since this device uses the capacitance principle of operation, the dielectric of the water dissolved in the sample is the signature effect, as described in the gases section. The aluminum oxide sensors can be used in liquid hydrocarbons that have low dielectrics, are immiscible, and are noncorrosive with respect to gold or aluminum. The fluid must be immiscible in order to properly calibrate the sensor. The normal operating range for this type of application is from 0 to 5000 ppmw, depending on the chemical FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +9199994621
Mechanical BE (Measurement and Instrumentation)