When the level to be measured is in a pressurized tank at elevated temperatures, a number of additional consequences must be considered. As the temperature of the fluid in the tank is increased, the density of the fluid decreases. As the fluid's density decreases, the fluid expands, occupying more volume. Even though the density is less, the mass of the fluid in the tank is the same. The problem encountered is that, as the fluid in the tank is heated and cooled, the density of the fluid changes, but the reference leg density remains relatively constant, which causes the indicated level to remain constant. The density of the fluid in the reference leg is dependent upon the ambient temperature of the room in which the tank is located; therefore, it is relatively constant and independent of tank temperature. If the fluid in the tank changes temperature, and therefore density, some means of density compensation must be incorporated in order to have an accurate indication of tank level. This is the problem encountered when measuring pressurizer water level or steam generator water level in pressurized water reactors, and when measuring reactor vessel water level in boiling water reactors.

Pressurizer Level Instruments

Figure 13 shows a typical pressurizer level system. Pressurizer temperature is held fairly constant during normal operation. The OP detector for level is calibrated with the pressurizer hot, and the effects of density changes do not occur. The pressurizer will not always be hot. It may be cooled down for non-operating maintenance conditions, in which case a second AP detector, calibrated for level measurement at low temperatures, replaces the normal OP detector. The density has not really been compensated for; it has actually been aligned out of the instrument by calibration.

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Figure 13 Pressurizer Level System

Density compensation may also be accomplished through electronic circuitry. Some systems compensate for density changes automatically through the design of the level detection circuitry. Other applications compensate

for density by manually adjusting inputs to the circuit as the pressurizer cools down and depressurizes, or during heatup and pressurization. Calibration charts are also available to correct indications for changes in reference leg temperature.

Steam Generator Level Instrument

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Figure 14 Steam Generator Level System

Figure 14 illustrates a typical steam generator level detection arrangement. The OP detector measures actual differential pressure. A separate pressure detector measures the pressure of the saturated steam. Since saturation pressure is proportional to saturation temperature, a pressure signal can be used to correct the differential pressure for density. An electronic circuit uses the pressure signal to compensate for the difference in density between the reference leg water and the steam generator fluid.

As the saturation temperature and pressure increase, the density of the steam generator water will decrease. The OP detector should now indicate a higher level, even though the actual OP has not changed. The increase in pressure is used to increase the output of the OP level detector in proportion to saturation pressure to reflect the change in actual level.

Summary

Density compensation is summarized below.

Density Compensation Summary

If a vapor with a significant density exists above the liquid, the hydrostatic pressure that it will add may need to be considered if accurate transmitter output is required.

Density compensation is accomplished by using either:

Electronic circuitry Pressure detector input Instrument calibration