Pressurizer Level Instruments

Level Detectors DENSITY COMPENSATION Pressurizer Level Instruments Figure 13 Pressurizer Level System Figure 13 shows a typical pressurizer level system. Pressurizer temperature is held fairly constant during normal operation. The DP 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 DP detector, calibrated for level measurement at low temperatures, replaces the normal DP detector. The density has not really been compensated for; it has actually been aligned out of the instrument by calibration. 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 Figure 14 illustrates a typical steam generator level Figure 14 Steam Generator Level System detection arrangement. The DP 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 DP detector should now indicate a higher level, even though the actual DP has not changed. The increase in pressure is used to increase the output of the DP level detector in proportion to saturation pressure to reflect the change in actual level. Rev. 0 Page 13 IC-03