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Reference Leg Temperature Considerations
Density Compensation Summary - h1013v1_78

Instrumentation and Control Volume 1 of 2
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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







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