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
