Patent Number: 050248022
Section: summary

FIELD OF THE INVENTION This invention relates to a method for water level measurement in a steam generator such as used in pressurized water reactors and, more particularly, water level measurements using differential-type transmitters for sensing water level in a steam generator in a nuclear power plant. The current steam water level measurement methodology is built around the use of differential-type transmitters for comparing the steam generator water level with a reference leg pressure input. The transmitter responds to a differential of water pressures inputted to it, and provides an output representative of the difference between a reference leg pressure and the pressure to the height of the liquid in the steam generator. This differential is a good correlation to water level when the lower pressure tap, for measuring the water level in the steam generator, is located in a relative low velocity region of the steam generator. However, if the lower pressure tap is moved to a higher velocity region, an error is introduced due to the effective velocity head of the moving water. In such circumstances, a level measurement penalty must be taken, meaning that the steam generator operating level margin, i.e., a range of permissible water levels, needs to be reduced. Present steam generator design calls for the lower level tap to be relocated in the high velocity region so as to minimize well known shrink/swell phenomenon. Accordingly, there is a need for an improved methodology to account for the velocity head effects in the transmitter calibration, so as to maximize the steam generator level operating margin. SUMMARY OF THE INVENTION It is an object of this invention to provide, in a steam generator used in a pressurized water reactor nuclear power system, an improved method of measuring the water level in the steam generator and taking into account a velocity head effect for determining the operating margin, i.e., the range of water levels within which operation is controlled. In accordance with this object, calibration of the differential pressure transmitter includes subtracting a bias factor from the differential pressure at maximum water level, the bias factor being said equal to the velocity head at 100% power level, or another determined fraction of maximum velocity head, thereby to adjust the transmitter reading at maximum water level (and minimum meter reading) to account for a velocity head. The high level trip setpoint is calculated by determining the net water level, e.g., corresponding to the top of the riser minus a bias, or adjustment for velocity head due to the calibration at the riser, which is some percentage of the span between the lower and upper taps.