Trapped air and the onset of cavitation in piping systems

Repetitive Reactor Water Clean Up (RWCU) pump trips were experienced at VYNPC, particularly during reactor cooldown to shutdown conditions. A study of various RWCU trips occurred over the last 10 years established that, for cases where no system isolation signal or known pump trip signals existed, the pump trips were caused by system low flow linked to cavitation induced flow oscillations. However, theoretical relationships at pump inlet indicated a reasonable margin to maximum allowable flow to prevent cavitation. It was then determined that air was present in solution in the form of either dispersed air or large pockets. Air pockets at system high points could move to the pump inlet causing low suction pressure and subsequent system low flow while small air bubbles distributed in the fluid impact the pressure of vaporization, creating better conditions for the onset of cavitation. This paper discusses the implications of air entrapped in piping systems and provides some recommendations to restore margin to cavitation conditions. (author)