Performance analysis of gas purging operation in volume control tank

The Volume Control Tank (VCT) is designed to provide for control of hydrogen concentration in the coolant and the means for the removal of radioactive gases by purging the accumulated gases in the tank. NRC notified the licensees that the charging pump with the minimum bypass line could be damaged by the gas binding in the suction piping. It is caused by the evolution of hydrogen gas at the point where the local pressure is less than the saturated pressure. The purging operation results in the pressure reduction of the VCT. The computer code is developed to evaluate the capacities and set points of the pressure regulating valves which are installed on the gas control system of the VCT. In order to exclude the hydrogen evolution during the purging operation with the supply capacity of 20 SCFM, the set points of the regulating valves for nitrogen gas and hydrogen gas shall not be higher than 45 psig and 30 psig, respectively. The capacities shall be more than 25 SCFM for nitrogen gas and 55 SCFM for hydrogen gas to get the adjustable set point of 20 through 50 psig. To minimize the purging time and the wasted gas mass during the purging operation, the set point shall be reduced as low as possible within this set point range