Root Cause Evaluation on Suction Piping Vibration of Auxiliary Feedwater Pump

Abnormal vibration was observed in the suction piping of Auxiliary Feedwater (AF) pump during a pre-operational test at an operating nuclear power plant in Korea. The vibration and noise were initiated when a recirculation valve located downstream of the pump was gradually closed to about 80%. The vibration reached its peak when the recirculation valve was in a fully closed position. After the test, it was found that the bearing of AF pump was damaged. The vibration of suction line was measured during a retest and the cause of vibration was evaluated. This paper presents a root cause of the vibration problem experienced in the pump suction piping. It also reviews the relationship between the vibration of suction piping and minimum flow operations. In conclusion, the pulsation frequency generated from the pump internal during a low flow operation of the pump caused the suction piping vibration.