Investigation of the Root Cause of Flow Dips at Pickering B

Abstract

Pickering Nuclear Generating Station (PNGS) B has recently raised its flow trip-point and is now experiencing a number of low-flow alarms. These alarms are caused by "flow dips" which are very short but fairly large drops in measured differential pressure (DP) across the flow-measuring orifice plates. Similar flow dips have been reported at other CANDU stations. The purpose of this present study is to determine the root cause of these flow dips. A set of high-bandwidth pressure and accelerometer measurements were made at PNGS unit 7. The data shows that the high and low sides of the DP cell do receive large (>100 kPa) pressure pulses and exhibit lightly damped resonant responses, except that the resonant response of the low-side impulse line becomes heavily damped after the second cycle, coincident with a build-up of high-frequency mechanical motion, especially in the low-side impulse line. Several possible theories to explain the cause of flow dips are given. On an operating reactor such as PNGS B, it is recommended that the flow transmitters be changed to the adjustable-damping type, and that their time constants be increased. This should be acceptable to safety and licensing, since the current DP cell time constant is much shorter than the licensing requirement. Recommendations are made on how to avoid similar flow dip problems in new plants.