Vibration Induced Failure Investigation and Evaluation for Main Steam Moisture Separator Drain Tanks

Main Steam Moisture Separator Reheater drain tank nozzles and associated piping at the commercial nuclear power plant have shown a history of tank wall, nozzle and pipe weld failures over an approximate four year cycle. Reports reflect weld failures at the top and bottom nozzles of these drain tanks and in the piping systems associated with the level and instrument trees attached to these tanks. The tanks experience high temperature transient conditions of steam dumped into the tanks. Stresses were induced in the piping and nozzles by vibration during the steam dump and other system transients. Time history field vibration data taken with accelerometers at various points of the pipe system configuration were incorporated and used to develop and benchmark a finite element model to evaluate the stress fields in the piping and the tank. The results of these analyses were then used to recommend modifications to eliminate the cracking issues.