Root Cause Failure Analysis of a Circulating Water Pump Impeller

Circulating Water Pump and Motor Assembly (Left) and Failed Impeller (Right)

A pump in the circulating water system of a nuclear plant experienced excessive vibration. The pump was removed from service and inspection showed that one of the six impeller vanes had failed.

The root cause of a circulating water pump impeller failure was attributed to the change in the discharge expansion joint installation practice. This change allowed up to 1-inch of expansion and 1-1/2-inch of compression clearance in the 6 axial expansion joint control rods.

This change altered the fluid force transmitted to the pump discharge flange affecting the impeller to shroud running clearance. Additionally, the replaced joint had a different stiffness that altered the dynamic response of the pump. KEI provided recommendations to restore the previous pump discharge expansion joint control rod configuration.