Determining The Cause Of Bearing Failures In Circulating Water Pump Motors

Abstract

Eight vertical circulating water pumps are used to circulate condenser water at the generation station in a nuclear power plant. The pump installation is pictured in Figure 1. The motor pump system is arranged vertically with the drive motor above the pump. Each pump is driven by a 28-pole, 2500 hp synchronous motor at 271 rpm. The motors sit on a platform that projects out over a large reservoir and each one delivers 275,000 gpm at about 12 psig. The pumps are location below the motors in the reservoir. There are four pumps per generating unit. Each unit can run at full capacity drops about 10 percent. Three lower motor bearing failures occurred shortly after sudden ambient temperature drops. Analysis showed that there was adequate lubrication but the original bearing design was very sensitive to clearance. A finite element analysis of the motor proved that the case would pinch the bearing when cooled from 70 degrees F to 25 degrees F. This reduced the bearing clearance significantly and caused the failures. A new profiled bearing was developed that would provide better rotor support with larger clearances that would be more tolerant of thermal clearance changes. The bearing failure analysis was wide in scope and was conducted by a large team including engineers from the power plant, turbomachinery consultants, repair shops, and bearing manufacturers. Many different theories were considered and investigated. This paper examines the investigations of the bearings lubrication with a computational fluid dynamics (CFD) analysis, the motor rotordynamics, the effects of ambient temperature drop, and the manufacture of a new bearing design.