Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/359954
44 July - August 2014 | www.machinerylubrication.com INDUSTRY FOCUS bearing would have been more distressed and would have initiated the event. Air ingestion also would have caused extreme transient axial loads that would have exercised the thrust bearings and left indications or damage on the inboard and/or outboard thrust shoes. If air ingestion produced high radial loads, then air would have resided in the wear ring annulus locations, signifi- cantly reducing the stiffness and damping of these water-lubricated bearings (Lomakin effect). This would have resulted in more contact between the impeller hub and wear rings than was found during the rotating element inspection. The loss of wear ring stiffness would have led to increased rotor motion with a resulting increase in vibration amplitude. The only increase in vibration during the event was at vane pass frequency, which was caused by the exiting air impacting the volute cutwater. Air ingestion was addressed by revising the procedure for venting the pump casing prior to startup. The Root Cause The pump experienced a significant equipment failure due to the introduc- tion of foreign material into the lube oil system. Debris Size and Type Predictive maintenance (PdM) anal- ysis revealed debris that was consistent with machine (metal) cuttings. The foreign material was possibly intro- duced into the pump bearing through the lube oil supply/return lines, bearing housings and oil reservoir. The piece was analyzed with a scanning electron microscope using energy dispersive x-ray spectroscopy semi-quantitative analysis. The composition of the debris was consis- tent with stainless steel. Furthermore, the morphology resembled a piece of cold drawn wire. Prior to this event, the primary PdM technologies in use consisted of surface-mounted accelerometers, thermography and oil analysis. Vibra- tion data was somewhat useful during the event. However, oil analysis was the main technology utilized to iden- tify the problem. This led to further investigation. Subsequent to this event, proximity probes and bearing resistance temperature detectors (RTDs) were installed on this and three other similar pumps. In reviewing the damaged compo- nents, it was discovered that the motor bearings and pump radial/ thrust bearings were all affected by the foreign material. Conclusion The nuclear plant relied upon existing maintenance activities for lube oil system cleanliness control. These cleanliness controls were insuf- ficient for ensuring the health of the feed water pump when the lube oil system was completely disassembled and its components reused. Upper bearing housing Lower bearing housing Pump casing oil drain The cleanliness controls were insufficient for ensuring the health of the feed water pump when the lube oil system was completely disassembled and its components reused.