Residual life of an aluminum-bronze valve

This paper describes a recent service experience with ASTM B148 alloy C 95400 aluminum bronze cast material used in the body of class 150, 1 1/2 inch and 2 inch valves. These valves were in service for about eight years in the service water system of Surry Nuclear Power Station. This portion of the system is in use for cooling lubricating oil or the Charging Pump. During a routine inspection of the system, sweating and weeping were identified on a portion of the valve surface even though there was no evidence of a through-wall crack on the valve body. There were 44 valves of this type in the system and the inspection indicate that 11 valves had through-wall seepage and an equal number exhibited some form of discoloration on the surface from through-wall moisture. An assessment of the residual life of the components became important to justify continued safe operation of the plant. The two worst valves were taken out of the system for evaluation of degradation which included visual inspection, metallurgical and mechanical property examination and the physical measurements of the extent of degradation of components. The degradation of the valve body was due to dealloying (dealuminification) of aluminum bronze cast material. Based upon the results of an examination of sample valves, a representative cross section of the valve body was characterized for structural evaluation. The results of the evaluation established that the residual life of the component was at least till the next refueling outage for the units, during which an orderly replacement of valves could be possible without interrupting the plant operation. The results of the study provided insight into the behavior of a type of aluminum bronze material in a service water environment