Safety related valves in the nuclear industry are designed to meet the requirements of the design specifications for the systems in which they are to be installed. In developing valve specifications, systems and valve engineers collaborate to craft the essential requirements needed to support the procurement of the valves that meet the design requirements, and thereby provide reliable service during plant life. The specification requirements, together with the ASME Boiler and Pressure Vessel Code and Standards, provide a strong basis for assuring both structural integrity and functionality of the valve assemblies.
The functional requirements cover the duties of the valves. As these valves are safety related, they are generally subjected to preoperational testing and possibly additional qualification testing during manufacture, to ensure that the valves can perform their safety related functions in service. The nuclear experience of engineered products such as valves shows that considerable amount of analysis and documentation of component stresses are performed to ensure compliance with the ASME code and specification requirements.
The ASME Code requirements, together with the normal controls applied during manufacture of safety related valves, enhance the reliability of the valves. However, valve failures still occur during plant operation. In this paper, the failures of air operated valves (AOVs) used in nuclear applications were reviewed and the data compared against the failures predicted by valve suppliers based on weak link analysis of the valves. The study shows that there are significant differences between what the suppliers consider structurally likely to fail, what the purchaser expects to fail, and what really fails from field experience. The study shows that field failures are complex. They can be initiated by many factors, most of which are not obvious and cannot be controlled by the valve designer. The complexity of field failures of air operated valves is discussed in this paper.