METHODOLOGY FOR MODELING MANUAL VALVE PLUGGING FAILURES IN PRA

摘要

In an initiative to improve the process of analyzing manual valve plugging failures, a systematic approach to evaluate them is put together and comprehensively explained. This is an often overlooked process in PRA modeling, and if not evaluated, manual valve plugging failures cannot be properly placed in the model or screened based on various criteria. This failure mode, although a small contributor compared to other PRA failure modes, is required to be assessed per ASME/ANS Standard SRs SY-A11 and SY-A14. The approach detailed in this paper helps establish technical adequacy of a plant's PRA by improving the process for evaluation, analysis, documentation and screening of manual valve plugging failures. This systematic approach considers the frequency at which the valve is flowed through, the fluid medium, the type of manual valve, and potential factors contributing to stem/disc separation. Screening criteria are discussed; calculation methodologies are given. This analysis develops a basis for the exclusion of certain air/gas valve plugging failures. As such, this paper gives an introduction and overview of manual valves with a concentration on the plugging failure mechanism and the susceptibility of manual valves to plugging failures.