Evaluation of Maintenance Intervals for Spring-Operated Relief Valves Using a Risk-Based Inspection Technique

The Savannah River Site (SRS) spring-operated relief valve (SORV) maintenance intervals were evaluated using the American Petroleum Institute’s (API) inspection updating approach in API RP 581 for Risk-Based Inspection (RBI) technology. In addition, the impact of extending the inspection schedule was evaluated using Monte Carlo simulation (MCS). The API RP 581 approach is characterized as a Weibull analysis with modified Bayesian updating. Initial Weibull parameter estimates were updated using SRS’s historical proof test records contained in the Center for Chemical Process Safety Process Equipment Reliability Database. The API RP 581 methodology was used to estimate the SORV’s probability of failing on demand and the annual expected risk. An illustration of the inspection costs versus the associated risks is provided. Current practices are reviewed, and recommendations are made for extending maintenance intervals. A cost-effective maintenance frequency balancing both financial risk and inspection cost is demonstrated.