(78d) Validation of Spring Operated Pressure Relief Valve Time-to-Failure and the Importance of Statistically Supported Maintenance Intervals

The Savannah River Site operates a Relief Valve Repair Shop certified by the National Board of Pressure Vessel Inspectors to NB-23, The National Board Inspection Code. Local maintenance forces perform inspection, testing, and repair of approximately 1200 spring-operated relief valves (SORV) each year as the valves are cycled in from the field.

The Site now has over 7000 certified test records in the Computerized Maintenance Management System (CMMS); a summary of that data is presented in this paper. In previous studies, various statistical techniques were used to investigate failure on demand and failure rates including a quantal response method for predicting the probability of failure in the interval {0, t} time in service. The non-conservative failure mode for SORV is commonly termed ?stuck shut?; industry defined as the valve opening at greater than or equal to 1.5 times the cold set pressure. Actual time to failure is typically not known, only that failure occurred some time since the last proof test (censored data).

The resulting Monte Carlo Simulation (MCS) in this paper shows that previous conclusions on the statistical lifetime predictions and failure studies were not as conservative as initially thought. The MCS employs an aging model for lift pressure increase as a function of set pressure, valve manufacturer, and time. This paper attempts to answer two questions: ?what is the predicted failure rate over the chosen maintenance/ inspection interval?, and ?do we understand aging and variability sufficiently to estimate risk when basing proof test intervals on proof test results.?