Probability of Initial Failure for Spring Operated Relief Valves

We present clear and convincing evidence that, for new spring operated relief valves (SORV) that are not proof tested by the user shortly before installation, there is a non-trivial probability that the SORV will be installed in the fail-to-open (stuck shut) failure mode. Using the results of over 4800 new ASME Boiler and Pressure Vessel Code Section VIII SORV proof tests, we estimate the probability of initial failure (PIF) due to manufacturer/assembly anomalies, as well as PIF due to in-storage aging of SORV based on their material composition. We indicate how PIF can be reduced by various preinstallation activities that may be undertaken by the user. We show how to compute values of PIF to be used in calculating the average probability of fail danger (PFDavg) (as required by IEC61508 and similar safety standards in order to determine a safety integrity level (SIL)) which accounts for both the SORV material composition and the pre-installation activities undertaken. For four typical SORV of different material compositions we show how pre-installation activities influence the achievable SIL. We discuss the implication of these findings for estimating PIF for used (previously installed) SORV. We close with recommendations to further address PIF.