Using Root Cause Failure Analysis Results to Estimate Failure Rates and PFDavg for Spring Operated Pressure Relief Valves

In a companion paper, root cause of failure analysis (RCA) was used to estimate the times to failure of a set of spring operated pressure relief valves (SORV) that were found in the fail-to-open (FTO) condition upon proof test. We now use these estimated times to failure to estimate the failure rate, ?D(t), of the SORV population with respect to the FTO mode of failure. The average probability of failure on demand (PFDavg) is then computed using the estimate for ?D(t). Additionally, we compute PFDavg directly from the proof test data and estimated times to failure and explain why this new approach provides a more accurate characterization of process-wide safety than does PFDavg computed from ?D(t) and probability of initial failure (PIF). We also demonstrate that the changes instituted as a result of the RCA study at the data-source facility have lead to measurable improvements in plant safety.