(53a) 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, λ_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 λ_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 λ_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.