This article reports on our statistical analysis of pressure relief valve (PRV) proof test data for the failure mode, fail-to-open, i.e., the PRV remains closed when actual pressure reaches or exceeds 150% of set pressure. Three data sets, from two Fortune 500 operating companies, which met the intent of the quality assurance of proof test data as documented by the Center for Chemical Process Safety Process Equipment Reliability Database (CCPS PERD)1 initiative, were analyzed. Although the original intent of our analysis focused solely on estimating the failure rate during the “useful life”2 of the equipment, it became apparent that the probability of failure on initial installation or reinstallation after proof test, and the need to address what constituted end of useful life were very significant. This article provides three important findings that are summarized as follows:
- 1The statistical analysis of each data set predicted a 1–1.6% probability of initial failure where initial failure is understood to be at the time of initial installation or reinstallation after a proof test. This implies that most of the failures found during the useful life via proof test are pre-existing failures from the time of installation or reinstallation rather than failures that occurred randomly after installation or reinstallation of the PRV.
- 2Our calculations, based on the three independent data sets, led to consistent estimates of PRV useful-life failure rates between 10−8 and 10−7 failures/h. Additionally, we compared our estimates from data analysis to the prediction of useful-life failure rate for a particular PRV model using the Failure Modes Effects and Diagnostics Analysis (FMEDA) method. The prediction was consistent with the data estimates.
- 3The data further indicated that the low useful-life failure rate was not supported beyond a 4–5 year proof test interval as the threshold of wear-out seemed to be approached.
The importance of these finding cannot be overestimated. When taking credit for a PRV in a risk assessment and in calculating the probability of failure on demand, both the initial probability of failing to open, as well as the probability of PRV failure due to the useful-life failure rate, must be taken into account (the article discusses how to do this). Even then, the results are only defensible when it can be demonstrated that the proof test occurs before wear-out (i.e., end of useful life) begins. © 2009 American Institute of Chemical Engineers Process Saf Prog, 2009