(88e) Analysis of Pressure Relief Valve Proof Test Data: Findings and Implications

This paper reports on our statistical analysis of pressure relief valve (PRV) proof test data for the failure mode, fail to open, i.e., PRV remains closed when actual pressure reaches or exceeds 150% of set pressure. Three data sets, from Fortune 500 operating companies, that met the intent of the quality assurance of proof test data as documented by the CCPS PERD initiative were analyzed. Although the original intent of our analysis focused solely on estimation of the failure rate during the useful life of the equipment, it became apparent that the probability of failure upon initial installation or reinstallation after poof test, and the need to address what constituted end of useful life were very significant.

This paper provides three important findings that are summarized as follows.

? The statistical analysis of each data set predicted a 1% ? 1.5% 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.

? Our 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/hour. Additionally, we compared our estimates from data analysis to the prediction of failure rate for a PRV using the Failure Modes Effects and Diagnostics Analysis (FMEDA) method. The prediction was consistent with the data estimates.

? The data further indicated that the low random failure rate was not supported beyond a 4 to 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, both the initial probability of failing to open, as well as the probability of failure on demand (PFD) based upon the random failure rate must be taken into account. Even then, the results are only defensible when it can be demonstrated that the proof test interval occurs before wear-out (i.e. end of useful life) begins.