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Evaluating relief valve reliability when extending the test and maintenance interval

Presently, more and more programs appear that provide processes for evaluating and extending the time between major process shutdowns and maintenance. RBI (risk‐based inspections) and Six Sigma processes are two examples of methods currently in use to help drive shutdown extension programs. The relief valves on these processes often go along with the shutdown extension programs without a thorough understanding of the impact on relief valve reliability. Using an analysis for a “stand‐by system,” the change in PFD (probability of failure on demand) of the relief system as a function of change in test interval is shown. Human error is another factor that compromises relief valve reliability. Some arguments are offered that suggest reduced inspections increase reliability because there are fewer opportunities for human error. This study addresses the human error question and shows that the human error contribution is constant while the relief valve is in service. Finally, the increase in PFD when increasing the test interval is discussed in terms of the potential increase in risk. The increased risk as a percentage change should be communicated and understood by those deciding to extend shutdown intervals. © 2004 American Institute of Chemical Engineers Process Saf Prog 23: 191–196, 2004