For years, PHA’s seemed to always list vessel “relief systems” as a “safeguard” for ALL the HIGH-PRESSURE scenarios involving the vessel. In this article I want to challenge this practice; but to be up front, if you were to look at all my PHA’s I was guilty as sin for doing this very thing. But as I do more and more PHA’s on all kinds of interesting processes, I have come to look at relief valves (RVs) differently. As such, the ONLY time I believe the RV should be listed as anything more than a MITIGATING safeguard is when the scenario is the same as the “design basis” the RV was designed to protect against – and then it would ONLY rise to the level of a PROTECTION device. Scenarios like an external fire or an overfilled scenario (as long as the proper design is applied) are where the RV becomes a safeguard. In a scenario of “HIGH PRESSURE” in the process vessel, the RV lifting is the LOPC point; at best it would be considered a MITIGATING safeguard in this scenario and then ONLY if the overpressure system was indeed a device that would RESEAT-CLOSE. If a process had, for example, a rupture disc as it’s overpressure protection, then, and ONLY then could it be listed as a safeguard for a BLEVE scenario. Let’s explore this further…