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…
I have a HIGH-PRESSURE event in a process vessel. I go through all my layers of protection for PREVENTING the deviation that leads to the RV lifting; PROTECTING my assets should it lift; and then MITIGATING the consequences of the HIGH-PRESSURE event.
Does the RV play any role what so ever in PREVENTING my process entering into the area above the SAFE OPERATING LIMIT and below the SAFE DESIGN LIMIT? No, it does nothing to prevent my process from exceeding the 250 psi SAFE OPERATING ENVELOPE used in my scenario in my earlier post.
Does the RV play any role in protecting my assets once my process enters into the area above the SAFE OPERATING LIMIT and below the SAFE DESIGN LIMIT? No, it does nothing to aid in my attempts to correct my HIGH-PRESSURE DEVIATION. Now I will agree the RV is on the pressure vessel to PROTECT the vessel from overpressure events, but in this scenario, the RV is actually going to become the point where we lose our PRIMARY CONTAINMENT – the very outcome we are hoping never happens!
But, if all of our engineering and administrative controls fail us and now we have burned through our safety margins and the RV is starting the lift we are at a crossroads of sorts… yes the RV is PROTECTING the vessel from a catastrophic failure, but at the same time it is the point of release of our HHC/EHS. When we look at RVs (the type that RESEAT-CLOSE) in this light, we see them for what they really are… MITIGATING devices.
Now as I said above, there are a few scenarios where the RV would be a PROTECTION safeguard, but those are the scenarios that the RV is designed and sized to PROTECT the vessel against, such as an external fire. All Relief System(s) design and design basis have to start with the “scenario” they are intended to PROTECT against. Most relief systems are designed for an external fire, but not all are. This is why OSHA/EPA require the PSI to contain the:
Relief system design and design basis (1910.119(d)(3)(i)(D) and/or § 68.65)
The design basis has to include the scenario as to which the system was designed for and it should go without saying this would be the worst-case scenario for the vessel. An external fire is the most popular, as most of the HHC/EHS’s have high Vapor Pressures and such an external heat source would cause a sudden rise in pressure and without a properly designed, installed/maintained, and SIZED relief system the vessel could suffer dire consequences which in turn would impact us humans who happened to be in the area (and I might add we are talking about a BIG AREA). So in our HIGH-PRESSURE scenario that did NOT involve an external fire, the RV is at best a MITIGATING device. When we get the “cause” of HIGH-PRESSURE being an external fire, then we can move the RV up from “mitigation” to “protection.”
At least that’s my take on documenting an RV in a PHA. So the next time you participate in a PHA, use your PREVENT → PROTECT → MITIGATE methodology and make sure the team understands that an RV is at best a PROTECTION layer. If we want to speak in terms of PREVENTION, then we have to PREVENT the fire that the relief system is designed for.