I see severe risk-taking regarding “safety assumptions” in process safety in my travels. I guess I am just “old school” and believe that every Pressure Vessel (PV) deserves its very own Pressure Relieving Device (PRD). But in today’s economy, where “costs” can be a driver in decision-making, I have experienced numerous scenarios where a single PRD will be “assumed” to protect multiple PVs. This may be allowed by code/RAGAGEP; however, here are the most common failures I find in the Design Basis and Sizing Basis of these sole PRDs protecting multiple PVs…
First and foremost, ensure the lone PRD has a set point that is either at or LOWER than ALL the PVs it is intended to protect.
It is not uncommon to find multiple PVs with varying Maximum Allowable Working Pressures (MAWP) and one of the vessels has a MAWP lower than the PRD set point.
For example:
- PV #1 has a MAWP of 150 psig
- PV #2 has a MAWP of 125 psig
- PV #3 has a MAWP of 150 psig
The PRD has a set point of 150 psig.
This means the PV #2 design basis and set point are INCORRECT.
The second most common error we find:
The lone PRD sizing basis does NOT consider ALL the PVs needing relief protection simultaneously.
In every scenario where we find these lone PRD arrangements, the PVs are always near each other; hence, a single pool fire event will impact all the PVs simultaneously. But the sizing basis is ONLY for the PV to which the PRD is attached.
The sizing basis has not considered volumes of gas/vapor from the other vessels, which in some scenarios may be larger than the PV the PRD is attached to. This leads to an under-sized relief system that can be catastrophic in a large-scale event.
The last issue we see often in these arrangements involves “intervening valves”.
If we intend that single PRD to protect multiple PVs then there MUST ALWAYS be an OPEN pathway to that single PRV when the PV(s) are in service. This means that any “intervening valve(s)” MUST be included in a Car-Seal Program as required by ASME Section VIII, UG-135(d)(2).
(NOTE: Members can read my dozen or so articles on Car-Seal Programs and download different Written Car-Seal Programs from the Document Library)