I have written about this topic numerous times, and each time it causes controversy among the SAFTENG members. Some members are adamant that we should NEVER allow any “intervening valve(s)” in our pressure relief systems; others have seen it done many times. ASME VIII does indeed allow “intervening valves” before and/or after a Pressure Relieving Device (PRD), and in my previous articles, I mention the reasons why and discuss the need for such a design (e.g., PRDs connected to a treatment system or a header). Now we see that ASME B31.3, 2018 mentions using these “Stop Valves” while the equipment operates. WTH?!?!? I have never heard of anyone closing a valve in a PRD system “while the equipment is in operation,” I would STRONGLY RECOMMEND this, not even enter into a discussion. It is risky enough to have the valve in the PRD system, and just closing it while the equipment is out of service is risky enough (because these valves get forgotten and stay closed after the equipment is returned to service). But for the sake of openness, here is what ASME B31.3 says…