This is a topic that is near and dear to me, as it is one I learned very early on in my career as a means to safely perform PRD work on an RV or RD that is connected to a header/manifold. Although this design MUST be adequately done AND must be managed very tightly, it can add a level of safety for workers doing work associated with the relief system or header/manifold.
Let’s first squash the rumor that having an intervening valve(s) before or after a pressure relief device (PRD) such as a relief valve (RV) or a rupture disc (RD) is just a rumor and has no basis in fact or engineering design! In fact, the two most prevalent pressure vessel codes, ASME and NB, both allow this design, albeit with some stringent requirements.
Special Note to NH3 Refrigeration readers: IIAR-2, 2014 also allows this in 15.4.1.
So why would we even consider this?