In many processes, the use of a rupture disc (RD) between the vessel being protected and the Relief Valve (RV) is a fairly common practice. This is ALWAYS done for design necessities and not just “because”. Some processes could be corrosive to the RV internals, at least this is the most common reason for this type of arrangement. There are some process safety professionals that will swear this is a PROHIBITED practice; although it is one that makes me nervous (from my lack of being an expert in RV design basis) it is ALLOWED per ASME Section VIII Division UG-127 NONRECLOSING PRESSURE RELIEF DEVICES, which states: