I have shared the ASME and NBIC “code” requirements for these arrangements and it has already stirred a number of debates/questions; of which the API has offered some excellent guidance on this design.
(emphasis by me)
8.3.1 General
Butterfly valves and globe valves are NOT full area due to the presence of internal elements and typically are NOT DESIGNED for tight shut-off. In addition, there is the potential for internal failure of the butterfly valve causing an obstruction in the PRO inlet line. For these reasons, butterfly valves and globe valves are typically NOT used as PRD isolation valves.
While butterfly valves are not recommended for usage as PRO isolation valves, high-performance butterfly valves can be suitable for isolating large relief headers. Butterfly valves used for these applications should be specified to a rigorous set of standards which assure that the disk will not fail in a position that will restrict flow and that provides a tight shut-off.
Check valves SHALL NOT BE INSTALLED in PRD inlet or outlet lines since these devices are normally closed and the check valve can become stuck in the closed position or fail in a manner causing an obstruction in the PRD path.
8.3.2 Inlet and Outlet Isolation Valves
For PRO inlet and outlet isolation valves:
a) Valves shall be suitable for the line service classification.
b) Valves shall have the capability of being locked or car-sealed in the appropriate position.
c) When gate valves are used, they shall be installed with stems oriented horizontally or, if this is not feasible, the stem shall be oriented downward to a maximum of 45° from the horizontal to keep the gate from falling off and blocking the flow.
d) A bleed valve shall be installed between the isolation valve and the PRD to enable the system to be safely depressurized prior to performing maintenance. This bleed valve can also be used to prevent pressure build-up between the PRD and the closed outlet isolation valve.
e) The outlet isolation valve should never be closed while the vessel is in operation without using an inlet isolation valve that has first been closed with the space between the inlet isolation valve and the PRV adequately depressed. If the PRV inlet pressure is higher than the design pressure of the PRV body or bellows, then closure of the discharge valve could lead to PRV component overpressure. Procedures or work practices can be used to facilitate proper sequencing, however consideration might also be given to using an interlocking system between the inlet and outlet isolation valves to assist with proper sequencing. These work practices or procedures can apply to relief device isolation valves in general.
f) Consideration should be given to painting the isolation valve a special color or providing other identification.