In the 2021 revisions to the National Board Part 4 – Pressure Relief Devices, the code formally recognizes the need for an “intervening valve” on the discharge side of a relief valve. The code, section 2.5.6 allows this in the following manner:
(emphasis by me)
A full area stop valve may be placed on the discharge side of a pressure relief device when:
- its discharge is connected to a common header for pressure relief devices to prevent discharges from these other devices from flowing back to the first device during inspection and repair.
This stop valve MUST be
- arranged so that it is LOCKED OR SEALED OPEN, and
- it shall NOT be closed except by an authorized person who shall remain stationed there during the period the valve remains closed.
The valve MUST be locked or sealed in the OPEN POSITION BEFORE the authorized person leaves the station.
Now I have to admit that I am puzzled by this next requirement/statement, as it reads as if the code also requires a stop valve on the inlet of the RV anytime there is one on the discharge side.
“This valve shall only be used when a stop valve on the inlet side of the pressure relief device is first closed.”