I have these debates on a regular basis: ” We can not put an isolation valve before or after a Pressure Relief Device (PRD).” And that would be an incorrect statement. Yes, placing an “intervening valve” in a PRD path (before or after the PRD) can be risky; however, in some situations, these valves are necessary for maintenance on the PRD without having to drain the entire system to achieve a Zero-Energy State. Below is an example of when a PRD needs an intervening valve.