A Relief Valve Cannot Protect Through a Closed Isolation

The path is part of the safeguard

A relief device is not protective merely because it appears on a drawing or carries a current inspection tag. Its inlet and discharge paths must be available in the operating configuration. Maintenance can require isolation, but the equipment then needs an approved safe state or an authorised alternative protection arrangement. The plant's own design basis and operating procedures govern the details.

The U.S. Chemical Safety Board investigated a fatal heat exchanger rupture at Goodyear in 2008. Workers had closed an isolation valve between the ammonia side of the exchanger and its relief device while replacing a rupture disk. The valve was not reopened. Subsequent heating and pressure rise contributed to rupture. This is a documented event, not a hypothetical example.

Make a closed valve visible to the whole team

Before an isolation is applied, identify what equipment loses protection and what other work or heat sources could pressurise it. Record the valve position and control it under the approved isolation and permit arrangements. If the relief path is unavailable, stop the affected operation or use a formally engineered and approved arrangement; a verbal promise to reopen it later is inadequate.

At handback, independently verify the required valve line-up, remove locks or tags only under the authorised sequence, update the plant status record and tell the incoming shift. Check the entire pressure boundary: a correct relief-valve set point is irrelevant if the connection to the protected volume is blocked.

A question for today's team

Which pressure relief device on our plant has a block valve or removable spool in its path, and how do we prove that its protected equipment cannot operate with that path unavailable?