Page 5 - Temporary Isolation of Relief Devices - CCPS Safe Work Practice

Fundamental Intent

An inherently safe design does not allow isolation valves on a PRD system. However, industry design standards allow
the use of isolation devices due to factors like online Preventive Maintenance, economics of operations and business
flexibility. It is impractical to shut down an entire unit for conducting PM of every PRD, especially at sites that have
hundreds of PRDs. Additionally, performing a shutdown has its own safety, environmental and financial repercussions.
A best practice in this regard is to pre-install redundant full-capacity PRDs in parallel, that will ensure complete
overpressure protection in case of unavailability of the other device. This might not be practical for large PRDs and/or
PRDs relieving hazardous materials that require special abatement (such as tall vent or flare stacks, thermal oxidizers, or
scrubbers) due to feasibility reasons.

A robust site Safety System Override or PRD Isolation Procedure is recommended, to ensure integrity of PRDs as the last
line of defense in preventing overpressure of equipment during upset scenarios. Therefore, this SWP provides guidance
on methods for temporary isolation of PRDs on operating processes for reasons of inspection, maintenance, repair,
testing, removal, or replacement while a process continues to operate. This SWP applies to situations where the pressure
relieving flow capacity of the process during temporary PRD isolation could be less than that required by the governing
pressure code.

SAFE WORK PRACTICE, TEMPORARY ISOLATION OF RELIEF DEVICES 2