Pressure Relief Valves

The Pressure-Relief Valve (PRV) is a self-contained, self-operating device, carefully selected and located to protect pressure equipment from an overpressure incident. They must be sized based on the worst-case scenario, accurately responding to system conditions and preventing catastrophic failure when other instrumentation fails to control process limits adequately.

One of the common questions we receive surrounding these devices is the confusion caused by the multiple names used to describe these devices. For those familiar with the European Norms, you will have recognised that all devices are referred to as safety valves which may or may not help.

The American codes, standards and recommended practices do not use the singular term as this describes the design and application of a valve used for compressible fluids. The PRV is a generic term for a spring-loaded reclosing type of device. The conventional spring-loaded PRVs include the ‘Safety’ valve used for compressible fluids, the ‘Relief’ valve used for non-compressible liquids and the ‘Safety-Relief’ valve used for multiservice applications. In contrast, the term Pressure-Relieve Device (PRD) should be considered an all-encompassing term used to include reclosing, non-releasing and vacuum types.

The relief and safety valve design types initially operate in the same manner when the disk of the valve remains in contact with the nozzle. However, when system pressure overcomes the balance of forces, the relief valves lift in proportion to overpressure, whereas the safety valves lift rapidly, commonly referred to as a ‘Pop Action.’

We often hear a misconception that when the safety valve pops, it opens fully. However, when the valve initially pops, the lift is approximately 60-70 full height, and system pressure must continue to increase to open the valve fully, achieving its rated capacity.

Although safety and relief valves differ in their opening and closing characteristics, both design types must be fully open at a maximum of 10% overpressure when a single PRV is protecting a vessel to the code requirements of ASME VIII and UKPSR/PED.

Don't hesitate to contact us at [email protected] if you want to learn about our three-day PRV maintenance and inspection courses.

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