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Fundamentals of Pressure Relief Valves

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October 10, 2019

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What Are They and Why Are They Needed
What: A stand-alone device that opens and recloses at a pre-selected pressure, containing an orifice sized to flow a required

capacity to prevent / avoid overpressure.

Why: All natural gas equipment (pipelines, pressure vessels, air-cooled heat exchangers, compressor cylinders, odorant tanks, instrument control lines, valves, underground storage, industrial-residential-commercial system supply) has a

maximum allowable operating pressure (MAOP) rating. Pressure ratings (MAOP) of each piece of equipment may be different. Pressure relief valves with proper application will prevent overpressure above MAOP. Set point is dictated by the lowest MAOP equipment in the system.

What Assurance Do I Have That the Chosen PRV Will Work and Prevent Overpressure?

Most, if not all, valve manufacturers have or had brilliant design engineers who developed product lines offering safe, reliable and accurate performance. Testing to ensure good PRV performance is done either by the manufacturer, by an independent third party, or both. Knowing that a PRV might not open at set point should concern all of us – if not send shivers down your spine. Look for and use only those valves that are tested by manufacturers and a third party and have a continual testing program.

Pressure relief valves certified to the ASME Code require stringent flow testing by the National Board of Boiler and Pressure Vessel Inspectors (NB) to guarantee valve capacities. Furthermore, following good engineering practice, ASME-certified PRV’s are designed to fail open. There are many utilities using regulators as relief valves that can fail closed without NB capacity testing, which is poor engineering.

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