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Overview of Pressure Relieving Devices (PRDs)

Pressure Relieving Devices (PRDs) are components used in refineries, chemical plants, and other similar facilities to prevent pressure vessels and other equipment from overpressurization by relieving excess pressure when necessary. They can be used release gas, steam, liquids, or vapors. Properly functioning pressure relief devices are essential for protecting plant personnel and equipment, since unexpected overpressure events can potentially cause equipment damage, loss of containment, and result in costly plant shutdowns.

Pressure relieving devices include mechanisms such as Pressure Safety Valves (PSV) and Pressure Relief Valves (PRV), although there are other types of pressure relieving devices as well, such as Rupture Disk Devices and Pin-Actuated Devices. These devices can come in many different sizes and shapes and allow pressurized fluids or gasses to escape through a secondary passage out of the system so that pressure cannot build up beyond safe operating limits.

Most pressure relieving devices work automatically, opening once certain internal pressure limits are reached or exceeded, and closing once the pressure has returned to an acceptable level. The level of pressure at which a valve will re-seal is known as its blowdown. Blowdown levels usually vary from around 2-20% and some valves have adjustable blowdowns.

In the United States and many other countries, industries are required to use PRDs on pressure vessels, piping, and other equipment. Several codes and standards that discuss the safe construction and operation of PRDs include API RP 520, API RP 521, API RP 526, API RP 527, API RP 576, ASME PD 583 and ISO 4126-1:2013. 


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Articles about Pressure Relieving Devices (PRDs)
  • March/April 2018 Inspectioneering Journal
    By Keith Lapeyrouse at Process Reliability Solutions

    This article demonstrates the process of converting API RP 576, Inspection of Pressure-Relieving Devices, into a work process that is suitable for conversion into a job procedure. It also explores some lessons learned that will make the document more relevant.

  • July/August 2016 Inspectioneering Journal
    By Grady Hatton at Versa Integrity Group, and Melissa Guerra at Chevron Products Company

    Many plants inspect PRDs on preset intervals as recommended by inspection code API 510, and implement inspection practices such as API RP 576. Just seeking PRD compliance with codes and standards usually results in mediocre mechanical integrity and process safety. Companies must go further to climb the ladder to “excellence.”

  • May/June 2010 Inspectioneering Journal
    By P.A. Henry, P.E. at The Equity Engineering Group, Inc., D. Baham at ConocoPhillips, and Valerie Magyari at The Equity Engineering Group, Inc.

    This article describes the API Risk-Based Inspection (RBI) methodology for pressure relieving devices (PRDs) as detailed in the soon to be released 2nd edition of API 581 "Base Resource Document On Risk-Based Inspection" This methodology has been successfully used at several petrochemical facilities to establish risk-based inspection intervals for PRDs.

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