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Pressure Relieving Devices (PRDs)

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Pressure Relieving Devices (PRD) 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 to 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.

Pressure Safety Valves (PSV)

A Pressure Safety Valve (PSV) is a type of valve used to quickly release gasses from equipment in order to avoid overpressurization and potential process safety incidents. PSVs are activated automatically when pressure exceeds prescribed pressure limits in order to return equipment pressure to a safe operating level.

Pressure Relief Valves (PRV)

A Pressure Relief Valve (PRV) is a type of valve used to release stored gas in various equipment in order to maintain an optimal pressure level. PRVs open gradually as pressure builds up in order to release the necessary amount of pressure. While the term PRV is sometimes used interchangeably with PSV, there is a difference between the two. A PRV opens gradually in relation to the pressure, while a PSV is opened suddenly once the pressure hits a certain level in order to avoid overpressurization and a potential process safety incident.

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Articles about Pressure Relieving Devices (PRDs)
July/August 2021 Inspectioneering Journal

A risk management approach for relief valves should be able to determine the proper interval of maintenance events and the necessity of installing isolation valves, rupture discs, or 100% spare capacity.

March/April 2021 Inspectioneering Journal

This article provides insights into the AIV damage mechanism, reviews approaches to calculate the probability of AIV failure in piping, provides design practices to prevent AIV failure, and introduces a RBI approach to evaluate risk of AIV failures.

March/April 2018 Inspectioneering Journal

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...

Authors: Keith Lapeyrouse
Online Article

The Valve World Americas Conference provides an opportunity for people from all sides of the flow control industry to come together to discuss experiences, share insights and work together to resolve common challenges. Sharing knowledge and...

July/August 2016 Inspectioneering Journal

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...

Partner Content

Traditional pressure vessel inspections often require taking assets out of service. However, MISTRAS’ Non-Intrusive Inspection (NII) programs provide data as good or better than internal inspections, while assets remain in service. NII is...

May/June 2010 Inspectioneering Journal

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...

January/February 2001 Inspectioneering Journal

Part 5 of this article continues to outline the 101 essential elements that need to be in place, and functioning well, to effectively and efficiently, preserve and protect the reliability and integrity of pressure equipment (vessels, exchangers,...

Authors: John Reynolds
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