ReliefValveCaptioned.PNGA pressure relief valve is a type of valve used to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits.[1][2][3] Such pressure exceedances (i.e., over-pressures) can result from a system upset, an instrument or equipment failure, or a fire. The relief valve is set to open at a predetermined pressure and allow the over-pressured fluid to flow through an auxiliary route out of the system.

For safety reasons, relieving of flammable or toxic fluids should not be routed directly to the atmosphere. Thus, in most cases, the diverted fluid (liquid, gas or liquid-gas mixture) is usually routed through a piping system known as a flare header or relief header to the top of an elevated flare stack where it is usually burned and the resulting combustion gases are released to the atmosphere.[4] As the fluid is diverted, the pressure inside the vessel will decrease. Once it decreases to the valve's re-seating pressure, the valve will the re-close.

In the petroleum refining, petrochemical and chemical manufacturing, |natural gas processing and electric power generation industries, a relief valve is also referred to as a pressure safety valve (PSV) or simply a safety valve.

Other similar applications

In some systems, a so-called bypass valve acts as a relief valve by being used to recycle all or part of the fluid discharged by a pump or gas compressor back to either a storage vessel or to the inlet of the pump or gas compressor. This is done to protect the pump or gas compressor and any associated equipment from excessive pressure. The bypass valve and bypass path can be an integral part of the pump or compressor or externally installed as an alternate path for the pump or compressor discharge fluid. Many fire engine trucks have such bypass systems to prevent over-pressuring the fire hoses.

In other systems, equipment must be protected against being subjected to an internal vacuum (i.e., low pressure) that is lower than the equipment can withstand. In such cases, vacuum relief valves are used to open at a predetermined low pressure limit and to admit air or an inert gas into the equipment so as control the amount of vacuum.

Legal and code requirements for pressure relief valves in industry

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also in most countries, equipment design codes such as those provided by the American Society of Mechanical Engineers(ASME), American Petroleum Institute]] (API) and other organizations like the European Union (EU) and the International Organization for Standardization (ISO) must be complied with and those codes include design standards for relief valves.

The main standards, laws or directives are:
  • American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code, Section VIII Division 1 and Section I
  • American Petroleum Institute (API) Recommended Practice 520/521
  • European Union (EU) standard based on Pressure Equipment Directive 97/23/EC
  • International Organization for Standardization (ISO) 4126


Formed in 1977, the Design Institute for Emergency Relief Systems[5] was a consortium of 29 companies under the auspices of the American Institute of Chemical Engineers (AIChE) that developed methods for the design of emergency relief systems to handle runaway reactions. Its purpose was to develop the technology and methods needed for sizing pressure relief systems for chemical reactors, particularly those in which exothermic reactions are carried out. Such reactions include many classes of industrially important processes including polymerizations, nitrations, sulphonations, aminations, esterifications, neutralizations and many others. Pressure relief systems can be very difficult to design, because what is expelled can be gas/vapor, liquid, or a mixture of the two.

DIERS investigated the two-phase vapor–liquid onset and disengagement dynamics and the hydrodynamics of emergency relief systems with extensive experimental and analysis work.[6] Of particular interest to DIERS were the prediction of two-phase flow venting and the applicability of various sizing methods for two-phase vapor-liquid flashing flow. DIERS became a user's group in 1985.

European DIERS Users’ Group (EDUG)[7] is a group of mainly European industrialists, consultants and academics who use the DIERS technology. The EDUG started in the late 1980s and has an annual meeting. A summary of many of key aspects of the DIERS technology has been published in the UK by the Health and Safety Executive (HSE).[8]


  1. R.W. Zappe (1998). Valve Selection Handbook, 4th Edition. Gulf Professional Publishing. ISBN 0-88415-886-1.
  2. C. F. Perry (1992). Relief Systems Handbook. Institution of Chemical Engineers. ISBN 0-85295-471-9.
  3. Bela G. Liptak (2003). Instrument Engineers' Handbook, 4th Edition. CRC Press. ISBN 0-8493-1083-0.
  4. M.R. Beychok (2005). Fundamentals of Stack Gas Dispersion, 4th Edition. author-published. ISBN 0964458802. See Chapter 11.
  5. Diers website home page Accessed November 1, 2013.
  6. H.G. Fisher, H.S. Forrest, S.S. Grossel, J.E. Huff, A.R. Muller, J.A. Noronha, D.A. Shaw and B.J. Tilley (1992). Emergency Relief System Design Using DIERS Technology: The Design Institute for Emergency Relief Systems (DIERS) Project Manual. ISBN 978-0-8169-0568-3.
  7. EDUG: European DIERS Users’ Group Accessed November 1, 2013.
  8. CRR 1998/136 Workbook for chemical reactor relief system sizing Accessed November 1, 2013.