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A relief valve or pressure relief valve (PRV) is a type of
safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...
used to control or limit the
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect
pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
s and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the " path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. In systems containing flammable fluids, the diverted fluid (liquid, gas or liquid-gas mixture) is either recaptured by a low pressure, high-flow vapor recovery system or is routed through a
piping Within industry, piping is a system of pipes used to convey fluid In physics, a fluid is a liquid, gas, or other material that continuously Deformation (physics), deforms (''flows'') under an applied shear stress, or external force. They ...
system known as a ''flare header'' or ''relief header'' to a central, elevated
gas flare A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit is a gas combustion device used in places such as oil refinery, petroleum refineries, chemical plants and natural gas processing plants, oil or gas extracti ...
where it is burned, releasing naked
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
gases into the atmosphere. In non-hazardous systems, the fluid is often discharged to the atmosphere by a suitable discharge pipework designed to prevent rainwater ingress which can affect the set lift pressure, and positioned not to cause a hazard to personnel. As the fluid is diverted, the pressure inside the vessel will stop rising. Once it reaches the valve's reseating pressure, the valve will close. The ''blowdown'' is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. The blowdown can vary roughly 2–20%, and some valves have adjustable blowdowns. In high-pressure gas systems, it is recommended that the outlet of the relief valve is in the open air. In systems where the outlet is connected to piping, the opening of a relief valve will give a pressure build-up in the piping system downstream of the relief valve. This often means that the relief valve will not re-seat once the set pressure is reached. For these systems often so-called "differential" relief valves are used. This means that the pressure is only working on an area that is much smaller than the area of the opening of the valve. If the valve is opened the pressure has to decrease enormously before the valve closes and also the outlet pressure of the valve can easily keep the valve open. Another consideration is that if other relief valves are connected to the outlet pipe system, they may open as the pressure in the exhaust pipe system increases. This may cause undesired operation. In some cases, a so-called ''bypass valve'' acts as a relief valve by being used to return all or part of the fluid discharged by a
pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the metho ...
or
gas compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...
back to either a storage reservoir or 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 internal (an integral part of the pump or compressor) or external (installed as a component in the fluid path). Many
fire engine A fire engine (also known in some places as a fire truck or fire lorry) is a road vehicle (usually a truck) that functions as a firefighting apparatus. The primary purposes of a fire engine include transporting firefighters and water to a ...
s have such relief valves to prevent the overpressurization of fire hoses. In other cases, equipment must be protected against being subjected to an internal
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "Void (astronomy), void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Ph ...
(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 An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
into the equipment to control the amount of vacuum.


Technical terms

In the
petroleum refining An oil refinery or petroleum refinery is an industrial process Industrial processes are procedures involving chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some ref ...
,
petrochemical Petrochemicals (sometimes abbreviated as petchems) are the product (chemistry), chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural ...
and chemical manufacturing,
natural gas processing Natural-gas processing is a range of industrial processes designed to purify raw natural gas by removing impurities, contaminants and higher molecular mass volatile organic compound, hydrocarbons to produce what is known as ''pipeline quality'' d ...
and
power generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmi ...
industries, the term relief valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and
safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...
: * Pressure relief valve (PRV) or Pressure Release valve (PRV) or pressure safety valve (PSV): The difference is that PSVs have a manual lever to activate the valve in case of emergency. Most PRVs are spring operated. At lower pressures some use a diaphragm in place of a spring. The oldest PRV designs use a weight to seal the valve. * Set pressure: When the system pressure increases to this value, the PRV opens. The accuracy of the set pressure may follow guidelines set by the
American Society of Mechanical Engineers The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing ...
(ASME). * Relief valve (RV): A valve is used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure. *
Safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...
(SV): Used in gas service. Most SVs are full lift or snap-acting, in that they pop completely open. * Safety relief valve (SRV): A relief valve that can be used for gas or liquid service. However, the set pressure will usually only be accurate for one type of fluid at a time. * Pilot-operated relief valve (POSRV, PORV, POPRV): A device that relieves by remote command from a pilot valve which is connected to the upstream system pressure. * Low-pressure safety valve (LPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small and near the atmospheric pressure. * Vacuum pressure safety valve (VPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative, and near the atmospheric pressure. * Low and vacuum pressure safety valve (LVPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative, or positive, and near the atmospheric pressure. * Pressure vacuum release valve (PVRV): A combination of vacuum pressure and a relief valve in one housing. Used on storage tanks for liquids to prevent implosion or overpressure. * Snap acting: The opposite of modulating, refers to a valve that "pops" open. It snaps into a full lift in milliseconds. Usually accomplished with a skirt on the disc so that the fluid passing the seat suddenly affects a larger area and creates more lifting force. * Modulating: Opens in proportion to the overpressure.


Legal and code requirements 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 The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing ...
(ASME),
American Petroleum Institute The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in extraction of petroleum, production, oil refinery, refinement, pipeline ...
(API) and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves. The main standards, laws, or directives are: * ''AD Merkblatt'' (German) *
American Petroleum Institute The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in extraction of petroleum, production, oil refinery, refinement, pipeline ...
(API); Standards 520, 521, 526, and 2000 *
American Society of Mechanical Engineers The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing ...
(ASME); Boiler & Pressure Vessel Code, Section VIII Division 1 and Section I * American Water Works Association (AWWA),
storage tank Storage tanks are containers that hold liquids, compressed gases (gas tank; or in U.S.A "pressure vessel", which is not typically labeled or regulated as a storage tank) or mediums used for the short- or long-term storage of heat or cold. The t ...
s * EN 764-7; European Standard based on pressure Equipment Directive 97/23/EC * Eurocode EN 1993-4-2, storage tanks. *
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Art ...
; ISO 4126 * Pressure Systems Safety Regulations 2000 (PSSR); UK


DIERS

Formed in 1977, the Design Institute for Emergency Relief Systems 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, diazotizations, sulphonations, epoxidations, aminations, esterifications, neutralizations, and many others. Pressure relief systems can be difficult to design, not least because what is expelled can be gas/vapor, liquid, or a mixture of the two – just as with a can of carbonated drink when it is suddenly opened. For chemical reactions, it requires extensive knowledge of both chemical reaction hazards and fluid flow. DIERS investigated the two-phase vapor-liquid onset/disengagement dynamics and the hydrodynamics of emergency relief systems with extensive experimental and analysis work. 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) 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 HSE.


See also

* Blowoff valve * Rupture disc *
Safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...
* Surge control


References


External links

*
PED 97/23/EC
Pressure Equipment Directive – European Union. {{DEFAULTSORT:Relief Valve Piping Pressure vessels Safety valves ru:Сбросной клапан (сантехника)