Relief valve
Encyclopedia
The relief valve is a type of valve
used to control or limit the pressure
in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire.
The pressure is relieved by allowing the pressurised 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
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. 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 a central, elevated gas flare
where it is usually burned and the resulting combustion
gases are released to the atmosphere. As the fluid is diverted, the pressure inside the vessel will drop. 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 from 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 openings area 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 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
or gas compressor
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 engines 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
(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.
, petrochemical
and chemical manufacturing
, natural gas processing
and power generation
industries, the term relief valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and safety valve.
In practice, people often do not stick to the technical distinctions between the most common abbreviations: SRV, PRV, SV and RV
Pressure relief valve (PRV) or pressure safety valve (PSV). The difference being that PSVs have a manual lever to activate the valve in case of emergency. Most PRV 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 increasing system pressure reaches this value the PRV opens. Accuracy of set pressure often follows guidelines set by the ASME
.
Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure.
Safety valve
(SV): Used in gas service. Most SV are full lift or snap acting, they pop open all the way.
Safety relief valve (SRV): A PRV that can be used for gas or liquid service. But set pressure will usually only be accurate for one type of fluid at a time (the type it was set with).
Pilot-operated relief valve
(POSRV, PORV, POPRV): device that relieves by remote command from a pilot valve that is connected to the upstream system pressure.
Low-pressure safety valve (LPSV): automatic system that relieves by static pressure on a gas. The pressure is small and near the atmospheric pressure.
Vacuum pressure safety valve (VPSV): automatic system that relieves by static pressure on a gas. The pressure is small, negative and near the atmospheric pressure.
Low and vacuum pressure safety valve (LVPSV): automatic system that relieves by static pressure on a gas. The pressure is small, negative or positive and near the atmospheric pressure.
Snap acting: The opposite of modulating, refers to a valve that "pops" open, it goes into 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 formed.
(ASME), American Petroleum Institute
(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:
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.
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
used to control or limit the pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire.
The pressure is relieved by allowing the pressurised 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
Pressure vessel
A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
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
Path of least resistance
The path of least resistance describes the physical or metaphorical pathway that provides the least resistance to forward motion by a given object or entity, among a set of alternative paths. The concept is often used to describe why an object or entity takes a given path.In physics, the path of...
" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. The diverted fluid (liquid, gas or liquid–gas mixture) is usually routed through a piping
Piping
Within industry, piping is a system of pipes used to convey fluids from one location to another. The engineering discipline of piping design studies the efficient transport of fluid....
system known as a flare header or relief header to a central, elevated gas flare
Gas flare
A gas flare, alternatively known as a flare stack, is an elevated vertical conveyance found accompanying the presence of oil wells, gas wells, rigs, refineries, chemical plants, natural gas plants, and landfills....
where it is usually burned and the resulting combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
gases are released to the atmosphere. As the fluid is diverted, the pressure inside the vessel will drop. 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 from 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 openings area 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 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
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
or gas compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
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 engines 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
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
(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
Inert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
into the equipment so as control the amount of vacuum.
Technical terms
In the petroleum refiningOil refinery
An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...
, petrochemical
Petrochemical
Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
and chemical manufacturing
Chemical plant
A chemical plant is an industrial process plant that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use special equipment,...
, natural gas processing
Natural gas processing
Natural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.-Background:...
and power generation
Power station
A power station is an industrial facility for the generation of electric energy....
industries, the term relief valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and safety valve.
In practice, people often do not stick to the technical distinctions between the most common abbreviations: SRV, PRV, SV and RV
Pressure relief valve (PRV) or pressure safety valve (PSV). The difference being that PSVs have a manual lever to activate the valve in case of emergency. Most PRV 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 increasing system pressure reaches this value the PRV opens. Accuracy of set pressure often follows guidelines set by the ASME
American Society of Mechanical Engineers
The American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering....
.
Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure.
Safety valve
Safety valve
A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits....
(SV): Used in gas service. Most SV are full lift or snap acting, they pop open all the way.
Safety relief valve (SRV): A PRV that can be used for gas or liquid service. But set pressure will usually only be accurate for one type of fluid at a time (the type it was set with).
Pilot-operated relief valve
Pilot-operated relief valve
Like other pressure relief valves , pilot operated relief valves are used for emergency relief during overpressure events . The distinction between PORV and conventional PRV is that pilot valves use system pressure to seal the valve...
(POSRV, PORV, POPRV): device that relieves by remote command from a pilot valve that is connected to the upstream system pressure.
Low-pressure safety valve (LPSV): automatic system that relieves by static pressure on a gas. The pressure is small and near the atmospheric pressure.
Vacuum pressure safety valve (VPSV): automatic system that relieves by static pressure on a gas. The pressure is small, negative and near the atmospheric pressure.
Low and vacuum pressure safety valve (LVPSV): automatic system that relieves by static pressure on a gas. The pressure is small, negative or positive and near the atmospheric pressure.
Snap acting: The opposite of modulating, refers to a valve that "pops" open, it goes into 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 formed.
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 EngineersAmerican Society of Mechanical Engineers
The American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering....
(ASME), American Petroleum Institute
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
(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:
- ASME (American Society of Mechanical EngineersAmerican Society of Mechanical EngineersThe American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering....
) Boiler & Pressure Vessel Code, Section VIII Division 1 and Section I - API (American Petroleum InstituteAmerican Petroleum InstituteThe American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
) Recommended Practice 520/521, API Standard 2000 et API Standard 526 - ISO 4126 (International Organisation for Standardisation)
- EN 764-7 (European Standard based on pressure Equipment Directive 97/23/EC)
- AD Merkblatt (German)
- PED 97/23/EC (Pressure Equipment Directive – European Union)
DIERS
Formed in 1976, 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/vapour, 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 valveBlowoff valveA blowoff valve or dump valve is a pressure release system present in most turbocharged engines.Its purpose is to prevent compressor surge, and reduce wear on the turbocharger and engine...
- Pilot-operated relief valvePilot-operated relief valveLike other pressure relief valves , pilot operated relief valves are used for emergency relief during overpressure events . The distinction between PORV and conventional PRV is that pilot valves use system pressure to seal the valve...
- Rupture discRupture discBurst disc redirects here. For the human back ailment see Spinal disc herniation.A rupture disc, also known as a bursting disc or burst diaphragm, is a non-reclosing pressure relief device that, in most uses, protects a pressure vessel, equipment or system from overpressurization or potentially...
- Safety valveSafety valveA safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits....
External links
- Pressure relief valve sizing calculator PSV sizing calculator for liquid blocked outlet case.
- PSV sizing calculator for blocked gas outlet case. PSV sizing calculator for blocked gas discharge.
- PSV sizing calculator – fire case for gas filled vessel
- PSV sizing calculator – fire case for liquid filled vessel