The
relief valve is a type of
valveA valve is a device that regulates 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
pressurePressure 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 vesselA 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 potentially dangerous and many fatal accidents have occurred in the history of their development and operation...
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 resistanceThe 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
pipingWithin 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 flareA gas flare or flare stack is an elevated vertical stack or chimney found on oil wells or oil rigs, and in refineries, chemical plants and landfills used for burning off unwanted gas or flammable gas and liquids released by pressure relief valves during unplanned over-pressuring of plant equipment...
where it is usually burned and the resulting
combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
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
pumpA pump is a device used to move fluids, such as gases, liquids or slurries. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure. Pumps alone do not create pressure; they only displace fluid, causing a flow. ...
or
gas compressorA 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
vacuumIn 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," but in reality, no volume of space can ever be perfectly empty...
(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 gasAn inert gas is any gas that is not reactive with elements.Unlike noble gases, inert gas is not necessarily elemental and is often a compound gas. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases...
into the equipment so as control the amount of vacuum.
Technical Terms
In the
petroleum refiningAn oil 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...
,
petrochemicalPetrochemicals are chemical products made from raw materials of petroleum or other hydrocarbon origin. Although some of the chemical compounds that originate from petroleum may also be derived from coal and natural gas, petroleum is the major source...
and
chemical manufacturingA 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 processingNatural gas processing plants, or fractionators, are used to purify the raw natural gas extracted from underground gas fields and brought up to the surface by gas wells...
and
power generationA power station is an industrial facility for the generation of electric power....
industries, the term
relief valve is associated with the terms
pressure relief valve (PRV),
pressure safety valve (PSV) and
safety valve.
It should be noted that in practice people often do not stick to the technical distinctions between the most common abbreviations: SRV, PRV, SV and RV - they just use the term they are comfortable with.
Pressure Relief ValveA pressure relief valve is a safety device that relieves in case of overpressure in a vessel or piping. The generic terms are pressure relief valve or pressure safety valve .Main types of pressure relief valves include:...
(PRV) or Pressure Safety Valve (PSV): the most generic terms. 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
ASMEThe 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 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. It is part of a bigger set named pressure safety valves or pressure relief valves...
(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 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...
(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.
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
ASMEThe American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering....
,
APIThe American Petroleum Institute, commonly referred to as API, is the main U.S trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum industry...
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 Engineers
The 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 Institute
The American Petroleum Institute, commonly referred to as API, is the main U.S trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum 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)
Pressure relief valves in oil hydraulics
Whereas pressure relief valves in gas pressure systems are always used to protect the system, in
oil hydraulicHydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
systems a pressure relief valve can act as part of the control system. The easiest use of the relief valve is as a sort of
check valveA check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction. Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave...
, a seat with a ball and an adjustable spring. More sophisticated relief valves are pilot operated, so that the pressure can be set at zero (by-pass) and sometimes at 2 or 3 other pressures. In these cases, the highest pressure acts as the maximum working pressure and the others as a set pressure during a certain operation of the installation.
See also
- 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. It is part of a bigger set named pressure safety valves or pressure relief valves...
- Blowoff valve
A blowoff valve is a pressure release system present in turbocharged engines. Its purpose is to prevent compressor surge and reduce wear on the engine.- Definitions :...
- Rupture disc
A rupture disc or bursting disc is a pressure relief device that protects a vessel or system from overpressurization. Rupture discs are a type of sacrificial part because it has a one-time-use membrane that fails at a predetermined pressure, either positive or vacuum. The membrane is usually a...
- 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...
External links