Relief valve

Relief valve

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 pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the pressure setting 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 re-seating pressure, the valve will re-close. This pressure, also called "blowdown", is usually within several percent of the set-pressure.

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. [cite book|author=Beychok, Milton R.|title=Fundamentals Of Stack Gas Dispersion|edition=4th Edition|publisher=author-published|year=2005|id=ISBN 0-9644588-0-2 See Chapter 11, "Flare Stack Plume Rise".]

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.

Legal and code requirements in industry

In the petroleum refining, 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.

The generic term is Pressure relief valve (PRV) or Pressure safety valve (PSV)

Relief valve (RV): A Relief valve is an automatic device used on a liquid service, which relieves pressure proportionally (slowly) as the increasing pressure overcomes the spring pressure.

Safety valve (SV): automatic system that relieves due to static pressure by a gas. It specifically open almost straight to full lift after a pop sound.

Safety relief valve (SRV): automatic system that relieves both gas and liquid.

Pilote operated safety relief valve (POSRV): automatic system that relief by remote command from a pilote on which the static pressure (from equipment to protect) is connected.

Low pressure safety valve (LPSV): automatic system that relief by static pressure on a gas. The pressure is small and near the atmospheric pressure.

Vacuum pressure safety valve (VPSV): automatic system that relief 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 relief by static pressure on a gas. The pressure is small, negative or positive and near the atmospheric pressure.

RV, SV and SRV are spring operated (even said spring loaded).LPSV and VPSV are spring operated or weight loaded.

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 ASME, API and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves. [ [ List of countries accepting the ASME Boiler & Pressure Vessel Code] ] [ [ API 5210-1, Sizing and Selection of Pressure-Relieving Devices] ]

The main standards, laws or directives are:
* ASME (American Society of Mechanical Engineers) Boiler & Pressure Vessel Code, Section VIII Division 1 and Section I
* API (American Petroleum Institute) 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 Merckblatt (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 hydraulic 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 valve, 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.

ee also

*Safety valve
*Blowoff valve
*Rupture disc


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Look at other dictionaries:

  • Relief valve — Relief Re*lief (r? l?f ), n. [OE. relef, F. relief, properly, a lifting up, a standing out. See {Relieve}, and cf. {Basrelief}, {Rilievi}.] 1. The act of relieving, or the state of being relieved; the removal, or partial removal, of any evil, or… …   The Collaborative International Dictionary of English

  • relief valve — n. SAFETY VALVE …   English World dictionary

  • relief valve — [1] A valve in the air pump of an air injection system which dumps part of the air at high pump speeds to prevent pump damage. It may be combined with the air gulp valve. [2] Safety device on a sealed system. It opens to release fluids before… …   Dictionary of automotive terms

  • relief valve — noun a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level • Syn: ↑safety valve, ↑escape valve, ↑escape cock, ↑escape • Hypernyms: ↑valve, ↑regulator * * * …   Useful english dictionary

  • relief valve — apsauginis vožtuvas statusas T sritis automatika atitikmenys: angl. pressure relief valve; relief valve; safety valve vok. Überdruckventil, n; Entlastungsventil, n; Sicherheitsventil, n rus. предохранительный клапан, m pranc. clapet de décharge,… …   Automatikos terminų žodynas

  • relief valve — A pressure control valve in any fluid system, which limits the pressure in the system by releasing it at a preset value. The pressure is not allowed to be built up above this preset figure. A typical oil pressure relief valve …   Aviation dictionary

  • relief valve — a device that, when actuated by static pressure above a predetermined level, opens in proportion to the excess above this level and reduces the pressure to it. Cf. safety valve (def. 1). * * * …   Universalium

  • relief valve — noun A safety valve that relieves pressure when it becomes high but before it becomes dangerous …   Wiktionary

  • Pressure relief valve — A pressure relief valve is a safety device that relieve in case of overpressure in vessel or piping. The generic term is or pressure relief Valve (PRV) or pressure safety valve (PSV).Main types of pressure relief valves include: *Relief valve… …   Wikipedia

  • negative pressure relief valve — A relief valve in an aircraft pressurization system that prevents the pressure in the cabin from becoming lower than the ambient outside pressure. The cabin air pressure safety valve combines functions of over pressure relief valve, vacuum relief …   Aviation dictionary

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