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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. 
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).
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.
Additional recommended knowledge
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 equipent 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.
The main standards, laws or directives are:
The main manufacturers of pressure safety valves are:
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Relief_valve". A list of authors is available in Wikipedia.|