User:Milton Beychok/Sandbox
A relief valve is a type of valve used to to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. Such pressure exceedances (i.e., over-pressures) can result from a system upset, an instrument or equipment failure, or a fire. The relief valve is set to open at a predetermined pressure and allow the over-pressured fluid to flow through an auxiliary route out of the system.
For safety reasons, relieving of flammable or toxic fluids should not be routed directly to the atmosphere. Thus, in most cases, 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 an elevated flare stack 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 decrease. Once it decreases to the valve's re-seating pressure, the valve will re-close.
Other similar applications
In some systems, a so-called bypass valve acts as a relief valve by being used to recycle all or part of the fluid discharged by a pump or gas compressor back to either a storage vessel or to 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 an integral part of the pump or compressor or externally installed as an alternate path for the pump or compressor discharge fluid. Many fire engines have such bypass systems to prevent the overpressuring the fire hoses.
In other systems, 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.[1][2]
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.