Safety valve
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2008-07-10T16:30:16Z
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{{Refimprove|date=May 2007}}
[[Image:Oxygen-Safetyvalve.jpg|thumb|200px| Oxygen Safety Valve]]
[[Image:Valvole di sicurezza di grandi dimensioni.jpg|thumb|200px|ND250-Safety Valves]]
A '''safety valve''' is a [[valve]] mechanism for the automatic release of a gas 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 Valve]]s (PSV) or [[Pressure Relief Valve]]s (PRV). The other parts of the set are named [[relief valve]]s, safety relief valves, pilot-operated safety relief valves, low pressure safety valves, vacuum pressure safety valves.
Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental [[boiler explosion|explosion]] when the operator allowed the pressure to become too high, either deliberately or through incompetence.
== Function and design ==
[[Image:Proportional-Safety Valve.jpg|thumb|200px|Proportional-Safety Valve]]
The earliest and simplest safety valve used a weight to hold the pressure of the steam, (this design is still commonly used on [[pressure cooker]]s); however, these were easily tampered with or accidentally released. On the [[Stockton and Darlington Railway]], the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure, but these (based on Salter spring balances) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase performance of a steam engine. In 1856 [[John Ramsbottom]] invented a tamper-proof spring safety valve which became universal on railways.
Safety valves also evolved to protect equipment such as [[pressure vessel]]s (fired or not) and [[heat exchanger]]s. Safety valve term should be limited to compressible fluid application (gas, vapor, steam).
The two general types of protection encountered in industry are ''thermal protection'' and ''flow protection''.
For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the safety valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.
Flow protection is characterized by safety valves that are considerably larger than those mounted in thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline.
==Legal and code requirements in industry==
{{Wikify|date=November 2007}}
In the [[oil refinery|petroleum refining]], [[petrochemical]] and [[chemical plant|chemical manufacturing]], [[natural gas processing]] and [[power station|power generation]] industries, the term '''safety valve''' is associated with the terms '''pressure relief valve''' (PRV), '''pressure safety valve''' (PSV) and '''relief valve'''.
The generic term is or [[Pressure Relief Valve]] (PRV) or [[Pressure Safety Valve]] (PSV)
[[Relief Valve]] (RV): automatic system that relief by static pressure on a liquid. It specifically open proportionally with pressure increasing.
[[Safety Valve]] (SV): automatic system that relief by static pressure on a gas. It specifically open almost straight to full lift after a pop sound.
[[Safety Relief Valve]] (SRV): automatic system that relief by static pressure on both gas and liquid.
[[Pilot-Operated Safety Relief Valve]] (POSRV): automatic system that relief by remote command from a pilot 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 [[American Society of Mechanical Engineers|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.<ref>[http://www.onetb.com/asme_code_countries.htm List of countries accepting the ASME Boiler & Pressure Vessel Code]</ref><ref>[http://www.techstreet.com/cgi-bin/detail?product_id=235758 API 5210-1, Sizing and Selection of Pressure-Relieving Devices]</ref>
The main standards, laws or directives are:
* [[ASME]] ([[American Society of Mechanical Engineers]]) Boiler & Pressure Vessel Code, Section VIII, Division 1
* [[API]] ([[American Petroleum Institute]]) Recommended Practice 520 and API Standard 526, API Standard 2000 (low pressure -Storage tank)
* [[ISO 4126]] (originated from from European Union directives)
* EN 764-7 (from CEN - European Committee for Standardization - originate from European Union directives)
* AD Merkblatt (German)
* [http://ec.europa.eu/enterprise/pressure_equipment/ped/index_en.html PED 97/23/EC] (Pressure Equipment Directive - European Union)
== Water heaters ==
[[Image:WH safety valve.JPG|thumb|right|Temperature and Pressure safety valve on a water heater.]]
They are required on [[water heater]]s, where they prevent disaster in certain configurations in the event a [[thermostat]] should fail. There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be levelled by the force of the blast.<ref> Elaine Porterfield, Paul Shukovsky, and Lewis Kamb (Saturday, July 28, 2001). [http://seattlepi.nwsource.com/local/33094_boom28.shtml Four hurt as water heater explodes]. seatlepi.nwsource.com, Retrieved on Saturday, May 12, 2007 </ref>
== Pressure cookers ==
[[Pressure cooker]]s are pots for cooking with a pressure proof lid. Cooking at pressure allows the temperature to rise above the boiling point of water (100 degrees Celsius at [[sea level]] ) which speeds up the cooking and makes the cooking more thorough.
Pressure cookers usually have two safety valves. One in a hole upon which a weight sits. The other is a sealed rubber [[grommet]] which is ejected in a controlled explosion if the first valve gets blocked.
The term '''''safety valve''''' is also used [[metaphor]]ically.
== See also ==
* [[Boiler explosion]]
* [[Relief valve]]
* [[Pilot-operated relief valve]]
* [[Safety shutoff valve]]
==External links==
'''[http://www.spiraxsarco.com/resources/steam-engineering-tutorials/safety-valves.asp Safety Valves]''' Why are safety valves required? What different types are available and how are they selected, sized and installed? - Spirax Sarco<br />
The main manufacturers of pressure safety valves are:
* [http://www.weirvalve.com Sarasin-RSBD] (France) ... SV SRV POSRV LPSV VPSV LVPSV
* Sapag (France) ... SV SRV
* Crosby (USA) ... SV SRV POSRV
* Consolidated (USA) ... SV SRV POSRV
* Anderson-Greenwood (USA) ... POSRV
* Farris (Canada) ... SV SRV POSRV
* Fukui (Japan) ... SV SRV POSRV
* [http://www.ari-armaturen.com ARI-Armaturen (Germany)]
* [http://www.leser.com Leser (Germany], [http://www.us.leser.com USA], [http://www.fr.leser.com France)] ... SRV
* Bopp & Reuter (Germany) ... SV SRV
* [http://www.besa.it Besa (Italy)] ... SRV
* [http://www.tecvalexport.com Tecval S.A (Colombia)]
* [http://www.motherwelltankprotection.co.uk Motherwell Tank Protection] (UK) ... LPSV POSRV
==References==
{{reflist}}
[[Category:Valves]]
[[Category:Safety equipment]]
[[cs:Pojistný ventil]]
[[de:Sicherheitsventil]]
[[es:Válvula de alivio de presión]]
[[it:Valvola di sicurezza]]
[[he:שסתום בטיחות]]
[[nl:Veiligheidsventiel]]
[[ja:安全弁]]
[[pl:Zawór bezpieczeństwa]]
[[ru:Предохранительный клапан]]
[[fi:Varoventtiili]]
[[sv:Säkerhetsventil]]