Safety valve

Safety valve

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 valves (PSV) or pressure relief valves (PRV). The other parts of the set are named relief valves, 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 explosion.

Function and design

The earliest and simplest safety valve on the steam digester in 1679 used a weight to hold the pressure of the steam, (this design is still commonly used on pressure cookers); 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 vessels (fired or not) and heat exchangers. 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. This protection can alternatively be achieved by installing a High Integrity Pressure Protection System (HIPPS).

Legal and code requirements in industry

In the petroleum refining, petrochemical and chemical manufacturing, natural gas processing and 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 ASME, API and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves. [ [http://www.onetb.com/asme_code_countries.htm List of countries accepting the ASME Boiler & Pressure Vessel Code] ] [ [http://www.techstreet.com/cgi-bin/detail?product_id=235758 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
* 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

They are required on water heaters, 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. [ 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 ]

Pressure cookers

Pressure cookers 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 metaphorically.

See also

* Boiler explosion
* Flow limiter
* Relief valve
* Pilot-operated relief valve
* Safety shutoff valve

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • safety valve — ˈsafety valve noun [countable] a small part on a machine that opens to let out steam or pressure when it becomes too great: • the safety valve device on the boiler * * * safety valve UK US noun [C] ► PRODUCTION a part of a machine that opens to… …   Financial and business terms

  • Safety valve — Safety chain Safety chain (a) (Railroads) A normally slack chain for preventing excessive movement between a truck and a car body in sluing. (b) An auxiliary watch chain, secured to the clothes, usually out of sight, to prevent stealing of the… …   The Collaborative International Dictionary of English

  • safety valve — safety valves 1) N COUNT A safety valve is a device which allows liquids or gases to escape from a machine when the pressure inside it becomes too great. Residents heard an enormous bang as a safety valve on the boiler failed. 2) N COUNT A safety …   English dictionary

  • safety valve — n 1.) a part of a machine that allows gas, steam etc to escape when the pressure becomes too great 2.) something that allows you to get rid of strong feelings without doing any harm ▪ Being able to express emotion is a healthy safety valve for… …   Dictionary of contemporary English

  • safety valve — ► NOUN 1) a valve that opens automatically to relieve excessive pressure. 2) a means of giving harmless vent to feelings of tension or stress …   English terms dictionary

  • safety valve — n. 1. an automatic valve for a steam boiler, pressure cooker, etc., which opens if the pressure becomes excessive 2. any outlet for the release of strong emotion, energy, etc …   English World dictionary

  • safety valve — safety ,valve noun count 1. ) a part of a piece of equipment that allows gas, air, or liquid to escape when there is too much pressure inside 2. ) a way of getting rid of strong emotions without harming anyone …   Usage of the words and phrases in modern English

  • safety valve — UK / US noun [countable] Word forms safety valve : singular safety valve plural safety valves 1) a part of a piece of equipment that allows gas, air, or liquid to escape when there is too much pressure inside 2) a way of getting rid of strong… …   English dictionary

  • safety 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

  • safety valve — A form of pressure relief valve that opens to relieve the pressure in the container any time the pressure rises above a predetermined value. Safety valve: it ensures that the fluid bypasses anytime the pressure builds up beyond designated value …   Aviation dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”