For other uses, see Safety valve (disambiguation).
An oxygen safety relief valve
A safety valve is a valve that acts as a fail-safe in a thermal-hydraulics plant. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve.
Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion.
Vacuum safety valves (or combined pressure/vacuum safety valves) are used to prevent a tank from collapsing while it is being emptied, or when cold rinse water is used after hot CIP (clean-in-place) or SIP (sterilization-in-place) procedures. When sizing a vacuum safety valve, the calculation method is not defined in any norm, particularly in the hot CIP / cold water scenario, but some manufacturers [1] have developed sizing simulations.
Contents
- 1 Function and design
- 2 Technical terms
- 3 Legal and code requirements in industry
- 4 Water heaters
- 5 Pressure cookers
- 6 See also
- 7 References
- 8 External links
Function and design
A cross-section of a proportional-safety valve
The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure (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 a Salter spring balance) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase the performance of a steam engine. In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways.
Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers. The term safety valve should be limited to compressible fluid applications (gas, vapor, or 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 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 for 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).
Technical terms
In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics and pharmaceuticals 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 Pressure relief valve (PRV) or pressure safety valve (PSV) It should be noted that PRVs and PSVs are not the same thing, despite what many people think; the difference is that PSVs have a manual lever to open the valve in case of emergency.
- Relief valve (RV): an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure[citation needed].
- Safety valve (SV): an automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound[citation needed].
- Safety relief valve (SRV): an automatic system that relieves by static pressure on both gas and liquid.[2]
- Pilot-operated safety relief valve (POSRV): an automatic system that relieves on remote command from a pilot, to which the static pressure (from equipment to protect) is connected[clarification needed].
- Low pressure safety valve (LPSV): an automatic system that relieves static pressure on a gas. Used when the difference between the vessel pressure and the ambient atmospheric pressure is small.
- Vacuum pressure safety valve (VPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference between the vessel pressure and the ambient pressure is small, negative and near to atmospheric pressure.
- Low and vacuum pressure safety valve (LVPSV): an automatic system that relieves static pressure on a gas. The pressure is small, negative or positive, and near to atmospheric pressure.
RV, SV and SRV are spring-operated (even spring-loaded). LPSV and VPSV are spring-operated or weight-loaded.
Legal and code requirements in industry
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. These codes include design standards for relief valves and schedules for periodic inspection and testing after valves have been removed by the company engineer or axelpetugrantist.[3][4]
Today, the food, drinks, cosmetics, pharmaceuticals and fine chemicals industries call for hygienic safety valves, fully drainable and Cleanable-In-Place. Most are made of stainless steel; the hygienic norms are mainly 3A in the USA and EHEDG in Europe.
Water heaters
Temperature and Pressure safety valve on a water heater.
Safety valves are required on water heaters, where they prevent disaster in certain configurations in the event that a thermostat should fail. There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be leveled by the force of the blast.[5][full citation needed]
Pressure cookers
Main article: Pressure cooking
Pressure cookers are cooking pots with a pressure-proof lid. Cooking at pressure allows the temperature to rise above the normal boiling point of water (100 degrees Celsius at sea level), which speeds up the cooking and makes it more thorough.
Pressure cookers usually have two safety valves to prevent explosions. On older designs, one is a nozzle 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. On newer generation pressure cookers, if the steam vent gets blocked, a safety spring will eject excess pressure and if that fails, the gasket will expand and release excess pressure downwards between the lid and the pan. Also, newer generation pressure cookers have a safety interlock which locks the lid when internal pressure exceeds atmospheric pressure, to prevent accidents from a sudden release of very hot steam, food and liquid, which would happen if the lid were to be removed when the pan is still slightly pressurised inside (however, the lid will be very hard or impossible to open when the pan is still pressurised).
The term safety valve is also used metaphorically.
See also
- Ball valve
- Butterfly valve
- Control valves
- Flow limiter
- Globe valve
- Needle valve
- Pilot-operated relief valve
- Relief valve
- Safety shutoff valve
References
- ^ Safety valve sized regarding hot CIP -> Cold water conditions
- ^ The Safety Relief Valve Handbook, Hellemans, M. (2009), Elsevier Science, ISBN 9780080961187, p.6
- ^ List of countries accepting the ASME Boiler & Pressure Vessel Code
- ^ API 520-1, Sizing and Selection of Pressure-Relieving Devices
- ^ Elaine Porterfield, Paul Shukovsky, and Lewis Kamb (Saturday, July 28, 2001).
External links
|
Wikimedia Commons has media related to Safety valves. |
- Schematical overview working principle Safety Relief Valve
- Pressure relief valve sizing calculator PSV sizing calculator for liquid blocked outlet case.
- PSV sizing calculator for blocked gas outlet case. PSV sizing calculator for blocked gas discharge.
- PSV sizing calculator - fire case for gas filled vessel
- PSV sizing calculator - fire case for liquid filled vessel
- Safety relief valve Sizing criteria, code requirements, pressure relieving methods, general types of safety relief valve design.
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Steam engines
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| Operating cycle |
- Atmospheric
- Watt
- Cornish
- Compound
- Uniflow
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| Valves |
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Valves
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- Slide
- Piston
- Drop
- Corliss
- Poppet
- Sleeve
- Bash
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Valve gear
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- Gab
- Stephenson link
- Joy
- Walschaerts
- Allan
- Baker
- Corliss
- Lentz
- Caprotti
- Gresley conjugated
- Southern
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| Mechanisms |
- Beam
- Cataract
- Centrifugal governor
- Connecting rod
- Crank
- Crankshaft
- Hypocycloidal gear
- Link chain
- Parallel motion
- Plate chain
- Rotative beam
- Sun and planet gear
- Watt's linkage
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| Boilers |
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Simple boilers
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- Haystack
- Wagon
- Egg-ended
- Box
- Flued
- Cornish
- Lancashire
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Fire-tube boilers
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Water-tube boilers
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- Babcock & Wilcox
- Field-tube
- Sentinel
- Stirling
- Thimble tube
- Three-drum
- Yarrow
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Boiler feed
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- Feedwater heater
- Feedwater pump
- Injector
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| Cylinder |
- Locomotive
- Oscillating
- Single- and double-acting
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| Condenser |
- Condensing steam locomotive
- Jet
- Kirchweger
- Watt's separate
- "Pickle-pot"
- Surface
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| Other |
- Crosshead
- Cutoff
- Expansion valve
- Hydrolock
- Piston
- Reciprocating engine
- Return connecting rod engine
- Six-column beam engine
- Steeple engine
- Safety valve
- Steeple compound engine
- Stroke
- Working fluid
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| History |
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Precursors
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Newcomen engine
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- Newcomen Memorial Engine (1725)
- Fairbottom Bobs (1760)
- Elsecar Engine (1795)
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Watt engine
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Beam
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- Kinneil Engine (1768)
- Old Bess (1777)
- Chacewater Mine engine (1778)
- Smethwick Engine (1779)
- Resolution (1781)
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Rotative beam
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- Soho Manufactory engine (1782)
- Bradley Works engine (1783)
- Whitbread Engine (1785)
- National Museum of Scotland engine (1786)
- Lap Engine (1788)
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High-pressure
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- Richard Trevithick
- Puffing Devil (1801)
- London Steam Carriage (1803)
- "Coalbrookdale Locomotive" (1803)
- "Pen-y-Darren" locomotive (1804)
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Compound
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- Woolf's compound engine (1803)
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Murray
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- Murray's Hypocycloidal Engine (1805)
- Salamanca (1812)
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High-speed
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| See also |
- Glossary of steam locomotive components
- History of steam road vehicles
- Cugnot's fardier à vapeur (1769)
- Murdoch's model steam carriage (1784)
- Lean's Engine Reporter
- List of steam technology patents
- Modern steam
- Stationary steam engine
- Timeline of steam power
- Water-returning engine
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