Safety valve
A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve, 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. A leak tight, lower cost, single emergency use option would be a rupture disk.
Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion unless carefully operated.
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 or SIP 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 have developed sizing simulations.
The term safety valve is also used metaphorically.
Function and design
The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure ; 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 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. The Ramsbottom valve consisted of two plug-type valves connected to each other by a spring-laden pivoting arm, with one valve element on either side of the pivot. Any adjustment made to one of valves in an attempt to increase its operating pressure would cause the other valve to be lifted off its seat, regardless of how the adjustment was attempted. The pivot point on the arm was not symmetrically located between the valves, so any tightening of the spring would cause one of the valves to lift.
Only by removing and disassembling the entire valve assembly could its operating pressure be adjusted, making impromptu "tying down" of the valve by locomotive crews in search of more power impossible. The pivoting arm was commonly extended into a handle shape and fed back into the locomotive cab, allowing crews to 'rock' both valves off their seats to confirm that they were set and operating correctly.
Safety valves also evolved to protect equipment such as pressure vessels and heat exchangers. The term safety valve should be limited to compressible fluid applications.
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.
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, pressure safety valve and relief valve.The generic term is Pressure relief valve or pressure safety valve. 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 : an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure.
- Safety valve : an automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound.
- Safety relief valve : an automatic system that relieves by static pressure on both gas and liquid.
- Pilot-operated safety relief valve : an automatic system that relieves on remote command from a pilot, to which the static pressure is connected.
- Low pressure safety valve : 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 : 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 : an automatic system that relieves static pressure on a gas. Used when the pressure difference is small, negative or positive and near to atmospheric pressure.
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 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.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 US and EHEDG in Europe.
Development of the safety valve
Deadweight lever valves
The first safety valve was invented by Denis Papin for his steam digester, an early pressure cooker rather than an engine. A weight acting through a lever held down a circular plug valve in the steam vessel. By using a "steelyard" lever a smaller weight was required, also the pressure could easily be regulated by sliding the same weight back and forth along the lever arm. Papin retained the same design for his 1707 steam pump.Early safety valves were regarded as one of the engineman's controls and required continuous attention, according to the load on the engine. In a famous early explosion at Greenwich in 1803, one of Trevithick's high-pressure stationary engines exploded when the boy trained to operate the engine left it to catch eels in the river, without first releasing the safety valve from its working load. By 1806, Trevithick was fitting pairs of safety valves, one external valve for the driver's adjustment and one sealed inside the boiler with a fixed weight. This was unadjustable and released at a higher pressure, intended as a guarantee of safety.
When used on locomotives, these valves would rattle and leak, releasing near-continuous puffs of waste steam.
Direct-acting deadweight valves
Although the lever safety valve was convenient, it was too sensitive to the motion of a steam locomotive. Early steam locomotives therefore used a simpler arrangement of weights stacked directly upon the valve. This required a smaller valve area, so as to keep the weight manageable, which sometimes proved inadequate to vent the pressure of an unattended boiler, leading to explosions. An even greater hazard was the ease with which such a valve could be tied down, so as to increase the pressure and thus power of the engine, at further risk of explosion.Although deadweight safety valves had a short lifetime on steam locomotives, they remained in use on stationary boilers for as long as steam power remained.
Direct spring valves
Weighted valves were sensitive to bouncing from the rough riding of early locomotives. One solution was to use a lightweight spring rather than a weight. This was the invention of Timothy Hackworth on his Royal George of 1828. Owing to the limited metallurgy of the period, Hackworth's first spring valves used an accordion-like stack of multiple leaf springs.These direct-acting spring valves could be adjusted by tightening the nuts retaining the spring. To avoid tampering, they were often shrouded in tall brass casings which also vented the steam away from the locomotive crew.
Salter spring balance valves
The Salter coil spring balance for weighing, was first made in Britain by around 1770. This used the newly developed spring steels to make a powerful but compact spring in one piece. Once again by using the lever mechanism, such a spring balance could be applied to the considerable force of a boiler safety valve.The spring balance valve also acted as a pressure gauge. This was useful as previous pressure gauges were unwieldy mercury manometers and the Bourdon gauge had yet to be invented.