A safety valve is a valve mechanism for the automatic release of a substance 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 v alves (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 Darlinfton 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. The term safety valve 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 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).

Technical terms

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 Pressure relief valve (PRV) or pressure safety valve (PSV) It should be noted, as most people think PRV and PSV are the same thing. Is that PSV's have a manual lever to open the valve in case of emergency.

• Relief valves (RV): automatic system that is actuated by static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure.

• Safety valve (SV): automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound.

• Safety relief valve (SRV): automatic system that relieves by static pressure on both gas and liquid.

• Pilot-operated safety relief valve (POSRV): automatic system that relieves 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 relieves static pressure on a gas. Used when difference between vessel pressure and the ambient atmospheric pressure is small.

• Vacuum pressure safety valve (VPSV): 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 the atmospheric pressure.

• Low and vacuum pressure safety valve (LVPSV): automatic system that relieves 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.