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.
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.
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).
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.
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.
The main standards, laws or directives are:
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.
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.