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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. They are often called by more specific names such as pressure relief valves, T&P valves, or temperature and pressure relief valves.

Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental explosion when the operator allowed the pressure to become too high, either deliberately or through incompetence.

Contents 1 Function and design 2 Water heaters 3 Pressure cookers 4 See also

The earliest and simplest safety valve used a weight to hold the pressure of the steam, but 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 better valve 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. 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 and heat exchangers. The two general types of protection encountered in industry are thermal protection and flow protection.

Thermal relief valves are generally characterized by the relatively small size of the safety valve necessary to provide protection from thermal expansion pressure increases in liquid-packed vessels. As most liquids are considered fairly incompressible, it takes a relatively small amount of fluid discharged through the relief valve to provide an adequate level of protection.

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.

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.^{[citation needed]}

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.

• Boiler explosion
• Relief valve
• Valve