A generic name for a device used for the automatic release of a substance from a vessel or system when the pressure or temperature exceeds preset limits. Safety valves were first used on steam boilers during the Industrial Revolution in which boilers were prone to accidental explosion. Examples of safety valves include pressure relief valves (PRV), pressure safety valves (PSV), and relief valves (RV). PSVs differ from PRVs by having a manual lever to open in the event of an emergency. Vacuum safety valves are used to prevent a vessel from collapsing when emptying or when a vacuum may accidentally form due to a trapped vapour that then condenses.