A safety valve is a valve which has the function of increasing the safety of a thermal-hydraulics plant. An example of safety valve could be a pressure safety valve (PSV), i.e. a pressure relief valve (PRV) which automatically releases a substance from aboiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Also pilot-operated relief valves could have the function of safety valves.
Safety valves were first used on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to accidental explosion.
Vacuum safety valves (or combined pressure/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 (clean-in-place) or SIP (sterilization-in-place) 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.