Definition of a safety valve according to DIN EN ISO 4126-1
A safety valve is a valve that can automatically, without the help of any other than that of the fluid concerned, discharge an amount of liquid to prevent it from exceeding a predetermined safe pressure, and that is designed to close again and prevent further flow of liquid after recovery from normal pressure.
The opening characteristic curve of a safety valve (last resort to prevent an explosion due to overpressure) is shown below. A safety valve should never be used, only in case of emergency. We recommend that you also replace the valve after it has been used in order to continue to guarantee safety.
Fixed points are: 1, 3, 4 and 6, there are no 'standards' for the others.
Where point 1 is the specified setting pressure.
The opening curve of an overpressure valve, overflow valve, pressure control valve, pressure relief valve is shown below. An overpressure valve may in principle blow off continuously and is therefore often used as an overflow on pumps to prevent the pump from continuing to pump against a tight valve.