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Safety valve vs Overpressure valve

What is a safety valve?

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.

Opening characteristic safety valve

  1. Contact pressure is reached; valve is still closed
  2. Contact pressure is exceeded; diaphragm opens
  3. If the response pressure is exceeded by 10%, the valve opens quickly and completely.
  4. Maximum opening is reached – the valve discharges the total overpressure.
  5. The pressure is again lower than the response pressure – the valve slowly starts to close again.
  6. At a pressure of 10% (gases) (20% for liquids) under the response pressure, the valve is closed again.

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.

Opening characteristic overflow valve

  1. Contact pressure is reached; pressure relief valve is still closed.
  2. Contact pressure is exceeded; the pressure relief valve is opened in proportion to the excess pressure.
  3. The pressure relief valve adjusts itself to the required amount of overflow.
  4. When the system pressure decreases, the pressure relief valve starts to close again.
  5. The system pressure decreases and the pressure relief valve continues to close.
  6. If the pressure is slightly lower than the response pressure, the pressure relief valve is closed again.

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