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Safety valve knowledge, please learn
Apr 21, 2018

The safety valve is a device for safety protection on equipment, installations and pipelines, to prevent the pressure of the medium in equipment, installations and pipelines exceeding the prescribed value. When the working pressure in the equipment, equipment and pipe exceeds the specified value, the safety valve opens automatically and releases the excess medium; when the pressure is restored to normal, the safety valve closes itself and prevents the medium from continuing the external flow.

Because the safety valve is an automatic valve, there are many differences in structure and performance parameters with general valves. Some special terms are easily confused. In order to make the readers more clearly understand the safety valve, we can correctly select the following main terms.

1. terminology of the safety valve

1) safety valve: a kind of automatic valve, it does not use any external force, but uses the force of the medium itself to discharge a rated quantity of fluid to prevent the pressure in the system from exceeding the predetermined safety value. When the pressure is restored to normal, the valve is closed again and prevents the medium from continuing to flow out.

2) direct load safety valve: a safety valve that uses mechanical loads such as a heavy hammer, a lever plus a hammer, or a spring to overcome the force produced by the pressure of the dielectric under the valve disc.

3) a safety valve with a power auxiliary device: the safety valve can be opened under a power auxiliary device under normal opening pressure. Even if the auxiliary device fails, the valve should still meet the standard requirements.

4) safety valve with additional load: this safety valve always maintains an additional sealing force until its inlet pressure reaches the opening pressure. The additional force (supplementary load) can be supplied by external energy, and should be released reliably when the safety valve reaches the opening pressure. The size should be set so that, assuming that the additional force is not released, the safety valve can still reach the rated displacement on the premise that the inlet pressure is not more than the percentage of the opening pressure prescribed by the state regulations.

5) pilot operated relief valve: a safety valve driven or controlled by a pilot valve. The pilot valve itself should be a direct load safety valve that meets the standard requirements.

6) open pressure (setting pressure): the inlet pressure of the valve disc when it starts to rise under operating conditions. Under this pressure, it begins to have a measurable opening height, and the medium can be continuously discharged by visual or auditory perception.

7) discharge pressure: the valve reaches the inlet pressure when the height is prescribed. The upper limit of discharge pressure shall be subject to the relevant national standards or specifications.

8) over pressure: the difference between the discharge pressure and the opening pressure is usually expressed by the percentage of the opening pressure.

9) back pressure: after discharge, the disc recontacts with the valve seat, that is, the inlet pressure when the height changes to zero.

10) opening and closing differential pressure: the difference between the opening pressure and the return pressure is usually expressed by the percentage of the opening pressure. Only when the opening pressure is very low, it is indicated by MPa.

11) back pressure: pressure at the exit of the safety valve.

12) rated discharge pressure: the standard specifies the upper limit of discharge pressure.

13) seal test pressure: inlet pressure for sealing test, and the leakage rate of sealing surface through closed parts under this pressure.

14) opening height: the actual lift of the disc from the closed position.

15) flow area: the minimum cross section between the valve inlet and the sealing surface of the closure part, which is used to calculate the theoretical displacement without any resistance.

16) channel diameter: the diameter corresponding to the area of the channel.

17) curtain area: cylindrical or conical channel area formed between the sealing surfaces of the disc when the disc is raised above the valve seat.

18) discharge area: the minimum section area of the fluid channel when the valve is discharged is equal to the full opening safety valve, the discharge area is equal to the area of the flow channel; for the micro start type safety valve, the discharge area is equal to the curtain area.

19) theoretical displacement: the calculated displacement of ideal nozzle which is equal to the area of the runner and the area of the safety valve.

20) discharge coefficient: the ratio of actual discharge to theoretical displacement.

21) rated discharge coefficient: product of discharge coefficient and reduction factor (0.9).

22) rated discharge: the part that is allowed to use as a safety valve in actual discharge capacity.

23) equivalent calculation displacement: refer to the calculation displacement of the safety valve when the conditions of pressure, temperature, medium nature and the applicable conditions of rated discharge are the same.

24) frequency hopping: the relief valve disc moves back and forth quickly and normally, and the valve contacts the valve seat in motion.

25) flutter: the relief valve disc moves back and forth quickly and unusually, and the disc does not touch the valve seat during movement.