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Summary, terminology and selection principles of safety valves
May 02, 2018

First, an overview of the safety valve

The safety valve is used as an overpressure protection device on pressure equipment, containers or pipelines. When the pressure in the equipment, container or pipeline is higher than the allowable value, the valve is automatically opened and then fully discharged to prevent the pressure in the equipment, container or pipeline to continue to rise. When the pressure is reduced to the specified value, the valve should be automatically closed in time so as to protect the safe operation of the equipment, the container or the pipeline.

The safety valve can be driven directly by the system pressure imported by the valve. In this case, the mechanical load is provided by a spring or a heavy hammer to overcome the pressure of the medium under the valve disc. They can also be driven by one mechanism, which opens or closes the safety valve by releasing or applying a closing force. Therefore, according to the above driving mode, the safety valve is divided into direct acting mode and pilot type.

The safety valve can be opened in proportion to the wide open range or in a considerable open height, and may be opened in a small open height range, and then suddenly open to full open position. Therefore, the safety valve can be divided into proportional and full open type.

The structure, application of the safety valve and the determination of the nominal access shall be subject to the regulation or the consent of the statutory body. Among the different specifications, the terms and definitions may be different. When applying safety valve, it must follow the requirements of its applicable specifications. Because the safety valve is an automatic valve, there are many differences between the safety valve and the general valve in terms of structure and performance parameters.

Some special nouns are easy to confuse. In order to make the majority of users understand the safety valve more clearly and be able to choose correctly, some of the main nouns are explained below.

Two. The terminology of the safety valve

(1) safety valve: a kind of automatic valve. Instead of using any external force, it uses the force of the medium itself to discharge a nominal amount 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 with a direct acting mechanical load, such as a heavy hammer, a lever plus a hammer, or a spring, to overcome the force produced under the pressure of the disc under the valve.

(3) safety valve with power assist device: the safety valve can be opened under normal opening pressure by means of a power auxiliary device. Even if the auxiliary device is out of order, the safety valve can still meet the standard requirements.

(4) safety valve with additional load: this kind of safety valve always maintains an enhanced seal pressure 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 does not exceed the percentage of the national regulations to open the pressure percentage.

(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) proportional safety valve: a safety valve that opens or closes in a wide opening range or at a fairly high opening height.

(7) full start safety valve: a safety valve that opens only at a small opening height and then suddenly opens to full open position. The opening height is not less than the 1/4 channel diameter.

(8) micro start safety valve: it is a direct acting safety valve used only in liquid medium. The opening height is in the range of 1/40 ~ 1/20 runner diameter.

(9) open pressure (rated pressure): the inlet pressure of the valve valve when the valve begins to rise under operating conditions. Under this pressure, the opening height of the valve begins to be measured and the medium can be discharged continuously from the visual or auditory trunk.

(10) emission pressure: the pressure of the disc reaches the required inlet height. The upper limit of discharge pressure should be subject to relevant national standards or specifications.

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

(12) seat back pressure: after discharge, the disc re contacts with the valve seat, that is, the inlet pressure when the height changes to zero.

(13) open and close pressure difference: the difference between the opening pressure and the back pressure is usually expressed as the percentage of the back seat pressure and the percentage of the opening pressure. Only when the opening pressure is low, the two pressure difference is used.

(14) back pressure: pressure at the exit of the safety valve.

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

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

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

(18) flow area: the minimum cross-sectional area between the inlet of the disc and the sealing surface of the closure, which is used to calculate the theoretical displacement without any resistance.

(19) the diameter of the runner: the diameter corresponding to the area of the runner.

(20) curtain area: when the disc is above the valve seat, the cylindrical or conical channel area formed between its sealing surfaces.

(21) emission area: the minimum cross section of the fluid channel when the valve is discharged. For the total revelation safety valve, the emission area equals the flow area; for the micro start safety valve, the emission area is equal to the curtain area.

(22) theoretical displacement: the sectional area of the runner and the flow of the safety valve.