1. Terminology for safety valves

(1) Safety valve. An automatic valve. It does not rely on any external force, but uses the force of the medium itself to discharge a rated amount of fluid to prevent the pressure inside the system from exceeding the predetermined safe value; After the pressure returns to normal, the valve will close again and prevent the medium from continuing to flow out.

(2) Direct load safety valve. A safety valve that overcomes the force generated by the pressure of the medium under the valve disc by a directly acting mechanical load, such as a heavy hammer, lever weighted hammer, or spring.

(3) Safety valve with power assist device. The safety valve can be opened with the help of a power assist device at a pressure lower than the normal opening pressure. Even if the auxiliary device fails, this type of safety valve can still meet the standard requirements.

(4) Safety valve with supplementary load. This safety valve maintains an enhanced sealing pressure at its inlet until the pressure reaches the opening pressure. The additional force (supplementary load) can be provided by external energy sources, and should be reliably released when the safety valve reaches the opening pressure. The size should be set in such a way that, assuming the additional force is not released, the safety valve can still achieve the rated displacement under the premise that the inlet pressure does not exceed the percentage of the opening pressure specified by national regulations.

(5) Pilot operated safety valve. A safety valve driven or controlled by discharging medium from 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 proportionally throughout the entire range of opening heights or within a considerable range of opening heights. (7) Fully open safety valve. A safety valve that opens proportionally only within a small opening height range and then suddenly opens to the fully open position. The opening height shall not be less than 1/4 of the channel diameter.

(8) Micro opening safety valve. It is a direct acting safety valve used only for liquid media. The opening height is within the range of 1/40 to 1/20 of the channel diameter.

(9) Open pressure (rated pressure). The inlet pressure at which the safety valve disc starts to rise under operating conditions, and at this pressure, there is a measurable opening height, and the medium is in a continuous discharge state that can be supported by visual or auditory branches.

(10) Emission pressure. The inlet pressure when the valve disc reaches the specified opening height. The upper limit of emission pressure shall comply with the requirements of relevant national standards or regulations.

(11) Exceeding the pressure. The difference between discharge pressure and opening pressure is usually expressed as a percentage of opening pressure.

(12) Return pressure. After discharge, the valve disc comes into contact with the valve seat again, that is, the inlet pressure when the opening height becomes zero.

(13) Opening and closing pressure difference. The difference between the opening pressure and the reseating pressure is usually expressed as a percentage of the reseating pressure and the opening pressure. Only when the opening pressure is very low, the pressure difference between the two is used to represent it.

(14) Back pressure. The pressure at the outlet of the safety valve.

(15) Rated discharge pressure. The standard specifies the upper limit of emission pressure.

(16) Sealing test pressure. Measure the leakage rate through the sealing surface of the closure component at the inlet pressure for conducting a sealing test.

(17) Open the height. The actual lift of the valve disc from the closed position.

(18) Flow channel area. The minimum cross-sectional area of the flow channel between the inlet end of the valve disc and the sealing surface of the closing component is used to calculate the theoretical displacement without any resistance effect.

(19) Channel diameter. The diameter corresponding to the flow channel area.

(20) Curtain area. The cylindrical or conical channel area formed between the sealing surfaces of the valve disc above the valve seat.

(21) Emission area. The minimum cross-sectional area of the fluid channel during valve discharge. For fully inspired safety valves, the discharge area is equal to the flow channel area; For micro opening safety valves, the discharge area is equal to the curtain area.

(22) Theoretical displacement. The calculated displacement of an ideal nozzle with a cross-sectional area equal to the flow area of the safety valve.

(23) Displacement coefficient. The ratio of actual displacement to theoretical displacement.

(24) Rated displacement coefficient. The product of displacement coefficient and reduction coefficient (taken as 0.9).

(25) Rated displacement. The part of the actual displacement that is allowed to be used as a benchmark for safety valves.

(26) Equivalent calculation of displacement. When the conditions of pressure, temperature, medium properties, etc. are the same as the applicable conditions of the rated displacement, the calculated displacement of the safety valve.

(27) Frequency hopping. The safety valve disc rapidly and abnormally moves back and forth, and during the movement, the disc contacts the valve seat.

(28) Flutter. The safety valve disc moves rapidly and abnormally back and forth, without touching the valve seat during movement.

2. Selection principles

(1) Steam boiler safety valves generally use fully open spring safety valves.

(2) Safety valves for liquid media generally use micro opening spring safety valves.

(3) Safety valves for liquefied petroleum gas tank trucks or liquefied petroleum gas railway tank trucks generally use fully open internal safety valves.

(4) The safety valve used for the outlet of oil production wells (Christmas trees) is generally a pilot operated safety valve. The high-pressure bypass safety valve of steam power generation equipment generally uses a pilot operated safety valve with dual functions of safety and control.