The pressure relief valve is a valve that adjusts the inlet pressure to a required outlet and relies on the energy of the medium itself to keep the outlet pressure stable automatically. From the point of view of fluid mechanics, the pressure relief valve is a throttle element that can change the local resistance, that is, by changing the throttle area so that the flow rate and the kinetic energy of the fluid change, resulting in different stress losses, thus achieving the objective of decompression. Then rely on the control and adjustment system, so that the pressure fluctuation of the valve and the spring force phase equilibrium, so that after the valve pressure in a certain range of error to maintain constant.

The pressure relief valve according to the structure form may divide into the membrane type pressure reducing valve, the spring film type pressure reducing valve, the piston type reducing valve, the lever type pressure reducing valve and the bellows type pressure reducing valve; According to the seat number can be artificial single seat and two-seater type; According to the position of the valve can be Pilot-type pressure reducing valve when the output pressure is higher or larger diameter, with the pressure regulator Spring direct pressure, the spring stiffness is bound to be too large, flow changes, output pressure fluctuation, valve structure size will increase. In order to overcome these disadvantages, the pilot type decompression valve can be used. The working principle of the pilot type pressure relief valve is basically the same as that of the direct motion type. The pressure-regulating gas used by the pilot pressure reducing valve is supplied by a small direct action pressure reducing valve. If the small direct-action pressure reducing valve installed in the body inside, it is called the internal pilot-type pressure reducing valve, if the small direct-action valve installed in the main body outside, it is called the external pilot-type pressure relief valves.

Pressure relief valve test method:

The strength test of ① pressure relief valve is usually assembled after single test, and can be assembled after test. Duration of strength test: The lmin;dn65~150mm of dn<50mm is greater than that of 2min;dn>150mm and is larger than 3min. After the bellows and assembly welding, the use of pressure relief valve after the maximum 1. 5 times times, the intensity test with air.

The ② seal test is carried out according to the actual working medium. When using air or water test, 1 of nominal pressure. 1 time times the test and the maximum working pressure allowed at operating temperature in the steam test. The difference between inlet pressure and outlet pressure is not less than 0.2MPa. The test method is as follows: After adjusting the inlet pressure, the adjusting screws of the valve are gradually adjusted, so that the outlet pressure can change sensitively and continuously in the maximum and minimum range, without stagnation or blocking phenomenon. On the steam relief valve, when the inlet pressure is transferred, after closing the valve, the outlet pressure is the highest and the lowest value, in 2min, the appreciation of its export pressure should conform to the provisions in table 4.176-22, at the same time, the valve after the pipeline volume conforms to table 4.18 as stipulated as qualified; for water, air pressure relief valve, when the import pressure is set, Outlet pressure is zero, close valve for sealing test, in 2min no leakage for qualified.