Special valve is a very important protection valve, which is widely used in various pressure vessels and pipeline systems. When the pressure in the pressurization system exceeds the specified value, it can be automatically opened to discharge the excess medium into the atmosphere to ensure the safe operation of the pressure vessel and pipeline system and prevent accidents. When the pressure in the system drops or is slightly lower than the working pressure, it can be automatically closed. The reliability of special valves is directly related to the safety of equipment and personnel, so attention must be paid to it.
1、 Leakage of valve body joint surface of special valve
Refers to the leakage at the joint surface of the upper and lower valve bodies. The main reasons for this leakage are: first, the tightening force of the bolts on the joint surface is insufficient or biased, resulting in poor sealing surface of the joint surface. Second, the toothed gasket on the joint surface of the valve body does not meet the standard. Third, the flatness of the joint surface of the valve body is too poor or hard impurities lead to seal failure.
2、 After the special valve acts, the main safety valve does not act
This phenomenon is usually referred to as main safety valve refusal. The refusal of the main safety valve is very harmful to the boiler in operation. It is a major hidden danger of the equipment and seriously affects the safe operation of the equipment. Once the medium pressure in the operating pressure vessel and pipeline exceeds the rated value, the main safety valve will not act. When the equipment operates under overpressure, it is easy to cause equipment damage and major accidents.
3、 Frequency jump of special valve
Frequency hopping refers to that when the pressure rises slightly after the special valve returns to the valve seat, the safety valve will open again. This phenomenon is called "frequency hopping" of the safety valve. The mechanical characteristics of the safety valve require that during the whole operation process, when the safety valve reaches the specified opening height, there shall be no jamming, vibration and frequency hopping. The frequency jump is extremely unfavorable to the sealing of the safety valve and easy to cause leakage of the sealing surface. The reason is mainly related to the high back pressure of the safety valve. When the back pressure is high, the excess medium in the container is discharged less, and the safety valve has returned to the valve seat. When the operator makes improper adjustment, the pressure in the container will rise rapidly again, which will cause the safety valve to act again. This situation can be eliminated by increasing the opening of the throttle valve. After the throttle valve is opened, the steam source to the piston chamber of the main safety valve is reduced, the force pushing the piston downward is small, the probability of the main safety valve action is small, and the continuous startup of the main safety valve is avoided.
4、 After the special valve is reseated, the reseating delay time of the main valve is too long

There are two main reasons for failure:
On the one hand, it is the steam leakage of the piston chamber of the main safety valve. Although the pulse safety valve returns to its valve seat, the steam pressure of the pipeline and the piston chamber is still very high, and the downward force of the piston is still very large, resulting in the main safety valve slowly returning to its valve seat. The method to eliminate this fault is mainly to increase the opening of the throttle valve and the diameter of the throttle hole. Whether the throttle valve is opened or the diameter of the throttle hole is increased, the steam left in the pulse tube is rapidly discharged, thus reducing the pressure in the piston and the downward thrust acting on the piston. Under the upward thrust of the steam medium in the steam header and the upward tension of the main safety valve spring, the valve core quickly returns to its valve seat. On the other hand, the reason is that the friction between the moving parts and the fixed parts of the main safety valve is too large, which will also cause the main safety valve to slowly return to its seat. The solution to this problem is to control the fit clearance between the moving parts and the fixed parts of the main safety valve within the standard range.