Pilot operated safety valve is an indirect load safety valve. It is composed of main valve and pilot valve (also known as “auxiliary valve”). The main valve is driven by the medium discharged from the pilot valve. Because the pressure in the system is in the form of pulse, it is also known as “pulse safety valve“. From a separate point of view, the pilot valve itself is also a direct load safety valve. When the medium turn force reaches the opening pressure of the pilot valve, the pilot valve opens first, and the discharged medium enters the main valve from the bypass pipe.
Classification of pilot operated safety valves
Pilot operated safety valves are divided into vertical hammer type, lever type, spring type and pilot type (pulse type) according to their structural forms; According to the structure of the valve body, it can be divided into closed type and non closed type. Closed safety valve means that the excluded medium will not leak out, and all will be discharged to the designated place along the outlet. It is generally used in toxic and corrosive media. For air and steam safety valves, non closed safety valves are mostly used. The selection of safety valve products shall be determined according to the actual sealing pressure. For spring-loaded safety valves, for springs with several working pressure levels within a nominal pressure (PN), in addition to indicating the model, name, medium and temperature of the safety valve, the sealing pressure of the valve body shall also be indicated when ordering, otherwise they shall be supplied according to the maximum sealing pressure.
Characteristics of pilot operated safety valve
Pilot operated safety valve is suitable for high pressure and large diameter occasions. The pilot operated safety valve has good sealing performance, and its action is rarely affected by back pressure. The disadvantage of this safety valve is that its reliability is related to the main valve and pilot valve. Its action is not as fast and reliable as the direct acting safety valve, and its structure is complex.
Working principle of pilot operated safety valve
When the protected system is in normal operation. The slide valve in the pilot valve chamber is open, and the system pressure is transmitted from the main valve inlet to the air chamber above the main valve disc (piston) through the lower guide pipe, pilot valve chamber and upper guide pipe. At this time, the gas pressure in the upper air chamber is equal to the system pressure at the main valve inlet. The sealing surface of the pilot valve is sealed to ensure that the gas in the air chamber above the main valve cannot be discharged from the pilot valve discharge port. As the piston area is larger than the sealing surface area of the valve disc, the system pressure produces a downward resultant force on the main valve disc, so that the main valve is closed and sealed. When the system pressure rises to the set pressure, the gas pushes up the valve core of the pilot valve at the sealing surface of the pilot valve to make the sealing surface open. At the same time, the slide valve in the pilot valve chamber moves upward to close the air passage in the pilot valve chamber. The medium in the air chamber above the main valve disc is discharged through the open pilot valve sealing surface, reducing the pressure above the main valve disc. The main valve disc opens under the push of inlet pressure to relieve the pressure of the system. When the system pressure drops to a certain value, the pilot valve core returns to its seat under the action of the spring, the sealing surface of the pilot valve is closed, the slide valve in the pilot valve chamber is opened, and the valve chamber is unblocked. The system pressure is transmitted to the air chamber above the main valve disc through the pilot valve chamber again, and the main valve disc is pushed to close.
Installation of pilot operated safety valve
Precautions for installation of pilot operated safety valve
Various safety valves shall be installed vertically.
There shall be no resistance at the outlet of the safety valve to avoid pressure.
Before installation, the safety valve shall be specially tested and its tightness shall be checked.
The safety valve in use shall be inspected regularly.
Installation requirements for pilot operated safety valves
(1) Boilers with rated evaporation capacity greater than 0.5t/h shall be equipped with at least two safety valves: Boilers with rated evaporation capacity less than or equal to 0.5t/h shall be equipped with at least one safety valve. Safety valves must be installed at the outlet of separable economizer and steam superheater.
(2) The safety valve shall be installed vertically at the highest position of the boiler supplier and header. The outlet pipe and valve for steam shall not be installed between the safety valve and the drum or header.
(3) The pilot operated safety valve shall be equipped with a device to prevent the weight from moving by itself and a guide frame to limit the derailment of the lever. The spring-loaded safety valve shall be equipped with a lifting handle and a device to prevent the adjustment screw from being screwed casually.
(4) For boilers with rated steam pressure less than or equal to 3.82MPa, the throat diameter of safety valve shall not be less than 25mm; for boilers with rated steam pressure greater than 3.82MPa, the throat diameter of safety valve shall not be less than 20mm.
(5) The cross-sectional area of the connecting pipe between the safety valve and the boiler shall not be less than the inlet cross-sectional area of the safety valve. If several safety valves are installed on a short pipe directly connected to the drum, the passage cross-sectional area of the short pipe shall not be 1.25 times of the exhaust area of all safety valves.
(6) Generally, the safety valve shall be equipped with steam exhaust pipe, which shall be directly connected to the safe place and have sufficient cross-sectional area to ensure smooth steam exhaust. The bottom of the exhaust pipe of the safety valve shall be pretended to have a drain pipe connected to a safe place. Valves are not allowed to be installed on the exhaust pipe and drain pipe.