Analysis Of Working Principle And Structural Characteristics Of Pilot Operated Safety Valve

Pilot operated safety valve is a valve for safety protection. Its opening and closing parts are normally closed under the action of external force. When the medium pressure in the equipment or pipeline rises and exceeds the specified value, it will open automatically. By discharging medium outside the system, the medium pressure in the pipeline or equipment will be prevented from exceeding the specified value. Pilot operated safety valves are widely used in natural gas, petroleum gas and chemical pressure vessel equipment and industrial pipelines. They have the outstanding advantages of fire prevention, explosion-proof, anti-static, reliable sealing, rapid opening and closing and large exhaust volume.

Structural features of pilot operated safety valve

Pilot operated safety valve is composed of main valve, pilot valve, relief valve, connector and conduit. Under normal working conditions, the medium pressure of the system acts on both the lower part and the upper part of the main valve disc.

Since the medium action area of the air chamber above the main valve disc is lower than that of the medium below, the main valve disc is closed under the action of pressure difference. With the rise of pipeline medium pressure, the sealing specific pressure obtained by sealing will also rise, which is just opposite to the spring-loaded safety valve, that is, the higher the medium pressure is, the more reliable the sealing is. When the pressure of the medium in the pipeline is abnormal and reaches or exceeds the set pressure, the upper valve disc of the pilot valve (Fig. 2) opens rapidly, and the medium in the air chamber above the main valve disc is discharged rapidly through the exhaust joint of the pilot valve. The main valve disc opens instantaneously under the strong action of the lower medium pressure to discharge the medium rapidly and in large quantities from the outlet of the main valve, so as to restore the medium pressure in the pipeline to the normal working pressure and ensure the safety of the system.

When the pipeline medium pressure drops to about 90% of the set pressure of the safety valve set by the system, the valve disc on the pilot valve is closed, and the pipeline medium enters the air chamber above the main valve disc through the pilot valve. Under the action of pressure difference, the main valve is closed to ensure the normal operation of the system.

Working principle of pilot operated safety valve

The pilot valve is composed of valve body, lower valve disc, ejector rod, vent joint, upper valve disc, spring and relief valve.

Under normal working conditions, the upper disc of the pilot valve is closed and the lower disc of the pilot valve is open. The pipeline medium enters the air chamber above the main valve disc through the lower valve disc to close the main valve disc. When the pipeline medium pressure is abnormal and reaches or exceeds the set setting force, the lower valve disc of the pilot valve is closed under the action of the medium pressure, cutting off the channel above the main valve disc. At the same time, the upper valve disc of the pilot valve is jacked up by the ejector rod, that is, the pilot valve is opened, and the medium in the air chamber above the main valve disc is quickly discharged through the exhaust joint. Under the strong action of the medium pressure below, the main valve disc opens instantaneously to discharge a large amount of pipeline medium from the outlet of the main valve quickly, so as to restore the pipeline medium pressure to the normal working pressure and ensure the safety of the system. When the pipeline pressure drops to about 90% of the set pressure set by the system, the upper valve disc of the pilot valve is closed under the action of the spring, and the lower valve disc of the pilot valve is pushed open by the ejector rod. The pipeline medium enters the air chamber above the main valve disc through the lower valve disc, and the main valve disc is closed under the action of differential pressure to ensure the normal operation of the system.

When it is found that the medium pressure in the pipeline is abnormal and the pilot valve fails to open, the relief valve on the pilot valve can be opened manually to release the medium in the air chamber above the main valve disc, so that the main valve disc can be opened to release the abnormal pressure of the medium in the pipeline and ensure the safety of the system.

Installation requirements of pilot operated safety valve

(1) Boilers with rated evaporation capacity greater than 0.5t/h shall be equipped with at least two pilot safety valves: Boilers with rated evaporation capacity less than or equal to 0.5t/h shall be equipped with at least one safety valve. Pilot operated safety valves must be installed at the outlet of separable economizer and steam superheater.

(2) The pilot operated safety valve shall be vertically installed at the highest position of the boiler 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 self movement of the heavy hammer 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 pilot 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 pilot 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) The pilot operated safety valve shall generally be equipped with a 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.