Pressure relief valve when the pressure inside the device exceeds the specified operating pressure and valve opening pressure is reached, the internal role of the media in trouble flap above loading mechanism force is greater than the force exerted on it, and then leave the valve seat, safety valve is open, the media inside the device is discharged through the valve seat. Valve play a security role in the system. When the system pressure exceeds a predetermined value, the safety valve open Ting, the system part of the gas into the atmosphere, so that the system pressure does not exceed the allowable value, in order to ensure that the system pressure is too high due to an accident does not occur. Also known as a safety valve relief. Several typical structure is illustrated as a safety valve. Figure A is a piston valve, the valve is a flat. Air pressure acting on the piston A, when the pressure exceeds a safe value determined by the spring force, piston A is top open, a portion of the compressed air that is discharged into the atmosphere from the valve port; when the air pressure falls below the safety value, the drive spring piston down, closes the valve port.