Definition: A device used to control the direction, pressure and flow of a fluid in a fluid system. A valve is a device that makes the medium (liquid, gas, powder) in the pipe and equipment flow or stop and can control its flow. Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media.

The valve is a control component in the pipeline fluid transmission system, which is used to change the channel section and the direction of the medium flow, with the functions of diversion, cut-off, throttling, check, shunt or overflow pressure relief. Valves for fluid control range from simple globe valves to valves used in very complex automatic control systems in a wide variety of varieties and specifications, with nominal diameters ranging from very small instrument valves to industrial pipeline valves with diameters up to 10m. The valve can be used to control the flow of water, steam, oil, gas, mud, various corrosive media, liquid metal and radioactive fluids and other types of fluids. The working pressure of the valve can be from 0.0013MPa to 1000MPa of ultra-high pressure, and the working temperature can be from ultra-low temperature of -269℃ to high temperature of 1430℃. The control of the valve can adopt a variety of transmission methods, such as manual, electric, hydraulic, pneumatic, turbine, electromagnetic, electromagnetic hydraulic, electro-hydraulic, gas-hydraulic, spur gear, bevel gear drive, etc. Under the action of pressure, temperature or other forms of sensing signals, according to the predetermined requirements of the action, or do not rely on the sensing signal and simple open or close, the valve depends on the drive or automatic mechanism to make the opening and closing parts for lifting, sliding, swinging or rotating movement, so as to change the size of its flow area to achieve its control function.