A valve is a device used to control the direction, pressure, and flow of fluids in a fluid system. It enables or stops the flow of media (liquids, gases, powders) within piping and equipment, and can control their flow.

A valve is a control component in pipeline fluid transportation systems, used to change the cross-section of the passageway and the direction of the medium's flow. It performs functions such as diverting, shutting off, throttling, checking, diverting, or providing relief and pressure relief. Valves used for fluid control come in a wide variety of types and specifications, from simple globe valves to valves used in highly complex automatic control systems. Valve diameters range from tiny instrument valves to valves for industrial pipelines with diameters up to 10 meters. Valves can be used to control the flow of various fluids, including water, steam, oil, gas, slurry, various corrosive media, liquid metal, and radioactive fluids. Valves can operate at pressures ranging from 0.0013 MPa to ultra-high pressures of 1000 MPa, and at temperatures ranging from -270°C to 1430°C. Valves can be controlled by 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.; they can act according to predetermined requirements under the action of pressure, temperature or other forms of sensor signals, or simply open or close without relying on sensor signals. The valve relies on a drive or automatic mechanism to make the opening and closing parts move up and down, slide, swing or rotate, thereby changing the size of its flow area to achieve its control function.