A valve is a device used within a fluid system to control the direction, pressure, and flow rate of a fluid; it serves to initiate or halt the flow of a medium (such as liquid, gas, or powder) within piping and equipment, while also regulating its flow volume.
Valves act as control components within pipeline fluid transport systems. They are utilized to alter the cross-sectional area of the flow passage and redirect the flow of the medium, performing functions such as flow guidance, shut-off, throttling, backflow prevention, diversion, or pressure relief. Valves employed for fluid control-ranging from the simplest shut-off valves to the highly sophisticated valves utilized in complex automated control systems-exist in a vast array of types and specifications. Their nominal diameters span a wide spectrum, extending from minuscule instrumentation valves to massive industrial pipeline valves with diameters reaching up to 10 meters. Valves can be used to control the flow of various types of fluids, including water, steam, petroleum products, gases, slurries, corrosive media, liquid metals, and radioactive fluids. Their operating pressures range from ultra-low levels of 0.0013 MPa to ultra-high pressures of 1000 MPa, while their operating temperatures span from cryogenic extremes of -270°C to high-temperature environments reaching 1430°C.
Valve control can be achieved through a variety of actuation methods, such as manual, electric, hydraulic, pneumatic, turbine-driven, electromagnetic, electro-hydraulic, pneumatic-hydraulic, spur gear-driven, or bevel gear-driven mechanisms. Valves can be configured to operate according to predetermined requirements in response to sensing signals-such as pressure, temperature, or other parameters-or they can perform simple opening and closing operations independently of such signals. By utilizing actuation mechanisms or automatic assemblies, valves cause their opening and closing elements to execute linear lifting, sliding, oscillating, or rotary movements, thereby altering the cross-sectional area of the flow channel to fulfill their specific control functions.