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A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.

The simplest, and very ancient, valve is simply a freely hinged flap which swings down to obstruct fluid (gas or liquid) flow in one direction, but is pushed up by the flow itself when the flow is moving in the opposite direction. This is called a check valve, as it prevents or “checks” the flow in one direction. Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.

Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosol spray cans have a tiny valve built in. Valves are also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, valves are instead called dampers. In compressed air systems, however, valves are used with the most common type being ball valves.

There are many valve designs, types and models, with a wide range of industrial applications, such as Solenoid Valve, Pneumatic Pulse Valve, Explosion Proof Solenoid Valve, Namur Solenoid, Solenoid Valve Parts, Pneumatic Valve, Fluid Solenoid Valve, etc. All satisfy one or more of the functions identified above. Valves are expensive items, and it is important that a correct valve is specified for the function, and must be constructed of the correct material for the process liquid. Regardless of type, all valves have the following basic parts.. the body, bonnet, trim (internal elements), actuator, and packing. The basic parts of a valve are illustrated in the image on the right.