Valve

A valve is a device or natural object that regulates, directs or controls the flow of a <a href="/facts/Fluid/wLnMhBC0">fluid</a> (gases, liquids, fluidized solids, or <a href="/facts/Slurry/erSTjthc">slurries</a>) by opening, closing, or partially obstructing various passageways. Valves are technically <a href="/facts/Piping_and_plumbing_fitting/8saV82HD">fittings</a>, 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 <a href="/facts/Check_valve/FPdCapdD">check valve</a>, as it prevents or "checks" the flow in one direction. Modern <a href="/facts/Control_valve/7cE7Jt09">control valves</a> may regulate <a href="/facts/Pressure/eyc37GRG">pressure</a> or <a href="/facts/Fluid_dynamics/y7uzSCfg">flow</a> downstream and operate on sophisticated <a href="/facts/Automation/3YlF2K5q">automation</a> systems.
Valves have many uses, including controlling water for <a href="/facts/Irrigation/RWX5PEgK">irrigation</a>, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Valves are also used in the military and transport sectors. In <a href="/facts/Heating%2c_ventilation%2c_and_air_conditioning/gm1UQdR6">HVAC</a> <a href="/facts/Duct_(flow)/34ThHzCt">ductwork</a> and other near-atmospheric air flows, valves are instead called <a href="/facts/Damper_(flow)/VpgWD5No">dampers</a>. In compressed air systems, however, valves are used with the most common type being ball valves.

Valve open-in-new

Valve