A valve is a device that regulates the flow of gases, liquids, fluidized solids, or slurries by opening, closing, or partially obstructing various passageways. In an open valve, fluid flows in a direction from higher pressure to lower pressure.
Valves are used in a variety of contexts, including industrial, military, commercial, residential, and transportation. Plumbing valves, such as taps for hot and cold water are the most noticeable types of valves. Other valves encountered on a daily basis in residential context include gas control valves on cookers, small valves fitted to washing machines and dishwashers, and safety devices fitted to hot water systems.
Valves may be operated manually, either by a hand wheel, lever or pedal. Valves may also be operated automatically, driven by changes in pressure, temperature, or flow. These changes may act upon a diaphragm or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water or steam systems.
More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an actuator. An actuator will stroke the valve depending on its input and set-up, allowing the valve to be positioned accurately, and allowing control over a variety of requirements.