Valves are defined as devices to control or regulate the commencement, termination, direction and also the pressure or rate of flow of fluid under pressure which is delivered by a compressor or vacuum pump or is stored in a vessel. 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.
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 aerosols have a tiny valve built in. Valves are also used in the military and transport sectors.
Valves are found in every industrial process. They are used everywhere including sewage processing, boiler rooms, gas stations, food manufacturing, chemical and plastic manufacturing etc.
Valves vary widely in form and application. Sizes typically range from 0.1 mm to 60 cm. Special valves can have a diameter exceeding 5 meters.Disposable valves may be found in common household items including mini-pump dispensers and aerosol cans.
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Valves are quite diverse and may be classified into a number of basic types. They may also be classified by how they are actuated:
- Solenoid valve
Many valves are controlled manually with a handle attached to the stem. If the handle is turned ninety degrees between operating positions, the valve is called a quarter-turn valve. Butterfly, ball valves, and plug valves are often quarter-turn valves. If the handle is circular with the stem as the axis of rotation in the center of the circle, then the handle is called a handwheel.
These can also be controlled by actuators attached to the stem. They can be electromechanical actuators such as an electric motor or solenoid, pneumatic actuators which are controlled by air pressure, or hydraulic actuators which are controlled by the pressure of a liquid such as oil or water.
Actuators can be used for the purposes of automatic control such as in washing machine cycles, a remote control such as the use of a centralised control room, or because manual control is too difficult such as when the valve is very large. Pneumatic actuators and hydraulic actuators need pressurised air or liquid lines to supply the actuator: an inlet line and an outlet line. Pilot valves are valves which are used to control other valves. Pilot valve in the actuator lines control the supply of air or liquid going to the actuators.
The fill valve in a toilet water tank is a liquid level-actuated valve. When a high water level is reached, a mechanism shuts the valve which fills the tank.
In some valve designs, the pressure of the flowing fluid itself or pressure difference of the flowing fluid between the ports automatically controls flow through the valve.