A valve is a mechanical device that regulates the flow of fluids (either gases, fluidised solids, slurries or liquids) by opening, closing, or partially obstructing various passageways.

Valves are used in a myriad of industrial, military, commercial, and residential applications. There are many different types of valves:

• Ball cock, often used as a water level controller (cistern);
• Ball valve, which is good for on/off control;
• Bibcock ;
• Butterfly valve, particularly in large pipes;
• Cock, colloquial term for a small valve;
• Gate valve, mainly for on/off control;
• Globe valve, which is good for regulating flow;
• Hydraulic valve (diaphragm valve);
• check valve or Non-return valve, allows the fluid to pass in one direction only;
• Pilot valves regulate flow or pressure to other valves;
• Plug Valve , for on/off control;
• Solenoid valve, an electrically controlled hydraulic or pneumatic valve;
• Tap (British English), Faucet (American English) is the common name for a valve used in homes to regulate water flow;
• Rotary valves and piston valves are parts of brass instruments used to change their pitch.

Different valves may be used for a variety of purposes:

• A flow control valve maintains a constant flowrate through the valve.
• A heart valve regulates blood flow through the heart in many organisms.
• A poppet valve is commonly used in piston engines to regulate the fuel mixture intake and exhaust. The sleeve valve is another valve type used for this purpose.
• A pressure reducing valve reduces pressure to a preset level downstream of the valve.
• A pressure sustaining valve maintains pressure at a preset level upstream of the valve.
• A regulator is used in SCUBA diving equipment and in gas cooking equipment to reduce the high pressure gas supply to a lower working pressure
• A safety valve or relief valve operates automatically at a set differential pressure to correct a potentially dangerous situation, typically over-pressure.
• A three way valve routes fluid from one direction to another.
• Vacuum breaker valves prevent the back-siphonage of contaminated water into pressurized, potable water supplies.

Other uses of the term

• in electronics, a valve (British English) = vacuum tube (American English)

See also

• Valve Software
• Plumbing
• Backwater valves