A valve is a device 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.

Valves are quite diverse and may be classified into a number of basic types. Valves may also be classified by how they are actuated:

• Hydraulic
• Pneumatic
• Manual
• Solenoid valve
• Motor

Basic Valve types

Valves can be categorized into the following basic types:

• Ball valve, for on/off control without pressure drop, and ideal for quick shut-off, since a 90° turn offers complete shut-off angle, compared to multiple turns required on most manual valves.
• Butterfly valve, for flow regulation in large pipe diameters.
• Ceramic Disc valve, used mainly in high duty cycle applications or on abrasive fluids. Ceramic disc can also provide Class IV seat leakage
• Clapper valve, Used in appliances like the Siamese fire appliance to allow only 1 hose to connected instead of two (the clapper valve blocks the other side from leaking out.
• Check valve or non-return valve, allows the fluid to pass in one direction only.
• Choke valve, a valve that raises or lowers a solid cylinder which is placed around or inside another cylinder which has holes or slots. Used for high pressure drops found in oil and gas wellheads.
• Diaphragm valve, which controls flow by a movement of a diaphragm. Upstream pressure, downstream pressure, or an external source (e.g., pneumatic, hydraulic, etc.) can be used to change the position of the diaphragm.
• Gate valve, mainly for on/off control, with low pressure drop.
• Globe valve, good for regulating flow.
• Knife valve, similar to a gate valve, but usually more compact. Often used for slurries or powders on/off control.
• Needle valve for accurate flow control.
• Pinch valve, for slurry flow regulation and control.
• Piston valve, for regulating fluids that carry solids in suspension.
• Plug valve, slim valve for on/off control but with some pressure drop.
• Poppet valve, commonly used in piston engines to regulate the fuel mixture intake and exhaust.
• Spool valve, for hydraulic control
• Thermal expansion valve, used in refrigeration and air conditioning systems.
• Pressure Reducing Valve
• Sampling valves
• Safety valve