Valves

Valves are mechanical devices that control the flow of fluids such as gas, liquid, fluidised solids, slurries, and so on, by opening, closing or partially obstructing a passage.

Some common household examples of valves include:

• A tap that when turned opens a valve to release pressurised water.
• A toilet that when flushed opens two valves – one that allows water to escape, and another that refills the toilet with more water.
• A gas cooker hob that opens a valve to allow more gas to increase the flame size.

Simple valves are typically made up of several parts; a solid metal outer casing, a rubber or plastic inner seal, and a body which opens and closes, fitting into a seat.

An automated or manual mechanism is used to open and close valves. For high-pressure flow pipework, opening and closing manually can be physically difficult, often requiring a long lever or large wheel. Big valves can be operated using hydraulic rams.

There are several different types of valves, including:

• Ball: A hollowed-out ball sits inside a pipe obstructing the flow and swivelling through 90-degrees when turned.
• Butterfly: A disc sits inside a pipe and swivels sideways or upright.
• Cock or plug: A cone-shaped plug moves to one side when a wheel or handle is turned.
• Gate or sluice: Metal gates are lowered across pipes, usually only capable of being fully open or fully closed.
• Globe: When turned a valve is screwed in, such as with a tap.
• Needle: A long, sliding needle regulates fluid flow, such as in central heating systems.
• Spool: Valves slide backwards and forwards to direct fluid flow around a pipe circuit, such as in hydraulic systems.

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