Last edited 09 Jul 2021



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 valve: A hollowed-out ball sits inside a pipe obstructing the flow and swivelling through 90-degrees when turned.
  • Butterfly valve: 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.

For more information see: Types of valve.

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