A valve is any device that can be used to control the flow of a liquid or gas. It is an essential component of pneumatic cannons, where a wealth of different designs are used depending on price, complexity and performance.
The efficiency of a valve is indicated by it's Flow coefficient or CV.
For a current list of articles on different valves, see Category:Valves
Also, take a look this page on Manual Valves that describes the different types as well as the physical characteristics and principles of valves.
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