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In an open valve, fluid flows from higher pressure to lower pressure. The simplest kind of valve is simply a freely hinged flap that drops to obstruct flow one way but is pushed open by fluid flowing the opposite way. This ancient valve is called a check valve, as it stops or "checks" the flow in one direction. Modern valves may regulate pressure or flow downstream and operate on sophisticated automation systems.http://www.tt-fire.com/fire-valve/