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 valves fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure.
In the coffee maker industry there are many devices that are equipped with a valve. The use of a valve is different though from a coffee brewing method to another. Immersion coffee makers such as coffee drippers, use a valve to control the steeping time. A simple valve is often found on automatic drip coffee machines as a way to temporarily stop the dripping for pouring a cup.
Some espresso machines are also equipped with a valve for regulating the pressure during brewing, (Over Pressure Valve, or OPV). Many advanced heat exchanger espresso machines are equipped with vacuum breaker valves. The purpose of the vacuum breaker valve is to release the vacuum pressure that is created when the espresso machine boiler cools from a very hot state to room temperature. Almost all semiautomatic espresso machines have a steam valve. The purpose of the valve is to open and close the steam on the milk frother. A leaking steam valve will cause the pressure to drop during espresso brewing.