Valve is a term used for various mechanical devices which open and close to control the flow of fluids in pipes and vessels. The term also is used in physiology for natural growths in the body which serve much the same purpose as mechanical valves. Among these are the valves of the heart, which open and close to control the flow of blood through the chambers of the heart.
Mechanical valves include check valves, relief valves, manual valves, and automatic control valves. Check valves and relief valves allow flow in one direction only. In a check valve, a gate or disk covers an opening called the seat. Gravity or a spring holds the disk in place. Pressure on one side of the disk forces the valve open. But pressure on the other side pushes the disk into the seat, preventing flow. In a relief valve, the disk is held closed by a compressed spring on one side. It opens only when pressure against the other side of the disk is greater than that of the spring.
Manual valves are controlled by hand. The common water faucet is a manual valve. Automatic control valves are controlled by some external power source, such as electricity or compressed air. These valves have become important in industry because hundreds of them can be operated from a central location by a computer or a device called a controller.
