Valve Definition
By definition, a valve is a specially designed mechanical device used to direct, start, stop, mix, or regulate the flow, pressure, or temperature of process fluids. Valves can be designed to handle either liquids or gases in operation. Depending on the design, function, and application characteristics of the valve, there are many types, sizes, and pressure ratings available. The smallest industrial valves can weigh as little as 1L (0.45kg) and can fit comfortably in one's hand. On the other hand, the largest ones can weigh over 10 tons (9070kg) and stand taller than 24 feet (6.1m). Industrial process valves can be used for pipeline sizes ranging from nominal diameter DN15 to over DN2000.
Today, the field of existing valves has evolved from simple faucets to control valves equipped with information processors that provide single-loop control of the process. The most commonly used types today include gate valves, plug valves, ball valves, butterfly valves, check valves, relief valves, and globe valves.
Valves can be made from many materials; most valves are made from steel, iron, plastic, copper, or a wide range of special alloys.