1. Definition of a Valve

By definition, valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid. Valves can be designed to handle either liquid or gas applications.
 By nature of their design, function, and application, valves come in a wide variety of styles, sizes, and pressure classes.The smallest industrial valves can weigh as little as 1 lb(0.45 kg) and fit comfortably in the human hand, while the largest can weigh up to 10 tons (9070kg) and extend in height to over 24 ft (6.1m). Industrial process valves can be used in pipeline sizes from 0.5 in [nominal diameter (ON) 15] to beyond 48 in (ON 1200), although over 90 percent of the valves used in process systems are installed in piping that is 4 in (ON 100) and smaller in size. Valves can be used in pressures from vacuum to over 13,000 psi (898 bar).
 Today's spectrum of available valves extends from simple water faucets to control valves equipped with microprocessors, which provide single-loop control of the process. The most common types in use today are gate, plug, ball, butterfly, check, pressure-relief, and globe valves.
 Valves can be manufactured from a number of materials, with most valves made from steel, iron, plastic, brass, bronze, or a number of special alloys.
By Philip L. Skousen (Valve Handbook)