is a device which controls the fluid's flow, pressure and direction, which is a control component in the piping system. The controlled fluid may be a liquid, a gas, a gas-liquid mixture or a solid-liquid mixture. The control function of the valve is fulfilled by relying on the driving mechanism or the fluid which changes areas of flow channels through rises and falls, drifts, swings and rotational motion of valve discs. Valves have a wide range of applications, from the simplest globe valves to the various valves
, used in extremely complex automatic control systems. Valves are closely related to people's daily lives, and faucets which are used for water pipes and reducing valves that are used for liquefied petroleum gas stoves all belong to valves. Valves are also indispensable parts for various mechanical equipment such as ships, vehicles, aircrafts, pumps, compressors, steam engines, pneumatic actuators, internal combustion engines and hydraulically driving arrangements.
The earliest valve
originated in China, and it appeared as early as two thousand years AD. During the Spring and Autumn Period and the Warring States Period, a person from the Shu Dynasty removed one end of the bamboo joint, and put the plunger valve at the end of the bamboo barrel to prevent the leakage of the bamboo barrel. In 1800 BC, the ancient Egyptian also used the similar wooden cock to control the water flow.
Then, with development of smelting technology and hydraulic machinery, Europe gradually put more requirements on valves. When copper and aluminum plug valves
were invented, valves could be made of metals. In 1769, Watt invented a steam engine, which officially made a valve enter the mechanical industry, because plug valves, safety valves, check valves and butterfly valves are much adopted for steam engines.
From the 18th century to the 19th century, steam engines were rapidly popularized in mining, smelting, textile, machinofacture and other industries, which needed more valves
and put more requirements on valves' quality; therefore, slide valves were invented. The successive appearance of the glove valve with the threaded valve stem and the wedge gate valve with the trapezoidal threaded valve stem was a major breakthrough in the development of the valve. The appearance of these two types of valves not only met requirements which were constantly put on valves' pressure and temperatures by various industries at that time, but also preliminarily met requirements of flow control. Since then, all kinds of high and medium pressure valves have developed rapidly due to rise of electricity, oil, chemical and shipbuilding industries.
After the Second World War, with development of special materials, including polymeric materials, lubricating materials, stainless steel and cobalt base hard alloys, the ancient plug valves and butterfly valves
gained new applications, and ball valves and diaphragm valves which were evolved from plug valves developed rapidly. The increasing variety and the quality improvement of the globe valve, the gate valve and other valves have made the valve manufacturing industry gradually become an important sector of the machinery industry.