Valves are defined as devices used to control the direction, pressure and flow of fluid in a fluid system. Valves are devices that allow or stop the flow of media (liquids, gases, powders) in piping and equipment and control their flow. Valve is the control device of fluid pipeline. Its basic function is to connect or cut off the flow of pipeline media, change the flow direction of media, regulate the pressure and flow of media, and protect the normal operation of pipeline equipment.

After the invention of steam engine by Watt, a large number of industrial valves have been applied. In the past twenty or thirty years, due to the needs of petroleum, chemical industry, power station, metallurgy, ship, nuclear energy and astronautics, higher requirements have been put forward for valves. People have been urged to research and produce valves with high parameters. Their working temperature ranges from ultra-low temperature-269 C to high temperature 1200 C, even up to 3430 C, and their working pressure ranges from ultra-low temperature to high temperature. Ultra-vacuum 1.33x10-8Mpa (1x10-1_Hg) to ultra-high pressure 1460MPa, valve diameter from 1mm to 600mm, even to 9750mm, valve materials from cast iron, carbon steel to titanium and titanium alloy, high strength corrosion resistant steel, etc., valve driving mode from manual to electric, pneumatic, hydraulic, programmable control, numerical control, remote control and so on.

With the continuous development of modern industry and the increasing demand for valves, a modern petrochemical plant needs tens of thousands of valves of all kinds, and the use of valves is large. Frequent opening and closing, but often due to improper manufacturing, use and maintenance, running, falling, dripping and leaking occur, which may cause flames, explosions, poisoning, scalding accidents, or lead to poor product quality, increased energy consumption, equipment corrosion, increased material consumption, environmental pollution, and even shutdown accidents, has been common, so people hope to obtain high-quality valves. At the same time, it also requires improving the use and maintenance level of valves. At this time, new requirements are put forward for valve operators, maintenance personnel and engineers. In addition to careful design, reasonable selection and correct operation of valves, timely maintenance and repair of valves are also required to minimize the "run, run, drop, leak" and various accidents of valves.

Generally, valves for low-pressure non-corrosive fluids are made of cast iron or copper, cast steel or forged steel for high and medium-pressure valves, alloy steel for high or high-pressure valves, stainless steel, plastics, corrosion-resistant alloys (such as copper-nickel-molybdenum alloy, titanium alloy, lead alloy, etc.) or corrosion-resistant materials for cast iron and steel linings.

Valves can be divided into six categories according to their functions: cut-off valves, control valves, check valves, diversion valves, safety valves and multi-purpose valves.

Interception valves are mainly used to cut off the flow path, including globe valves, gate valves, cock valves, ball valves, butterfly valves, diaphragm valves, pinch valves, etc. Regulating valves are mainly used to regulate the pressure and flow of fluids, including control valves, throttle valves, pressure relief valves and floating ball regulating valves, check valves are used to prevent reverse flow of fluids, diversion valves are used to distribute the direction of fluids, or two-phase regulating valves. Fluid separation, including sliding valves, multi-way valves, traps and air exhaust valves; safety valves are mainly used for safety protection, to prevent the destruction of boilers, pressure vessels or pipelines due to overpressure; multi-purpose valves are valves with more than one function, such as stop-check valves, which can both stop flow and check back.

Industrial pipeline valves can also be divided into vacuum valves, low pressure valves, medium pressure valves, high pressure valves and ultra-high pressure valves according to nominal pressure; valves can also be divided into normal temperature valves, medium temperature valves, high temperature valves and low temperature valves according to working temperature; valves can also be classified according to the type of driving devices, the connection mode with pipelines and the material used in the valve body. Valves can be named individually or in combination according to various classification methods, or according to the structural characteristics or specific uses of hoists.