Valves are devices that control fluid flow, pressure and flow. The fluid to be controlled may be a liquid, a gas, a gas-liquid mixture or a solid-liquid mixture. Valve usually by the valve body, valve cover, seat, opening and closing pieces, drive mechanism, seals and fasteners and other components. Valve control function is to rely on the drive mechanism or fluid to drive the hoist up and down, slipping, swing or rotary motion to change the size of the flow area to achieve. Valves are used for a wide range of purposes. They are closely related to people's daily life. For example, faucets for water pipes and pressure reducing valves for liquefied petroleum gas are all valves. Valves are also indispensable components for various mechanical equipment such as internal combustion engines, steam engines, compressors, pumps, pneumatic transmissions, hydraulic transmission vehicles, ships and aircraft. Two thousand years ago BC, Chinese used bamboo and cork valves on the water pipelines, and later used sluices on irrigation channels. They used plate check valves on smelting bellows and used bamboo in well salt mining Tubular and plate check valves for extraction of brine With the development of metallurgy and hydromechanics, copper and lead plug valves have emerged in Europe. With the use of boilers, lever heavy hammer safety valves appeared again in 1681. Plug valves and check valves have always been the main valve until the Watt steam engine appeared in 1769. The invention of the steam engine made the valve into the field of machinery industry. In addition to the use of plug valves, safety valves and check valves, Watts steam engines also use butterfly valves to regulate flow. With the steam flow and pressure increases, the use of plug valves to control the steam inlet and exhaust steam can not meet the needs, so there has been a slide valve. Around 1840, one after another with the closure of threaded stem valve, and wedge gate valve with trapezoidal thread, which is the valve in the development of a major breakthrough. The emergence of these two types of valves, not only to meet the then a variety of industries on the pressure, temperature rising requirements, and initially meet the flow regulation requirements. Since then, with the development of power industry, petroleum industry, chemical industry and shipbuilding industry, various high and medium pressure valves have been rapidly developed. After the Second World War, due to the development of polymer materials, lubricating materials, stainless steel and cobalt-based cemented carbide, old plug valves and butterfly valves gained new applications, and ball valves and diaphragm valves developed rapidly. Cut-off valves, gate valves and other valves increased variety, improve quality. Valve manufacturing industry has gradually become an important sector of the machinery industry. Valves by using the function can be divided into cut-off valve, control valve, check valve, divert valve, safety valve, multi-purpose valve six categories. Shut-off valve is mainly used to cut off the fluid path, including the cut-off valve, gate valve, plug valve, ball valve, butterfly valve diaphragm, pinch valve, etc .; regulating valve is mainly used to regulate fluid pressure, flow, etc., including regulating valve, Pressure reducing valve and float valve, etc .; check valve is used to stop the reverse flow of fluid; diverter valve is used to distribute the passage of the fluid, or separate the two-phase fluid, including the slide valve, multi-way valve, steam trap and row Air valve, etc .; safety valve is mainly used for safety protection to prevent damage to the boiler, pressure vessel or pipe due to overpressure; multi-purpose valve is a valve with more than one function, such as the end of the check valve can play a role in both flow and start Back to function. Industrial piping valves by nominal pressure can be divided into vacuum valve, low pressure valve, medium pressure valve, high pressure valve, high pressure valve; valve by operating temperature can be divided into room temperature valve, temperature valve, high temperature valve, cryogenic valve; According to the type of drive, with the pipe connection and the use of valve body material classification. The valve can be named individually or in combination according to various classification methods and can also be named according to the structural features or specific uses of the opening and closing element.The basic parameters of the valve are working pressure, working temperature and caliber. For a large number of industrial valves used in various valves, nominal pressure and nominal diameter as the basic parameters. Nominal pressure refers to a material valve, at the specified temperature, the maximum allowable working pressure. Nominal diameter refers to the nominal inner diameter of the valve body and pipe coupling end. Valves according to their types and uses have different requirements, the main seal, strength, regulation, circulation, opening and closing and other properties. In the design and selection of valves, in addition to consider the basic parameters and performance, but also consider the fluid properties, including the fluid phase gas, liquid or solid particles, corrosive, viscosity, toxicity, flammable, valuable Rare and radioactive. Sealing performance and strength performance of all the most basic valve, the most important performance. Valve sealing points within the outer seal and two parts. The inner seal is the seal between the valve disc and the valve seat. The outer seal is the seal between the valve seat and the bonnet, between the valve body and bonnet and between the valve body and the pipe. Valve in use requires not only good sealing performance, but also to ensure safety. Leakage due to poor sealing or damage to parts due to insufficient strength can result in varying degrees of economic loss, such as the transport of toxic, flammable, explosive or highly corrosive fluids and can lead to serious accidents. In order to ensure the valve seal and strength, in addition to the relevant provisions of the standard must be reasonable to carry out structural design, to ensure the quality of the process, but also must be the correct choice of materials. Normally, low pressure non-corrosive fluid valves are made of cast iron or cast copper; cast or forged steel for high and medium pressure valves; alloy steel for high temperature or high pressure valves; valves for corrosive fluids are made of stainless steel, plastic, Alloy such as copper nickel molybdenum alloy, titanium alloy and lead alloy or cast iron, cast steel lined with corrosion-resistant materials. Usually, the sealing surface of the low-pressure valve mostly uses brass or bronze, most of the high-pressure valves are made of stainless steel, and cobalt-based hard alloys that require high high, medium pressure valves or high-temperature valves are used. Polymeric materials in the valve has been widely used, such as the ball valve seat is mainly PTFE plastic, butterfly valve seals and diaphragm valve diaphragm using a variety of rubber materials. These materials have a better seal than metals over the temperature range that can be used. With the development of the modern nuclear industry, the petrochemical industry, the electronics industry and the aerospace industry, as well as the automatic control of the process technology and the development of long-distance fluid transportation, modern low-temperature valves, vacuum valves, nuclear industrial valves and various regulating valves are promoted development of. Valve drives for remote control and program control are used more and more. The future development of the valve to expand the product parameters, the development of energy saving, labor-saving and self-control valve, improve the structure, the use of new materials and new processes to improve the service life of the valve, and the development of special valve series such as hair development, Liquid hydrogen and liquefied natural gas cryogenic valves, vacuum valves, nuclear industrial valves, safety valves, regulating valves, steam traps and valve drives.