The valve is the control device of the fluid pipeline. Its basic function is to connect or cut off the circulation of the pipeline medium, change the circulation of the medium, change the flow direction of the medium, adjust the pressure and flow of the medium, and protect the normal operation of the pipeline equipment.
The large number of applications of industrial valves is after Watt invented the steam engine. In the past two or three decades, due to petroleum, chemical,
The needs of power stations, metallurgy, ships, nuclear energy, aerospace and other aspects put forward higher requirements for valves, prompting people to research and produce valves with high parameters. Its working temperature ranges from ultra-low temperature -269 ℃ to high temperature 1200 ℃, even 3430 ℃, and its working pressure ranges from ultra-vacuum 1.33 x10-8Mpa(1x10-1㎜Hg) to ultra-high pressure 1460MPa, valve diameter from 1mm to 600mm, even up to 9750mm, valve materials from cast iron, carbon steel development to titanium and titanium alloy, high strength corrosion-resistant steel, valve driving mode from manual development to electric, pneumatic, hydraulic, program control, numerical control, remote control, etc.
     With the continuous development of modern industry, the demand for valves is growing, and a modern petrochemical plant requires tens of thousands of various valves, and the use of valves is large. Frequent opening and closing, but often due to improper manufacturing, use, selection, maintenance, running, dripping, leakage phenomenon, resulting in flame, explosion, poisoning, scald accidents, or resulting in poor product quality, energy consumption, equipment corrosion, material consumption increase, environmental pollution, and even cause shutdown and other accidents, has been common, so people want to obtain high-quality valves, but also require to improve the use of valves, maintenance level, then engaged in valve operators, maintenance personnel and engineering and technical personnel, put forward new requirements, in addition to careful design, reasonable selection, correct operation of the valve, but also timely maintenance, repair of the valve, so that the valve "run, run, drip, leak" and all kinds of accidents to a minimum.
    Classification of valves:
    Valves are widely used, a wide variety of classification methods are also more. The total can be divided into two categories:
   The first type of automatic valves: valves that rely on the ability of the medium (liquid, gas) to act on its own. Such as check valve, safety valve, regulating valve, trap, pressure reducing valve, etc.
   The second type of drive valve: with manual, electric, hydraulic, pneumatic to manipulate the action of the valve. Such as gate valve, globe valve, throttle valve, butterfly valve, ball valve, plug valve, etc.
   In addition, the classification of the valve has the following methods:
According to the structural characteristics, the 1. can be divided according to the direction of movement of the closing part relative to the valve seat:
1. Cut-off shape: the closing member moves along the center of the valve seat.
2. Gate shape: the closing member moves along the center of the vertical seat.
3. Cock and ball: The closing member is a plunger or ball that rotates around the center line of its own.
4. Swivel-opening shape; the closing member rotates around an axis outside the valve seat.
5. Disc: The disc of the closing piece that rotates around the axis in the valve seat.
6Slide valve shape: the closing member slides in the direction perpendicular to the channel.
2. by use, according to the different uses of the valve can be divided:
1Breaking use: used to connect or cut off the pipeline medium, such as stop valve, gate valve, ball valve, butterfly valve, etc.
2Non-return use: used to prevent the media back flow, such as check valve.
3Regulation: used to adjust the pressure and flow of the medium, such as regulating valve, pressure reducing valve.
4Distribution: used to change the flow direction of the medium and distribute the medium, such as three-way cock, distribution valve, slide valve, etc.
5. Safety valve: when the medium pressure exceeds the specified value, it is used to discharge excess medium to ensure the pipeline system and equipment.
Safety, such as safety valves, accident valves.
6He special purpose: such as trap, vent valve, drain valve, etc.

3. according to the driving mode, according to different driving mode can be divided:
1. Manual: With the help of handwheel, handle, lever or sprocket, etc., it is driven by manpower. When driving large torque, it is equipped
Worm gears, gears and other deceleration devices.
2· Electric: Driven by means of a motor or other electrical device.
3. Hydraulic: with the help of (water, oil) to drive.
4Pneumatic; driven by means of compressed air.
According to the 4. pressure, according to the nominal pressure of the valve can be divided:
1Vacuum valve: absolute pressure <0.1Mpa, that is, 760mm mercury high valve, usually mm mercury or mm
The water column indicates pressure.
2Low pressure valve: nominal pressure PN≤ 1.6Mpa valve (including PN≤ 1.6MPa steel valve)
3Medium pressure valve: valve with nominal pressure PN2.5-6.4MPa.
4High pressure valve: valve with nominal pressure PN10.0-80.0MPa.
5. Ultra-high pressure valve: valve with nominal pressure PN≥ 100.0MPa.
5. according to the temperature of the medium, according to the valve when the medium temperature can be divided:
1Ordinary valve: suitable for medium temperature -40 ℃ ~ 425 ℃ valve.
2High temperature valve: suitable for medium temperature 425 ℃ ~ 600 ℃ valve.
3Heat-resistant valve: suitable for medium temperature above 600 ℃ valve.
4Low temperature valve: suitable for medium temperature -40 ℃ ~ -150 ℃ valve.
5Ultra-low temperature valve: suitable for medium temperature -150 ℃ below the valve.
According to the nominal diameter of the 6., according to the nominal diameter of the valve can be divided:
1Small diameter valve: nominal diameter DN<40mm valve.
2Medium diameter valve: valve with nominal diameter DN50 ~ 300mm.
3Large diameter valve: nominal diameter DN350 ~ 1200mm valve.
4Extra-large diameter valves: valves with nominal diameter DN≥ 1400mm.
The 7. is divided according to the connection mode of the valve and the pipeline, and can be divided according to the connection mode of the valve and the pipeline;
1Flanged valve: valve body with flange, and pipe flange connection valve.
2Threaded connection valve: valve body with internal thread or external thread, and pipe using threaded connection valve.
3Welded connection valve: the valve body is provided with a welded joint, and the valve is connected with the pipeline by welding.
4Clamp connection valve: the valve body is provided with a clamp, and the pipe is connected by a clamp.
5Valve connected by ferrule: the valve connected by ferrule and pipeline.