Valve is used to control the direction, pressure and flow of fluid in fluid system. The valve is a device that makes the medium (liquid, gas, powder) in the piping and equipment flow or stop and can control its flow.
　　Valve is the control component in the pipeline fluid transmission system, which is used to change the channel section and medium flow direction, and has the functions of diversion, cut-off, throttling, check, diversion or overflow relief. The valves used for fluid control range from simple globe valves to various valves used in complex automatic control systems, with various varieties and specifications. The nominal diameter of the valves ranges from small instrument valves to industrial pipeline valves with a diameter of 10m. The valve can be used to control the flow of water, steam, oil, gas, mud, various corrosive media, liquid metal and radioactive fluid. The working pressure of the valve can be from 0.0013mpa to 1000MPa, and the working temperature can be from - 270 ℃ to 1430 ℃. The valve can be controlled by a variety of transmission modes, such as manual, electric, hydraulic, pneumatic, turbine, electromagnetic, electromagnetic hydraulic, electro-hydraulic, gas-liquid, spur gear, bevel gear drive, etc. under the action of pressure, temperature or other forms of sensing signals, the valve can be controlled by a variety of transmission modes, The valve operates according to the predetermined requirements, or simply opens or closes without depending on the sensing signal. The valve relies on the driving or automatic mechanism to make the lifting, sliding, swinging or rotary movement of the opening and closing parts, so as to change the size of its flow channel area to realize its control function

　　By function and purpose
　　(1) Block type: such as gate valve, globe valve, plug valve, ball valve, butterfly valve, needle valve, diaphragm valve, etc. Cut off valve, also known as closed-circuit valve, stop valve, its role is to connect or cut off the medium in the pipeline.
　　Check valve, also known as one-way valve or check valve, check valve is an automatic valve, its function is to prevent the medium in the pipeline from backflow, prevent the pump and drive motor from reversing, and the leakage of the container medium. The bottom valve of pump suction valve also belongs to check valve.
　　Explosion proof valve, accident valve, etc. the function of the valve is to prevent the medium pressure in the pipeline or device from exceeding the specified value, so as to achieve the purpose of safety protection.
　　The function of regulating valve, throttle valve and pressure reducing valve is to regulate the pressure, flow and other parameters of medium.
　　(2) Vacuum: such as vacuum ball valve, vacuum baffle valve, vacuum charging valve, pneumatic vacuum valve, etc. Its function is in the vacuum system, used to change the direction of air flow, adjust the size of air flow, cut off or connect the pipeline vacuum system components called vacuum valve.
　　(3) Special purpose: such as pigging valve, vent valve, blowdown valve, vent valve, filter, etc. Exhaust valve is an indispensable auxiliary component in pipeline system, which is widely used in boiler, air conditioning, oil and gas, water supply and drainage pipeline. It is often installed at the commanding height or elbow to eliminate the excess gas in the pipeline, improve the efficiency of the pipeline and reduce the energy consumption.
　　By main parameters
　　According to nominal pressure
　　(1) Vacuum valve: refers to the valve whose working pressure is lower than the standard atmospheric pressure.
　　(2) Low pressure valve: refers to the valve with nominal pressure PN ≤ 1.6Mpa.
　　(3) Medium pressure valve: refers to the valve with nominal pressure PN of 2.5MPa, 4.0Mpa and 6.4Mpa.
　　(4) High pressure valve: refers to the valve with nominal pressure PN of 10.0MPa ~ 80.0mpa.
　　(5) High pressure valve: refers to the valve with nominal pressure PN ≥ 100.0mpa.
　　(6) Filter: refers to the nominal pressure PN of 1.0MPa, 1.6Mpa
　　According to working temperature
　　(1) Ultra low temperature valve: it is used for the valve with medium working temperature T < - 101 ℃.
　　(2) Normal temperature valve: used for the valve with medium working temperature - 29 ℃ < T < 120 ℃.
　　(3) Medium temperature valve: used for medium working temperature 120 ℃ < T < 425 ℃.
　　(4) High temperature valve: used for the valve with medium working temperature T > 425 ℃.