, the definition of the valve: The valve is the control device of the fluid pipeline, which plays an important role in the process of petrochemical production. It has several major functions: connecting and truncating the medium; preventing the reverse flow of the medium; regulating the pressure and flow of the medium; separating, mixing or distributing the medium; The pressure of the medium exceeds the specified value to ensure the safe operation of the pipe or equipment. 2, the classification of valves: Classification by purpose and role: Truncated type: mainly used for cut off or connected to medium flow. Such as gate valve, globe valve, ball valve, butterfly valve, plug valve, diaphragm valve Check: used to prevent the media back. Including all kinds of structure of the check valve. Adjustment: adjust the medium pressure and flow, such as valve, pressure regulating valve, throttle valve Safety: in the medium pressure exceeds the set value, used to discharge the excess medium, to ensure the safety of piping systems and equipment. Distribution: change the medium to flow distribution medium, such as the three way cock distribution valve slide valve etc. Special purpose: such as drain valve vent valve, drain valve etc. According to the classification of pressure: Vacuum valve working pressure below standard atmospheric pressure valve. – low pressure valve nominal pressure PN less than 1.6MPa valve. – medium pressure valve nominal pressure PN 2.5~6.4MPa valve. High pressure valve – the valve nominal pressure PN10.0~80.0MPa. Ultra high pressure valve nominal pressure PN greater than 100MPa valve. According to the classification of medium temperature: High temperature valve t 450 deg. valve. The temperature valve 120 degrees Celsius to 450 degrees Celsius valve. Valve -40 to 120 DEG C valve. Low temperature valve -100 DEG to -40 DEG C valve. Ultra low temperature valve – less than -100 C valve. Classification according to body material: Nonmetal valves: ceramic valves, fiberglass valves, plastic valves Metal valves: such as cast iron valves, carbon steel valves, cast steel valves, low-alloy steel valves, high alloy steel valves and copper alloy valves etc.. According to nominal path Small caliber valve: valve nominal diameter DN<40mm. Medium caliber valve: valve nominal diameter DN50~300mm. Large caliber valves: valves with nominal diameter DN350~1200mm. Large diameter valves: nominal diameter DN = 1400mm valve According to the way of connection with the pipe, it can be divided into: Flange connection valves: valves with flanges and flanged connections to pipes. Threaded connection valve: valve with thread and threaded connection with pipe. Welded joint valve: valve with welded joint and welded joint with pipe. Clamp connection valve: valve with a clamp on the valve body and a clamp connected to the pipe. Jacket connection valve: a valve connected by a jacket and a pipe. The general classification method is the most commonly used classification method at present in the world and in China, which is divided according to the principle, the function and the structure. General: gate valve, globe valve, throttle valve, instrument valve, plunger valve, diaphragm valve, plug valve, ball valve, butterfly valve, check valve, pressure relief valve safety valve, drain valve, valve, valve, filters, drain valve etc.. The selection steps of the valve Use clear the valve in the device or device, determine the working conditions of the valve: for medium, working pressure, temperature and so on. Determine the valve connected with the nominal pipe diameter and connections: flange, thread, welding etc.. Determine the valve operation mode: manual, electric, electromagnetic, pneumatic or hydraulic, electrical or electro-hydraulic linkage etc.. According to pipeline transportation medium, working pressure and working temperature, the material of shell and inner part of the selected valve is determined: gray iron, malleable iron, ductile iron, carbon steel, alloy steel, stainless acid resisting steel and copper alloy. Type selection valve: closed valves, regulating valves, safety valves etc.. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam traps, etc.. The basis for selecting valves is to understand and master the steps of the valve, and to further understand the basis of the selection of the valve. The use of the selected valves, operating conditions and control mode. The nature of the working medium is working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive medium, the viscosity of the medium and so on. Requirements of the valve flow characteristics: flow resistance, discharge capacity, flow characteristics, seal level and so on. Installation size and dimension connection requirements: nominal diameter, and pipe and connection size, size or weight restrictions. Additional requirements of the valve reliability, service life and performance of explosion-proof electrical equipment etc.. When selecting parameters, it should be noted that if the valve is to be used for control purposes, the following additional parameters must be determined: operation method, maximum and minimum flow requirement, normal flow pressure drop, pressure drop at closing time, maximum and minimum inlet pressure of valve. According to the basis and steps of selecting valve above, we must know more about the internal structure of various valves when we choose the valve correctly and correctly, so that we can make the right choice for the priority valve. The final control of the pipe is the valve. The valve opening and closing part controls the flow way of the medium in the pipeline, and the shape of the valve passage has a certain flow characteristic for the valve. When we choose the valve that is most suitable for installing the pipeline system, we must take this into account.