Valve is the control part of the fluid conveying system. It has functions such as cut-off, regulation, diversion, prevention of countercurrent, steady pressure, diversion or overflow pressure relief. Valves used for fluid control systems, ranging from the simplest cut-off valves to the most complex automatic control systems, have a wide range of valves and specifications. Valves can be used to control the flow of various types of fluids such as air, water, steam, corrosive media, mud, oil, liquid metal and radioactive medium.

The "valve" is defined as a device used to control the direction, pressure and flow of fluid in a fluid system. Valve is the device to make the medium (liquid, gas, powder) flow and stop and control its flow in piping and equipment.
The valve is the control part in the pipeline fluid conveying system. It is used to change the channel section and the direction of the flow of the medium. It has the functions of diversion, cut-off, throttling, check, diversion, overflow and pressure relief. Valves used for fluid control, from the simplest cut-off valve to extremely complex automatic control systems, have various types and specifications. The nominal diameter of the valve is from a very small instrument valve to an industrial pipeline with a diameter of 10m. The valve can be used to control the flow of various types of fluid, such as water, steam, oil, gas, mud, various corrosive media, liquid metal and radioactive fluid. The operating pressure of the valve can be from 0.0013MPa to 1000MPa, and the working temperature is from the ultra low temperature of -269 C to the high temperature of 1430. Valves can be controlled by a variety of transmission modes, such as manual, electric, hydraulic, pneumatic, worm, electromagnetic, electromagnetic, hydraulic, electro-hydraulic, pneumatic, liquid, positive gear, and bevel gear drive, etc., and can be operated under the action of pressure, temperature or other form of sensing signals, or not. Depending on the sensing signal, the valve is simply opened or closed, and the valve depends on the drive or automatic mechanism to make the opening and closing work as a lift, slip, swing or rotary motion, so as to change the size of the channel area to achieve its control function.
Classification of valves

1. classification by role and use

(1) cut off valve: the truncated valve is also called closed circuit valve, and its function is to connect or cut off the medium in the pipeline. The cut-off valve includes gate valve, globe valve, cock valve, ball valve, butterfly valve and diaphragm valve.

(2) check valve: check valve, also known as check valve or check valve, its role is to prevent the flow of media in the pipeline backwards. The bottom valve of the water pump is also a kind of check valve

(3) safety valve: the function of the safety valve class is to prevent the pressure of the medium in the pipeline or device from exceeding the prescribed value, so as to achieve the purpose of safety protection.

(4) regulating valve: the regulating valve includes control valve, throttle valve and pressure reducing valve. Its function is to regulate the pressure and flow rate of the medium.

(5) shunt valve: shunt valve includes all kinds of distributing valve and steam trap, etc. its function is to distribute, separate or mix medium in pipeline.

(6) exhaust valve: exhaust valve is an essential auxiliary element in the pipeline system, and is widely used in boilers, air conditioners, oil and natural gas, and water supply and drainage pipelines. It is often installed at the commanding point or elbow, eliminating the excess gas in the pipeline, improving the efficiency of the pipe road and reducing the energy consumption.

2. classification by nominal pressure

(1) vacuum valve: a valve whose working pressure is below the standard atmospheric pressure.

(2) low pressure valve: the valve with nominal pressure PN or less than 1.6Mpa.

(3) medium pressure valve: the valve with a nominal pressure PN of 2.5, 4 and 6.4Mpa.

(4) high pressure valve: the valve with a pressure PN of 10 to 80Mpa.

(5) extra high pressure valve: the valve with nominal pressure PN or more than 100Mpa.

3. classification by working temperature

(1) cryogenic valve: valve for medium working temperature of - 100 degrees.

(2) cryogenic valve: valve for medium working temperature - 100 degrees t or less - 29 degrees.

(3) normal temperature valve: valve for medium working temperature - 29 degrees T < 120 degrees.

(4) medium temperature valve: valve for medium working temperature less than 120 t or less than 425 degrees centigrade.

(5) high temperature valve: valve for medium working temperature t>450 C.

4. classification by drive

(1) automatic valve is a valve that does not need external driving, but relies on the energy of the medium itself to make the valve move. Such as safety valve, pressure reducing valve, steam trap, check valve, automatic regulating valve and so on.

(2) power driven valve: power driven valve can be driven by various power sources.

Electric valve: power driven valve.

Pneumatic valve: valve driven by compressed air.

Hydraulic valve: valve driven by liquid pressure such as oil.

In addition, there are more than one combination of driving modes, such as gas electric valve.

(3) manual valve: manual valve uses manual wheel, handle, lever and chain wheel to control valve action by manpower. When the valve opening and closing moment is larger, the wheel or worm gear reducer can be set between the handwheel and the stem. If necessary, remote operation can also be made by using universal joints and transmission shafts.

5. according to nominal path classification

(1) small diameter valve: nominal diameter DN less than 40mm valve.

(2) medium diameter valve: the valve with a nominal diameter of DN is 50 to 300mm.

(3) large diameter valve: the valve DN is 350 to 1200mm.

(4) extra large diameter valve: the nominal diameter DN is equal to or equal to 1400mm.

6. classification according to structural features

(1) door valve: the closure is moved along the center of the valve seat.

(2) stopcock: the closing part is a plunger or ball, revolving around the centerline of itself.

(3) gate shape: the closure parts move along the center of the vertical valve seat.

(4) swing valve: the closing part revolves around the axis outside the valve seat.

(5) butterfly valve: the disc that closes the part revolves around the axis in the valve seat.

(6) spool valve: the closure is sliding in the direction perpendicular to the channel.

7. classification by connection method

(1) threaded connection valve: the valve body has internal thread or external thread, and is threaded with the pipeline.

(2) flange connection valve: valve body with flange, connected with pipe flange.

(3) welding connection valve: valve body has welding groove and welded connection with pipeline.

(4) the hoop connection valve: the valve body is provided with a clamp and connected with the pipe clamp.

(5) card sleeve connection valve: it is connected with pipe by card sleeve.

(6) to clamp the connection valve: use bolts to directly connect the valve and two pipes together.

8. classification by body material

(1) metal material valve: its valve body and other parts are made of metal materials. Such as cast iron valve, carbon steel valve, alloy steel valve, copper alloy valve, aluminum alloy valve, lead alloy valve, titanium alloy valve, Monel alloy valve, etc.

(2) non-metallic material valve: its valve body and other parts are made of non-metallic materials. Such as plastic valve, ceramic valve, valve, glass steel valve and so on.

(3) metal valve lining valve: the body shape is metal, and the inner surface of the body is lined with the main surface of the medium, such as the lining valve, the lining plastic valve, the potting valve and so on.