Valves are control components in fluid conveying systems that have functions such as diversion, interception, regulation, throttling, prevention of backflow, diversion, or overflow pressure relief. Valves are important control components in pipelines. It is important to choose the most suitable valve for the pipeline system, which is related to the safety, flow loss, cost, etc. of the pipeline. It is necessary to be familiar with the characteristics of the valve and the steps and basis for selecting the valve. At present, China's valve industry can produce a complete range of 12 categories, including gate valves, globe valves, check valves, ball valves, butterfly valves, throttle valves, plug valves, diaphragm valves, safety valves, control valves, drain valves, and special valves; The maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200 ℃, and the minimum working temperature is -196 ℃. Suitable media include water, steam, oil, natural gas, highly corrosive media (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable media (such as benzene, ethylene, etc.), toxic media (such as hydrogen sulfide), explosive media, and radioactive media (such as metallic sodium, primary circuit pure water, etc.). Valve pressure parts are made of cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid resistant steel, Hastelloy, Inconel, Monel alloy, duplex stainless steel, titanium alloy, etc. And it can also produce various hydraulic, pneumatic, and electric valve driving devices. Faced with so many valve varieties and such complex working conditions, to choose the valve product that is most suitable for installation in the pipeline system, I believe that the first step is to understand the characteristics of the valve; Secondly, one should master the steps and basis for selecting valves; Furthermore, the principle of selecting valves should be followed.

1、 The characteristics of valves include usage characteristics and structural characteristics

The usage characteristics determine the main performance and scope of use of the valve. The categories that belong to the usage characteristics of the valve include: closed circuit valve, regulating valve, safety valve, etc; Product type (gate valve, globe valve, butterfly valve, ball valve, etc.); The material of the main parts of the valve (valve body, valve cover, valve stem, valve disc, sealing surface); Valve transmission mode, etc. Structural characteristics determine some of the structural characteristics of valve installation, repair, maintenance, and other methods. The structural characteristics include: the structural length and overall height of the valve, the connection form with the pipeline (flange connection, threaded connection, clamp connection, external thread connection, welding end connection, etc.); The form of sealing surface (insert ring, threaded ring, overlay welding, spray welding, valve body); The structural form of the valve stem (rotating rod, lifting rod), etc.

2、 Steps for selecting valves

1. Clarify the purpose of the valve in the equipment or device, determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc

2. Determine the nominal diameter and connection method of the pipeline connected to the valve: flange, thread, welding, etc

3. Determine the method of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electro-hydraulic linkage, etc

4. Determine the materials of the selected valve casing and internal components based on the medium, working pressure, and working temperature transported by the pipeline: cast steel, carbon steel, stainless steel, alloy steel, stainless acid resistant steel, gray cast iron, malleable iron, ductile iron, copper alloy, etc.

5. Choose the type of valve: closed circuit valve, regulating valve, safety valve, etc

6. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap valve, etc

7. Determine the parameters of the valve: For automatic valves, determine the allowable flow resistance, discharge capacity, back pressure, etc. according to different needs, and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole

8. Determine the geometric parameters of the selected valve: structural length, flange connection form and size, dimensions in the height direction of the valve after opening and closing, size and quantity of connecting bolt holes, overall valve size, etc

9. Utilize existing information such as valve product catalogs and valve product samples to select appropriate valve products

3、 Basis for selecting valves

1. The purpose, operating conditions, and control methods of the selected valve

2. Nature of working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable or explosive, viscosity of the medium, etc

3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc

4. Requirements for installation dimensions and external dimensions: nominal diameter, connection method and size with pipeline, external dimensions or weight limitations, etc

5. Additional requirements for the reliability, service life, and explosion-proof performance of valve products (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: operating method, maximum and minimum flow requirements, normal flow pressure drop, pressure drop when closing, maximum and minimum inlet pressure of the valve.)

Based on the basis and steps for selecting valves mentioned above, it is necessary to have a detailed understanding of the internal structure of various types of valves when selecting valves reasonably and correctly, in order to make the correct decision on the preferred valve. The final control of the pipeline is the valve. The valve opening and closing components control the flow direction of the medium in the pipeline, and the shape of the valve flow channel endows the valve with certain flow characteristics. This must be considered when selecting the valve that is most suitable for installation in the pipeline system.