In the fluid piping system, the valve is the control element, and its main function is to isolate the equipment and the piping system, adjust the flow, prevent backflow, adjust and drain the pressure. Since it is very important to choose the most suitable valve for the piping system, it is also important to understand the characteristics of the valve and the steps and basis for selecting the valve. The valve industry has so far been able to produce a complete range of gate valves, globe valves, throttle valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, steam traps and emergency shut-off valves. 12 categories, more than 3000 models, and more than 4000 specifications of valve products; the maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200℃, the minimum working temperature is -196℃, and the applicable medium is water , Steam, oil, natural gas, strong corrosive media (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable media (such as benzene, ethylene, etc.), toxic media (such as), explosive media and radioactive media (,- Circuit pure water, etc.). Valve pressure parts material: 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, duplex stainless steel , Titanium alloys, etc. And can produce various electric, pneumatic, hydraulic valve drive devices. Faced with so many valve varieties and such complex various working conditions, to choose the most suitable valve product for the pipeline system, I think that we should first understand the characteristics of the valve; secondly, we should master the steps and basis for selecting the valve; Follow the principle of selecting valves. 1. There are generally two types of valve characteristics, use characteristics and structural characteristics. Use characteristics: It determines the main performance and scope of use of the valve. The characteristics of the valve are: valve category (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, bonnet, valve stem, valve clack, sealing surface); valve transmission mode, etc. Structural characteristics: It determines some structural characteristics of the valve installation, repair, maintenance and other methods. The structural characteristics include: the structural length and overall height of the valve, and the connection form with the pipeline (flange connection, threaded connection, clamp connection , External thread connection, welding end connection, etc.); the form of the sealing surface (inlaid ring, threaded ring, surfacing, spray welding, valve body); valve stem structure (rotating rod, lifting rod), etc. 2. The steps and basis for selecting valves are roughly as follows: ⑴ Selecting steps ① Clarify the purpose of the valve in the equipment or device, and 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 connecting with the valve: flange, thread, welding, etc. ③ Determine the method of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electric linkage or electro-hydraulic linkage, etc. ④ Determine the material of the selected valve shell and internal parts according to the medium, working pressure and working temperature of the pipeline: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, 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, etc. ⑦ Determine the parameters of the valve: For automatic valves, first 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, valve height dimension after opening and closing, connecting bolt hole size and number, overall valve outline size, etc. ⑨ Use existing information: valve product catalog, valve product sample, etc. to select appropriate valve products. ⑵ The basis for selecting a valve While understanding the steps of selecting a valve, it is also necessary to further understand the basis for selecting a valve. ① The purpose, operating conditions and operation control mode of the selected valve. ② The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is a flammable or explosive medium, the 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 to the pipeline and connection dimensions, external dimensions or weight restrictions, etc. ⑤ Additional requirements for the reliability and service life of valve products and the explosion-proof performance of electric devices. (Note when selecting parameters: If the valve is to be used for control purposes, the following additional parameters must be determined: operation method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, maximum and minimum inlets of the valve pressure.) According to the above-mentioned basis and steps for selecting the valve, the internal structure of various types of valves must be understood in detail when selecting the valve reasonably and correctly, so that the correct choice can be made on the preferred valve. The ultimate control of the pipeline is the valve. The valve opening and closing parts control the flow mode of the medium in the pipeline. The shape of the valve flow channel makes the valve have a certain flow characteristic. This must be considered when selecting the most suitable valve for the pipeline system. The following are the principles to be followed when selecting a valve: ⑴ The valve flow path for the cut-off and open medium is a straight-through valve, and its flow resistance is small, and the valve for the cut-off and open medium is usually selected. The downward closing valve (stop valve, plunger valve) is less used because of its tortuous flow path and higher flow resistance than other valves. Where a higher flow resistance is allowed, a closed valve can be used. ⑵ The valve used to control the flow is usually the valve that is easy to adjust the flow as the control flow. Downward closing valves (such as globe valves) are suitable for this purpose because the size of its seat is proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flex-body valves (pinch valves, diaphragm valves) can also be used for throttling control, but they are usually only applicable within a limited range of valve diameters. The gate valve uses a disc-shaped gate to cross-cut the circular valve seat opening. It can only control the flow better when it is close to the closed position, so it is usually not used for flow control. ⑶ Valves used for reversing and diversion, according to the needs of reversing and diversion, this kind of valve can have three or more channels. Plug valves and ball valves are more suitable for this purpose. Therefore, most of the valves used for reversing and splitting select one of these valves. However, in some cases, other types of valves can also be used for reversing and shunting as long as two or more valves are properly connected to each other. ⑷ Valves for medium with suspended particles When there are suspended particles in the medium, it is most suitable to use a valve with a wiping effect on the sliding of its closing part along the sealing surface. If the back and forth movement of the closing member to the valve seat is vertical, it may hold particles. Therefore, this valve is only suitable for basic clean media unless the sealing surface material allows particles to be embedded. Ball valves and plug valves have a wiping effect on the sealing surface during the opening and closing process, so they are suitable for use in media with suspended particles. At present, whether in the pipeline system of petroleum, chemical industry, or other industries, valve applications, operating frequencies and services are ever-changing. To control or prevent even minor leaks, the most important and critical equipment is the valve. The ultimate control of the pipeline is the valve. The service and reliable performance of the valve in various fields is unique.
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