Principles to be followed in the selection of the

2022-07-27
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The principle of valve selection

in the fluid pipeline system, the valve is the control element, which is mainly used to isolate equipment and pipeline system, regulate flow, prevent backflow, regulate and discharge pressure. Since it is very important to select the most suitable valve for the pipeline system, it is very important to understand the characteristics of the valve and the steps and basis for selecting the valve

so far, the valve industry has been able to produce a complete range of valve products in 12 categories, more than 3000 models and more than 4000 specifications, including gate valve, stop valve, throttle valve, plug valve, ball valve, electric valve, diaphragm valve, check valve, safety valve, pressure reducing valve, steam trap and emergency shut-off valve; The maximum working pressure is 600MPa smooth oil is not allowed for the rotating bearing of the internal shaft of the dynamometer main shaft and the rotating bearing in the pointer structure. The maximum nominal diameter is 5350mm, the maximum working temperature is 1200 ℃, and the minimum working temperature is -196 ℃. The applicable media are 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 hydrogen sulfide) Explosive media and radioactive media (metal sodium, primary pure water, etc.). Materials of valve pressure parts: cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless and acid resistant steel, Hastelloy, Inconel, Monel, duplex stainless steel, titanium alloy, etc. And can produce all kinds of electric, pneumatic, hydraulic valve driving devices. In the face that the mechanical properties of such metals are the design of parts or structural parts, according to numerous valve varieties and such complex operating conditions, it is necessary to select the valve products most suitable for installation in the pipeline system. In my opinion, first of all, we should understand the characteristics of the valve; Secondly, master the steps and basis for selecting valves; Moreover, the principle of valve selection shall be followed

1. there are generally two types of valve characteristics, service characteristics and structural characteristics

service characteristics: it determines the main service performance and service range of the valve. The service characteristics of the valve include: the category of the valve (closed-circuit valve but no doubt, regulating valve, safety valve, etc.); Product type (gate valve, globe valve, butterfly valve, ball valve, etc.); Materials of main parts of the valve (valve body, bonnet, valve rod, valve disc and sealing surface); Valve transmission mode, etc

structural characteristics: it determines some structural characteristics of 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, welded end connection, etc.); Form of sealing surface (insert ring, threaded ring, surfacing, spray welding, valve body); Valve stem structure (rotating rod, lifting rod), etc

2. the steps and basis for selecting the valve are generally as follows:

⑴ selection steps

① specify 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 mode of the pipeline connected with the valve: flange, thread, welding, etc

③ determine the mode of valve operation: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc

④ determine the materials of the shell and internals of the selected valve according to the medium conveyed by the pipeline, working pressure and working temperature: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid resistant steel, copper alloy, etc

⑤ select the type of valve: closed-circuit valve, regulating valve, safety valve, etc

⑥ determine the type of valve: gate valve, stop valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc

⑦ determine valve parameters: for automatic valves, it is difficult for domestic instruments to go abroad according to different needs. First determine the allowable flow resistance, discharge capacity, back pressure, etc., 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, size in the height direction of the valve after opening and closing, size and quantity of connected bolt holes, overall size of the whole valve, etc

⑨ use the existing data: valve product catalog, valve product samples, etc. to select appropriate valve products

⑵ basis for selecting valves

while understanding the steps for selecting valves, we should also further understand the basis for selecting valves

① purpose, operating conditions and operation control mode of the selected valve

② properties 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 grade, etc

④ requirements for installation dimension and overall dimension: nominal diameter, connection mode with pipeline, connection dimension, overall dimension or weight limit, etc

⑤ additional requirements for reliability, service life of valve products and 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 of normal flow, pressure drop at closing, maximum and minimum inlet pressure of the valve.)

according to the above basis and steps for selecting valves, when selecting valves reasonably and correctly, it is also necessary to have a detailed understanding of the internal structure of various types of valves, so as to make a correct choice for the preferred valves

the final control of the pipeline is the valve. The valve opening and closing parts control the flow beam mode of the medium in the pipeline. The shape of the valve channel enables the valve to have certain flow characteristics. This must be taken into account when selecting the valve most suitable for installation in the pipeline system

the following are the principles to be followed in selecting valves:

⑴ valves for cut-off and open media

the flow passage is a straight through valve with small flow resistance, and the valves for cut-off and open media are usually selected. Downward closed valves (stop valve and plunger valve) are less selected because of their tortuous flow path and higher flow resistance than other valves. Closed valves can be used where high flow resistance is allowed

⑵ valves for flow control

generally, valves that are easy to adjust flow are selected for flow control. A downward closing valve (such as a globe valve) is suitable for this purpose because its seat size is proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flex body valves (clamp valves, diaphragm valves) can also be used for throttling control, but they are usually only applicable to a limited range of valve diameters. The disc-shaped gate valve makes a cross cutting movement to the circular valve seat. It can better control the flow only when it is close to the closed position, so it is usually not used for flow control

⑶ valve for reversing and shunting

according to the needs of reversing and shunting, this 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 shunting are one of these valves. However, in some cases, other types of valves, as long as two or more valves are properly connected with each other, can also be used for reversing and shunting

⑷ valve for medium with suspended particles

when the medium has suspended particles, it is most suitable to use the valve with wiping effect by sliding its closing part along the sealing surface. If the back and forth movement of the closing part to the valve seat is vertical, particles may be clamped. Therefore, this valve is only suitable for basically clean media unless the sealing surface material allows the insertion of particles. Ball valve and plug valve can wipe the sealing surface during opening and closing, so they are suitable for use in media with suspended particles

at present, the application, operation frequency and service of valves in the pipeline system of petroleum, chemical industry and other industries are ever-changing. To control or eliminate even low leakage, the most important and critical equipment is valves. The final control of the pipeline is the valve. The service and reliable performance of the valve in various fields are unique. (end)

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