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Basis for selecting valves

In understanding the steps to master the selection of valves, should further understand the basis for the selection of valves. 

    ① the purpose of the selected valve, the use of working conditions and manipulation and control mode. 

    ② the nature of the working medium: working pressure, working temperature, corrosive properties, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive media, the viscosity of the medium and so on.

    ③ Requirements for the fluid characteristics of the valve.

    ④ Installation size and external dimension requirements: nominal diameter, connection mode and connection size with pipeline, external dimension or weight limit, etc. 

    ⑤ Additional requirements for valve product reliability, service life and explosion-proof performance of electric devices, etc. 

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    (When selecting parameters should be noted: If the valve is to be used for control purposes, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, maximum and minimum inlet pressure of the valve.) 

    Based on the above basis and steps for selecting valves, a reasonable and correct selection of valves must also be based on a detailed understanding of the internal structure of the various types of valves so that the right choice can be made for the preferred valve. 

    The final control of the pipeline is the valve. Valve openings and closings control the way the medium flows in the pipeline, the shape of the valve flow channel so that the valve has certain flow characteristics, which must be taken into account when selecting the most suitable valve for installation in the pipeline system. 

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    The following are the principles to be followed in selecting valves. 

    (1) stop and open media with the valve 

    The flow path for the straight-through valve, the flow resistance is small, usually selected as the valve for the shut-off and open media. Downward closing type valve (globe valve, plunger valve) because of its flow path zigzag, flow resistance than other valves, so less choice. In the occasion of allowing a higher flow resistance, closed type valves can be used. 

    (2) control the flow of valves 

    Usually choose easy to adjust the flow of the valve as a control flow. Downward closing type valve (such as shut-off valve) is suitable for this purpose, because its seat size is proportional to the relationship between the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flexural body type valves (clamping valves, diaphragm valves) can also be used for throttling control, but usually only in a limited range of valve bore. Gate valve is a disc-shaped gate to the circular seat mouth to do cross-cutting movement, it is only close to the closed position, it can better control the flow, so it is usually not used for flow control. 

    (3) Valve for reversing and diverting 

    According to the need for directional flow diversion, this valve can have three or more channels. Plug and ball valves are more suitable for this purpose, therefore, most of the valves used for directional flow diversion are selected as one of these types of valves. But in some cases, other types of valves, as long as two or more valves are properly interconnected, can also be used for directional diversion. 

    (4) with suspended particles in the media with the valve 

    When the medium with suspended particles, the most suitable for the use of its closing member along the sealing surface of the sliding with wiping effect of the valve. If the back and forth movement of the closing member to the seat is vertical, then the particles may be trapped, so this valve unless the sealing surface material can be allowed to embed particles, or only for basic clean media. Ball valve and plug valve in the opening and closing process on the sealing surface are wiped, so it is suitable for use in the media with suspended particles. 

    At present, whether in the petroleum, chemical, or pipeline systems in other industries, valve applications, operating frequencies and services vary widely, to control or eliminate even the low leakage, the most important and critical equipment is still the number of valves. The ultimate control of pipelines is the valve, and valves are unique in their service and reliable performance in all areas.