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Butterfly valve selection, first of all, according to successful engineering practice, relevant laws, and regulations, choose the appropriate structure, material, and pressure level to ensure that the butterfly valve is safe and reliable.

Structure type

①The flow characteristics of the butterfly valve are approximately linear, whether it is midline type or eccentric type, the difference is not very big. The midline type is suitable as a regulating and small diameter process valve.

②How to choose double eccentric and triple eccentric butterfly valves, both structures have the characteristics of being able to disengage quickly and close tighter and tighter, so how to choose the two structures? As a soft seal with double eccentric, it is more convenient in processing and manufacturing. The triple eccentric is more complicated to process and manufacture, but it is more prominent in the tightening characteristics, and it is not easy to produce interference, so it is more suitable for metal hard seals. At present, there is no comprehensive analysis in the market, that what is recommended to use three eccentric metal hard seal is not right, especially on the large-diameter room temperature water system is recommended use, there is no need. Because the metal hard seal is much more difficult to replace than the soft seal, it is appropriate to use a soft seal in a large-diameter waterworks butterfly valve. Only those who are not easy to repair and replace and have higher temperatures and harsh working conditions are forced to use the metal hard seal. In order to facilitate the replacement of metal seals under severe conditions, the valve body is generally lengthened by a section, and opened a mid-port, so as to facilitate the disassembly of the seal.

③The choice of the vacuum butterfly valve. The vacuum butterfly valve has two kinds of rubber soft seal and metal hard seal, from the structure, can be midline type, eccentric type, and lever type, for the requirement of high vacuum degree, is generally soft seal. For those who require a high vacuum degree, it is generally a soft seal, but only a metal seal is required for a certain temperature. In order to get a good effect, the surface that can be processed in general should be processed and have a good surface roughness and cleanliness, can not be painted and coated with general anti-rust oil, only allowed to use vacuum grease.

Caliber

Butterfly valve caliber selection is mainly determined by the media flowing through the valve, flow, or flow rate.

For liquids, the general flow rate should not exceed 5m/s, the maximum does not exceed 7-8m/s. Usually: the economic flow rate of water 2-3m/s, low pressure gas flow rate recommended 2-10m/s, medium pressure gas flow rate recommended 10-20m/s; for steam flow rate, recommended low pressure steam 20-40m/s, medium pressure steam 40-60m/s, high pressure steam 60-80m/s.

Of course, sometimes in order to reduce pressure losses, the valve will be selected larger, so that the flow rate is lower, but if you want to regulate at low flow rates can not do so.

The liquid in a very short period of time flows through a very low absolute pressure region of rapid evaporation and recondensation phenomenon, known as cavitation, this phenomenon is not possible in the gas flow, because the gas in the low pressure of its state is not changed. When the fluid flows into the region of higher pressure, the vapor bubbles will suddenly condense and the bubbles will collapse or burst together. The flow velocity of such a storm may reach 110m/s and high pressure of 50MPa, producing impact damage to the wall. In order to avoid cavitation, a number of measures should be taken.

① Make the flow velocity not so large as to produce such low pressure.

② Introduce the atmosphere into the low-pressure area.

③reduce the differential pressure △Pt≤FL(P1-Pvc), where Pvc indicates the pressure of the generated blockage flow on the shrinkage section.

When the differential pressure on the valve is p < 1.5MPa even if the cavitation phenomenon, the damage to the material is not serious, so no special measures are required.

Considering the material, generally speaking, the harder the material, the stronger the ability to resist cavitation.

When the butterfly valve does not produce cavitation, the general cavitation coefficient ∂ ≥ 2.5.

Materials

Butterfly valve body material should be determined according to the nature of the medium and working temperature and flow conditions (whether there are particles, two-phase flow, etc.). Under normal circumstances, the user is required to propose the main body material of the valve, especially the main body material of the valve for special working conditions or severe working conditions. In most cases, the valve body material is the same as or slightly higher than the pipeline material.

Steel valves can be adapted to the temperature of ANSI B16. 34, carbon steel is undoubtedly a common economic material, which can withstand temperatures up to 425C, cast iron, and ductile iron in low-pressure valves are used a lot.

For seawater-resistant valves, due to different requirements, depending on the working conditions and important to take the following methods:

① using titanium alloy manufacturing;

②Adopt duplex stainless steel;

③Adopt rubber lining or PTFE;

④Apply epoxy or ceramic coating;

⑤ using low-nickel cast iron or low-alloy cast iron.

Sometimes also use the cross-composite of several materials. Usually, light quality is required on ships, and titanium alloy is appropriate, while ordinary power plants and chemical plants should be lined or low-alloy ductile iron with coating.