Selection of 1. structure type

One of the main disadvantages of the double seal control valve is the large leakage. Single seal type control valve shall be selected. Sealing issues, reliability issues, maintenance and spare parts issues were all resolved. Now people began to look for a simpler structure, more reliable seal cut-off valve. At this time, it was found that the angle stroke valve is more superior than the straight stroke valve, so three eccentric cut-off butterfly valve, full-function cut-off valve and so on were produced.

Selection of 2. sealing surface

(1) Face-to-face sealing:

Common plunger valve core, its sealing surface is 60 ℃ small conical surface, the valve seat is also 60 ℃ small conical surface, the width of the small conical surface is usually between 0.5mm ~ 2mm, to seal well must ensure good contact between the two conical surfaces.

(2) Spherical seal:

The spherical rotation of the valve core is tangent to the small conical surface of the fixed valve seat, and the line contact between them is better than the above-mentioned face-to-face sealing effect.

Selection of 3. sealing materials

(1) Soft seal

In addition to the corrosion-resistant lining valve door, the general soft sealing valve refers to the valve core or valve seat of one of the two non-metallic materials used in the form of sealing valve. Soft sealing valve sealing effect is the best, but in the pipeline installation, system cleaning are likely to leave more or less unclean debris (such as welding slag, iron, etc.), they flow through the control valve, easy to the soft seal seat or spool scratches, so that the leakage increased, the reliability of the seal is poor. Therefore, the choice of soft sealing structure must consider the cleaning of the medium and strict flushing of the pipeline before putting into operation.

(2) Hard seal

Hard seal and surfacing wear-resistant alloy is the best choice for cut-off valve. This way in considering the sealing at the same time also consider the service life and reliability, although the factory index is only 10-6~10-8, not up to the soft seal zero leakage effect, but enough to meet the requirements of strict cut-off, and it is durable, from the economic point of view more cost-effective.

Consideration of 4. Seal Reliability

(1) Soft sealing, good cutting effect, but not suitable for granular media, once scratched, leakage will increase sharply. Therefore, when conditions permit, try to choose a hard seal.

(2) the wear resistance of the seal: in addition to the selection of hard seal, the leakage requirements of more than 10-6 cut-off valve, should be surfacing wear-resistant alloy to improve the wear resistance of the seal, erosion resistance, scratch resistance, improve the life of the valve.

(3) The cut-off control valve for high-temperature media is assembled at room temperature and works at high temperature, and the temperature changes by several hundred degrees. Thermal expansion changes the specific seal pressure established during assembly. If it is too tight, the sealing surface will get stuck, and it will be severely scratched and strained during movement; if it is too loose, the sealing pressure is not enough, and the leakage will increase sharply in both cases. The best solution is to use the elastic valve seat to absorb the difference caused by this change. There are usually three kinds of elastic scheme: ① spring type; ② sheet type flexible metal ring; ③ rigid flexible elastic valve seat. The latter is best, that is, there is flexibility to overcome thermal expansion, but also stiffness and pile wear-resistant alloy to improve the reliability of the wear-resistant surface.

(4) for the erosion medium containing particles, it is necessary to consider the direct erosion of the valve core to avoid the high-speed medium. Straight stroke valve, butterfly valve does not have this function, and full-featured ultra-light valve eccentric rotation has this feature. In this case, the latter two valves should be selected.

(5) For hard seals, there must be sufficient hardness difference, and materials of the same hardness "do not give way to each other" and are easy to scratch.

(6) The thrust and rigidity should be increased to make the actuator move freely.

Main valve type of 5. shut-off valve

According to the above analysis, the main type of cut-off valve is as follows. The selection of cut-off valve should comprehensively consider various parameters such as leakage rate, medium type, medium temperature, flow coefficient and cost performance.

Selection of 6. high temperature, high pressure, large pressure difference and large diameter shut-off valve

High temperature, high pressure, large pressure difference and large diameter cut-off are relatively common problems. The main reason is the lack of valves with large allowable pressure difference, good cut-off and high temperature resistant expansion cards. For example, high pressure steam regulation and cut-off, due to large pressure difference (1 ~ 3MPa or more) and large diameter (DN200 ~ 300), balanced double sealing structure (such as double seat valve or double sealing sleeve valve) has to be adopted, after special treatment, the leakage rate can reach 10-4, and then improve the difficult, and full-function ultra-light control valve comprehensive performance is good, can better solve this problem, the leakage rate can reach 10-6~10-7 above.