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Comprehensive understanding of the cut-off valve selection skills

2023-01-04

The shut-off valve is a kind of actuator in the automation system. It is composed of a multi-spring pneumatic film actuator or a floating piston actuator and a regulating valve. It receives the signal of the regulating instrument and controls the cut-off, connection or switching of the fluid in the process pipeline. Can be widely used in petroleum, chemical, metallurgy and other industrial production departments.

In the field of use, the cut-off valve often cut off bad, leakage and other faults. In addition to the plug card, allow the pressure difference is too small to be top open and other reasons caused by the increase in leakage, most of them are due to the structural defects of the control valve and improper selection.

 

Selection of 1. structure type

Double seal control valve (such as double seat valve, double seal sleeve valve, etc.) is one of the main disadvantages of large leakage. But before the 1980 s, due to the more popular sleeve valve, however, many manufacturers in the sleeve valve up and down a lot of effort to improve the structure to reduce leakage, so that it evolved into a very complex structure.

As a result, this has brought many shortcomings: many parts, poor reliability, difficult spare parts, difficult maintenance, unsatisfactory cutting effect, etc. In fact, this is the design ideas have fallen into a misunderstanding, rather than in the "heart" of the valve body to make such complex changes, it is better to achieve through external adjustment. Therefore, the single seal type regulating valve should be selected.

 

In this way, sealing problems, reliability problems, maintenance and spare parts problems are solved one by one. At this time, the focus of solving the contradiction is transformed into the problem that the unbalanced force of the medium on the valve increases, and it only needs to be solved with a strong piston actuator and a thick valve stem.

This external solution is obviously much easier than internal complex improvements. It is not so much a question of method as a question of thinking. In the 1990 s, people began to look for a simpler structure and a more reliable seal. At this time, it was found that the angle stroke valve is more superior than the straight stroke valve (single seat valve, sleeve valve, gate valve), so three eccentric cut-off butterfly valve, full-function cut-off valve and hard seal cut-off ball valve were produced.

Selection of 2. sealing surface

1 Face to face seal:

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.

But in fact, it is always affected by the processing error (such as concentricity, roundness, inclination, etc.), and its sealing effect is not very ideal. This type of valve leakage rate is usually 10-4, after precision grinding up to 10-6, can only achieve a better seal grade.

 

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. Hualin company launched a full-featured ultra-light valve, spherical sealing butterfly valve is the use of this idea to produce, the leakage rate of up to 10-6~10-8, high-performance three eccentric butterfly valve can be as high as 10-8, or zero leakage.

 

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 (mainly PTFE, and rubber, etc.) 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. Full-featured ultra-light valve hard seal up to 10-7~10-8, three eccentric butterfly valve up to zero leakage.

(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 (or must) 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 actuator should be equipped with increased thrust and stiffness, so that the movement 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. In the following order of preference, a hard seal is considered first, followed by a soft seal.

(1) Full-function ultra-light control valve

(2) three eccentric metal hard seal butterfly valve

(3) double eccentric spherical sealing butterfly valve

(4) Ball valve

(5) Eccentric rotary valve

(6) Single seat valve

(7) Single seat sleeve valve

(8) soft sealing valve

6. "high and large" shut-off valve selection

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, the full-function ultra-light control valve has good comprehensive performance, which can better solve this problem, and the leakage rate can reach 10-6~10-7.