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How to choose the cut-off function of the regulating valve

The regulating valve is also known as the control valve. By accepting the control signal output by the regulating control unit, the regulating valve is operated by power to change the fluid flow. The regulating valve generally consists of an actuator and a valve. According to the power used by the actuator, the control valve can be divided into three types: pneumatic control valve, electric control valve and hydraulic control valve. In addition, according to their functions and characteristics, linear characteristics, equal percentage characteristics and parabolic characteristics are three kind.

1. The choice of structure type

One of the main disadvantages of double-sealed control valves (such as double-seat valves, double-sealed sleeve valves, etc.) is the large leakage. However, before the 1980s, due to the prevailing sleeve valve at that time, many manufacturers made great efforts to improve the structure to reduce the leakage of the sleeve valve, so that it evolved into a very complex structure. As a result, this brings many shortcomings: many parts, poor reliability, difficult spare parts, difficult maintenance, unsatisfactory cutting effect and so on. In fact, this is a misunderstanding of the design ideas. Rather than making such complex changes in the "heart" of the valve body, it is better to achieve it through external adjustments. Therefore, a single-sealed control valve should be selected. In this way, sealing problems, reliability problems, maintenance and spare parts problems are all solved. At this time, the focus of solving the contradiction is transformed into the problem of increasing the unbalanced force of the medium on the valve, which only needs to be equipped with a strong piston actuator and a thicker valve stem. This external solution is obviously much easier than internal complex improvements. It is not so much a matter of methods as it is a matter of ideas. In the 1990s, people began to look for shut-off valves with simpler structures and more reliable sealing. At this time, it was found that the angular stroke valve is more superior than the straight stroke valve (single seat valve, sleeve valve, gate valve), so the triple eccentric cut-off butterfly valve, the full-function cut-off valve, and the hard-sealed cut-off ball valve were produced.

2. Choice of sealing surface

1. Face to face sealing:

The common plunger valve core, its sealing surface is a small cone surface at 60°C, and the valve seat is also a small cone surface at 60°C. The width of this small cone surface is usually between 0.5mm and 2mm. The cones are in good contact. But in fact, it is always affected by processing errors (such as concentricity, out-of-roundness, inclination, etc.), and its sealing effect is not very ideal. The leakage rate of this type of valve is usually 10-4, and it can reach 10-6 after precision grinding, which can only achieve a good sealing level.

2. Spherical seal:

The spherical rotation of the valve core is tangent to the small conical surface of the fixed valve seat, and there is a line contact between them, which is better than the above-mentioned face-to-face sealing effect. The full-function ultra-light valve and spherical sealing butterfly valve launched by Hualin Company are made with this idea. Zero leakage.

3. Choice of sealing material

1. Soft seal

In addition to the corrosion-resistant lined valve, the general soft-sealed valve refers to a valve in which one of the valve core or valve seat adopts a non-metallic material (mainly polytetrafluoroethylene, and rubber, etc.) in the form of sealing. The soft sealing valve has the best sealing effect, but it is possible to leave more or less unclean debris (such as welding slag, iron filings, etc.) in the pipeline installation and system cleaning. The soft sealing valve seat or valve core is scratched, which increases the leakage and causes poor sealing reliability. Therefore, the selection of the soft sealing structure must consider the cleaning of the medium and the strict flushing of the pipeline before putting into operation.

2. Hard seal

Hard sealing and re-surfacing wear-resistant alloys are the best choices for shut-off valves. In this method, the service life and reliability are also considered while considering the sealing performance. Although the factory index is only 10-6 to 10-8, which cannot achieve the effect of zero leakage of the soft seal, it is sufficient to meet the requirements of strict shutoff, and it Durable and more cost-effective from an economical point of view.
4. The consideration of sealing reliability

(1) Soft sealing, good cutting effect, but not suitable for medium containing particles, once scratched, the leakage will increase sharply. Therefore, when conditions permit, try to use hard seals. Full-featured ultra-light valve hard seal can reach 10-7 ~ 10-8, triple eccentric butterfly valve can achieve zero leakage.

(2) Consideration of wear resistance of seals: In addition to the selection of hard seals, for shut-off control valves with leakage requirements of 10-6 or more, wear-resistant alloys should (or must) be surfacing to improve the wear resistance and resistance of seals. Erosion, scratch resistance, improve valve life.

(3) The shut-off regulating valve for high temperature medium is assembled at room temperature, works at high temperature, and the temperature changes by several hundred degrees. Thermal expansion will change the specific pressure of the seal established during assembly. If it is too tight, the sealing surface will be stuck, and it will be severely scratched and strained during movement; if it is too loose, the sealing pressure will not be enough, and the leakage will increase sharply in both cases. The best solution is to use a resilient seat to absorb the difference caused by this change. There are usually three elastic schemes: ① spring type; ② sheet type flexible metal ring; ③ elastic valve seat that combines rigidity and flexibility. The latter is the best, that is, it has flexibility to overcome thermal expansion, and it has rigidity and builds wear-resistant alloys to improve the reliability of wear-resistant surfaces.

(4) For the erosion medium containing particles, the valve core should be considered to avoid the direct erosion of the high-speed medium. Linear travel valves and butterfly valves do not have this function, but 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 enough difference in hardness, and materials of the same hardness "do not give way to each other" and are easy to be scratched.

(6) The thrust and stiffness of the actuator should be increased to make it move freely.
5. Main valve type of shut-off valve

According to the above analysis, the main valve types of the shut-off valve are as follows. The selection of the shut-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, consider the hard seal first and then consider the soft seal.

(1) Full-featured ultra-light control valve

(2) Triple eccentric metal hard sealing 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

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