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First, the choice of structure type
　　
One of the major drawbacks of double seal type regulators (eg double seat valves, double seal sleeve valves, etc.) is the large leak. However, prior to the 1980s, due to the prevalence of sleeve valves at that time, however, many manufacturers made great efforts on sleeve valves to improve the structure to reduce leakage so that it evolved into a very complicated structure. As a result, this has brought many deficiencies: more parts, poor reliability, spare parts is difficult to maintain, the cutting effect is not ideal and so on. In fact, this is a design concept into a misunderstanding, rather than in the "heart" of the valve so complicated changes, it might as well through external adjustment to achieve. Therefore, should choose a single sealed type regulator. In this way, sealing, reliability, maintenance and spare parts issues are all solved. At this point, the focus of resolving contradictions into the medium on the valve unbalanced force increases, to solve it only with strong piston actuator, bold stem can be. This external solution to the problem is obviously much easier than the internal complex improvement. Rather it is a matter of solution rather than an issue of thinking. Into the nineties, people began to find the structure is simpler, more reliable sealing valve. At this time, it was found that the angular stroke valve was more advantageous than the straight stroke valve (single seat valve, sleeve valve, gate valve ), so that a triple eccentric cutoff butterfly valve, a fully functional cutoff valve and a hard seal cutoff ball valve were produced .
　　
Second, the choice of sealing surface
　　
(1) face to face seal:
　　
Common plunger spool, the sealing surface of 60 ℃ small cone, the valve seat is 60 ℃ small cone, the width of this cone is usually between 0.5mm ~ 2mm, to be sealed, we must ensure that two A cone of good contact. In fact, it is always affected by machining errors (concentricity, out-of-roundness, inclination, etc.) and its sealing effect is not very satisfactory. Leakage rate of such valves is usually 10-4, after grinding up to 10-6 precision, only to achieve a better seal level.
　　
(2) spherical seal:
　　
The use of the spherical surface of the valve seat and the fixed seat cone taper tangent between them for the line contact, which is better than the above face to face seal effect. Hualin company launched a fully functional ultra-light valve, spherical sealing butterfly valve is to use 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, and even Zero leakage.
　　
Third, the choice of sealing material
　　
(1) soft seal
　　
In addition to corrosion-resistant liner valves, a typical soft-seal valve is a valve in the form of a seal of a non-metallic material (mainly Teflon, rubber, etc.) on either the valve core or the valve seat. Soft sealing valve sealing the best, but in the pipeline installation, cleaning the system may leave more or less clean debris (such as welding slag, iron filings, etc.), they flow through the control valve, easy to put Soft seal seat or spool scratches, so that increased leakage, poor reliability of the seal. Therefore, the choice of soft seal structure must consider the media cleaning and rigorous flushing pipeline before commissioning.
　　
(2) hard seal
　　
Hard seal and then surfacing wear-resistant alloy is the best choice for shut-off valve. This approach takes into account the sealing of the same time, but also consider the service life and reliability, although the factory index of only 10-6 ~ 10-8, less than the zero leakage of soft sealing effect, but sufficient to meet the strict cut-off requirements, and it Durable, from an economic point of view more cost-effective.
　　
Fourth, seal reliability considerations
　　
(1) soft seal, cut off the effect is good, but not suitable for containing granular media, once scratched, the leak will increase dramatically. Therefore, when possible, try to use hard seal. Full-featured ultra-light valve seal up to 10-7 ~ 10-8, three eccentric butterfly valve up to zero leakage.
　　
(2) consideration of seal wear resistance: In addition to the selection of hard seal, the leakage of more than 10-6 cut-off control valve, should (or must) surfacing wear-resistant alloy to improve seal wear resistance, resistance Erosion, scratch resistance, improve valve life.
　　
(3) cut off the high temperature medium Regulating valve assembled at room temperature, working at high temperatures, temperature changes of several hundred degrees. Thermal expansion changes the seal's specific pressure at assembly. Too tight, the sealing surface will be stuck, sports will be a serious abrasion, strain; too loose, the pressure is not enough, the two cases of leakage will increase dramatically. The best solution is to use flexible seat to absorb the changes caused by this difference. There are usually three kinds of flexible solutions: â‘ spring type; â‘¡ sheet flexible metal ring; â‘¢ flexible and flexible valve seat. The latter is the best, that is flexible to overcome thermal expansion, but also stiffness and heap wear-resistant alloy to improve the reliability of wear-resistant surface.
　　
(4) on the erosion of media containing particles, to consider the valve core to avoid high-speed direct erosion of media. Straight travel valve, butterfly valve does not have this feature, and full-featured ultra-light valve eccentric rotation has this feature. In this case, after the election should be two kinds of valves.
　　
(5) For a hard seal, there must be sufficient hardness difference, the same hardness material "to each other", easy to scratch.
　　
(6) The actuator should be equipped with increased thrust and stiffness, so that action freely.
　　
Fifth, the main valve shut-off valve
　　
According to the above analysis, the main valve shut-off valve is as follows, select the shut-off valve should take into account the leakage rate, medium type, medium temperature, flow coefficient and other parameters and cost. In the following preferred order, first consider the hard seal, then consider the soft seal.
　　
(1) full-function ultra-light control valve
　　
(2) three eccentric metal 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 seal valve
　　
Sixth, high temperature, high pressure, pressure, large diameter cut off valve options
　　
High temperature, high pressure, large pressure, large diameter cut off is a more common problem, the main reason is the lack of both the pressure to allow large, cut off, high temperature expansion of the card valve, such as high pressure steam regulation and cut off, due to large pressure (1 ~ 3MPa or more), large diameter (DN200 ~ 300), had to use a balanced double seal structure (such as double seat valve or double seal sleeve valve), the special treatment, the leakage rate of up to 10-4 , And then increase on the difficult, and full-featured ultra-light control valve comprehensive performance, can better solve this problem, the leakage rate of up to 10-6 ~ 10-7 above.
Shut-off valve to buy methods and types
In demanding applications will put forward the cut-off valve requirements, so the cut-off valve is an important function and use of internal quality indicators (cut-level classification and standards, see the relevant information). In the field of use, there will often appear to cut off the control valve is not good, such as large leaks and other failures. In addition to plugging, allowing the pressure is too small to be open and other causes caused by increased leakage, the vast majority are due to the structural defects of the control valve and the selection caused by improper.