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, often there will be a good cut off the control valve, leakage 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. First, the choice of structure type Double seal type regulator (such as double seat valve, double seal sleeve valve, etc.) One of the major drawbacks is a large leak. However, prior to the 1980s, due to the prevalence of sleeve valves at that time, however, many manufacturers made great efforts to improve the structure of the sleeve valve 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 difficult, 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 clearly much easier than the complicated internal 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 plug, the sealing surface is 60 ℃ small cone, the valve seat is 60 ℃ small cone, the width of this small cone usually 0.5mm ~ 2mm, to be sealed, you must ensure good contact between the two cones. 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 fixed seat taper tangent between them for the line contact, which is more effective than the face to face seal. 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 selection of sealing materials (1) soft seal In addition to corrosion-resistant lining valve, the general soft-seal valve refers to the valve core or valve seat of either one of the non-metallic materials (mainly PTFE, rubber, etc. ) Sealed form of the valve. 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 to cut off the 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, consider the reliability of the seal (1) soft seal, cut off well, 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: ①spring type; ② sheet flexible metal ring; ③ 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, media types, media 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 hard sealing butterfly valve (3) Double eccentric spherical sealing butterfly valve (4) Ball valve (5) Eccentric rotary valve (8) soft seal valve 6, high temperature, high pressure, large pressure, large diameter cut-off valve selection High temperature, high pressure, pressure, large diameter cut is a more common problem, mainly due to the lack of large pressure drop, Cut off, high temperature expansion card valves, such as high-pressure steam regulation and cut off, due to large pressure (1 ~ 3MPa or more), large diameter (DN200 ~ 300), had to adopt a balanced double seal structure 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 Up to 10-6 ~ 10-7 above. More keyword search: Regulating valve
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