Cut off the control valve on the cut-off valve regulator is the most important requirements of the use of functional and intrinsic quality. The cut-level classification and standards in Table 1. Control valve leakage encountered in the use of large leakage is one of the most common, except as a result of plug-in leakage increases, the vast majority are the defects of the valve structure and selection Caused by improper 1) Blocking caused by increased leakage This problem naturally from the anti-blocking considerations, see anti-blocking type regulator (7.5) section. 2) structural defects caused by increased leakage (1) double-sealed valve can not be used as a shut-off valve double seal control valve, such as double seat valve, sleeve valve, one of the major drawback is the big leak. However, many manufacturers have made great efforts to improve the structure of the sleeve valve to reduce leakage; make it evolve into a very complex structure, see Figure 4-35. As a result, there are many shortcomings: many parts, poor reliability; spare parts difficult to maintain; cutting is not satisfactory. In fact, this is a design concept into a misunderstanding, rather than in the "heart" of the valve for such a complex change, it would be better to achieve through the outside. Therefore, I believe that should be used single-seated sleeve valve, so that the sealing, reliability, maintenance and spare parts are one by one to solve the problem, this time to resolve the key contradictions into the medium on the valve unbalanced force increases The problem, to solve it only with strong piston actuator 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. (2) design of the sealing surface ①face to face seal: Common plunger spool, the sealing surface is 60 ℃ small bevel, the valve seat is 60 ℃ small bevel, the width of this small bevel is usually between 0.5mm ~ 2mm, To be sealed, the two bevels must be in good contact, but in fact, it is always affected by machining errors, such as concentricity, out-of-roundness and inclination, and the sealing effect is naturally not very satisfactory. The leak rate of such valves is usually 10, through the precision grinding up to 10, only to achieve a better seal level. ② spherical seal: the use of the spherical surface of the valve spool rotation and fixed small incline 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 produced by this method, the leakage rate of up to 10, than face to face seal can be increased two orders of magnitude. (3) the consideration of the sealing material ①soft seal "Good factory is not good": the soft seal is the best sealing effect, but who can guarantee the use of a period of time, the leakage is how much? In the pipeline installation, system cleaning will leave more or less unclean debris, such as welding slag, iron filings, they flow through the control valve, easy to scratch the soft sealing material, so that leakage increased, So "good factory not easy to use." ② hard seal re-surfacing wear-resistant alloy is the best choice for shut-off valve: This approach also takes into account the sealing and reliability, although the factory target is 10, not up to the soft sealing effect, but it is sufficient to meet the strict seal Cut off the request, more importantly, it can be durable. (4) Poor reliability of diaphragm valves Back in the 1950s, diaphragm valves were often considered the preferred shut-off valves and have been used until now, such as desalinated water control in water treatment systems. It should be noted here that both the earlier rubber lined diaphragm valve and the subsequent fluorine plastic diaphragm valve have a common disadvantage that the diaphragm is not reliable and can be easily broken so that the valve Very short life. (5) The preferred sequence of the shut-off valve According to the above analysis, the following sequence of shut-off valves is preferred: ①Full-function ultralight valve → ② Spherical sealing butterfly valve → ③ Eccentric valve → ④V-type ball valve → ⑤ Single seat valve → ⑥ Single seat Sleeve valve → ⑦ soft seal valve (O-ball valve, rubber lined butterfly valve, lined diaphragm valve, soft seal single-seat valve).
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