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原文
The choice of structure type shut-off valve Double seal type valve (eg, two-seat valve, double-sealing sleeve valve, etc.) is one of the major shortcomings of large leaks. But before the 1980s, because of the more popular sleeve valve, however, many manufacturers are in a great effort from top to bottom sleeve valve to improve the structure to reduce leakage, it evolved into a very complex structure. So, this brings a lot to be desired: many parts, poor reliability, spares difficult, difficult to maintain, cut off the result is not satisfactory, and so on. In fact, this is the design ideas are misunderstanding, rather than in the body of the "heart" in making such complex changes, it would be better achieved through the external adjustment. Therefore, we should choose a single seal type valve. Thus, sealing problems, reliability issues, maintenance and spare parts are all the solutions of the problem. At this point, the focus of conflict resolution into the media's unbalanced force on the valve increases the problem, solve it only with a powerful piston actuator stem can be bold. External to this solution is obviously complicated by internal improvements than the much easier way which is not so much the problem, as it is thought the problem. Into the nineties, people began to look for structure is more simple, more reliable sealing shut-off valve. At this time, found that rotary valve than the straight travel valve (single-seat valves, sleeve valves, gate valves) is more superior, so have cut off three eccentric butterfly valve, all-shut-off valve, seal off valve.
原文来自：http://www.gemofa.org

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译文：
双密封类调节阀（如双座阀、双密封套筒阀等）主要的缺点之一就是泄漏大。但上世纪80年代以前，由于当时较盛行套筒阀，然而，不少厂家却在套筒阀上下了很大功夫以改进结构来降低泄漏量，使其演变成非常复杂的结构。于是，这又带来了许多不足：零件多、可靠性差、备件难、维护难、切断效果不理想等等。实际上，这是设计思路都陷入了误区，与其在阀体的“心脏”内作如此复杂的改动，倒不如通过外部调整来实现。因此，应选择单密封类调节阀。这样，密封问题、可靠性问题、维修和备件问题均被一一解决了。此时，解决矛盾的重点转化为介质对阀的不平衡力增大的问题，解决它仅需配强力活塞执行机构、加粗阀杆即可。这种通过外部来解决的办法显然比通过内部的复杂改进要容易得多，与其说这是办法问题，还不如说这是思路问题。进入到九十年代，人们开始寻找结构更简单、密封更可靠的切断阀。这时，发现角行程阀比直行程阀（单座阀、套筒阀、闸阀）更具有优越性，于是产生了三偏心切断蝶阀、全功能切断阀、硬密封切断球阀。

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