Origin and Development of Plug Valves
In the 230 B.C, the first description of three-way plug valves
is noted in the Byzantine era. At that time, a three-way valve
was is made of bamboos and had a wooden cork between two bamboo strips.
According to historical documents, people in the 15th and 16th century used copper plug valves
a lot. In 1837, British had its first patent plug valve. Since then, gradually, the ancient style plug valves were replaced by the switching valves which we still use uses today. In German, the traditional alcohol barrels still adopt the archaic style plug valves. Moreover, in modern times, draught beer cabinets still adopts the so called "beer valve", which is also a kind of plug valves. But, instead of being connected to the barrels directly, the valves are installed on beautiful decorative pipes which connects the barrels. Plug valves
for beers can be disassembled completely, and we can clean every surface of the parts. We can't leave any liquid(beer) in the valves, or the flavor of the beer and our health will be affected. We refer this characteristic of not leaving any liquid inside the valves as no cavity.
Since the plug valves appeared in the world, they have gone through processes of self improvement and development. The plug valves includes two sealing types: hard seal and soft seal. But the hard sealed plug valves are out of market gradually with very little amount of usage while the soft sealed plug valves keep over 99 percent of market share in the whole world, Hard sealed plug valves have the shortcomings of high costs and poor sealing performance.
Long distance crude oil transportation pipelines of foreign countries such as Canada, Brazil, Russia, Poland, etc., always used plug valves with metal hard seals, but later they were replaced by plug valves with soft seals, and ball valves with hard seals are hardly used. Because leakages of media like coal slurry, cinder and crude oil can not only have an impact on the environment, but also waste money due to the relatively high material costs.
Especially after 1980s, long-distance crude oil transportation pipelines, about hundreds of kilometers long, are not built in foreign countries any more and soft sealed plug valves began to be used for crude oil transportation not over one hundred kilometers. However, China has still built the long-distance crude oil transportation pipelines and Chinese traditional designing institutions also would like to choose hard sealed plug valves for the pipelines. Therefore, there are more hard sealed plug valves are used in China compared with soft sealed plug valves.
As for valve functions, plug valves can not only block mediums, but also can distribute and regulate mediums. Now, the pulg valve has developed two-way, three-way, four-way, five-way, six-way and seven-way plug valves. The plug type regulating function is neither exact regulation of flow nor for obtaining accuracy, but for making valves have big fluctuations in flow, within the required distance. Obviously, plug valves can meet this requirement.
In the aspect of valve lining, there are more and more liners in the market these days. In order to make molds have good flowability, plastic liners such as FEP, PFA, PVDF, etc. are adopted. We usually have the injection process for linings. PTFE is difficult to be injected into moulds and it must use compression moulding process, after which, it also need to be sintered. Due to the complex structures of valve bodies, uneven heating of PTFE may occur during the process of sintering, which will make PTFE brittle and not polymerized well. Therefore, there is no PTFE lining of valve in the world. PTFE is used for valve bushings generally and is seldom injected into the valve bodies.