The structure of the center line butterfly valve is characterized by the same position of stem axis, disc center and body center. Simple structure and convenient manufacture. The common rubber - lined butterfly valve belongs to this category. Disadvantages are due to disc and valve seat always in the extrusion, scraping state, large resistance, wear fast. In order to overcome the extrusion, scraping, guarantee the sealing performance, the valve seat basically USES the rubber or the polytetrafluoroethylene and so on the elastic material, but also because of the sealing material in the use receives the temperature limit, this is why traditionally people thought the butterfly valve not to be able to bear the high temperature the reason.