The structure of the centerline 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.Double eccentric butterfly valve on the basis of single eccentric butterfly valve further improved, the application is also the most widely. Its structure is characterized by deviation from both the center of the disc and the center of the body in the shaft of the valve stem. The effect of double eccentricity makes the valve be opened after the butterfly plate can quickly off the valve seat, greatly eliminate the butterfly plate and the valve seat unnecessary excessive extrusion, scraping phenomenon, reduce the open resistance distance, reduce wear, improve the valve seat life. Scraping is greatly reduced, but also makes the double eccentric butterfly valve can also use the metal seat, improve the butterfly valve in the field of high temperature applications. But because its sealing principle position belongs to the sealing structure, namely the butterfly plate and valve seat sealing surface of line contact, through the butterfly plate extrusion seat sealing effect caused by the elastic deformation, therefore, to the closed position requirement is very high (especially metal seated), low bearing capacity, which is why people traditionally butterfly valve leakage in high pressure resistance, no major reasons.