The revolving center of the butterfly plate (i.e. the center of the stem) is located on the center line of the valve body and the section of the sealing surface of the butterfly plate. The valve seat is made of rubber. When the butterfly plate is closed, the outer sealing surface of the butterfly plate extrudes the synthetic rubber seat, which causes the elastic deformation of the seat, thus forming the elastic force as the sealing specific pressure to ensure the sealing of the butterfly valve. Flange butterfly valve seals are composed of PTFE and synthetic rubber. Its characteristics are that the elasticity of the valve is still provided by synthetic rubber and the low friction coefficient, wear resistance and ageing resistance of PTFE are utilized to improve the service life of butterfly valve.