The turning center of the butterfly plate is on the centerline of the valve body and the sealing face section of the butterfly plate. In closing, the ex-circle sealing face of the butterfly plate will extrude the valve seat made of synthetic rubber, generating elastic deformation on the seat to form an elastic force used as a sealing specific pressure so as to guarantee the tightness of the butterfly valve.

During valve opening or closing, the butterfly plate sealing face will be deviated gradually from and pressed into gradually the valve seat sealing face, thus reducing the mechanical abrasion and damage between the butterfly plate and the valve seat during opening or closing. For the double-eccentricity butterfly valve, when the butterfly plate rotates from 0° to 8°-12°, the butterfly plate sealing face will break away from the valve seat sealing face completely, thus reducing the mechanical abrasion and the extrusion corner travel during opening or closing the butterfly valve. With the reduction of mechanical abrasion and extrusion, the sealing performance and service life of the butterfly valve will be improved greatly.

For the structure of the triple-eccentricity seal butterfly valve, when the valve is under the complete open state, its butterfly plate sealing face will break away from the valve seat sealing face completely; when it is opened from 0 to 90°, the butterfly plate sealing face will break away from the valve seat sealing face upon opening; when it is closed from 90° to 0°, and only upon closing, the butterfly plate sealing face can contact and press onto the valve seat sealing face tightly.