The typical Butterfly Valves heretofore known comprises an approximately cylindrical casing having an annular valve seat on its inner peripheral surface, a valve stem rotatably extending through the casing, a valve disk attached to the valve stem by a stem mount portion and having an outer peripheral edge portion opposed in its entirety to the valve seat when the disk is in its closed position, and an annular valve seal fixedly attached to the outer peripheral edge portion so as to be in pressing contact with the valve seat when the valve disk is in its closed position. With this butterfly valve, the valve seat, the valve stem, the valve disk and the valve seal must be prepared with high precision,BUTTERFLY VALVES and the valve stem, the valve disk and the valve seal need to be installed in place also with high precision. Otherwise, it is impossible to assure a uniform pressure (seal pressure) of contact between the valve seat and the valve seal over the entire circumference when the valve disk is closed. Accordingly butterfly valves of this type which are satisfactory in performance are invariably expensive to make.

To overcome the above drawback, a Butterfly Valves has been proposed which includes clamp means at the outer peripheral edge portion of the valve disk for holding an annular valve seal. The valve seal is adjustably deformed by being compressed by the clamp means to realize the desired seal pressure despite the dimensional errors of the parts concerned and errors involved in installation. The butterfly valve nevertheless has the drawback that the valve seal breaks within a short period of time owing to fatigue deterioration because the seal is used as deformed by compression.

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