An annular valve body, a disk-shaped valve member having a rotation axis and pivotally mounted on said annular valve body at each end of said rotation axis to open or shut a fluid path defined by said annular valve body,BUTTERFLY VALVES and a seat ring having an annular sealing surface disposed between said valve body and said valve member to ensure shutting of the fluid path by said valve member, said sealing surface having two first surfaces each being in the vicinity of the corresponding end of said rotation axis and two second surfaces each extending from one of said first surfaces to the other first surface, each of said second surfaces having substantially a maximum width in the middle position between said first surfaces,Butterfly Valves the width of each of said second surfaces gradually enlarging from the position adjacent to said first surfaces to said middle position of the said second surface wherein peripheral edges formed on each side of each second surface define said width, Butterfly Valvessaid peripheral edges each having a curve defined with a part of a trigonometric function.

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