Abstract: A butterfly valve comprising a cylindrical valve body (1), a sheet ring (5) fitted into the inner periphery of the valve body (1), and a discoid valve discoid valve disk (3) for opening or closing the valve. When the valve is opened by rotating the valve disk (3), a gap forms at the half of the valve disk (3) that is rotated downstream between the sheet ring (5) and the sealing surface (3c) of the valve disk (3). The gap between the sheet ring (5) and the sealing surface (3c) of the valve disk (3) expands and a second gap is formed at the half of the valve disk (3) that is rotated upstream between the sheet ring (5) and the sealing surface (3c) as the valve disk (3) further opens. The sheet ring (5) is not influenced by turbulent flow produced due to a narrowed channel (4) because the sheet ring (5) is positioned to the upstream side of a position where the channel (4) is most narrowed when a valve opening rate is approximately 15% in which the load applied to the sheet ring (5) due to fluid is maximized.

Abstract: At the outer-periphery contact portion of a butterfly disk of a butterfly valve, the sealing area closest to the shaft is formed smaller than other sealing areas. Thus, when the valve is closed by the butterfly disk, the outer-periphery contact portion of the butterfly disk almost simultaneously starts contacting a seat ring throughout the periphery because the area of the portion is smaller than other sealing areas. Therefore, the sealing effect for a long time is expected because the shaft is hardly scratched and the seat ring is hardly abraded.