BUTTERFLY VALVE

PROBLEM TO BE SOLVED: To realize a minute amount control, and to prevent an easy breaking of a seat ring, by generating a clearance between the seat ring and a valve body seal surface, from the side end side rotating at the downstream side of a seal body, at the valve opening condition, and expanding this clearance following the opening of the valve. SOLUTION: When a shaft 2 is driven and a valve body 3 is rotated from a blocking condition, a seal surface 3c at the end of the other side 3b of the valve body 3 is separated from a seat ring 5, a clearance is generated, and the valve opening is started. When the rotation is advanced further, the clearance is expanded gradually to one side 3a of the valve body 3, and finally, the seal surface 3c at the end of one side 3a is separated from the seat ring 5, and the seat ring 5 is made in the full opening condition at about 90 deg.. The load by the flowing applied to the seat ring 5 is made maximum immediatly after the starting of the opening of the valve, and immediately before the blocking, near the valve opening 15%, but since the seat ring 5 is positioned at the upstream from the position where the flow passage 4 is narrowed mostly, it does not receive the influence of a jet spout or a turbulant flow. When the valve opening is less than 90 deg., the flow draws an about linear line or a loose curve, and the flow rate reaches to 0%, there is no large inflection point, and the flow rate less than 10% can be also controlled.