The butterfly valve known up until now is the one that by attaching an elastic seat ring to the inside of the hollow cylindrical body made of rigid material and inserting the valve plug, opens and closes by the seat ring crimping and separating due to the rotating stem. For the butterfly valve, what is used is a seat ring having a circular outer circumferential shape and an inner circumferential diameter formed smaller than the outer circumferential diameter of the valve plug by one size. However, there was a drawback to this butterfly valve. When the valve plug is crimped and sealed to the inner circumferential part, since the available surface pressure is low near the stem through-hole, the sealing effect is small with the same compression rate, making it vulnerable to leakage.
In order to solve the above problem, the seat ring as described in the Japanese Unexamined Utility Model Publication No. 3-62271 (FIG. 1, Pages 4 to 5) was developed. This is a seat ring for the butterfly valve to improve the sealing performance near the stem through-hole. As shown in FIGS. 5 to 7, it has a circumferential shaped protrusion 24 represented by a trajectory with a radius R1 and O1 as its center on its body circumference attaching surface 23 which is smaller in width than the body attaching surface 23. Furthermore, it is distinctive in that it has a circular protrusion 25 which is smaller in width than the above protrusion 24 and is decentered toward the stem axial direction X′ on the surface of protrusion 24, has a center at a point O2 decentered to the stem axial direction X′ with respect to the center O1, and is represented by a trajectory with a radius R2 smaller than the above radius R1. Due to this technology, a crushing margin near the stem through-holes 26 and 27 is made larger when the valve is blocked, allowing for the improvement of sealing performance.
However, with this type, as shown in a circular mark portion in FIG. 6 and in FIG. 7, with an outer circumferential shape of a seat ring 21, two circular outer circumferences with different diameters and center positions are brought into contact with the seat ring 21 and the body attaching portion, and a crossing portion 28 of the two outer circumferences is inflected, and not smooth. Therefore, the sealing pressure at the outer circumferential crossing portion 28 is smaller in the fully closed state, which leads to the possibility of leakage from the portion on the flow passage side. Moreover, at the pressure contact portion between the valve plug 22 and the seat ring 21 near the stem through-holes 26 and 27, with double protrusions of 24 of 25 and thickness changing as shown in FIG. 8, the available surface pressure (shown by an arrow) is not even. That constituted a problem of fluid leaching through the gap between the stem through-holes 26 and 27 of the seat ring 21 and the stem.