Airtightness maintaining structure for butterfly valve

An airtightness maintaining structure for a butterfly valve comprising:

• a partition that includes an opening through which fluid passes;
• a valve element including a valve body and a shaft which is rotatable, the valve element being attached to the shaft so that rotation of the shaft results in rotation of the valve element, the valve element being rotatable by virtue of rotation of the shaft between a closed valve state in which the valve element is positioned to close the opening in the partition and prevent fluid flow through the opening and an open valve state in which the valve element is positioned to open the opening in the partition and permit fluid flow through the opening;
• a rotation supporting portion that includes a supporting member axially overlapping a portion of the shaft so that the supporting member pivotably supports the shaft;
• a shaft seal movable in axial and radial directions between the partition and the shaft, the shaft seal including an annular portion and contacting, in a fitted state, both:
• (i) an end surface of a member radially outwardly of the rotating shaft;
• and (ii) a surface of a member located on an inner side of either the rotation shaft or the pivotably supporting member;
• and
• in the closed valve state of the valve element, the shaft seal moves in the radial direction of the rotation shaft due to a difference of pressures between a high fluid pressure acting on one side of the valve element and a low fluid pressure acting on an opposite side of the valve element so that the shaft seal contacts both the valve element and the rotation supporting portion on the high fluid pressure side.