In BUTTERFLY VALVES, it is generally difficult to obtain a fluid-tight seal when the valve is in the closed position. The present invention satisfactorily overcomes the sealing problem by providing a unique valve structure in which an effective fluid seal is obtained between the valving member and the structure which it engages for sealing purposes, without the need of a relatively large force acting on the valving member. The present invention utilizes a one-piece pre-formed liner between the valving member and the body of a Butterfly Valves, with a resilient back-up member or expander disposed between the liner and the body. The back-up member exerts a yielding force against the liner to enforce formation of a fluid seal between the liner and the valving member when the valve is in the closed position.

Since the valving member of a Butterfly Valves is pivotally mounted in the body or housing for movement between open and closed positions, fluid may tend to leak past the pivotal mounting means. Furthermore, in order to actuate the valving member between open and closed positions, it is necessary that a stem of the valving member extend exteriorly of the valve body, with the risk of permitting fluid to escape along the stem from the passage or chamber of the valve body or housing.

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