A BUTTERFLY VALVES assembly for installation in a pipe line between flanges having spaced parallel planar annular faces extending transversely of and centered upon the lengthwise axis of said pipe line comprising:Butterfly Valves a tubular valve body made of rigid material and having parallel planar annular end walls centered upon said axis of the pipe line in which said assembly is to be installed adapted to make face-to-face contact with said faces of said flanges, said valve body having an inner circumferential valve-seat-supporting surface extending axially of said pipe line, said valve body having a pair of oppositely disposed annular ribs each of said ribs extending axially outwardly from said valve-seat-supporting surface and each of said ribs terminating, respectively, axially inwardly of the plane of one of said end walls at the high point of a convexly-curved rib surface, a valve seat insert made of elastomeric material having a tubular seat portion the exterior circumferential surface of which is shaped to engage said valve-seat-supporting surface and the interior circumferential surface of which is shaped to define a tubular fluid passageway extending axially through said valve assembly when said seat insert is fitted upon said valve body, said insert having radially outwardly extending from each axial end thereof a flange sealing portion with a concave recess shaped to fit upon the convexly-curved rib surface of one of said ribs, a first dimension constituted by the axial distance, Butterfly Valvesin said insert as molded and before it is fitted upon said valve body, between the point within the concave recess in said flange sealing portion at one axial end of said seat portion which is to fit upon the high point of one of said rib surfaces and the corresponding point in the concave recess in the flange sealing portion at the other end of said seat portion being smaller by a predetermined amount than a second dimension constituted by the axial distance between the high point of the convexly-curved rib surface of one of said pair of ribs and the high point of the convexly-curved rib surface of the other of said pair of ribs, whereby in order to fit said valve seat insert upon said valve body to prepare said butterfly valve assembly for installation in a pipe line it is necessary to stretch said tubular seat portion axially to such an extent that the concave recess of the flange sealing portion at each end of said seat portion may be forced over one of said ribs, the recovery forces exerted by said stretched seat portion thereafter being continuously effective to hold said seat portion firmly engaged with said valve-seat-supporting surface of said valve body.

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