Butterfly valve

A butterfly valve having a body pierced by a passage designed to come into position opposite to two adjacent portions of a piping system, a valve closure disc mounted for rotation within the valve passage about an axis which is transverse to said passage and displaced with respect to its axis of rotation, the valve disc having at its pheriphery an annular edge portion having a convex profile associated with an annular seal which is mounted so as to project within the valve passage, said seal comprising a removable metallic ring coated at least partially with plastic material, the ring having a substantially cylindrical plastic-coated metal mouthpiece mounted within one end portion of the valve passage, the internal surface of said mouthpiece being such as to have a profile which is flared in the direction of the axis of the valve disc whose peripheral edge portion is capable of bearing against said internal surface in fluid-tight manner, retaining means for maintaining the ring within said valve passage in a predetermined position with respect to said valve disc, said ring having an L-section profile metal radial flange integral with the mouthpiece and having a larger diameter than said mouthpiece, the two faces of said flange being capable of bearing in fluid-tight manner against the valve body at the end of the valve passage and against the flange of an adjacent pipe section, the metal mouthpiece of the metallic ring which is mounted within the valve passage being slotted to provide radial elasticity of said mouthpiece, an outer cylindrical surface of said metal mouthpiece being plastic coated and in fluid-tight contact with said one end portion of the valve passage.