Butterfly Valve Seat Construction:

Stoppage of flow is accomplished through the valve disk sealing against a seat which is on the inner diameter periphery of the valve body. Several butterfly valves have an elastomeric seat against which the disk seals. The other butterfly valves have a seal ring arrangement that uses a clamp-ring and backing-ring on a serrated edged rubber ring. This design avoids extrusion of the O-rings.

In previously designs, a metal disk was used to seal against a metal seat. This arrangement did not give a leak-tight closure, other than did provide sufficient closure in some applications (that is water distribution lines).