Abstract: A disc valve having a valve housing with a conduit therethrough and a seal ring mounted to the housing. A disc can be pivoted about a pivot axis that is offset from the main valve axis and perpendicular thereto between an open position and a closed position in which a seating surface of the disc contacts a sealing surface of the seal ring in a sealing plane that is perpendicular to the valve axis. The seating surface of the disc is a conically shaped surface that is concentric with the valve axis when the disc is in the closed position. At least a portion of the sealing surface of the seal ring at the intersection between the sealing surface and the sealing plane has the same conical shape. The seal ring can be deflected by moving the disc beyond the closed position to thereby increase the sealing pressure between them.