A valve including: a flow channel and a closure member. A first sealing surface is provided along walls of the flow channel and a second sealing surface is provided along outer edges of a sealing section of the closure member. A rotation axis is offset from the sealing section by a first predetermined distance and from a centerline of the closure member by a second predetermined distance. To ensure smooth operation of the closure member at least one of the first and second sealing surfaces has a non-conical shape that is at least in one area asymmetric with respect to the center axis of the closure member.