A damper assembly comprising a casing having an inner surface defining a flow channel, a closure member mounted for pivotal movement about a pivot axis in and out of the flow channel between a closed position in which it closes the flow channel and an open position, and a sealing arrangement for sealing the flow channel in the closed position of the closure member, wherein the sealing arrangement comprises a first ring member of metal provided at the inner surface of the casing so as to prevent flow of fluid between the first ring member and the inner surface of the casing, a first surrounding abutment ring of metal provided at the first ring member of metal, and a second surrounding abutment ring of metal provided at the closure member so as to prevent flow of fluid between the second surrounding abutment ring and the closure member, wherein the first surrounding abutment ring and the second surrounding abutment ring are configured to abut each other in the closed position of the closure member, the damper assembly being characterized in that the sealing arrangement further comprises a second ring member of metal provided at the second surrounding abutment ring, wherein the second ring member and the first surrounding abutment ring are configured to abut each other in the closed position of the closure member so form a surrounding gas space that is defined by the first surrounding abutment ring, the second surrounding abutment ring and the second ring member, and gas feeding means configured to feed pressurized gas into the surrounding gas space, wherein the closure member is pivotably attached to the first ring member by means of at least one arm member, wherein the closure member is pivotably attached at the inner surface of the casing by means of a linkage arrangement provided between the closure member and the inner surface of the casing, wherein the damper assembly further comprises a drive means functionally connected to the linkage arrangement to effect pivotal movement of the closure between the closed position and the open position, and wherein the flow channel is essentially unobstructed by the closure member, the second ring member and the second surrounding abutment ring in the open position of the closure member. a casing having an inner surface defining a flow channel, a closure member mounted for pivotal movement about a pivot axis in and out of the flow channel between a closed position in which it closes the flow channel and an open position, and a sealing arrangement for sealing the flow channel in the closed position of the closure member, wherein the sealing arrangement comprises a first ring member of metal provided at the inner surface of the casing so as to prevent flow of fluid between the first ring member and the inner surface of the casing, a first surrounding abutment ring of metal provided at the first ring member of metal, and a second surrounding abutment ring of metal provided at the closure member so as to prevent flow of fluid between the second surrounding abutment ring and the closure member, wherein the first surrounding abutment ring and the second surrounding abutment ring are configured to abut each other in the closed position of the closure member, the damper assembly being characterized in that the sealing arrangement further comprises a second ring member of metal provided at the second surrounding abutment ring, wherein the second ring member and the first surrounding abutment ring are configured to abut each other in the closed position of the closure member so form a surrounding gas space that is defined by the first surrounding abutment ring, the second surrounding abutment ring and the second ring member, and gas feeding means configured to feed pressurized gas into the surrounding gas space, wherein the closure member is pivotably attached to the first ring member by means of at least one arm member, wherein the closure member is pivotably attached at the inner surface of the casing by means of a linkage arrangement provided between the closure member and the inner surface of the casing, wherein the damper assembly further comprises a drive means functionally connected to the linkage arrangement to effect pivotal movement of the closure between the closed position and the open position, and wherein the flow channel is essentially unobstructed by the closure member, the second ring member and the second surrounding abutment ring in the open position of the closure member.