A support plate mounted on the inner side wall of an inner drum has a through hole corresponding to drain openings formed in the wall of the drum, and a gap between the support plate and the side wall of the inner drum forms a space for valve plugs of sealing valves to move up and down; a mounting sleeve installed on the upper side of the support plate has an inner through hole coaxially arranged with the through hole in the support plate, so that the valve plugs of the sealing valves are coaxially installed in the inner through hole and can move up and down. A drain opening formed in the side wall of the inner drum is provided with a sealing valve installed on the inner drum via the sealing valve structure.