When the impeller is rotated in the casing with a due amount of sealing water added in the casing, the sealing water, which is greater in the specific gravity, is pressed against the inner wall of the casing by the centrifugal force, making a concentric ring as shown in the diagram. Utilizing the changing volume of the space surrounded by the inner wall of the sealing water ring and the impeller blades as the impeller turns, the pump performs actions of intake, compression, and delivery continuously, with the gas coming and going through the suction and exhaust ports provided in the side wall or inner wall of the impeller. Rotation of higher speed can attain more efficient exhaust. In addition, the sealing water functions as the seal so that sucked gas will be positively exhausted.
Owing to the sealing water, the pump can exhaust even flammable gases and corrosive gases. The water-sealed structure is very suitable for exhausting gases containing water vapor (or steam) and/or water droplets. As the pump is designed in two stages, it exerts high performance at a pressure of 3 ×104Pa or lower.