Multistage pump is a centrifugal pump, but also rely on the rotation of the impeller to obtain centrifugal force, thus the material. Until the density of the gas reaches the working range of the mechanical vacuum pump and is withdrawn to gradually obtain a high vacuum. In the multi-stage pump body with the right amount of water as the working fluid. When the impeller rotates clockwise, the water is thrown around by the impeller. Due to the centrifugal force, the water forms a closed circle of approximately equal thickness depending on the shape of the pump chamber. The inner surface of the lower part of the water ring is just tangential to the hub of the impeller. The upper inner surface of the water ring is just in contact with the top of the blade (in fact, the blade has a certain insertion depth in the water ring). At this moment, a crescent-shaped space is formed between the impeller hub and the water ring, and the space is divided into a plurality of small cavities equal to the number of the blades by the impeller. If the lower part of the impeller 0 ° as a starting point, then the impeller 180 ° before the rotation of the small chamber volume from small to large, and the end of the suction port communicating, when the gas is inhaled, when the inhalation end of the small cavity When the impeller continues to rotate, the small cavity becomes smaller and the gas is compressed; when the small chamber communicates with the exhaust port, the gas is discharged outside the pump. In summary, multi-stage pump is based on changes in the volume of the pump to achieve suction, compression and exhaust, so it is a variable volume centrifugal pump. Multistage pumps can be used in many industrial sectors such as petroleum, chemical, machinery, mining, light industry, medicine and foodstuffs. In many industrial processes, such as vacuum filtration, vacuum water diversion, vacuum feed, vacuum evaporation, vacuum concentration, vacuum resurgence and vacuum degassing, multistage pumps are widely used. Can be evacuated flammable, explosive gas, in addition to pumping dust, water-containing gas, therefore, water ring pump applications are increasing.