Cavitation and gas entrapment in centrifugal pumps

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Cavitation in centrifugal pumps: When the impeller of a centrifugal pump rotates at high speed, it generates a strong centrifugal force. Under the influence of this force, a vacuum lower than atmospheric pressure is created at the pump’s inlet. When the inlet pressure reaches the vaporization pressure of the liquid at that temperature, the liquid begins to vaporize and form bubbles. In this way, the bubbles formed in the moving liquid flow along with the liquid. When the bubble reaches a region where the static pressure exceeds the saturated vapor pressure, it collapses rapidly. The surrounding liquid moves at high speed toward the center of the bubble, resulting in a high-frequency water hammer effect that strikes the surface of the impeller, thereby generating noise and vibration. The repeated formation and collapse of such bubbles cause damage to the impeller surface in that area, resulting in a decrease in pump flow rate, a reduction in head, and a lower efficiency. This phenomenon is known as cavitation. Air locking in centrifugal pumps: Due to the presence of gas inside the pump, when the impeller rotates at high speed, the low density of the gas means that its centrifugal force is not sufficient to create enough vacuum, preventing the liquid from being drawn up. From the perspective of the different causes of cavitation and air locking: air locking occurs when there is air inside the pump, usually during startup, and is mainly manifested by the failure to remove all the air from within the pump ； Cavitation occurs when a liquid reaches its vaporization pressure at a certain temperature; it is therefore closely related to the medium being transported and the operating conditions. The main causes of cavitation are: 1. Excessive resistance in the inlet pipeline or pipes that are too narrow; 2. Excessively high temperature of the medium being transported ； 3. Excessive flow rate, that is, the outlet valve is opened too wide ； 4. The installation height is too high, affecting the pump’s suction capacity ； 5. Selection issues, including the selection of the pump and the material used for the pump. Solutions: 1. Remove any debris from the inlet pipeline to ensure unobstructed flow, or increase the diameter of the pipeline ； 2. Lower the temperature of the conveying medium ； 4. Reduce installation height ； 5. Rechoose the pump, or improve certain components of it, such as using materials resistant to cavitation. The problem can be resolved by filling the pump chamber with liquid or by adding a buffer tank at the inlet