Centrifugal Pump Fluid Dynamics

Working Principle of Centrifugal Pumps The working principle of centrifugal pumps is based on the action of centrifugal force. When the impeller rotates at high speed, the liquid is thrown from the center of the impeller to the outer edge under the influence of centrifugal force, thereby gaining kinetic energy and pressure energy. The specific working process is as follows: 1.Liquid enters the central area of the impeller through the pump's suction inlet. 2.The rotation of the impeller generates centrifugal force, causing the liquid to move from the center of the impeller to the outer edge along the blade passages. 3.The liquid gains kinetic energy and pressure energy within the impeller and is then discharged into the pump casing. 4.Inside the pump casing, part of the liquid's kinetic energy is converted into pressure energy, and the liquid is ultimately discharged through the outlet. During the operation of a centrifugal pump, the impeller does work by converting mechanical energy into the energy of the liquid. As the liquid flows through the impeller, both its pressure and velocity increase. According to Bernoulli's equation, the increase in the total energy of the liquid is primarily manifested as an increase in pressure energy, enabling the centrifugal pump to transport the liquid to a higher elevation or overcome greater system resistance. It is important to note that the prerequisite for the normal operation of a centrifugal pump is that the pump cavity must be filled with liquid. This is because centrifugal force can only act on liquids and not on gases. If air is present in the pump cavity, the pump will be unable to build up pressure normally, resulting in "vapor lock," which ultimately leads to cavitation. Analysis of Causes for Centrifugal Pump Cavitation  1.Inadequate Inlet Medium or Insufficient Inlet Pressure Inadequate inlet medium is one of the most common causes of centrifugal pump cavitation. The following situations may lead to insufficient inlet medium: a. Low Liquid Level: When the liquid level in a pool, tank, or storage container falls below the pump's suction pipe or the minimum effective level, the pump may draw in air instead of liquid, resulting in cavitation. b. Excessive Suction Lift: For non-self-priming centrifugal pumps, if the installation height exceeds the allowable suction lift, even if the suction pipe is immersed in the liquid, the pump will be unable to draw the liquid up, leading to a lack of liquid inside the pump. According to physical principles, the theoretical maximum suction lift for non-self-priming centrifugal pumps is approximately 10 meters of water column (atmospheric pressure value). However, considering various losses, the actual suction lift is typically below 6-7 meters. c. Insufficient Inlet Pressure: In applications requiring positive inlet pressure, if the provided inlet pressure is lower than the required value, the pump may experience inadequate liquid supply, causing cavitation. d. Poor System Design: In some system designs, if the suction pipeline is too long, the pipe diameter is too small, or there are too many bends, the pipeline resistance increases, reducing the inlet pressure and preventing the centrifugal pump from drawing liquid properly. Case studies show that approximately 35% of centrifugal pump failures in the petrochemical industry are caused by inadequate inlet medium or insufficient inlet pressure. This issue is particularly common in oil transportation systems due to the high viscosity and vapor pressure of oil products.    2.Blockage in the Inlet Pipeline Blockage in the inlet pipeline is another common cause of centrifugal pump cavitation. Specific manifestations include: a. Clogged Screens or Filters: During long-term operation, screens or filters in the inlet pipeline may become gradually blocked by impurities or sediments, restricting liquid flow. b. Scale Formation Inside the Pipeline: Particularly when handling hard water, water with high calcium and magnesium ion content, or specific chemical liquids, scale or crystalline deposits may form on the inner walls of the pipeline, reducing the effective diameter over time. c. Foreign Object Entry: Accidental entry of objects such as leaves, plastic bags, or aquatic plants into the suction pipeline can block elbows or valves, obstructing liquid flow. d. Partially Closed Valves: Operational errors, such as failing to fully open valves in the suction pipeline, or internal valve malfunctions, can also lead to insufficient flow. e. Foot Valve Failure: In systems equipped with foot valves, if the foot valve malfunctions (e.g., spring deformation or sealing surface damage), it can affect the pump's ability to draw liquid properly. Statistical data indicate that approximately 25% of centrifugal pump cavitation cases in municipal water supply and drainage systems are caused by inlet pipeline blockages. This issue is especially common in wastewater treatment systems with high levels of suspended solids.      3.Incomplete Air Removal from the Pump Cavity Incomplete air removal from the pump cavity is a significant cause of centrifugal pump cavitation. Key manifestations include: a. Inadequate Priming Before Initial Startup: After initial installation or prolonged shutdown, centrifugal pumps must be primed to remove air from the pump body. If priming is insufficient, residual air can prevent the pump from establishing normal working pressure. b. Insufficient Self-Priming Capability: Non-self-priming centrifugal pumps cannot expel air on their own and rely on external priming. While some self-priming pumps have a certain self-priming capability, improper startup methods or excessive self-priming height can lead to poor air expulsion. c. Air Leaks in the Pipeline System: Minor cracks in suction pipeline connections, sealing points, or aging pipes can allow air to enter the system under negative pressure. This is particularly hazardous because even if the pump is initially primed correctly, air can accumulate over time, eventually causing cavitation. d. Seal Failure: Worn or improperly installed shaft seals (e.g., mechanical seals or packing seals) can allow external air to enter the pump, especially when the suction side pressure is below atmospheric pressure. In industrial applications, approximately 20% of centrifugal pump cavitation cases are caused by incomplete air removal from the pump cavity. This issue is particularly common during initial startup after installation or maintenance.    4.Other Causes In addition to the main causes mentioned above, other factors can also lead to centrifugal pump cavitation: a. Liquid Vaporization: When handling high-temperature or highly volatile liquids, if the suction pipeline pressure falls below the liquid’s saturation vapor pressure at that temperature, the liquid may vaporize, forming bubbles. This can prevent the pump from drawing liquid or cause cavitation. b. Operational Errors: Human factors, such as incorrect valve operation or failure to follow startup procedures, can lead to pump cavitation. c. Control System Malfunctions: In automated control systems, failures in level sensors, pressure sensors, or errors in PLC programming logic may cause the pump to start or operate under inappropriate conditions, resulting in cavitation. d. Power or Motor Issues: Incorrect power phase sequence causing motor reversal can prevent the pump from drawing liquid properly. Voltage instability causing motor speed fluctuations can also disrupt normal pump operation. e. Temperature Effects: In extreme environmental conditions, such as cold regions, inadequate insulation may cause liquid in the pipeline to freeze, obstructing flow. In high-temperature environments, liquids may vaporize, forming vapor locks. Research indicates that these other causes account for approximately 20% of centrifugal pump cavitation cases. Although the proportion is relatively small, they can be significant factors in specific scenarios or conditions and should not be overlooked.