Cavitation analysis and solutions for the impeller of circulating water pumps

Thread Content

Analysis and Solutions for Cavitation in the Impeller of Circulating Water Pumps. Analysis of cavitation problems in the impellers of circulating water pumps: Cavitation is a phenomenon related to fluid mechanics; it is influenced by the fluid dynamic properties as well as the physical properties of materials, and it is specific to fluid machinery. As the water flows over the surface of the turbine’s flow components, the kinetic and potential energy of the water is converted into mechanical energy, resulting in a local decrease in pressure. If this local pressure is lower than the vaporization pressure of the liquid at that temperature, impurities present in the liquid, tiny solid particles, as well as gas trapped in the gaps at the interface between the liquid and solids can form vapor nuclei. These nuclei grow in the areas of low pressure within the liquid; when their radius R exceeds the critical radius RC, they become unstable and rapidly develop into visible bubbles. These bubbles move with the water flow, simultaneously subjected to hydrodynamic pressure and their own surface tension. When the dynamic water pressure exceeds the surface tension that maintains the bubble in a spherical shape, the bubble is forced to change its shape and split into many tiny bubbles. A large number of tiny bubbles gather into clusters and move along with the fast-flowing water; when they enter high-pressure areas, they burst instantly. The instantaneous collapse of the bubble is accompanied by two types of water hammer pressure: one is the impact pressure generated by the fluid dynamics filling that space in an instant, and the other is the concentrated pressure resulting from the free collapse of the spherical bubble. At the moment the bubble collapses, these two types of water hammer pressure together create an extremely high jet velocity, resulting in a high-speed impact on the flow boundary. According to Terlin’s calculations, the maximum pressure when cavities collapse can reach 2200 atmospheres; if this pressure acts on the surface of solid components, it will cause damage to that surface, and this is what is known as cavitation. The mechanism by which cavitation causes damage to the surface of solid components is highly complex. Experiments have shown that the water hammer effects resulting from the formation, growth, and collapse of bubbles occur at frequencies of up to 100,000 to 200,000 times per second, subjecting the flow-facing surfaces of turbines to repeated impact loads. Under repeated impact loads, metal materials suffer from fatigue failure, which is the main cause of cavitation. Secondly, bubbles release a certain amount of energy when compressed; meanwhile, the repeated impacts of water hammer pressure on the metal surface cause local temperature increases, allowing the gas contained within the bubbles to oxidize the metal surface. Furthermore, under high temperatures, bubbles cause discharge phenomena, that is, electrochemical reactions occur, thereby leading to electrolysis of the metal surface. Therefore, cavitation is the result of the combined effects of the aforementioned mechanical, chemical, and electrochemical processes. Cavitation occurs in water pumps, turbines, valves and gate valves, propellers, aero engines, and cleaning equipment. Cavitation damage can reduce the surface finish of flow-through components; in severe cases, the affected areas are eroded to form continuous, honeycomb-like holes, or even reach a spongy state ； Cavitation damage can lead to a reduction in the efficiency of equipment, or even cause damage to it, with significant consequences and impacts. Furthermore, when bubbles collapse, the instantaneous and periodic increase in pressure, as well as the collisions between water particles and the impacts on the pump casing and impeller, can cause cavitation to result in intense noise and vibration in the water pump. The vibration of the water pump can cause vibration in the unit’s foundation or base. Resonance can also occur when the frequency of cavitation vibration is close to the natural frequency of the water pump. Solutions for cavitation in circulating water pump impellers: Based on the above analysis, SD carbon nanopolymer material offers effective solutions by taking advantage of the properties of non-metallic materials. Specifically, SD carbon nanopolymer composites are high-performance composites modified with carbon elements, and they exhibit significant differences in performance compared to traditional epoxy quartz composites or ordinary polymer composites. The Sore carbon nanopolymer composite uses aviation-grade resin materials as its base ingredients, and through graphene modification, its overall performance is significantly enhanced. When applied to fluid devices such as turbines and pumps, its performance advantages are mainly reflected in the following: 1. The mirror-like surface of the material significantly reduces the flow resistance of the fluid ； This means that the bubbles generated by frictional resistance and high-speed turbulence will be significantly reduced, resulting in a lower cavitation capacity ； Secondly, the significant improvement in surface smoothness reduces the operating resistance of the pump, thereby greatly increasing its efficiency (under normal circumstances, after stainless steel pumps manufactured using the die-casting process are finished, applying a coating layer can increase their efficiency by at least 3%-8%) ； 2. The flexibility (yielding property) of the material can mitigate the impact of bubbles on the surface of the solid (metal), resulting in a decrease in cavitation resistance ； 3. The structure of the inorganic materials in the material significantly enhances its resistance to erosion ； 4. A bonding strength of over 20 MPa per square centimeter on the metal surface, which prevents the material from coming loose during operation ； 5. The lower specific gravity of the material reduces the weight borne by the rotor itself ； 6. Protection through coating ensures the long-term high-efficiency operation of the turbine (pump), reducing energy consumption ； 7. Extend the equipment’s service life, significantly reducing the frequency of maintenance and the associated costs ； 8. It reduces management complexity and the workload for workers. Case study on cavitation in the impeller of a circulating water pump: