Cavitation in centrifugal pumps and solutions

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Cavitation in centrifugal pumps and solutions 1. Cavitation phenomenon: When the liquid pressure near the inlet of the impeller blades is less than or equal to the saturated vapor pressure at the temperature at which the liquid is being transported, the liquid vaporizes; simultaneously, gases dissolved in the liquid may escape, forming numerous bubbles. These bubbles move with the liquid to areas within the blade channel where the pressure is higher, where they then instantly condense and collapse. At the moment when the bubble coalesces and collapses, the liquid surrounding the bubble rushes into the cavity formed by its disappearance, creating a strong localized high-frequency high-pressure water hammer effect, which causes erosion of the metal surface due to fatigue. At the same time, the presence of reactive gases (such as oxygen) and the local high temperatures generated during bubble condensation lead to electrochemical corrosion of the metal surface. The aforementioned process is known as cavitation. 2. Factors affecting cavitation: Factors that affect the liquid pressure and saturated vapor pressure can all influence the occurrence of cavitation. Factors affecting it: (1) Structural parameters of the pump inlet: including the shape of the impeller inlet, the width of the blade inlet edge, the position of the blade inlet edge, and the shape of the front cover, etc. (2) Operating conditions of the pump: These include the pump’s flow rate, head, and speed. (3) Pump installation location: It includes the hydraulic loss in the pump’s suction pipeline and the installation height. (4) Environmental factors: It includes the atmospheric pressure at the location where the pump is installed. (5) Factors affecting it include the properties of the medium itself and the operating temperature of the medium. 3. Several solutions to the cavitation problem in centrifugal pumps: (1) Improving the structural parameters of the pump inlet. This solution is suitable for the design and manufacturing stage of centrifugal pumps, and it is rarely used in actual production settings. (2) Installing an inducer at the pump’s suction inlet is very effective in improving the cavitation resistance of centrifugal pumps and in resolving cavitation problems. Moreover, it has a simple structure that makes it easy to manufacture and install, is convenient for operation and maintenance, and has a low cost. It can be installed and calibrated without affecting production, making it particularly suitable for widespread use in production sites. (3) Reasonably designing the suction pipeline and adjusting the installation height. Although this method can completely eliminate cavitation problems, it is rarely used in production sites. This is because adjusting the pump’s suction pipeline and installation height involves significant work and high costs, and it is also restricted by the construction environment; such adjustments can only be carried out when the unit is shut down or during major maintenance ； At the same time, due to limitations in the process conditions, adjusting the pump’s suction pipeline and installation height will affect subsequent processes, resulting in a chain reaction. (4) Optimize process operating conditions: Where the process conditions permit, changing operating parameters such as the pump’s flow rate, head, speed, and the operating temperature of the medium can help prevent cavitation from occurring. However, due to limitations in process conditions, optimizing the process operating conditions has significant constraints, and in most cases the effects are not substantial. Therefore, this method can be used as an auxiliary approach to solving cavitation problems. 4. Determination of the solution: Through the analysis and comparison of the above options, in order to resolve the cavitation problem of the pump without affecting normal production, the preferred approach is to install an inducer at its inlet. 5. Design of the impeller: When liquid flows through the impeller, it does work on the liquid, which is equivalent to increasing the pressure of the liquid that enters the subsequent impeller, thereby raising the pressure. Although this method increases the load on the motor, since the power of motors is generally high (usually 20–30% higher than the shaft power of centrifugal pumps), and the size of the impeller is influenced by the diameter of the inlet pipe, its pressure increase range is limited. Under normal circumstances, the motor still meets the requirements, so there is no need to replace it. Since our understanding of the inducer wheel is still in its exploratory stage, there is no unified view on some theoretical issues. Therefore, the design of the inducer wheel is largely based on experience, in conjunction with the actual structure of the pump.