Cavitation phenomenon is generally ignored and not understood by people. The cavitation problem in the actual production is often the main cause of centrifugal pump impeller damage. Generally understood as impeller decay and aging. But how this happens is unclear. So it is necessary to understand how cavitation is caused and how to avoid it. Let’s talk about cavitation problems and preventive measures.
1. What’s the cavitation?
When there is air in the housing of the centrifugal pump, the density of air is much smaller than that of the liquid, resulting in a small centrifugal force. Thus, the pressure difference between the liquid level above the tank and the suction inlet of the pump is not enough to press the liquid in the tank into the pump, that is, the centrifugal pump has no self-priming ability, so that the centrifugal pump can not transport liquid, this phenomenon is called “cavitation phenomenon”. When the pressure of a liquid at a certain temperature is reduced to the pressure of vaporization at that temperature, the liquid produces a vapor bubble. This phenomenon of bubble formation is called cavitation. The bubbles produced by cavitation, when flowing to high pressure, their volume decreases so that they burst. This phenomenon of the bubble disappearing into the liquid as the pressure rises is called cavitation collapse.
Pump in operation, if the flow of local area (usually the impeller inlet somewhere later) for some reason, reduced to the absolute pressure of pumping liquid temperature of liquid pressure, liquid at the begin to boil, to produce a large amount of steam, bubble formation, when the liquid containing a large number of bubbles within the impeller forward high, The high-pressure liquid surrounding the bubble causes it to shrink so rapidly that it bursts. At the same time of bubble condensation and rupture, the liquid particle fills the hole at a very high speed, which produces a very strong water shock at this moment, and strikes the metal surface with a very high impact frequency. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second. When serious, the wall thickness will be broken down. The cavitation process in the water pump is the process of bubble formation and bubble rupture causing the flow parts to suffer damage. After the pump cavitation in addition to the flow components will produce damage, but also will produce noise and vibration, and lead to the performance of the pump decline, serious will make the pump in the liquid interrupted, can not work normally. In order to make the pump filled with liquid, usually at the bottom of the suction pipe installed at the bottom of the filter valve, the bottom valve until the reverse valve, the filter is to prevent solid substances into the pump damaged impeller or prevent the normal operation of the pump.
2. Why is the centrifugal pump the most prone to cavitation?
a. At the front cover plate of the impeller with the largest curvature, the low-pressure side near the blade inlet edge;
b. The low pressure side near the inlet edge of volute tongue and guide vane in the pressure outlet chamber;
c. The sealing gap between the outer circle of the blade tip of the high specific revolution impeller without the front cover plate and the shell, and the low pressure side of the blade tip;
d. The first stage impeller in the multistage pump.
3. How to solve the centrifugal pump cavitation phenomenon?
1. Measures to improve the cavitation resistance of the centrifugal pump itself
(1) Improve the centrifugal pump inlet to the impeller near the structure design. Increase the flow area; The radius of curvature of the inlet section of the impeller cover plate is increased to reduce the rapid acceleration and pressure reduction of liquid flow. Reduce the thickness of the blade inlet appropriately, and round the blade inlet, so that it is close to the flow line shape, can also reduce the acceleration and pressure reduction around the flow blade head; Improve the surface finish of impeller and blade inlet to reduce the drag loss; The blade inlet edge is extended to the impeller inlet to make the liquid flow accept work in advance and improve the pressure.
(2) The pre-induction wheel is used to make the liquid flow work in advance in the pre-induction wheel to improve the liquid flow pressure.
(3) the use of double suction impeller, so that the liquid flow from both sides of the impeller into the impeller at the same time, the inlet section is doubled, the inlet flow rate can be reduced by twice.
(4) A slightly larger positive Angle of attack is adopted in the design condition to increase the blade inlet Angle, reduce the bending at the blade inlet, and reduce the blade obstruction, so as to increase the inlet area; Improve working conditions at high flow to reduce flow losses. But the positive Angle of attack should not be too large, otherwise the efficiency will be affected.
(5) The use of cavitation resistant materials. The practice shows that the higher the strength, hardness and toughness of the material, the better the chemical stability and the stronger the resistance to cavitation.
2. Measures to improve the effective cavitation margin of the liquid intake device
(1) Increase the liquid level pressure in the liquid storage tank before the centrifugal pump to improve the effective cavitation allowance.
(2) reduce the installation height of the suction device pump.
(3) Change the upper suction device to the backdown device.
Reduce the flow loss in the pipeline in front of the centrifugal pump. For example, shorten the pipeline as far as possible in the required range, reduce the flow rate in the pipeline, reduce the elbow and valve, and increase the valve opening as far as possible. The above measures can be comprehensively analyzed according to the centrifugal pump type, material selection and pump use site conditions, and applied appropriately.
4. Conclusion
Cavitation is a phenomenon that occurs when air is present in the housing of a centrifugal pump, resulting in a lack of sufficient pressure to draw liquid into the pump. This inability to transport liquid is known as cavitation. When the pressure of a liquid is reduced to its vaporization pressure, vapor bubbles form, creating cavitation. These bubbles then collapse when they encounter high-pressure areas, causing damage to the pump’s flow components. Cavitation can lead to noise, vibration, decreased pump performance, and even the interruption of normal pump operation.
Centrifugal pumps are particularly prone to cavitation due to several factors. The impeller’s front cover plate, with its largest curvature and low-pressure side near the blade inlet edge, is a common location for cavitation. The volute tongue and guide vane in the pressure outlet chamber also experience cavitation near their inlet edge on the low-pressure side. Additionally, the sealing gap between the outer circle of the blade tip in high specific revolution impellers without a front cover plate and the pump’s shell is susceptible to cavitation. In multistage pumps, the first stage impeller is particularly prone to cavitation.
Understanding the causes and effects of cavitation is crucial for preventing and mitigating its damaging effects. Installing filter valves and reverse valves at the bottom of the suction pipe can help prevent solid substances from entering the pump and damaging the impeller or disrupting pump operation. By addressing these factors and implementing preventive measures, the occurrence of cavitation in centrifugal pumps can be minimized.
To solve the phenomenon of centrifugal pump cavitation, various measures can be implemented. Firstly, improving the cavitation resistance of the pump itself is crucial. This can be achieved by optimizing the pump’s inlet and impeller structure design, increasing the flow area, and adjusting the impeller cover plate’s radius of curvature. Additionally, using a pre-induction wheel can enhance the liquid flow pressure. The utilization of a double suction impeller allows for simultaneous entry of liquid from both sides, reducing the inlet flow rate. Adopting a slightly larger positive angle of attack in the design condition increases the blade inlet angle and improves the inlet area. It is important to use cavitation-resistant materials with high strength, hardness, toughness, and chemical stability.
Furthermore, measures can be taken to improve the effective cavitation margin of the liquid intake device. This includes increasing the liquid level pressure in the storage tank, reducing the installation height of the suction device, and changing the upper suction device to the backdown device. Reducing flow loss in the pipeline in front of the centrifugal pump is also crucial, achieved by shortening the pipeline, decreasing flow rate, minimizing the use of elbows and valves, and maximizing valve opening.
These measures should be comprehensively analyzed and applied according to the specific centrifugal pump type, material selection, and pump usage site conditions. By implementing these measures effectively, the issue of centrifugal pump cavitation can be resolved.
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