Cavitation
Thread Content
When the liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, the liquid will generate bubbles. This phenomenon of bubble generation is called cavitation. When the bubbles generated during cavitation flow to high pressure, their volume decreases and they burst. This phenomenon of bubbles disappearing into the liquid due to rising pressure is called cavitation collapse. When the pump is running, if for some reason the absolute pressure of the pumped liquid is reduced to the vaporization pressure of the liquid at the current temperature in the local area of the overflow part (usually somewhere behind the impeller blade inlet), the liquid will begin to vaporize there, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes forward through the high-pressure area in the impeller, the high-pressure liquid around the bubbles will cause the bubbles to shrink sharply and even burst. At the same time as the bubbles condense and collapse, the liquid particles fill the holes at a very high speed. At this moment, a very strong water hammer effect is produced, and the metal surface is hit at 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. In severe cases, the wall thickness will be broken down. The process of generating bubbles and bursting bubbles in the water pump, causing damage to the flow-passing components, is the cavitation process in the water pump. After cavitation occurs in a water pump, in addition to damaging the flow-passing components, it will also produce noise and vibration, which will lead to a decrease in pump performance. In severe cases, the liquid in the pump will be interrupted and the pump will not work properly.
The original poster is right, it is an important issue when operating the pump.
Yes, we have a tower bottom pump that has a similar phenomenon. It is very difficult to deal with. Please pay attention!
It’s very detailed. Do we still need to discuss it?
Agree with the fourth floor statement: Basic knowledge of chemical engineering
The cavitation phenomenon and air binding of centrifugal pumps first look at the difference in definitions: When the centrifugal pump works, a vacuum is generated in the center area of the impeller to form a low pressure and suck the liquid up. If the low pressure formed is very low, the suction capacity of the centrifugal pump is stronger, which is reflected in the higher suction height. However, if the pressure in the vacuum zone is too low, lower than the saturated vapor pressure of the body, the sucked liquid will vaporize in large quantities in the vacuum zone to produce bubbles. Liquid containing bubbles condenses or bursts sharply after being squeezed into a high-pressure area. Due to the disappearance of the bubbles, a partial vacuum is generated, and the surrounding liquid flows to the center of the bubbles at a very high speed, instantly generating a huge local impact force, causing impact on the impeller and pump casing, causing damage to the material. The process in which the impeller material is damaged due to the formation and collapse of bubbles in the pump is called cavitation. When a centrifugal pump is started, if there is air in the pump, the air density is very small and the centrifugal force generated after rotation is small. Therefore, the low pressure formed in the center area of the impeller is not enough to suck in the liquid. In this way, the centrifugal pump cannot complete the transportation task even if it is started. This phenomenon is called air binding. This means that the centrifugal pump has no self-priming ability, so the pump must be filled with the liquid to be transported before the centrifugal pump is started. Of course, if the suction port of the centrifugal pump is placed below the surface of the liquid being transported, the liquid will automatically flow into the pump. This is a special situation. The suction line of the centrifugal pump is equipped with a bottom valve to prevent the liquid poured in before starting from flowing out of the pump. The filter screen can prevent the solids in the liquid from being sucked in and blocking the pipe and pump casing. The regulating valve installed in the discharge line is used to start, stop the pump and adjust the flow rate. From the different causes of cavitation and gas binding: Air binding means there is air in the pump body. It usually occurs when the pump is started. It is mainly manifested in that the air in the pump body is not drained out. ； Cavitation is caused by the liquid reaching its vaporization pressure at a certain temperature, which can be seen to be closely related to the transport medium and working conditions. The main causes of cavitation are: 1. The inlet pipeline resistance is too large or the pipeline is too thin ； 2. The temperature of the conveying medium is too high ； 3. The flow rate is too large, which means the outlet valve is opened too large. ； 4. The installation height is too high, which affects the suction volume of the pump. ； 5. Selection issues include pump selection, pump material selection, etc. Solution: 1. Clean the foreign matter in the inlet pipe to make the inlet unobstructed, or increase the diameter of the pipe ； 2. Reduce the temperature of the conveyed medium ； 3. Reduce traffic ； 4. Lower installation height ； 5. Re-select the pump, or improve some parts of the pump, such as using cavitation-resistant materials, etc. Air binding is caused by the presence of air in the pump body, the pump body is not filled with liquid, or there is too much gas entrained in the liquid. This can be solved by filling the pump body with liquid or adding a buffer tank at the inlet.
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