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Cavitation is a fascinating physical phenomenon that occurs when a liquid is subjected to rapid changes in pressure, leading to the formation of small vapor-filled cavities in regions where the pressure is relatively low. When these cavities, known as "bubbles" or "voids," collapse or implode, they can generate shock waves that are powerful enough to cause significant damage to materials such as metal surfaces of propellers, pumps, and pipes. This process is not just a concern for engineers; it's also a natural occurrence in the animal kingdom. For instance, the snapping shrimp exploits cavitation to stun prey with a bubble that collapses with a loud snap!
Understanding cavitation is crucial in various industries to prevent equipment degradation and improve efficiency. According to a study published in "Wear" (Volume 390–391, 2017), cavitation causes an estimated annual cost of $6 billion in damages in industrial equipment. To mitigate these effects, engineers design machinery to operate outside of cavitation-prone conditions and use materials resistant to cavitation-induced wear. Beyond its destructive power, cavitation has beneficial applications too, such as in ultrasonic cleaning devices and in medical treatments like lithotripsy, where it helps to break down kidney stones without invasive surgery.
Cavitation occurs in liquid when bubbles form and implode in pump systems or around propellers. Pumps put liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to vaporize, just like boiling water. In a small, sensitive system, however, the bubbles can't escape so they implode, causing physical damage to parts of the pump or propeller.
A combination of temperature and pressure constraints will result in cavitation in any system. No manufacturer or industrial technician wants to run pumps that keep getting affected by this problem, however, as it will permanently damage the chambers of the device. The vaporization actually causes a loud, rocky noise because the bubbles are imploding and making the liquid move faster than the speed of sound.
Inside every pump, there is a propeller that draws liquid from one side of the chamber to the other. The liquid normally continues out through a valve so it can do another job in a different part of the machine. Sometimes, this device is called an impeller. Even though the total chamber stays under the same pressure, and the materials are temperature regulated, cavitation manages to occur right next to the surface of the propeller.
A propeller rotates through a liquid and actually creates localized differences in pressure along the blades. This can even occur underwater on a submarine or ship's propeller. Bubbles appear in low-pressure areas but then immediately want to implode with such force that they make dings and pits in metal. A propeller exposed to these bubbles resembles the surface of the moon, with tiny, scattered craters.
There are two types of this phenomena that can occur in the different stages of pumping, but both are results of the same phenomenon. Suction or classical cavitation occurs around the impeller as it is drawing liquid through the chamber. The propeller's motion creates the changes in pressure necessary for vaporization.
Discharge or recirculation cavitation is the result of changing pressure at the point of exit, the discharge valve. The valve is not able to let all the liquid through as fast as it should, so the currents' different velocities create miniature changes in the uniform pressure. Even such small variations are enough to create the ideal circumstances for this problem.