What Does Cavitation Mean?
Cavitation is the formation of vapor phase cavities, or bubbles, within a liquid, usually due to rapid changes in localized pressure. Various mechanical parts such as pumps, control valves and propellers can cause cavitation to occur.
There are two types of cavitation:
- Inertial cavitation refers to the formation of vapor cavities, which proceed to rapidly collapse, causing a shockwave.
- Non-inertial cavitation is the oscillation of the size and shape of the vapor cavities.
The most critical aspect of cavitation are the shockwaves that form in inertial cavitation. These shockwaves can cause various types of damage such as cavitation damage, wear and corrosion. To avoid the damaging aspects of cavitation, marine mechanical parts and liquid flow components must be engineered to minimize the formation of cavitation.
Corrosionpedia Explains Cavitation
Cavitation occurs when a pressure drop occurs within a region of a fluid to a point below the vapor pressure of the fluid at the current temperature. At this point, the state change from liquid to gas occurs, creating a bubble. The pressure drop can be achieved in numerous ways including the acceleration of the liquid, as in a waterfall or propeller, or from subjecting the liquid to an energy input, such as laser pulses, electrical discharges or acoustic fields.
After the cavities are created, the low-pressure bubbles cannot sustain their form because they are surrounded by a higher pressure fluid, and they subsequently collapse. The collapse of the bubble causes the gas inside to pressurize and heat up, and a shockwave forms with high pressure and temperature. Noise and light may also form. As mechanical parts may cause cavitation, these shockwaves can occur close to the part’s surface, causing damage and pitting. The pitted surface causes more fluid turbulence and in turn causes more cavitation. Thus, the cavitation continuously damages the surface and facilitates the corrosion of the metal.
Given the dangers of cavitation, engineers have created methods to reduce its occurrence and extend the lifetime of mechanical parts. One option is to redesign the highest risk components based on huge pressure drops and high temperatures. In some cases the difference between the static pressure and the vaporization pressure can be increased, which decreases the chance for the liquid to reach the vaporization pressure and form low-pressure bubbles. Finally, placing mechanical components in colder areas of the system or lowering the temperature of the system lowers the likelihood of cavitation because the vapor pressure is temperature dependent.