What is Cavitation and How it Occurs?

Cavitation is a phenomenon of formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid becomes equal or less than the vapor pressure. When these vapor bubbles reaches into the region of higher pressure, they collapse and creates high impact pressure. These high impact pressure created by the vapor bubbles eroded the materials from metallic surface and produces cavity.

What is Cavitation and How it Occurs?

For better explanation of the above cavitation phenomenon one must have knowledge of vaporization and vapor pressure.

What is Vaporization?

The change of liquid phase into gaseous phase is called vaporization. The vaporization depends upon the prevailing temperature and pressure condition. It occurs due to the continuous escaping of the liquid molecules from free surface of the liquid.

What is Vapor Pressure?

In order to understand vapor pressure, let’s take a closed vessel in which a liquid (say water) is present. Let the temperature of the water is 20 oC and pressure is atmospheric. In this situation the vaporization of water takes place at 100 oC. As the vaporization of water starts, the molecules of liquid (vapor) start escaping out from the free surface of the liquid. The vapor molecules escaping out from the liquid free surface gets collected between the free surface of the liquid and top of the vessel. These vapor molecules exerts pressure on the free surface of liquid. This pressure exerted by the vapor is called vapor pressure. Vapor pressure is also defined as the pressure at which the liquid changes into vapor

Now, again take a closed vessel filled with a liquid (say water). Let the temperature of the water is 20 oC and the pressure in the vessel is reduced by some external source. When the pressure is reduced, the vaporization temperature is also reduced. Let the pressure inside the vessel is reduced to such an extent that it becomes equal or less than the vapor pressure. In this situation the boiling of water takes place even though the temperature of liquid is 20 oC. Thus the boiling of water takes place even at ordinary temperature; if the pressure is reduced as such it becomes equal to or less than the vapor pressure.

Explanation of Cavitation

Considered a system in which a liquid (say water) is flowing. When the flowing water enters into a region where the pressure becomes equal to or less than that of the vapor pressure. Than the vaporization of water starts and vapor bubbles are formed in the water. When these vapor bubbles are carried by the flowing liquid in the region of higher pressure they explode. The explosion of vapor bubbles creates impact pressure of high intensity. Since the liquid flows over the metallic surface, the high pressure produced by explosion of vapor bubbles erodes the material from the metallic surface and creates cavity. This phenomenon is called cavitation. The bursting of the vapor bubbles creates noise and vibration.

For better explanation watch the video given below:


Cause

Cavitation occurs when pressure of flowing liquid in any region becomes equal to or less than the vapor pressure.

Effect

  • It results in damage to the metallic surfaces and creates cavity.
  • Creates noise and vibration due to sudden collapsing of vapor bubbles.
  • It reduces the efficiency of hydraulic machines like turbine and pumps.

Precaution

  • The pressure of the liquid in any part of the hydraulic system should not be reduced below its vapor pressure.
  • The special materials or coating such as stainless steel and aluminium bronze, which are cavitation resistance should be used.
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