It means that cavities are forming in the liquid that we are pumping. When these cavities form at the suction of the pump several things happen all at once.

Cavities result due to five basic reasons and it is common practice to lump all of them into the general classification of cavitation. Learn to correct each of these conditions & understand why they occur and how to fix them. Here they are in no particular order:

Vaporization

A fluid vaporizes when its pressure gets too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization that is supplied to us by the pump manufacturer.

Since there are losses in the piping leading from the source to the suction of the pump the head must be determined after these losses are calculated. In other words, this is the Net Positive Suction Head (NPSH) required to prevent the fluid from vaporizing.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature or decrease the NPSH required.

Air Ingestion

A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous and air gets into a system in several ways that include:

Both vaporization and air ingestion have an affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization both occur they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

Internal Recirculation

As the name implies the fluid recirculates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. 

With an open impeller pump you can usually correct the problem by adjusting the impeller clearance to the manufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution seems to be to contact the manufacturer.

Turbulence

It is preferred to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions can change the velocity of the liquid and any time there is a change the velocity of a liquid, there is a change in its pressure.

The Vane Passing Syndrome

At times you may have noticed damage to the tip of the impeller caused by its passing too close to the pump cutwater. The velocity of the liquid increases if the clearance is too small, lowering the pressure and causing local vaporization. The bubbles collapse just beyond the cutwater and there is where you should look for volute damage. 

The damage is limited to the center of the impeller and does not extend into the shrouds. You can prevent this problem if you keep a minimum impeller tip to cutwater clearance of 4 % of the impeller diameter in the smaller impeller sizes (less than 355 mm.) and 6% in the larger impeller sizes (greater than 355 mm.)