Cavitation

An implosion of vapor bubbles in a liquid inside a pump caused by a rapid local pressure decrease occurring mostly close to or touching the pump casing or impeller. As the pressure reduction continues these bubbles collapse or implode. Cavitation may produce noises that sound like pebbles rattling inside the pump casing and may cause the pump to vibrate and to lose hydrodynamic efficiency. This effect contrasts with boiling, which happens when heat builds up inside the pump.

Continued serious cavitation may destroy even the hardest surfaces. Avoiding cavitation is one of the most important pump design criteria. 

Cavitation limits the upper and lower pump sizes, as well as the pump’s peripheral impeller speed. Cavitation may be caused by any of the following conditions:

  1. Discharge heads are far below the pump’s calibrated head at peak efficiency.
  2. Suction lift is higher or suction head is lower than the manufacturer’s recommendation.
  3. Speeds are higher than the manufacturer’s recommendation.
  4. Liquid temperatures (thus, vapor pressure) are higher than that for which the system was designed.

Source: Handbook of water and wastewater treatment plant operations

Wikipedia:  https://en.wikipedia.org/wiki/Cavitation