Cavitation at pumps

Cavitation refers to the formation and breakdown of steam bubbles in fluids. In this case, the evaporation is caused by a reduction in the static pressure, e.g. due to acceleration of the medium at the impeller inlet in the pump. The steam bubbles break down in the flow path as the increase in external pressure causes the bubbles to implode (microscopic steam hammer). This can cause significant damage and even complete destruction of the inner workings of the pump.

There are many possible causes of cavitation in pumps. Installation height too low, fluctuating pressures in the intake side or fluctuating medium temperatures. The feed pump has often not been correctly throttled, as is also the case with this specific issue. At the start of operation there is no or only very little prevailing back-pressure in the boiler. The pump therefore delivers a significantly higher quantity of water. This causes the acceleration at the inlet to the pump impeller to increase considerably and a significant drop in the static pressure occurs. If the feed water is already hot, cavitation can occur at this point. The pump was damaged beyond repair after only a few minutes.

Everybody can hear cavitation: At the start of cavitation the sound is like rain on a metal roof and full cavitation sounds like hail on a metal roof.

Problem:

• Cavitation at pumps

Cause:

• Pumps not throttled

Remedy:

• Throttle pumps as specified in operating instructions
• Observe installation conditions

1 Denting at pump impeller

2 Blown off pump impeller blades being blown off

3 Damage/destruction of further pump components due to pump impeller blades