Cavitation and NPSH

Cavitation
Cavitation begins as the formation of vapor bubbles at the impeller eye due to low pressure. The bubbles form at the position of lowest pressure at the pump inlet which just prior the fluid being act upon by the impeller vanes, they are then rapidly compressed. The compression the of the vapor bubbles produce a small shock wave that impact the impeller surface and pits away at the metal creating over time large eroded areas and subsequent failure.



NPSH
Net Positive Suction Head Available (NPSHA) is total energy per unit weight or head at the suction flange of pump less the vapor pressure head of the fluid. Why we need to calculate the NPSHA? This value is required to avoid cavitation of the fluid. Cavitation will be avoided if the head at the suction is higher than the vapor pressure head of the fluid. In addition, the pump manufactures require a minimum NPSH to guarantee proper operation of the pumps, they call this Net Pressure Suction Head Required (NPSHR).


Condensate Extraction Pump is very risk to get cavitation due to vacuum condition at its suction line, to get satisfied of NPSH, the pump should be install deep below condensor hotwell
The pump is usually connected through rubber or copper expansion joint to the condenser hot well, and in order to prevent the pump from becoming vapor-bound the pump casing is vented back to the vapor space of the condenser.

Example from Johnston Pump
Fox, McDonald INTRODUSTION TO FLUID MECHANIC John Wiley & Sons 1992
Jacques Chaurette FLUID DESIGN Inc 2003
Johnston Pump













click here for boiler feed pump chart.