Centrifugal, helical-centrifugal and axial-flow pumps: cavitation

Cavitation is still a major concern for anyone involved with rotodynamic pumps, whether they are users, installers or, of course, manufacturers. The rapid evolution of the pump industry, and recent advances in the study and understanding of the many mechanisms governing cavitation, make it all the more necessary to update our knowledge of the subject. The aim of this article is to provide the reader with explanations and elements that can be used directly in practice, while at the same time highlighting the multiplicity and complexity of the many elements to be taken into account.

The cost of a pumping station is a direct function of two parameters: the speed of rotation of the pumping units selected, and the level of the station invert with respect to the minimum level of fluid to be pumped at suction. These two parameters refer to the Net Positive Suction Head (NPSH): (NPSH) _req of the pump and (NPSH) _disp linked to the installation; it is in the value of the safety margin to be taken between (NPSH) _req and (NPSH) _disp to ensure satisfactory operation for the user with regard to cavitation that a major part of the cost of the pumping station lies.

A large part of the article defines the various aspects of cavitation in pumps, and provides guidelines for calculating the (NPSH) _disp related to installations. These basic definitions will help you make the right technical choices.

A small section has been devoted to noise generated by cavitation, as this subject of extreme importance in certain applications will be covered in a more general article devoted to turbomachinery noise. It seems more coherent to group together all the elements devoted to noise, as it is tricky to treat separately the part of noise attributable to cavitation from other sources of noise generated by pump units.

Given current industrial practice, NPSH has been expressed in meters of liquid column, whereas it would have been more scientifically sound to use the expression NPSE = g (NPSH), suction mass energy (in J / kg).