Cavitation noise and erosion damage

Q  -  We have five single stage, double suction, axial-split, horizontal pumps in
raw water service. When we run three pumps in parallel they operate smoothly.
But when we run four units, to get slightly more flow and head, they make loud
cavitation noise.

We disassembled one pump and found that:

  • The impeller high pressure vane tips had moderate erosion damage on
    its outboard side.

  • The volute lips also had erosion damage - about 1 square inch and 3/16
    inch deep. The damage was aligned with the damage on the impeller.

What was the likely cause of the damage, and how can we prevent it in the future?

A  -  Since the erosion damage is on one side of the double suction impeller , the
problem was likely caused by unbalanced flow and the cavitation-like noise was
likely caused by internal flow recirculation, rather than by classical cavitation.

In classical cavitation, caused by insufficient NPSHA, the noise dissipates as the
flow rate is reduced, and the damage is likely to be on the low pressure side of
the impeller. These area exactly the opposite of what the pumps were doing.

In an unbalanced flow, one side of the double suction impeller is getting more
flow whereas the other side is starving causing some discharge flow recirculation
to occur. The recirculation becomes more severe when more pumps operate
because each unit pumps at lower flow rate.

In double suction pumps the inlet flow on both sides of the impeller should be the
same making them more sensitive to improper suction piping compared to single
suction pumps. To prevent unbalanced flow make sure that their suction nozzles
piped properly.

Do not run the pumps below their recommended minimum stable flow. Install a
minimum flow by-pass line with ARV, if needed.

Find out if it were doable to re-rate the pumps for higher flow rate and head so that
only three pumps would be needed. Or, re-rate the pumps for lower flow rate to
avoid low flow recirculation when four units are in operation.

Check that the impeller outlet width is properly centered in the volute.

Tags: cavitation, erosion damage, internal flow recirculation
Sections: operation, installation, trouble-shooting, hydraulic re-rate
File: cavitation noise
Related topics

Introduction to cavitation
Impeller erosion
Noise and damage
Recirculation vs. insufficient NPSHA





Engineering data


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