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 are 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.