Cavitation: A Case of Nasty Bubbles

The popping, hissing, swishing and crackling noises you may have heard coming from a pump is cavitation — bubbles created from too much vapor in the hydronic loop that are collapsing and causing pressure waves to strike interior pump surfaces.  Aside from the annoying noise, the shock waves formed by cavitation can damage the pump the impeller and interior surfaces of the housing, reducing pump efficiency and eventually leading to a costly, premature replacement.  So, what next?

 

To avoid cavitation in pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure or the net positive suction head (NPSH). The net positive suction head available (NPSHA) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSHR) is the minimum net positive suction head necessary to avoid cavitation.

Here are a few remedies that can be applied to the pump to avoid cavitation:

  • Reduce delivery rate:  The NPSHR value will generally become smaller, and the NPSHA value greater. If necessary, split delivery to several pumps, e.g., operate standby pump as well.
  • Install larger impeller:  In many cases, the NPSHR is better, but power consumption is, of course, also greater.
  • Reduce speed:  Pumps running at lower speeds have better NPSHR values. In many cases, however, a larger pump also becomes necessary.
  • Install larger impeller and reduce speed:  If a relatively small impeller is installed in the pump, this solution is ideal from a hydraulic view-point (smoother running, less wear).
  • Operate pump with cavitation:  In individual cases, the pump supplier and the operator of the system can agree, that total delivery head drop should be more than 3%. This must be determined carefully, however, so delivery does not collapse completely.
  • Select pumps with better NPSH Value:  Larger pumps in many cases have better NPSH values at the same delivery rate. If necessary, special impellers designed specifically for good suction can be installed.

The next time you are walking by a pump and hear it cavitating – stop to investigate what is causing it and fix the problem. The bubbles you hear collapsing are eating away at the pump, so it’s pay attention now or pay a lot of money later. Your call.

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