Impeller cavitation-erosion damage



Q  -  We have a single stage, double suction, horizontal pump in cooling tower
service. The impellers has had cavitation-erosion damage at the inlet vanes so
severe that we have to replace the carbon steel impeller every six months. The
vendor advised that the pump NPSHR cannot be reduced because its suction
specific speed is already too high. We cannot increase the NPSHA and replacing
the pump with one of slower RPM, or with a vertical canned-type, is not an option
at this time. How can we prolong the impeller life beyond six months?




A  -  The cavitation-erosion damage is localized at the impeller inlet vanes and
does not show at the impeller outlet, or at the volute. We suspect the damage is
partly caused by suction flow recirculation possibly due to an oversized impeller
eye area which is typical of high suction specific speed impeller. Upgrading the
impeller material from carbon steel to the more cavitation-resistant ASTM A-743,
Grade CA6NM, may help prolong the useful life of the impeller. Or, consider
overlaying the carbon steel impeller with Stellite #6 material at the inlet vanes.


R0609-IMCA
File: cavitation erosion


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Introduction to cavitation
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