Impeller Diameter (in): in
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Pump Speed (rpm): rpm
Tip speed in feet per second: 251 ft/sec
Tip speed in feet per minute: ft/min
Speed kills. More specifically, high rotational and flow speeds kill slurry pumps.
A recent, inadvertent operator error gave us a wonderful data point to illustrate the destructive power of high speeds in slurry pumping. The pictured polyurethane impeller wore out in less than a week. This while the norm in the specific application is eight to ten weeks, possibly longer depending on the specific feed material being processed.
The dramatic reduction in wear life called for a site visit by HART Slurry Pumps' specialist application engineers to investigate and understand the cause of this marked negative deviation.
On investigation, it was found that the motor and drive pulley had been inverted, resulting in a drive ratio that sped up the pump by 20%, instead of reducing speed by 20%. This unintentional duty point (Duty Point A in the figure below), way right of the Best Efficiency Point (BEP), caused erratic and unpredictable pump performance and rapid degradation of wear parts.
To explain this in more detail, centrifugal pumps have a fairly narrow operating range around the point of optimal efficiency, called the Best Efficiency Point or BEP for short. Any deviation from the BEP is typified by efficiency losses and a major drop in MTBF. Due to the presence of abrasive solids, slurry pumps are even more sensitive to these effects, and optimally operate between BEP -10% and BEP +5%.
With the incorrect pump speed, operation oscillates between Points A and B. Operation at the design duty is stable (Point C).
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With the incorrectly installed pulleys, Duty Point A was at BEP +100%! It is beyond the drop-off point where recirculation dominates and the pump simply cannot operate effectively, let alone efficiently. Operating this far outside the design window is extremely unstable and may cause all sorts of undesirable operational effects such as:
This last point is important, as it causes a shift in operating conditions. When the pump goes past the drop-off point, discharge failure occurs. Discharge flow slows down below the deposition velocity of the solids being conveyed, particles start falling back into the pump causing an impossibly high solids concentration. When this happens the total dynamic head seen by the pump increases dramatically. At this time (Duty Point B) the pump approaches shut-off head at a duty around BEP -80%. If it manages to clear the slumped solids, it goes back to Duty Point A for the cycle to start again. If it does not clear the solids the pump will clog and the motor will trip on overload.
Fortunately, there was a quick and easy solution to this problem. Once the pump duty point had been restored by swapping the pulleys (Duty Point C), the user once again experienced design pump performance. For reference, Duty Point C is at BEP -4%, right in the sweet spot!
Clearly, the premature failure of the impeller was due to incorrect drive set configuration, and not as a result of defective design or materials. It could be due to a deliberate attempt to derail the performance of the pump but is more than likely the result of an honest mistake by maintenance personnel. Whatever the reason, the incorrect drive configuration had a devastating impact on the pump and its performance and reliability.
To prevent similar future events we suggest the following actions:
The lesson is clear: always know the appropriate pump speed and have a drive schedule handy to ensure the correct drive components are fitted, and they are fitted the right way.
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