My thinking on the coupling failure is that overheating in the pump could cause axial force imbalance within the pump envelope and axial shifting of the pump shaft greater than the coupling can handle.  Depending on the motor bearing types, there could be some axial shifting of the motor rotor as it finds its electrical center (differing from its axial position during alignment procedures).  All of this being based on wild speculation, of course, since we know precious little of the exact details of this situation.  Most likely, the coupling problem could be mitigated by replacement with a coupling Magnetic pumps that can accommodate a greater range of movement.Although the pump heating during blocked flow operation is surely less severe here than in the BFP's of my experience base, it is also likely that this pump has been operated in this mode far longer than may really be prudent to avoid internal damage regardless of the coupling damage.  I've seen plenty of smaller pumps turned into mechanical water heaters due to inadequate flow (usually by virtue of unfortunately mis-matched pumps operating in parallel).  These pumps never operated long in this mode without suffering serious internal damage (what a surprise!).  Valuable advice from a professor many years ago:  First, design for graceful failure.  Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs.  Only then can practicality and economics be properly considered. 
 

2011-08-05

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