This is very small pump compared to my experience with boiler feed pumps (multi-thousand hp each), but the principles are still the same.  If very low flow rates must be tolerated, it is essential that a by-pass line be installed to return a minimum flow of the discharge flow to an upstream feedwater heater (or a heat exchanger to cool the flow) to assure an adequate minimum flow through the pump to avoid both overheating and damaging unintended forces.  It will be necessary for you to determine what the tolerable minimum flow will be and how (and where) the temperature rise must be accommodated.Yes, the energy usage of the pump will be increased by this by-pass flow, but you will find Screw pumps it to be a trivial burden compared to the burden of pump system damage.  Also, since the problem has been severe enough to damage the shaft coupling, it is likely that there will be damage to the pump internals as well.  It is likely that pump performance and efficiency have suffered from this type of operation and the resulting damages.  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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