Hello, I do not have much practical experience with pumps and have a question as to what physically happens when a system curve is at a point beyond the rated pumps curve. In other words, what happens when a pump runs out on its curve. Do you simply extrapolate the curve to the lower TDH and higher flow rate? Or can the pump even operate at this point? Would it shut downIt's generally not a good place to be. I'm basing my response on centrifugal pumps - as psafety indicated, the power requirement continues to rise, and that may be enough to shut the pump off when the motor overloads, but that's not all.The pump is not designed to operate continuously off the end of the curve, and doing so can lead to higher vibration, higher temperature rise, high bearing loads, fluid surging, just to name a few effects. Efficiency drops off, and NPSHr increases (which can itself lead to unstable operation).During commissioning, when analysing results from pump runs, I have found a pump to be off the end of its curve and running quite happily on the extrapolation, but others exhibiting significantly more vibration and with lower flow and head than the extrapolation would predict. In short, the pump manufacturers don't want you operating in that region, and as indicated above, results can be unpredictable and there are no real benefits.
Basically there are 2 main occurences: 1) motor overload, fire pump unless of course an oversize motor has been fitted, 2)rising NPSHr and reducing NPSHa which can lead to severe cavitation and subsequent damage to the impeller.Other problems can be: unstable flow, high axial loads on the impeller / bearings, high levels of vibration and noise all which contribute to shortened pump life. If there is any chance of the pump "running-out" on its curve you could monitor and alarm power input or fit an orifice plate which will increase head as the flow increases and limit run-out. 

2011-08-23

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