Pump Curve Run-out
Pump Curve Run-out
(OP)
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 down???
Any clarification would be greatly appreciated.
Thanks,
Any clarification would be greatly appreciated.
Thanks,





RE: Pump Curve Run-out
RE: Pump Curve Run-out
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.
Just my thoughts,
John
RE: Pump Curve Run-out
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.
RE: Pump Curve Run-out
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pump Curve Run-out
But I absolutely do agree with the recommendation "don't go there."
rmw
RE: Pump Curve Run-out
At best efficiency point (BEP) flow, the designer designs the impeller exit blade angles to match passage flow angles. Often inlet blade angles are set optimally at a flow lower than BEP to get a "usable curve" and allow for some fluid pre-rotation in the suction pipe.
As you extrapolate the pump H-Q curve, the passage flow angles will become highly mismatched with blade angles. The problem becomes most acute with regards to suction pressure. NPSH requirements tend to go through the roof when you go significantly above the optimal suction flows. Additionally, cavitation damage is possible. Remember that NPSH and cavitation are two different issues. When there is head falloff because of insufficient NPSH, the cavitation is so advanced that it blocks the flow passages. Damage can occur well below stated NPSH values.
So, to get the pump operating "out on the curve" you may well need a charging pump in series with the pump to boost the suction pressure sufficiently. Increase the capacity of the charging pump enough, and the head can go negative.
If you need to run out on the curve for brief periods of time and have sufficient NPSH, ask the pump manufacturer for advice. I can't think of any case where you would want to order a pump where your normal duty point was way out on the curve. Better to pick another pump.
RE: Pump Curve Run-out
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pump Curve Run-out
By the way listen to the folks above DONT GO THERE if you want your pump to survive.
RE: Pump Curve Run-out
Don't get me wrong, operating here wouldn't make a lot of sense. But pump companies test the pumps under wierd conditions to assess pump behaviors, etc. during motor trips, surges, etc. When the pumps get very large, such information can become crucial.
RE: Pump Curve Run-out
To me, life is much more positive when 0 </= a </= (pi)/2.
You just have to make sure you don't go off on a tangent.
Regards,
SNORGY.
RE: Pump Curve Run-out
When the tank is at its lowest point, there will be less head required giving a higher flow rate from the pump. As the tank fills, the head increases decreasing the flow rate.
RE: Pump Curve Run-out
Limit end of curve run-out performance to 120% - 150% (max) of BEP flow if adequate NPSH is available. anything past this, consider re-rating the pump to a larger impeller dia (if possible) or increasing pump speed (if possible)
Did you know that 76.4% of all statistics are made up...
RE: Pump Curve Run-out
Operate over BEP to more than 120% you have:
1. More power consumption.
2. More NPSHr with cavitation failure risk.
3. Erosion in high velocity zones, Cut water, suction, discharge vanes zone, etc.
3. More radial load with bearing and shaft failure risk.
4. Global vibration levels increase dramatically.
5. Reliability going down to 30%.
6. Other unstable hydraulics issues.
So it is not recommended.