Pump operating outside the characteristic curve
Pump operating outside the characteristic curve
(OP)
We are looking at a situation where the pump will be selected for higher head, ~9-10 bar but at times it is required to operate at lower head ~2-3 bar. This is well outside the region of pump curve. What could be the potential problems with this design and operation. Are there any guidelines for extrapolating the charateristic curve.





RE: Pump operating outside the characteristic curve
Nothing like some self-education if for no other reason than to understand the likely replies to this posting.
As for your question, it is too brief and lacking any real information to give any specific advice as much will depend on how the system is to be configured.
Additional information might result in meaningful advice from the guys in this forum.
RE: Pump operating outside the characteristic curve
The problems come when you plan to operate at only one flowrate for each of the given discharge heads. You could experience differential pressures greatly different from those at which the pump was designed to operate, which besides having a very low operating efficiency, would no doubt drastically unbalance the usual loads on the impeller to create maintenance headaches. (search this forumn for "operating outside the curve"). A VSD might lower the energy costs associated with the poor efficiencies, but will not solve the unbalanced pressure issues.
You may do well to investigate the possibility of operating methods that are more evenly balanced, by pumping at higher heads and flows for shorter times, then turning the pump off, or pumping at lower heads and/or flows for longer periods and filling an elevated tank for times when high heads or flows are needed. Short of those, investigate the possibility of using two pumps.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pump operating outside the characteristic curve
RE: Pump operating outside the characteristic curve
I don't think use of VS pumping will allow similar flow capacities at different head except to the extent that you don't run off the right end of the curve to begin with, so if you are not on the curve at design speed, then no speed adjustment will move you to the left on the new curve.
If your low head pumping needs are occasional, and energy costs are calculated to be inexpensive enough, then induce head on the low head side so you get back on the curve... throttle a valve (which will likely cause cavitation at the valve and slowly disintegrate the valve, but thats cheaper than a pump) or reduce pipe diameter (how much and how far is a calc you can do), or add discharge restrictions....
Have fun.
RE: Pump operating outside the characteristic curve
RE: Pump operating outside the characteristic curve
Pump curves can be extrapolated within reasonable accuracy out to the 0 flow point. That is not to say that it is a good idea to operate at any point on the curve you are able to plot on a piece of graph paper. Operation near BEP is the key and, since BEP represents the optimum design point for all pump design variables, operating away from BEP for extended time periods should not be taken lightly.
On the other hane, pump curves should not be "affinitized" (adjusted for rpm variations, change in impeller diameter, etc.) by anything reaching a difference of more than 10 to a maximum of 20% from the original curve.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pump operating outside the characteristic curve
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pump operating outside the characteristic curve