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Article #37: How Much Energy can we Save by Redesigning (not simply replacing!) Impeller Hydraulics?


A question was presented to the pump users - how much energy do you waste when a 2000 hp pump operates at 50% flow? Tim Cross, a Sr. Mechanical Engineer at Georgia Gulf Chemicals & Vinyls responded:

There total wasted energy is a function of pump efficiency and TDH. Since TDH increases with decreased flow and hydraulic efficiency decreases when operating away from BEP both the estimated head increase and efficiency loss must be included in the energy loss calculation.  A typical 2,000hp cooling water pump operating at 75FT TDH and 89% efficiency requires approximately 50bhp per thousand gpm flow.  Reducing the flow by 50% increases the TDH to about 225FT and the efficiency drops from 89% to 76% which results in approximately 75bhp per thousand gpm flow.  The total power wasted is approximately 500hp and the BHP required to operate at 50% flow is 1,500hp.  At $0.05/kwh and 95% annual operation the annual electricity wasted is $155,660 or approximately 34% of the bhp required to operate the pump.



Goulds 30x36-42G, 3420















Design Load

50% flow










bhp/mgpm wasted








total bhp wasted








total kw wasted
















Annual Run Time








Annual kwh wasted








Energy Wasted

 $  155,660

Total Electrical Cost Wasted





Operating Cost

 $  463,509

Total Annual Electrical Cost







% wasted electricity



Tim Cross
Sr. Mechanical Engineer II
Georgia Gulf Chemicals & Vinyls  LLC


Tim, - I enjoyed the depth and detail of your analysis. You used a double suction split case pump for the analysis, and in fact any other pump type of that size and power, from any manufacturer, would show results very similar to what you got. I thought it would be helpful to a reader to actually see a performance curve, for ease of following along with the numbers. Goulds happens to have a nice computerized performance selection program, on line, so I picked it for illustration purpose, as shown below. I happened to pick a vertical turbine pump type, just to show similarities in results with your double suction unit. The numbers differ slightly from yours, but are reasonably close for the comparison purpose.



Your pump BEP point is 89%, which is close to 88% shown on the curve above. As a fine point, at these power levels, for vertical turbine pumps, most manufacturers show a “bowl efficiency”, not a true pump efficiency, not accounting for discharge piping losses, which typically amount to additional efficiency decrease by about 2-3%.


At 50% flow, you show 76% efficiency, and the pump I looked at had even greater decline, down to approximately 61%, i.e. the cost difference situation you presented would be even more dramatic for this pump. Also, you used $0.05 per KW-HR energy cost, which is probably generous, and I usually apply about $0.08 value. With energy prices going up as they have been, this would doubtfully get any better, i.e. your point will be making even more and more sense as time goes on.


The good news is that many people are beginning to realize this rather grim picture of pump energy troubles. Unfortunately, few of those who do realize have gone the distance beyond generalities, - by actually evaluating the numbers, as you did, and this is where another problem is. My article, from which you took the queue to evaluate the actual cost degradation, talks about the need of training, and education of maintenance, operating and management personnel of these facts. Energy does matter, and this is why.


Interestingly, few pump manufacturers actually act upon these revelations in the most effective way. The best what happens is a replacement of one pump with another, - a smaller one in this case. As you pointed, a 40,000 gpm (BEP) pump would be replaced by a 20,000 gpm (BEP) pump. It is true that such smaller pump would then operate at improved efficiency, with savings as presented. However, the investment by the end user is considerable – an expensive (new) pump, and, what’s even more dramatic, modifications and changes to the piping to accommodate a new pump. When the entire economics are evaluated, the payback period may be many years, thus negating, or significantly offsetting, the apparent energy savings.


A better approach is to keep the same pump, but to replace an impeller only with specially designed hydraulics to fit the proper required flow, and to account for the existing geometry of the casing. Such approach is much less expensive, quicker, and with no piping modifications. Of course, good knowledge of pump hydraulics, to accommodate such design retrofit is required, but that is why training needs are to be emphasized. In the end, it is no longer possible these days to achieve significant benefits, with regard to energy conservation, by quick, superficially appealing methods of the entire pump replacement. Clearly, a more professional, technically sound engineering design approach must be revitalized. There are several articles on this subject, which you may also enjoy reading at our web site (section Articles), with respect to how actually a redesign/replacement by a new impeller within the existing casing works. You will also find there articles on other methods of hydraulic BEP shifting, as well as application of such methods to engineered upgrade of pumps, under section Repairs and Upgrades.


Again, thank you for your most insightful evaluation of the challenge I presented. As promised, - you are a winner of a free pass to the pump training session I conduct – the next one is September 21-22, 2006 in Atlanta, GA, - an annual PumpTec hands-on pump conference. Details are on our web site:  www.PumpingMachinery.com.  I look forward to see you there!


Best regards,


Lev Nelik

Lev Nelik, Ph.D., P.E., APICS

Pump Magazine

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