SUBJECT: How efficient is your pump? 7-2
A few years ago, efficiency became "the name of the game". Automotive companies advertised "miles per gallon (liters per 100 kilometers) information in their advertisements and appliance manufacturers published kilowatt consumption numbers along with their pricing information.
Unfortunately high efficiency also means higher maintenance costs because you are required to maintain tighter tolerances and keep the flow passages smooth and free from obstructions. The demise of the double volute pump design in smaller size pumps, is a perfect example of the increase in mechanical seal problems as the efficiency of the volute pump was increased to satisfy consumer demand.
Maybe the "trade off" is acceptable as long as you are dealing with accurate numbers, but are you really doing that? Is the efficiency shown on the pump curve accurate? How was the data taken? What was included in the data, and more important, what was left out? As an example:
If you would like to keep the pump salesman honest, take the data from his pump curve and then make the following calculation:
In inch sizes : GPM x TDH / 3960 = WHP
If we refer to the above pump curve, and insert the numbers into our formula, we would get:
in inch size: GPM x TDH / 3960 = 250 X 300 / 3960 = 18.9
You then divide this number by the efficiency shown on the pump curve:
18.9 / .60 = 31.5 horsepower required to generate the WHP. If this number is lower than the horsepower shown on the performance curves, the efficiency date is questionable. As an example:
If the performance curve showed a requirement for 40 Horse power, the actual efficiency would be 18.9 water horse power40 pump horsepower = .47 or 47% actual efficiency.
Doing the same thing in the metric system we would get:
Referring to the above diagram, and putting in the numbers :
M3 / HR X TDH / 360 = 68 x 76 / 360 = 14.36 WKW. The curve shows a 60% efficiency so:
14.36 water kilowatts / 0.60 efficiency = 23.93 Kilowatts required. If this number is lower than shown on the pump performance curve, the efficiency of the plump is questionable. As an example:
If the pump performance curve showed a requirement for a 30 Kilowatt input, the actual efficiency would be:
14.36 water horse power / 30 Kilowatts required = 48 % actual efficiency.
The fact of the matter is that you seldom operate at the best efficiency point so the numbers become even more depressing. The point is that efficiency should only be one of the points taken into consideration when you purchase a centrifugal pump of a given head, material and capacity. Equally if not more important should be:
You can save money by lowering operating costs (efficiency) or increasing the time between repairs (design). Be sure you consider both when you make your pump buying decision.
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