| Reengineering the Impeller |
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| Written by John Kozel, Sims Pump | |
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Page 2 of 2
Figure 4. Operating away from the design point. This chart represents an average failure mode based on information compiled of pumps operating away from their design point.In other words, on average, this pump will have to be overhauled 5 times more than if it operated at BEP. With an estimated overhaul price of $2500, the operational cost is approximately $13,000 more than it should be! Redesigning the impeller and installing a new reengineered impeller will save electrical costs ($432 savings per year per pump in this example) and, more importantly, operational costs ($13,000 savings per pump in this example). Example 2: A Somewhat Larger PumpConsider a 4 x 6-10 pump operating in a plant system at 600-gpm and producing 100-ft of head. Again, the pump is off the efficiency peak. It operates at approximately 65 percent, whereas its peak efficiency at that diameter (10.25-in) should be 82 percent at the original specified performance of 1100-gpm at 88-ft.
Figure 5. Original hydraulic curve for a 4 x 6-10 pump.
Now the energy dollars become more pronounced. Its power consumption is approximately 25-hp (19-kW), according to horsepower lines in the proximity of the operating point: 19 x 24 x 360 x $0.10 = $16,416. The electrical cost to operate the pump would be substantially less if efficiency were increased by redesigning the impeller to operate at the system requirements of 600-gpm at 100-ft, instead of 1100-gpm at 88-ft. The efficiency at this new point would be increased to 78 percent: $16,416 x (65/78) = $13,680. The electrical savings would be: $16,416 - $13,680 = $2,736 per year, about 16 percent in this case. This pump is operating in the system at 600-gpm at 100-ft, instead of 1100-gpm at 88-ft, which is approximately 45 percent away from the original design point (600/1100 = 55 percent - 100 = 45 percent away from design point). From Figure 4 we can determine that when this pump operates 45 percent away from the original design point (1100-gpm at 88-ft red arrow in Figure 5), the pump failure rate has increased drastically, by 2.56 times. In other words, 833/325 = 2.56 times or 256 percent higher cost! This pump will have to be overhauled 2.5 times more than if it operated at BEP. With an estimated overhaul price of $5,500, the operational cost is approximately $13,750 more than it should be.
Figure 6. The performance curve of the 4 x 6-10 pump using a reengineered impeller.Reengineering the impeller will save $2736 in electrical costs per year per pump, and save $13,750 in operational costs per pump. Example 3: An Even Larger PumpAssume an 8 x 10-17 pump operates at 2000-gpm (280-ft head) instead of a peak point of 4000-gpm at 240-ft of head. The efficiency at the actual operating point is only 70 percent, rather than the potentially achievable 83 percent by this pump.
Figure 7. Original hydraulic curve for an 8 x 10-17 pump.The horsepower at the operating point is roughly 225-hp (168-kW), and the yearly energy bill is: 168 x 24 x 360 x $0.10 = $145,152. The electrical cost to operate the pump would be substantially less if efficiency were increased by redesigning the impeller to operate at the system requirements of 2000-gpm at 280-ft instead of 4000-gpm at 240-ft. The efficiency at this new point would be increased to 81 percent: $145,152 x (70/81) = $125,440. The electrical savings would be: $145,152 - $125,152 = $20,000 per year, or about 13.8 percent in this case. This pump is operating in the system at 2000-gpm at 280-ft instead of 4000-gpm at 240-ft, which is approximately 45 percent away from the original design point (2000/4000 = 50 percent - 100 = 50 percent away from design point). From Figure 4 we can determine that when this pump operates 50 percent away from the original design point (4000-gpm at 240-ft red arrow in Figure 7), the pump failure rate has increased drastically by 2.78 times. In other words, 833/300 = 2.78 times or 278 percent higher cost!
Figure 8. The performance curve of the 8 x 10-17 pump using a reengineered impeller.This pump will have to be overhauled 2.78 times more than if it operated at BEP. With an estimated overhaul price of $10,000, the operational cost is approximately $27,800 more than it should be. Reengineering the impeller will save $20,000 in electrical costs per year per pump, and save $27,850 in operational costs per pump. ConclusionsThe net savings to be gained from reengineering depends upon how far away from the BEP the pump operates, a problem that exists in many pumps that were purchased and installed years ago that no longer operate at their originally intended hydraulic conditions. As operating conditions change, the pump is simply throttled further and further away from the BEP, resulting in dollars that are literally "burned up," not to mention problems such as high loads, shaft breakage, premature rotating life wear, etc. Obtaining a smaller pump may not be a good answer, because it may still not (and usually does not) have the hydraulics sized to hit the operating point "dead on." A smaller pump may help somewhat, but it is expensive and not as efficient. This limits the choice to the standard pump sizes available from the pump manufacturer's catalog. Even with a large number of sizes in the catalog, it is virtually impossible to cover each and every variation of the operating conditions. This could force the user to settle for the "second best," but not the optimum. Even more important is the issue of economics and feasibility of piping changes to accommodate a proposed pump downsizing. Piping changes alone can often cost more than a pump. An alternative solution is to install a custom-designed, reengineered impeller sized for the operating conditions. This can essentially "shift" or "slide" the pump performance to the exact BEP. The net losses and radial loading each become zero. This approach can be effective and the investment is minimal, with a payback less than one year and often only a few months. Not only ANSI or single stage overhung-impeller pump designs can benefit from this approach. Split case and multistage pumps, vertical multistage, river intake pumps, condenser, circulating, etc. have all benefited greatly with improved impeller hydraulics. When a metal impeller is replaced with structural engineered composite impeller (often 85 percent lighter then metal), the combined effect of hydraulic fine-tuning with reduced weight (and thus load) can be dramatic and the rotor dynamic benefits are obvious. When analyzing the total cost of a pump over its entire lifetime, you will find only 4 percent of the total cost is the initial purchase price. 85 percent of the total cost is the operational cost (including the cost of energy) and 11 percent of the total cost is maintenance cost (see Figure 9).
Figure 9. Total cost of a pump.If a pump costs $15,000 and you increase its efficiency by 5 percent by installing a reengineered impeller, you will save $18,750 (more than the initial cost of the pump). John Kozel is the president of Sims Pump Valve Company, Inc., 1314 Park Ave, Hoboken, NJ 07030, 800-746-7303, Fax: 201-792-4803, http://www.simsite.com/. Comments (0)
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