Editor's Note: This is the fourth in a six-part series on centrifugal pump efficiency. For other articles in this series, click here.
The performance of a centrifugal pump with a trimmed impeller will follow the affinity laws as long as that trim is relatively small. Many experts recommend a maximum of 10 percent reduction from the full design diameter, but, when looking at a typical set of manufacturer’s catalog curves, trims as great as 30 percent to 35 percent can be found.
In Part 3 of this series, I examined the volute’s contribution to hydraulic efficiency. An important part of the volute is the tongue, or cutwater. Its purpose is to maintain flow into the throat while minimizing recirculation back into the case. The optimum clearance between the tongue and the impeller periphery is the smallest distance that does not give rise to pressure pulsations during vane tip passing. A well-designed pump will have a full-size impeller that meets these clearance criteria. When an impeller is trimmed, this distance increases and allows more fluid to recirculate back into the case. As recirculation increases, hydraulic efficiency decreases.
The Effect of Impeller Trim
Figure 1 shows the catalog curves for a centrifugal pump with a 9-inch, full size impeller and several trims.At the best efficiency point (BEP), hydraulic efficiency is 77 percent for the 9-inch impeller. When trimmed to 8 inches (11 percent), BEP efficiency drops to 74 percent. A 23 percent trim (7 inches) reduces BEP efficiency to 70 percent and a 33 percent trim (6 inches) lowers it to just 62 percent. Although not shown on the graph, if the trim were just 6 percent (8.5 inches), BEP efficiency would remain at 77 percent. The trims shown on themanufacturer’s catalog curves are the allowable trims, not necessarily the most prudent ones.
Although an efficiency reduction of 3 percent could be acceptable in certain applications, reductions of 7 percent and 15 percent should seldom be acceptable. If you have to trim an impeller that much to meet the requirements of the application, it is time to consider a smaller pump.
Figure 1. Catalog curves for a centrifugal pump with a 9-inch, full size impellar and several trims
Altered Speed
An alternative to trimming the impeller is to alter the impeller’s speed. This can be achieved in many ways. Figure 2 shows the same pump with four different speed curves that were selected to match the trims shown in Figure 1. These speed changes were produced by a variable frequency drive (VFD), so the curves are labeled in Hertz. Other speed change options include belt drives and adjustable, magnetic couplings.
The result is that BEP efficiency remains at the full-speed (diameter) efficiency (77 percent) across a broad range of speeds. This occurs because the impeller periphery to tongue clearance remains unchanged and recirculation is limited to its design conditions. The affinity laws also hold true across this range.
Next month, Part Five of this series will compare the impact of peak BEP efficiency versus a broad range of high efficiency. It will also show how a pump curve’s breadth of efficiency can affect both fixed and variable speed operation.
Figure 2. The same pump from Figure 1 with four speed curves
Note:
In Part 2 of this series, “Centrifugal Pump Efficiency—Specific Speed” (March 2012), I forgot to mention that parts of Europe use an alternative method when computing specific speed (Ns) for double suction pumps.
The European method uses half the BEP flow. In the U.S., we use full flow regardless of the pump design. When Ns is calculated using half the BEP flow, the result equals 0.707 that of the full flow calculation.
Pumps & Systems, May 2012
