In my opinion, the system curve is the single most important component of the pump selection process. After all, the system curve determines the operating point on a pump’s performance curve. Other aspects, such as materials of construction and special features, can be reviewed after the hydraulic selection. The system curve is even more important when an application requires multiple pumps operating in parallel.
Multiple pumps present a challenge to a system designer. If a system is designed for three pumps to operate at their best efficiency points (BEPs) when running in parallel, chances are that one or two pump operations will result in an operating point well to the right of BEP. If a three-pump system is designed for two pumps to run at BEP, the third pump will likely push all three to the left of BEP. As the number of pumps increase, it becomes even more difficult.
Wastewater Force Main
A good example is the wastewater force main. Seldom does any combination run at BEP. Fortunately, a combination of variable speed and across-the-line control can keep multiple clear water pumps at or near BEP. Unfortunately, this technique can be problematic with wastewater pumps because lower inlet velocities can lead to ragging at the vane entries.
The Parallel Pump Performance Analyzer—which can be found under the “Pump Evaluation, Selection & Testing Tools” page at www.PumpEd101.com—is designed to evaluate both across-the-line and variable-frequency operation. It allows users to select a pump that will operate at BEP when several are running and view their operating points when all or fewer are running. It also shows the effect of variable frequency drive (VFD) operation with single and multiple pumps. It is designed for use with up to eight identical pumps. Figure 1 is a screen shot of the data entry tab. The required data include the number of pumps operating in parallel; the flow, head and hydraulic efficiency points for one pump; and the flows and heads necessary to generate a system curve
In the example, the system curve is designed for maximum flow, and a maximum of three pumps were selected to run in parallel. The tabs at the bottom of Figure 1 plot pump performance for different pump combinations in across-the-line and variable-speed operation.
The “Traditional” tabs show across-the-line, parallel operation for two to eight pumps. Figure 2 is the Traditional 3 tab. It shows a three-pump parallel flow of 5,000 gallons per minute (gpm) at a hydraulic efficiency of 86 percent. Two pumps can produce about 3,750 gpm at 84 percent efficiency, and a single pump produces 2,000 gpm at about 78 percent efficiency. Based on this operating point, the manufacturer’s approval should be attained if a single pump will be running across the line for extended periods of time.
Figure 3 is the plot from the “Composite Curves” tab. It shows the variable frequency curves for the number of pumps entered on Line 2 of Figure 1 and assumes synchronous control. Under VFD control, the three pumps can provide flows down to 4,000 gpm while remaining at the BEP. They can maintain 84 percent efficiency at flows as low as 3,100 gpm. If the number of pumps entered in Line 2 of Figure 1 is changed to two, the Composite Curves tab will show the variable frequency curves for two pumps. Although these curves are not shown, two pumps can maintain 86 percent efficiency from 2,600 to 3,200 gpm and can maintain 84 percent efficiency down to 2,000 gpm.
Figure 4 is the plot from the “One Pump” tab. When operated across the line, this pump operates well to the right of its H/Q curve. Under VFD control, it will operate at BEP from 1,250 gpm to 1,550 gpm. The Parallel Pump Performance Analyzer provides the information required to properly select and operate multiple parallel pumps.