The article has been updated successfully.
Figure 1: Piping system with pumps running in parallel.
In order to determine whether or not there will be problems, it is imperative that you have pump performance curve data for both the existing and the new impeller diameters. Figure 2 shows a plot of two different pump curves. the curve on the bottom is for a centrifugal pump with a 25.5 inch impeller installed. The curve on the top represents the same pump with a 26.5 inch impeller.
Figure 2: Performance curves for two different impeller diameters.
If these two pumps are running simultaneously in parallel, then it is possible to determine what the flow will be from each of the pumps. The data points can be added "horizontally" and plotted. When doing this, you must choose a particular head value, and draw a straight line horizontally intersecting the two curves as shown in Figure 3.
Figure 3: Adding pump curves together horizontally.
Determine the flow rates at each of these intersecting points, and add the flows together. In this example, at a Total Head value of 140 feet, the flow rate of the smaller diameter pump is 17,200 gpm and the larger diameter pump is 19,500 gpm. You can then plot this point on a new curve as shown in Figure 4.
Figure 4: Combined parallel pump performance curves.
As you can see, the combined curve will follow the curve of the larger impeller pump at higher Total Head values until the Total System Head drops to roughly 168 feet. Then the curves are added together as the Head values drop. With the combined curve, for a given system flow rate, you can determine what the total pump head will need to be and what the flow rate will be through each pump.
Figure 5: Operating points with System Head at 168 ft.
The farther to the right or to the left of the pump's Best Efficiency Point, the higher the radial loads will be on the pump impeller, the bearings, and the seals. Thermal stresses due to over-heating as a result of running below the pump's minimum flow requirement can also damage the pump. Cavitation may also be a consideration if the pump is running too far out on its pump curve. If there are going to be problems occurring with the pumps, it would be as a result of operating too far to the left or right of the BEP. With these particular pumps, you can see from Figure 5 that if the System Resistance (System Head) is over 168 ft, then the smaller diameter pump will not be able to generate enough pressure to overcome this resistance. There will be no flow out of the smaller diameter pump in this case, and it will essentially be "dead-heading". In the absence of a check valve, you may even see reverse flow through the smaller pump. If the System Resistance is over 163 ft, then the pump will be running below the manufacturer's recommended minimum flow rate.
The key to running dissimilar pumps in parallel is to run the total system flow rate in a range that allows both pumps to run within a reasonable region on their pump curves to avoid issues due to excessive loads on the impeller, bearings, or seals. The pump manufacturer should be consulted to determine if parallel pump operation with dissimilar pumps is advisable for any particular application. When the pump curves are relatively flat as these are in the Figures, it becomes even more important to avoid excessive resistance. A small change in system resistance can take this pump from operating cleanly and efficiently to not operating at all.
