In designing multiple centrifugal pump installations it is necessary to keep in mind the interaction between the pump curves and the system curve. That is, throughput cannot be doubled by adding an identical pump in parallel, and head is not doubled by adding an identical pump in series.

The effect of adding two identical pumps in parallel can be seen in Figure 11-1. Curve A is the pump curve for one pump. Curve B is constructed by doubling the flow rate at a given head to show how the

pumps behave in parallel operation. Curve C shows a system curve where the addition of the second pump adds only about 50% to system throughput. Curve D shows a steeper system curve where the system throughput is only increased about 20%.

Figure 11-2 shows the effect of installing two pumps in series. Curve A is the head-flow-rate curve for one pump. The combined curve for both pumps, B, is constructed by doubling the head of Curve A at each value of flow rate. The benefit of the additional pump can be seen by inspecting the intersection of the system curves, C and D, with the pump curves.

The choice of whether to add an additional pump in series or in parallel is illustrated by Figure 11-3. If the system curve is shallow, more throughput is obtained from parallel operation. If the system curve is

steep, more throughput can be obtained by series installation.