Smaller "slave pump"

Save energy by installing a smaller "slave" pump
Often the sump and the pumps in a pump station are designed to handle a design flow that rarely occurs. For example a pump station could be designed for an inflow that appears during heavy rainfalls occurring only a few times a year during a short period of time.

The normal inflow that takes place during most of the time will cause the pumps to run with very short cycles and since they are dimensioned to handle big inflows the delivered flow will be significantly high compared to the inflow. Since the losses increases with the square of the flow this means that the system losses will be much higher than if a small pump would have pumped a smaller flow. It can be see the graph below:

Due to the shape of the system loss curve the head and therefore the consumed energy will be much higher for the large pump than for the small pump even if the large pump has a better efficiency in its duty point than the small one.
Specific energy calculation example

As one can also see, the difference in head between the small and the large pump depends on the shape of the system curve. In our example the system has characteristics of being a loss system compared to a lift system where the system curve is more flat (almost all head consists of geodetic head).

We therefore conclude that depending on the consumed energy, pump run times and system characteristics, it could be beneficial from a total cost perspective to invest in a smaller slave pump for a pump station. It is therefore important to emphasize that the whole system including the pumps and the pipe system has to be carefully analyzed in order to determine whether it is a good idea to use a smaller slave pump or not.