In many situations the use of a single pump will not work and it is necessary to use two or more units connected in parallel, or in series, to run a broad range of operating condition.
1. The flow range is too wide that a single pump will run below its allowable minimum stable flow when running at the minimum flow range, or it will run near the end of curve when running at the maximum flow range. The unit, at either point, will be very inefficient.
2. The horsepower (HP) size of available drivers determines the number of pumps needed. For example, the system capacity and head may require 3,750 HP but the available motors are 2,000 HP only. Two units, driven by 2,000 HP motors each, are needed.
3. A commercial utility company imposes a limit on the maximum power that can be drawn from its sub-station to ensure the equitable distribution of power among its customer specially if the sub-station is short of capacity. The power source will have to be spread across different sub-stations by using two, or more, pumps at different locations.
4. The pressure needed to move a liquid to its ultimate destination in a pipeline exceeds the maximum allowable working pressure (MAWP) of the pipeline. The required pressure is high due to change in elevation and the high friction loss across long distance. The pipeline is provided with intermediate pump stations spaced apart over long distance to prevent exceeding the pipeline MAWP.
Parallel, or series?
When two, or more, pumps are needed a decision has to be made whether the units should operate in parallel, or in series.
A parallel operation is one where two, or more, pumps are connected such that the pumps take their suction flow from a common source and discharge into a common header or tank. The total flow rate in the system is the total of the flow rates of the individual pumps which can be different from each other. Pumps in parallel should have stable or continuously rising curve to shut-off.
A series operation is one where two, or more, pumps are piped such that one pump discharges into the suction of the next pump in the series, and so on. The flow rate in the system is the same as the flow rate of each pump in the series but its total head is the total of the heads of the individual pumps which could be different from each other. Pumps in series do not need to have stable or continuously rising curve to shut-off.
In the four examples cited above, the first example requires pumps in parallel, the second and third examples require pumps in either parallel or series operation, and the fourth example requires pumps connected in series.