Parallel or series operation

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.

Examples:

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.

Next page:  When to run in parallel or series?

R-1109-PAOR
File: pump parallel series

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