Parallel or Series Pump Operation
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In many situations the use of a single pump will not be sufficient for the service
and it will be necessary to use two or more units connected in parallel, or in
series, to meet a broad range of operating conditions, or a unique situation.

When two, or more, pumps are needed for the service a decision has to be
made whether the units shall operate in parallel, or in series with one another.

Examples of need for multiple pumps

1. The operating 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 operation

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.
In many multiple pump application the choice between running the units in
parallel, or in series, is clear and easy. But in as many application the choice
can be difficult and the wrong choice can be problematic and costly.

Some guidelines in deciding which one is right for the application:

1. Generally pumps with higher specific speed (NS) have higher efficiency. For a
given system conditions, pumps selected to run in series will always have higher
NS compared to those selected to run in parallel, assuming the units will run at
the same speed (RPM.)   

2. In situations where the net positive suction head available (NPSHA) is tight,
units selected for parallel operation, due to their lower flow rate, may be able to
run at faster speed (RPM) and may result in smaller and cheaper pump size
being used.

3. Units selected for parallel operation must have a constantly rising curve to
shut-off for them to operate properly and to avoid the phenomenon known as
"hunting." They must have at least a 10%, preferably 15%, headrise to shut-off.

Units for series operation need not have a constantly rising curve, and will run
as well with a "hooking" curve. "Hooking" curves are typically produced by
impellers with more vanes or high discharge angle which tend to have higher
efficiencies than impellers with less vanes and lower discharge angle.

4. Units in parallel operation do not have to share the capacity load equally -
each can share a different percentage. This is ideal for optimizing the selection
where there is a wide flow range from minimum to maximum flow.

5. Overhang pumps in series operation need special consideration for axial
thrust balance which could be different from one pump to another because of
their varying suction pressure. This is not a concern for pumps in parallel.

6. Generally in multiple pumps application there will be a spare unit in case of an
unforeseen equipment breakdown. In case where there is no anticipated spare
unit for the service, it is recommended to draw a family of curves for both
parallel and series operations, and super-imposed then with the system-head
curves. Examine where would the family of curves and system-head curves
intersect if one pump becomes out of service. Determine which  operation,
parallel or series, would result in a performance the process can live with when
one pump becomes out of service.

7. Pumps in series operation, connected far apart over a long distance and
changing ground elevation, such as in pipeline application, will enable the use of
pumps with lower maximum allowable working pressure (MAWP) because the
total system pressure can be divided among the pumps in series. The use of
series operation may also be dictated by the pressure limitation of the pipeline

Pumps in parallel operation must be designed to take the full system pressure
and this may require the use of special heavy pump casings. The installation
may also require provisions to safeguard it from the risk of water hammer in the
event of an upset condition.

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Photo shows some single stage,  double suction,  horizontal
pumps connected in parallel for flood control service.