When to run in parallel or series?

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On page 1:  What is parallel or series pump operation?

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 costly.

Here are 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, spread apart over a distance, can allow units with
lower maximum allowable working pressure (MAWP) and hydrotest pressure.
Pumps in parallel operation must be designed to take the full pressure of the
system - this may require units with special heavy casing..

File: pump parallel series2

"Make it simple"
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