Selecting the right centrifugal pump for specific service can be challenging to a
pump application engineer, or rotating equipment engineer, more so of one who
is in an entry-level position in the pump industry. This article will explain the
process to simplify the selection.

First, determine the total system capacity and head for the specific service. Two
things need close attention when doing this:

It is not uncommon for pump engineers to determine not only the present normal
capacity but also to anticipate future pump capacity requirement in anticipation
of increased demand or production. To ensure that the pump and its driver are
sized to meet the future capacity some engineers specify the future capacity as
the pump rated capacity. As a result the pump would be operating at its normal
capacity with less optimum efficiency resulting in substantial energy loss and
costing the end-user. This practice is acceptable provided it is also specified
that the pump BEP shall be at, or closer to, to the normal flow.

There is a difference between the terms total dynamic head (TDH) and
differential head. TDH includes velocity head which is a head spent to increase
the liquid velocity when a pump has different sizes of suction and discharge
nozzles; velocity head is not measurable with a discharge pressure gauge. If a
pump has the same size of suction and discharge sizes the velocity head is zero
and TDH is numerically equal to differential head. Differential head is the actual
head needed to overcome the site system head.

Most pump standards such as Hydraulic Institute, ANSI, and API 610, specify an
acceptable pump head tolerance – typically in the range of +3% / -3%.

Then determine the number of pumps needed to meet those conditions.

Avoid customization. It is preferable to select a pump in a commercially available
size. Selecting a pump that requires a custom size will be very costly and will
have long lead times. The high cost will to extend to the price of driver and
controls that are likely to require size customization also.

In some locations serviced by local power company, the sub-station may be
limited in HP due to the power demand of others adjacent industries, and may
require another pump station powered by a different sub-station. Similarly, such
as in pipeline services, several pump stations located far apart may be needed
to avoid over-pressuring the pipeline.

In services requiring more than one pump a decision has to be made if the
pumps are to run in series, or in parallel.

It is preferable to select a pump to run at the highest advisable speed. Pumps
running at higher speed are generally more efficient and less expensive than
comparable pumps running at slower RPM. They are also smaller in size and will
require less real estate for installation. There are, of course, some exceptions
such as in slurry pumps where high speed is to be  avoided to prevent
accelerated erosion damage on the impeller and casing.

For example, if a pump were to be driven by an electric motor at 60 hertz power-
supply determine  if the pump can be selected to run at 2-pole speed, or 3600
RPM.

It is preferable to select the pump with the highest specific speed within the
same type of rotodynamic pump, example, within the radial flow type pumps.

Essentially this is going through the process of elimination. If, for instance, the
pump were intended for use in the oil and gas industry that requires an API
pump, then that would automatically excludes pumps that are non-API compliant.
If the requirement were for fire pumps, then it would automatically exclude non-
UL listed, non-FM approved pumps.

The liquid characteristics also determine the pump type or construction by
default. Typically, foot-mounted pumps can be used for pumped liquid up to 200
degree F, but near-centerline mounted pumps are needed for up to 400
degrees F. The pump casing can either be axially-split, or radially split. But for
above 400 degrees F radially-split casing is almost always required.
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Pumps of similar hydraulics are available in end-suction, top-discharge
configuration; top-top configuration, or side-side configuration. The selection of
nozzle configuration is dictated by the pump service, or piping arrangement in a
plant. End suction, top discharge, or top-top pumps are popular in refinery
service where pumps typically discharge into elevated pipes and columns, but in
pipeline service side-side nozzle pumps are always used.  

The use of end-suction pumps is to be avoided in services with high suction
pressure because of the resultant high axial thrust and potentially high shaft
deflection – in such application between-bearing pumps are almost always use.
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Making a centrifugal pumps selection