Lesson Two: Reading a Pump Performance Curve
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Let's take a look at a pump curve, the common way of showing a centrifugal
pump's performance.
Pump Size Number
The size of the pump, 1-1/2 x 3 - 6 is shown in the upper part of the
pump curve illustration. Note that the size number 1-1/2 x 3 - 6 indicates
that the pump has a 1-1/2 inch discharge port, a 3 inch suction port, and
a maximum nominal impeller size of 6 inches. This type of nomenclature
is common, with some companies putting the 3 in the first position instead
of the 1-1/2. In either case, standard procedure is that the suction port
is the larger of the first two numbers shown and the largest of the three
numbers is the nominal maximum impeller size.
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Pump Speed in RPMs
Also in the upper right hand corner notice that the curve indicates
performance at the speed of 3450 RPM (a common electric motorspeed in 60
hz countries). All the information given in the curve is valid only for
3450 RPM. Generally speaking, curves which indicate RPM to be between 3400
and 3600 RPM are used for all two pole (3600 RPM nominal speed) motors
applications.
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Flow
The pump's flow range is shown along the bottom of the performance curve.
Note that the pump, when operating at one speed, 3450 RPM, can provide
various flows. The amount of flow varies with the amount of head generated.
As a general rule with centrifugal pumps, an increase in flow causes a
decrease in head.
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Head
The left side of the performance curve indicates the amount of head a pump
is capable of generating.
Trimmed Impeller Curves
Notice that there are several curves which slope generally downward as
they move from left to right on the curve. These curves show that actual
performance of the pump at various impeller diameters. For this pump the
maximum impeller diameter is shown as 6 inches and minimum is 3 inches.
Impellers are trimmed in a machine shop to match the impeller to the head
and flow needed in the application.
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Duty Point
The point on the curve where the flow and head match the application's
requirement is known as the duty point. A centrifugal pump always
operates at the point on it's performance curve where its head matches
the resistance in the pipeline. For example, if the pump shown above was
fitted with a 6 inch impeller and encountered 100 feet of resistance in
the pipeline, then it would operate at a flow of approximately 240 gallons
per minute and 100 feet of head. It is important to understand that a centrifugal
pump is not limited to a single flow at a given speed. Its flow depends
on the amount of resistance it encounters in the pipeline. To control the
flow of a centrifugal pump it is normally necessary to restrict the discharge
pipeline, usually with a valve, and thus set the flow at the desired rate.
Note:
Generally speaking, do not restrict a pump's flow by putting a valve on
the suction line. This can cause damage to the pump!
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Brake Horsepower (BHP)
Along the bottom of this performance curve are brake horsepower lines sloping
upward from left to right. These lines correspond to the performance curves
above them (the top performance curve corresponds to the top BHP line,
and so on). These lines indicate the amount of driver horsepower which
is required at different points of the performance curve. The lines correspond
to a BHP horsepower scale on the lower right hand corner of the page. In
our example operating point at 120 gpm and 150 feet of head we observe
that the corresponding BHP line equals about 6.8 horsepower. See the chart
below.
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End of Curve Horsepower
When sizing a motor driver to fit an application it is necessary to consider
whether the pump will ever be required to operate at a flow higher than
the duty point. The motor will need to be sized accordingly. If the pump
may flow out to the end of the curve (if someone opens the restriction
valve all the way, for example) it is important that the motor does not
become overloaded as a result. Therefore it is normal practice to
size the motor not for the duty point, but for the end of curve (EOC) horsepower
requirements. In the example shown below, a 7-1/2 hp motor would
adequately power the pump at a duty point of 120 gpm at 150 feet. But notice
that the end of curve brake horsepower requires that a 10 hp motor
be used.
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Proceed to Lesson Three