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