Reading the Pump Curve - Intro to Pumps

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« Common Centrifugal Pump Designs NPSHr: What Is It and Why Does It Matter? »

Making a Selection the Old-Fashioned

Way

Reading the Pump CurveReading the Pump CurveReading the Pump CurveIn years past making a pump selection meant sitting down with large printed

catalogs and flipping through them until you reached a pump curve that fit the

projects hydraulic requirements. Today this process is made much easier through

the use of electronic pump curve catalogs. One of the most well-known developers

of electronic pump catalogs is Engineered Software and their pump selection

software pump-flo. All of the curves in this article were generated in their

web-based pump selection software.

A pump curve provides a wealth of information regarding the capabilities of a

pump. At it most basic level a pump curve is a graphical representation of the

performance characteristics of a pump. Information is plotted on an x-y graph

where the x-axis is measured in units of flow and the y-axis is measured in units of head, power, and NPSHr.

For the sake of example, today we’re going to look at a selection made for the following design condition: 1,000 GPM at 100 ft.

Here is a possible pump selection that might be a good fit for that operating condition.

Composite Pump Performance Curve

Pump Performance Curve: Head, Flow and Efficiency

The first piece of information provided by a pump curve is the flow that the pump will develop at any given head. The curve that

provides that information is called the pump performance curve. Some pump curves only provide a single pump performance

curve, but most will provide the maximum performance the pump is capable of achieving with a full-trim impeller, the minimum

performance the pump is capable of achieving with a minimum-trim impeller, and the performance provided by the design-trim

impeller. The design-trim impeller is the impeller trim the pump selection software has selected as the closest fit to the design

condition provided. In this case the design-trim is 16.3125”, the max-trim is 17” and the minimum-trim is 15”.

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Pump Performance Curve

Net Positive Suction Head Required Curve

Considering the design trim curve we see that at zero

flow, also known as shutoff, the pump will develop about

130 ft of head. This is the head the pump would develop

if it were operating against a closed valve. Keep in mind

that the actual pressure experienced between the pump

and the closed valve might exceed this value because a

pump ADDS head to the liquid being pumped. In other

words, if this pump were operating at shutoff with

suction pressure of 20 feet the total head experienced at

the pump discharge flange would be 150 ft (20 ft + 130

ft). So considering the design trim impeller we see that

shutoff occurs at about 130 ft, the design condition falls

very close to the pumps best-efficiency-point (BEP), and the pump will operate down to approximately 60 ft of head and

produce a flow of approximately 1380 GPM at 60 ft. The maximum and minimum trim curves also tell us the possible conditions

that the pump could be modified to meet in the future by installing an impeller of a different trim.

In addition to head and flow most pump performance curves will also provide efficiency information. A pump’s efficiency is the

relationship between the power required to drive the pump at a given operating condition and the water horsepower being

created by the pump. If a pump were 100% efficient then the input power required would be equal to the water horsepower

being generated by the pump. However, since no pump is 100% efficient every pump will require more input power than it will

generate in water horsepower. In the case of this pump the best-efficiency-point falls at approximately 1075 GPM at 95 Ft, and

efficiency at BEP is 84.1%.

Special attention should be paid to the location of BEP relative to the operating condition. Pumps run best at or near BEP. For

this reason the Hydraulics Institute has defined a pumps Preferred Operating Region (POR) as flows from 70% to 120% of flow

at BEP for most centrifugal pumps. This would mean that the POR for this pump would be from approximately 750 GPM to 1290

GPM (70% to 120% of 1075 GPM). For some pumps with high specific speed impellers the POR is a more-restrictive 85% to

110% of BEP. It is best to select a pump that will operate most of the time in the POR since this will have implications for pump

life and power consumption.

There is another region of operation that is defined by the pump manufacturer. This is the Allowable Operating Region (AOR)

and is made up of the portion of the curve shaded in light yellow. This is the region that the pump manufacturer has

determined comprises all of the points that this pump can operate at continuously. While it is preferable to select pumps to

operate within the POR, pumps should always be selected to operate within the AOR without exception. Very short-term

operation outside of the AOR might be acceptable, but the pump manufacturer should be consulted before selecting a pump

that will see even intermittent operation outside the confines of the AOR.

The pump performance curve above has two more items which should be mentioned. First, the red line on the left hand side of

the pump curve is the Minimum Continuous Stable Flow (MCSF) line. This is the point beyond which the pump manufacturer

has determined the pump should not be allowed to operate for any extended period of time. Second, the blue curve beginning

at 0 GPM and 0 Ft and extending through the design condition is the System Curve and represents the operation of the system

in which the pump is being applied. This curve can be manipulated by manually entering data points and is particularly useful

when evaluating the variable-speed performance of a pump.

NPSHr Curve

The next part of the pump curve is the Net Positive

Suction Head Required (NPSHr) curve. The NPSHr curve

provides information about the suction characteristics of

the pump at different flows. The x-axis is still measured in

flow units, but the y-axis is now measured in feet of

NPSHr. Each point along the curve identifies the NPSHr

required by the pump at that flow to avoid cavitation

issues that would be damaging to the pump and would have a negative impact on overall pump performance.

Looking back at our example design flow of 1,000 GPM we can see that this pump will require approximately 7 ft of NPSHr at

that condition. A typical safety margin between Net Positive Suction Head Available (NPSHa) and required (NPSHr) is 5 ft. So in

this case it would generally be recommended that this pump not be applied in applications where NPSHa at the design flow of

1,000 GPM is less than approximately 12 ft.


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Power Curve

« Common Centrifugal Pump Designs NPSHr: What Is It and Why Does It Matter? »

Generally speaking NPSHr does not vary dramatically between variations in impeller trim which is why we do not see separate

curves for the minimum and maximum impeller trims. Those curves are actually present, but they are overlaid by the

design-trim NPSHr curve.

Power Curve

The final portion of the pump curve is the power curve.

Once again the x-axis is measured in units of flow, but

the y-axis is now measured in power units. In this case

the unit of measurement is horsepower. This curve tells

us how much power the pump will demand at any

particular flow point. This information is useful in

ensuring the selected motor is suitably sized, and is also useful when calculating power consumption costs.

At our design flow, 1,000 GPM we can see that power demand is approximately 30 HP and that power demand is greatest at

approximately 1,300 GPM. Based on this information, if the pump were to be driven by an electric motor, most pump

manufacturer’s would recommend that the next largest motor rating be used. In this case that would be a motor rated for 40

HP.

Power demands do vary considerably depending on the impeller trim which is why separate power curves for the minimum and

maximum impeller trims can also be seen in the power curve. This information is useful if the customer would like to size the

drive unit to allow a future increase in capacity without requiring replacement of the drive unit. In that case the customer could

chose to size the drive unit for the maximum trim impeller power curve, and at a later date the capacity of the pump could be

increased by installing a maximum trim impeller without requiring replacement of the drive unit. In the case of our example

pump curve the motor required by the design trim impeller will be adequate to cover the power demanded by the maximum

trim impeller, but this will not always be the case.

Summary

When combined these three curves are called the composite pump curve, and they provide the information we need to

determine if a particular pump is a suitable selection for the hydraulic requirements of an application. The information provided

by each part of the curve is critical to ensuring the pump is a good fit for the application hydraulics.

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Reading the Pump Curve - Intro to Pumps

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