pump min flow calc

7/28/2019 Pump Min Flow Calc

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Reading a Pump Curve

Pump Name and Speed

The customary naming of a pump provides the discharge diameter, suction diameter, and maximum impeller diameter. Infigure 1 we see the pump curve for a 4x6-13 pump. That means the pump discharge nozzle diameter is 4 inches, thepump suction nozzle diameter is 6 inches, and the maximum impeller diameter is 13 inches. Some manufacturers list thesuction diameter before the discharge diameter, but this poses little problem since the suction diameter is always greaterthan or equal to the discharge diameter on a centrifugal pump.

The second bit of information on a pump curve is the pump rotational speed. Pump speed is measured in revolutions perminute. When a manufacturer tests a pump, the rotational speed is recorded, and the pump performance data is suppliedfor that speed. If the pump will be operating at a speed other than the test speed, the pump curve will be adjusted toreflect the change in pump speed.

Pump Head

The pump head is displayed on the Y-axis of the pump curve and is read in head of fluid (feet or meters). The pump head

varies with the volumetric flow rate, or capacity, through the pump suction. The pump capacity values are read on the X-axis.

Head is used in centrifugal pumps as the energy content of the liquid referred to datum elevation. It is expressed in unitsof energy per unit weight of liquid. The measuring unit for head is a foot (or meter) of liquid.

For this pump curve (with a 10.5-inch diameter impeller running at 1150 rpm, with no flow going through the pump) thehead is 55 feet. With a flow rate of 800 gpm through the pump (the end of the pump curve), the head is 24 feet. Noticethe pump head decreases as the flow rate increases. This is typical of a centrifugal pump.

The head value with no flow rate through the pump is referred to as the Shut Off head. A pump is typically not run atshutoff because of potential problems; always consult your pump manufacturer if you intend to run the pump with thepump discharge closed.

The red line indicated on the pump curve is the manufacturers specified minimum flow rate through the pump. A pump

should not be run below the minimum flow rate.

The crosshatched area on the pump curve is known as the manufacturers recommended operating window. This is therange of flow through the pump that the manufacturer recommends for continuous operation.

The next point of interest on the pump head curve is the rated condition point (the red bracket on the pump curve). Thisrated condition of the pump applies to the capacity, head, net positive suction head, and speed of the pump as specifiedby the customers order. This is sometimes referred to as the specified condition point.

The maximum flow rate specified by the pump manufacturer is displayed on the end of the pump curve. The maximumflow is often referred to as run-out. A pump should never be run past the end of the pump curve.

Two additional head curves are displayed on the pump curve in figure 1. The 13-inch curve is the head performance forthe maximum impeller diameter that the manufacturer provides. The 9-inch curve shows the head performance for theminimum impeller diameter that the manufacturer provides.

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Figure 1 Typical pump curve showing head, multiple trims and iso-efficiency lines.

The manufacturer can change the impeller diameter within the maximum and minimum values, which changes the pumpperformance. This is accomplished by trimming metal off the impeller, hence the term "trimming" the impeller. The pumpin this example has been trimmed to 10.5 inches.

Pump Efficiency

Pump efficiency is the ratio of the energy transferred to the liquid by the pump compared to the energy delivered to thepump shaft, expressed in percent. The pump efficiency can be displayed as a continuous curve as a function of the flowrate, where the efficiency value is on the Y-axis. Referring to figure 2 you can see the efficiency curve for the 10.5-inchimpeller.



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Figure 2 Pump curve for a single impeller trim showing head, efficiency, NPSHr, and power.

In Figure 1 you see the pump curve displayed as iso-efficiency lines. In looking at the 10.5-inch impeller you can seethat at 327 US gpm the pump efficiency is 65%, and is 70% at a flow rate of 391 US gpm. You must interpolate the pumpefficiency between the published iso-efficiency lines. With the Figure 2 pump curve you can read the efficiency for anyflow rate.

A major landmark on the pump curve is the Best Efficiency Point or BEP (pronounced beep). The BEP is the capacityand head at which the pump efficiency is maximized. The pump is designed by the manufacturer to be operated at ornear its BEP.

Net Positive Suction Head requiredThe Net Positive Suction Head required (NPSHr) is defined as the amount of suction head over vapor pressure requiredto prevent more than 3% loss in total head from the first stage of the pump at a specific capacity1. The NPSHr isdisplayed on the Y-axis of the pump curve as a function of the flow rate through the pump. The manufacturer typicallyprovides the NPSHr from the minimum recommended flow rate to the pumps run-out.

You should not extrapolate the NPSHr curve past the manufacturers defined range because the NPSHr value canchange rapidly outside the manufacturers range of recommended flow.

Power Curve

The Pump Input Power is defined as the power delivered to the pump shaft at the driver to the pump coupling. It is alsoreferred to as brake horsepower. The power value is displayed on the Y-axis and the input power typically increases withan increasing flow rate. In some cases the power increases then may decrease after the BEP flow. The maximum powerdraw on the power curve is used in sizing the motor needed to drive the pump.

Multiple Impeller Trims

As previously stated, impellers are trimmed to achieve a range of rated condition points. As a result manufacturers oftenshow performance data for multiple impeller diameters on the same pump curve. Figure 3 is an example of amanufacturers catalog pump curve. The head performance data is displayed for a variety of impeller diameters for thesame pump model or size.



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Figure 3 The Best Efficiency Point is displayed for the range of available impeller trim.

In figure 3 you will see the head and flow curves for five impeller diameters. Notice how the selection window provides anindication of the manufacturers recommended continuous operating range for this pump. The iso-efficiency lines showthe pump efficiency for all impeller diameters. The BEP for all the pumps can also be easily seen as a line connected bythe vertices of the iso-efficiency curves. (See dashed-line on figure 3).

Conclusion

The pump curves provided by the pump manufacturer tells how the pump works for different operating of conditions. Theinformation may be displayed in a variety of formats, but the same information is always displayed (head, flow, efficiency,and NPSHr). Once you know how to read a pump curve you can use that information to select a new pump or evaluatehow the existing pump is operating in the piping system.

The most important thing to remember about reading a centrifugal pump curve is that the pump should only be run in therange of flows specified by the manufacturer. Running the pump with a flow rate outside the manufacturersrecommendations will result in shorter pump life and higher operating cost.

Footnotes

ANSI HI 1.1-1.5 1994 Centrifugal Pumps for Nomenclature, Definitions, Application and Operation


pump min flow calc

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