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How to Read a Centrifugal Pump Curve

By Welcome / September 24, 2018

Centrifugal Pump curves show 8 important factors that are critical in selecting the right pump for your application.  These factors are: Flow, Head, Impeller Trim Diameter, Efficiency, Power, NPSHR, MCSF (Minimum Continuous Stable Flow) and RPM.  The Pump curve also displays the Pump model, the Pump size and number of stages for multi-stage pumps.

The Pump curve shows the Flowrate range on the horizontal scale.  As standard the Flowrate is from zero flow to 20% past the Best Efficiency Flow. The first point on the curve at zero flow is referred to as “Shut Off” and the last point at maximum flow is “Run Out”.

The vertical scale on the main curve is always differential head.   Differential Head, often referred to as “Head” or “Total Dynamic Head”, is a universal method of quantifying the pressure the pump is capable of developing.   Since pressure is relative to the process fluid density, virtually all pump curves display this metric in a unit of length (FT or Meters).  This makes the pump curve applicable to most liquid types.  Exceptions are liquids with solids or high viscosity.

Most centrifugal pumps are characterized by an elliptical head curve which is at a maximum at zero flow (Shut Off) and a minimum at Run Out.

Once the Rated Flow is plotted vertically, and the Rated Head plotted horizontally, where the two points meet, is Rated Duty Point, which is usually shown as a triangle.  Below is a pump Curve showing Flow, Head, HP,  NPSHR, Efficiency, Impeller diameter and the MCSF.

pump performance curve

how to read pump curve

Below we identify these factors on this Curve.

The triangle representing the Rated Duty Point is the location where the System Resistance Curve that the pump is feeding, crosses the pump Head-Flow Curve.

Pump curves are drawn showing the performance of the Rated Impeller Diameter.  Some curves are drawn to show the performance of several Impeller Diameters from the Maximum Impeller to the Minimum Impeller.  Centrifugal pump impellers can be machined to larger or smaller diameters to alter the performance to best match the application.  The Rated Diameter Impeller is the actual size of the impeller installed in the pump.

As shown on the above Pump curve, each red Efficiency line represents one Efficiency value, they are known as ISO-Efficiency lines. Some pump curves show diagonal HP lines crossing all the Head-Flow Impeller diameter curves, which are called ISO Power lines.  When trying to determine the Efficiency or Power of any duty point between the ISO-lines, the gap between all ISO-lines is linear.

If the duty point is midway between two ISO Power lines such as 100HP and 140HP ISO Power lines, the actual Power of the duty point is also midway between 100HP & 140HP, which is 120HP.

NPSHR is another important value that is plotted on most pump curves.  Most curves display NPSHR values from MCSF to Run Out.  NPSHr will generally increase exponentially as flow rate is increased.

 

Now you have a general understanding of how to read a centrifugal pump performance curve.  Stay tuned for our upcoming training blogs that explore more aspects of pump application, design and maintenance.

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