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How to Read a Pump Curve for Centrifugal Pumps | PumpWorks

What is a Pump Curve?

A centrifugal pump curve is a graphical representation of a pump’s performance characteristics. It allows users to evaluate how a pump will perform under different conditions of flow and pressure. By understanding the data presented in a pump curve, engineers and operators can ensure that a selected pump will meet the specific demands of their system, maintain efficiency, and operate within safe limits.

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 (Net Positive Suction Head Required)
• MCSF (Minimum Continuous Stable Flow)
• RPM (Revolutions Per Minute)

The Pump curve also displays the Pump model, the Pump size and number of stages for multi-stage pumps.

Understanding the Axes: Flow vs. Head

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.

Expert Tip: Avoid operating a pump continuously at Shut Off or Run Out. These are system extremes and can lead to overheating, cavitation, or premature wear. Always aim for operation near the Best Efficiency Point (BEP).

The Shape of the Curve

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.

Rated Duty Point

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.

The System Resistance 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.

Impeller Diameter and Trimming

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.

Efficiency and ISO-Efficiency Lines

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.

Power and ISO Power Lines

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 Curve

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.

Reminder: NPSHR is what the pump requires, while NPSHA is what your system provides. Always ensure NPSHA exceeds NPSHR (ideally with a safety margin of 3-5 feet) to avoid cavitation.

Expert Tips for Real-World Pump Selection

• Operate pumps close to the Best Efficiency Point (BEP) for long-term energy savings and reduced wear.
• Always ensure that NPSHA (available) is greater than NPSHR (required) to prevent cavitation.
• Trim impellers to fine-tune pump performance if your system requirements shift over time.
• Use ISO-Efficiency and ISO Power lines as a guide, but validate with actual system performance data.
• Ensure the pump curve intersects the system resistance curve in an optimal range to avoid unstable or damaging operation.

Wrapping Up

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.