How to Read a Pump Curve

First: Know Which Kind of Curve You Are Looking At

The phrase "pump curve" can describe different chart styles depending on the pump technology. If you do not identify the curve type first, it is easy to read the wrong variables and choose the wrong pump.

The two most common curve types your readers will run into are:

1. Centrifugal pump curves: usually focused on flow, head, efficiency, power, and NPSHr.

2. AODD pump curves: usually focused on flow, discharge pressure, air inlet pressure, and air consumption.

For a centrifugal example, see Griswold's® complete guide to centrifugal pumps. For an AODD-specific example, see How Understanding AODD Pump Performance Curves Optimizes Operations.

How to Read a Centrifugal Pump Curve

A centrifugal pump curve is usually the chart people picture first. Even if it looks busy, it becomes manageable once you break it into layers.

Here is the basic reading order:

• Step 1: Find the required flow rate on the horizontal axis (usually gpm).

• Step 2: Move vertically until you hit the pump's impeller trim curve.

• Step 3: Read horizontally to the left-hand axis to see how much head the pump produces at that flow.

• Step 4: Check where that same point falls against the efficiency lines to see how efficiently the pump runs there.

• Step 5: Check the power line to understand motor load.

• Step 6: Check the NPSHr line to see what suction margin the pump needs to avoid cavitation.

Griswold® explains this directly in its centrifugal pump guide: the left axis is head in feet, the bottom axis is flow, efficiency is shown on the right axis, power is shown on a separate axis, and NPSHr is plotted in feet below the chart.

The Most Important Centrifugal Terms on the Curve

If you want readers to understand a curve quickly, these are the terms that matter most:

• Head: the pressure energy the pump adds to the liquid, usually shown in feet of head.

• Impeller trim: the specific impeller diameter that defines that performance line.

• Efficiency: how effectively the pump converts input energy into useful hydraulic work.

• BHP or power: how much horsepower or kilowatts the motor must deliver at that point.

• NPSHr: the minimum suction margin the pump requires to avoid unacceptable cavitation.

Readers often assume the main flow/head line is the whole story. It is not. The supporting lines are what tell you whether the pump can run reliably at that point once real system conditions are considered.

What BEP Means (and Why It Is the 'Healthy Zone')

Best Efficiency Point, or BEP, is the operating point where the pump is most efficient. It is also usually the point where vibration, radial load, and mechanical stress are lowest. That makes BEP more than an energy metric - it is a reliability metric.

Griswold notes in its centrifugal pump guide that pumps running at BEP generally see the lowest radial loads and lowest vibration levels, while moving away from BEP increases shaft loading, vibration, and wear.

That is why a good pump selection is usually not the point where the curve says the pump can technically run. It is the point where the curve says the pump can run well.

What Composite Curves Show (Multiple Curves on One Chart)

Some pump curves show more than one impeller trim or more than one speed. These are composite curves. They help you compare multiple operating envelopes on one chart.

A composite curve usually adds:

• Multiple trim lines or speed lines.

• Iso-efficiency lines that arc between trim lines.

• Iso-power lines that help estimate motor load at different operating points.

• A separate NPSHr section tied to the selected trim.

PSG® covers this in Griswold's® composite performance curve article, which explains how published composite data helps estimate head, efficiency, power, and NPSHr for varying trim sizes or speeds.

How to Read an AODD Pump Curve

AODD curves look different because they reflect an air-driven pump, not a motor-driven centrifugal pump. That means the curve is not just telling you what the pump can move - it is also telling you what air supply you must deliver to make that performance possible.

A practical reading order for an AODD curve is:

• Step 1: Find your target discharge flow on the horizontal axis.

• Step 2: Move to the required discharge pressure on the vertical axis.

• Step 3: Use the curved air-pressure line to determine the inlet air pressure needed.

• Step 4: Use the air-consumption line to estimate SCFM required at that operating point.

PSG's® AODD curve article explains that AODD charts typically use flow on the horizontal axis, pressure on the vertical axis, a curved line for air inlet pressure, and another curved line for air consumption in SCFM.

The AODD Mistake People Make Most Often

One of the most common misunderstandings in AODD selection is assuming PSI alone determines output. In practice, air-operated pumps need both pressure and air volume. A curve helps reveal that because the operating point is tied not just to discharge pressure, but also to air consumption.

This is exactly why AODD curves are useful in troubleshooting. If the curve says the pump needs more SCFM than the compressor or air plumbing can actually deliver, the pump may never reach the expected flow even if the regulator shows high PSI.

For a broader AODD refresher, see the AODD technology page.