Pump Curves Made Simple
A pump curve shows how a pump performs at different flow rates. Learn to read pump curves, understand head, NPSH, efficiency, and how to match a pump to your system.
What is a Pump Curve?
A pump curve is a graph showing the performance characteristics of a pump. The horizontal axis is flow rate, and the vertical axis is head (pressure). The curve shows how much pressure the pump can generate at any given flow rate.
Reading a pump curve is the most fundamental skill in pump selection. It tells you whether a pump can meet your system requirements and at what efficiency.
Key Points on the Curve
- Shut-off head: The maximum pressure the pump can produce, at zero flow (discharge valve closed). The pump can never produce more pressure than this.
- Best Efficiency Point (BEP): The flow rate where the pump operates at maximum efficiency. This is the ideal operating point. Aim to operate within 70 to 120% of BEP.
- Duty point: Where the pump curve intersects the system resistance curve. This is the actual flow rate and head the pump will deliver in your system.
- NPSH required curve: Shows the minimum suction pressure required at each flow rate to prevent cavitation. Compare to NPSH available in your system.
- Power curve: Shows the motor power required at each flow rate. Ensures the motor is adequately sized.
System Curve
The system curve shows how much pressure the system requires at each flow rate. It has two components: static head (the fixed height difference the pump must overcome) and friction head (pipe and fitting losses, which increase with flow).
The pump duty point is where the pump curve and system curve intersect. At this point, the pump's output exactly matches the system's requirements.
How to find your duty point
Plot the system curve on the same graph as the pump curve. The intersection is the duty point, where the pump will actually operate. If this point is not near the BEP, consider a different pump or adjust the impeller diameter.
Using the Curve for Troubleshooting
- Low flow: If actual flow is less than the curve predicts, check for: blocked suction strainer, closed valve, air in the system, worn impeller, or higher system resistance than expected.
- Low pressure: If pump cannot reach required pressure, check for: worn wear rings (internal recirculation), wrong impeller diameter, wrong rotation direction, or air in the system.
- High motor current: If motor is overloaded at the operating point, the pump may be running too far right on the curve (too much flow). Throttle the discharge or reduce impeller diameter.
- Cavitation: If the pump is noisy and vibrating, check NPSH. The actual NPSH available must exceed the NPSH required by the pump curve at the operating flow.
Tip: Keep a copy of every pump's performance curve in the maintenance file. When troubleshooting, compare actual performance to the curve. The gap between expected and actual performance tells you what is wrong.