Pump Curve Explained: How to Read a Performance Curve

What is a Pump Curve?

A pump performance curve (also called a pump characteristic curve) is a graph that shows the relationship between flow rate and head for a specific pump at a specific speed and impeller diameter. It's the primary tool for selecting, operating, and troubleshooting centrifugal pumps.

The curve is generated by testing the pump at the manufacturer's test facility. The pump is run at a fixed speed, and the discharge valve is opened gradually from fully closed (shutoff) to fully open. At each point, the flow rate, head, power, and efficiency are recorded and plotted.

Reading the Curve

A typical pump curve has four important lines:

• Head-Flow curve (H-Q): The primary curve. Shows how the head (pressure) the pump can produce decreases as flow increases. At zero flow (shutoff), the head is maximum. At maximum flow, the head is minimum. The shape of this curve depends on the impeller design.
• Efficiency curve (η-Q): Shows the pump's efficiency at each flow rate. The peak of this curve is the Best Efficiency Point (BEP). Operating near BEP gives the lowest energy consumption and longest pump life.
• Power curve (P-Q): Shows the motor power required at each flow rate. For most centrifugal pumps, power increases with flow. The motor must be sized for the maximum power the pump will draw at the operating point.
• NPSHr curve: Shows the Net Positive Suction Head required at each flow rate. NPSHr increases sharply at high flow rates. The available NPSH (NPSHa) must always exceed NPSHr.

Key Points on the Curve

• Shutoff head: The maximum head the pump can produce, at zero flow. This is the pressure the pump develops when the discharge valve is fully closed. Useful for determining if the pump can overcome the system's static head.
• Best Efficiency Point (BEP): The flow rate where the pump operates at maximum efficiency. Operating at or near BEP gives the lowest energy consumption, least vibration, and longest bearing and seal life. Target: operate between 70% and 110% of BEP.
• Minimum continuous flow: The lowest flow rate at which the pump can operate without damage from overheating, recirculation cavitation, or bearing loads. Typically 30% of BEP flow. Below this, the pump must be bypassed.
• Run-out point: The maximum flow rate the pump can produce, at the far right of the curve. Operating here causes high motor load, high NPSHr, and cavitation. Never operate at or beyond run-out.

System Curve and Operating Point

The pump curve shows what the pump can do. The system curve shows what the system requires. Where these two curves intersect is the operating point — the actual flow and head the pump will produce in the system.

The system curve is calculated from:

Where: H_static = static height difference (m), H_friction = friction losses (proportional to flow²)

The system curve starts at the static head (the pump must overcome this even at zero flow) and rises with flow² (friction losses increase with the square of flow). The intersection of the pump curve and system curve is where the pump will actually operate.

Impeller Trimming

Pump curves typically show multiple curves for different impeller diameters. A smaller impeller produces less head and flow. If the pump is oversized, the impeller can be trimmed (machined to a smaller diameter) to reduce the operating point. This is far cheaper than replacing the pump or installing a VSD.

Where: Q = flow, H = head, P = power, D = impeller diameter

The affinity laws show that trimming the impeller reduces flow linearly, head by the square, and power by the cube. A 10% impeller trim reduces power by 27% — a significant energy saving.

Variable Speed Operation

When a Variable Speed Drive (VSD) is installed, the pump curve changes with speed:

Reducing speed by 20% reduces flow by 20%, head by 36%, and power by 49%. This is why VSDs are so effective for variable-flow applications — the energy savings are enormous.