Relationship Between Head and Flow Rate in Centrifugal Pumps

1. Basic Relationship

The relationship between head (H) and flow rate (Q) generally shows a declining trend:

As the flow rate increases, the head decreases; as the flow rate decreases, the head increases.

This curve is known as the pump performance curve (H–Q curve).

✅ Explanation:

When the flow rate increases:

• The liquid moves faster inside the pump;

• Flow resistance and friction losses increase;

• As a result, the discharge head drops.

When the flow rate decreases:

• The internal flow becomes smoother;

• Energy loss is reduced;

• The head increases accordingly.

? 2. Typical Head–Flow Curve

In a pump performance chart, the horizontal axis represents flow rate (Q), and the vertical axis represents head (H).
The curve slopes downward from left to right — showing that higher flow leads to lower head.

⚙️ 3. Operating Point

When a centrifugal pump is connected to a piping system, the system itself has a system resistance curve, which rises as flow increases.
The intersection of the pump performance curve and the system curve is called the operating point.

At this point, the pump’s output flow rate exactly matches the system’s resistance.

? 4. Other Related Performance Curves

• Power curve (P–Q): As the flow increases, the shaft power also increases.

• Efficiency curve (η–Q): The pump has a Best Efficiency Point (BEP) where it operates most efficiently.

• Operating far from the BEP (too high or too low flow) reduces efficiency and can cause vibration or noise.

? 5. Summary Table

? 6. Practical Tips

1. Select a pump whose operating point is close to the Best Efficiency Point (BEP).

2. Avoid running the pump for long periods at very high or very low flow rates.

3. Use valve adjustment or variable frequency control (VFD) to maintain balance between head and system requirements.