Centrifugal Pump Curves

How manufacturers summarize pump performance with head–flow data.

introfluidspumps

⏱️ About 18 min

A pump does not deliver “a flow rate” by itself—it delivers whatever flow the system allows, and its curve tells you what head it can supply at that flow.

💡

The big idea: A centrifugal pump curve (Head H vs Flow Q) is an empirical performance map provided by the manufacturer; you use it to predict head at a given flow and to match a pump to a system.

🎯 By the end, you'll be able to

Interpret a typical head–flow (H–Q) pump curve
Explain why H generally decreases as Q increases for a centrifugal pump
Use a simple algebraic pump-curve model to compute head at a given flow
Recognize that efficiency and power curves accompany the H–Q curve

📎 Helpful to know first

Orifice & Venturi Flow Measurement

What a pump curve is

A centrifugal pump curve most commonly plots head (often in meters of liquid) versus flow rate (often in m³/s or L/s). For a given pump speed and impeller diameter, the curve tells you the head the pump can supply at each flow.

In introductory courses, you typically do not derive this curve from first principles. Instead, you treat it as manufacturer data (or a fitted model) used to predict operating behavior.

🔑 Typical shape

For a centrifugal pump at fixed speed and impeller diameter, the H–Q curve typically falls with increasing Q: higher flow means more internal losses and less head rise available to the fluid.

Companion curves: efficiency and power

Datasheets often include additional curves alongside the H–Q curve:

Efficiency vs Q (often with a peak at the best efficiency point, BEP), and shaft power vs Q. These matter for energy cost and motor sizing, even if the first analysis focuses on head and flow.

\[ H_{\text{pump}}(Q)\approx H_0-aQ^2 \]

A common simple model: head decreases approximately with the square of flow.

✨ Why a Q² form shows up so often

Many hydraulic losses scale with velocity squared, and velocity is proportional to Q for a given cross-section. So a quadratic dependence is a simple way to approximate how head drops as flow increases.

📝 Worked example: A pump curve is approximated by H = 40 − 1000 Q², where H is in meters and Q is in m³/s. Estimate the pump head at Q = 0.10 m³/s.

Use the model H = 40 − 1000 Q².
Compute Q²: (0.10 m³/s)² = 0.0100 (m³/s)².
Compute the head drop term: 1000 × 0.0100 = 10.0 m.
Compute head: H = 40 − 10.0 = 30.0 m.

✓ 30.0 m

✏️ Practice: A different pump is modeled by H = 35 − 800 Q² (H in m, Q in m³/s). What head does it deliver at Q = 0.15 m³/s?

Solution

Compute Q²: (0.15)² = 0.0225.
Compute head drop: 800 × 0.0225 = 18.0 m.
Compute head: H = 35 − 18.0 = 17.0 m.

⚠️ Speed and impeller diameter matter

The pump curve is specific to a speed (rpm) and impeller diameter. Changing either shifts the curve. Always confirm the curve corresponds to the operating configuration.

Check your understanding

1. For a centrifugal pump at fixed speed, the H–Q curve usually:

Centrifugal pump head typically decreases with increasing flow at fixed speed/impeller due to internal losses and flow effects.

2. Why are pump curves usually taken from manufacturer data in an intro course?

The internal fluid mechanics are complex; manufacturers test and provide curves for practical use.

3. A pump modeled by H = H0 − aQ² implies that as Q increases:

The −aQ² term grows with Q², reducing the available head as flow increases.

✅ Key takeaways

A pump curve relates head H to flow Q for a specific speed and impeller diameter.
Centrifugal pump H–Q curves usually fall with increasing flow.
Efficiency and power curves often accompany the head curve.
Simple models like H = H0 − aQ² can approximate manufacturer data for calculations.

➡️ Now that you can compute head from a pump curve, the next step is to combine it with the system’s required head to find the operating (duty) point.

Want to test yourself on this? Try the Chemical Aptitude test →