A centrifugal pump curve (or performance curve) is a graphical representation of a pump's operating characteristics, showing how flow rate (Q), head (H), efficiency (η), and power (P) relate to each other. Understanding these curves is essential for proper pump selection and troubleshooting.
A typical centrifugal pump curve includes:
Head vs. Flow Rate (H-Q Curve)
Shows how pressure (head) decreases as flow increases.
Measured in meters (m) or feet (ft).
Efficiency Curve (η-Q Curve)
Indicates pump efficiency at different flow rates.
Peak efficiency = Best Efficiency Point (BEP).
Brake Horsepower (BHP) Curve
Shows power consumption (kW or HP) at different flows.
NPSHₐ (Net Positive Suction Head Available) Curve
Ensures the pump avoids cavitation.
Head (H)
|
| ____
| / \
|/ \______
|______________
Flow (Q)
Intersection of System Curve & Pump Curve = Operating Point.
System Curve represents the piping resistance (friction + static head).
The flow rate where the pump runs most efficiently.
Operating far from BEP causes:
Low efficiency → Energy waste
Recirculation → Vibration & damage
Maximum head at zero flow (pump discharge valve closed).
Pump performance changes with speed (RPM) or impeller diameter (D).
| Law | Formula | Effect |
|---|---|---|
| Flow ∝ RPM | Q₂/Q₁ = N₂/N₁ | Speed change → Linear flow change |
| Head ∝ RPM² | H₂/H₁ = (N₂/N₁)² | Speed change → Squared head change |
| Power ∝ RPM³ | P₂/P₁ = (N₂/N₁)³ | Speed change → Cubed power change |
If RPM increases by 10%:
Flow increases by 10%
Head increases by 21% (1.1² = 1.21)
Power increases by 33% (1.1³ = 1.331)
| Problem | Cause | Solution |
|---|---|---|
| Operating Far from BEP | Wrong pump selection | Trim impeller or adjust speed |
| Cavitation | NPSHₐ < NPSHᵣ | Increase suction pressure |
| High Power at Low Flow | Recirculation damage | Install a bypass line |
| Flat vs. Steep Curve | Impeller design issue | Choose correct pump type |
| Pump Curve | System Curve |
|---|---|
| Shows pump capabilities | Shows system resistance |
| Head decreases with flow | Head increases with flow |
| Fixed for a given pump | Depends on piping, valves, elevation |
Pump Curve: Head = 50m @ 0 L/s, 30m @ 100 L/s
System Curve: Needs 20m @ 0 L/s, 40m @ 100 L/s
Operating Point: Where the two curves intersect (~80 L/s @ 35m).
Determine Required Flow & Head (from system calculations).
Match to Pump Curve (ensure BEP aligns with duty point).
Check NPSH & Power (avoid cavitation & motor overload).
Consider Future Needs (trimming impeller or speed control).
Pump curves help select the right pump and troubleshoot issues.
BEP is the ideal operating point for efficiency & longevity.
Affinity laws predict performance changes with speed/diameter.
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