KEVOS® MAKEOS · Hydraulics
Pump Operating Point on the System Curve
Where will this centrifugal pump actually run on my system, how efficiently, and what speed gives the flow I want?
Operating point of a centrifugal pump on a pipe system: a quadratic pump curve through shut-off head and the best-efficiency point, a system curve from static head and one known duty point, their intersection (flow, head), efficiency there from a parabolic efficiency curve, absorbed power, the position relative to the best-efficiency flow, and the speed (by the affinity laws) that gives a target flow.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pump at rated speed | ||
| Shut-off head | m | Head at zero flow from the pump curve. |
| Best-efficiency flow | L/min | Flow at the best-efficiency point (BEP). |
| Head at best efficiency | m | Head at the BEP. |
| Best efficiency | % | Efficiency at the BEP. |
| Rated speed | rpm | Speed the curve applies to. |
| System | ||
| Static head (lift plus pressure difference) | m | Static head of the system: level difference plus pressure difference as head. |
| Known system flow | L/min | A flow at which the system head is known (from a pipe loss calculation). |
| System head at that flow | m | Total system head at that flow. |
| Liquid density | kg/m³ | Liquid density. |
| Target flow (for the speed change) | L/min | Flow you want; the tool finds the pump speed that gives it. |
Method and formulas
- Pump: H = H₀ − a Q², with a = (H₀ − H_BEP) ÷ Q_BEP²; system: H = H_s + k Q², with k = (H_d − H_s) ÷ Q_d².
- Operating flow Q = √[(H₀ − H_s) ÷ (a + k)].
- Efficiency η = η_BEP (2x − x²), x = Q ÷ Q_BEP; power = ρ g Q H ÷ η.
- Speed ratio s for a target flow (affinity laws): s² H₀ − a Q_t² = H_s + k Q_t².
- Example: the pump runs at 390.8 L/min and 40.455 m, 97.701% of its BEP flow.
Limitations
- Quadratic fits to the pump and system curves; use the maker’s curve near the ends of the range.
- Affinity laws hold for speed changes on the same impeller; static head limits how far speed can drop.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Predict the flow a pump will deliver on a new pipe system
- Check that a pump runs near its best efficiency point
- Set a variable-speed drive for a target flow
Who it is for
- Plant engineers
- Maintenance engineers
- Process engineers
What you get
- Operating flow and head
- Efficiency, power and position against BEP
- Speed for the target flow
Interpreting the result
- Run between about 70% and 120% of BEP flow for long seal and bearing life.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3021–3026. Topic reference; no source tables or recommended settings are reproduced.
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Open the Pump Operating Point on the System Curve calculator
