Finding the duty point instead of guessing a pump size
Reading a pump curve as a system curve, and matching the two to find where the system will actually operate.
The engineering question
The pump and the system were both chosen, but the flow is wrong. Where is the real operating point?
Worked example
The figures below were produced by running the Pipe Fitting Loss Calculator with the same inputs — during the build, from its authoritative engine.
| Head loss | 0.4893 m |
|---|---|
| Pressure drop | 4.8 kPa |
| Velocity head | 0.2039 m |
| Equivalent straight length | 12 m |
| Assessment | Through a K = 2.4 fitting the head loss is 0.49 m (≈ 5 kPa water). Equivalent straight length ≈ 12.0 m at f = 0.02. |
- Every fitting on the route contributes a loss coefficient that must be summed before the system curve is drawn.
- Frictional loss over the pipe run is added to those fitting losses — they are not alternatives.
- Read the intersection of the system curve with the manufacturer's pump curve; that intersection is the duty point.
- NPSH at that flow is checked against the curve, not against the shut-off value.
Sanity check
The duty point must lie on the pump curve. A calculated flow that does not appear anywhere on the curve means the system resistance was built wrong.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
- Pump Power Calculator Calculate hydraulic power, shaft power, and motor electrical power from head and flow rate.
- NPSH Calculator Net positive suction head available at the pump from pressure, vapour pressure, static head and suction friction.
- Pipe Fitting Loss Calculator Head loss, pressure drop and equivalent length for a pipe fitting from its K-value and flow velocity.
- Pump Head / Total Dynamic Head Calculator Compose total dynamic head from static lift, friction losses, minor losses, and residual pressure.
Engineering review required
This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.
References & standards
- Hydraulic Institute — pump system design
- Crane TP-410 — system curves
Related problems
Where the pressure in a pipe line actually goes
Separating frictional loss from fitting loss, and why small bore runs lose far more than intuition suggests.
Why a pump is cavitating and what NPSH margin it needs
Working out the net positive suction head available at the pump eye, and why a few centimetres of margin matter.
Also: Environmental