How to Find a Pump’s Operating Point
A desired flow and head is a design duty. The installed pump settles at the intersection for the actual pipework, valve position and speed. Finding that point explains why a pump may deliver more or less flow than the design target.
Start with curves for the right conditions
- Obtain the pump model, impeller diameter, speed and fluid conditions for the manufacturer’s curve. Keep flow and head units consistent.
- Build the system head curve from the static boundary and pipe, fitting, valve and equipment losses at several flows. An estimate at one flow is only one point.
- Overlay the curves on the same axes. Trace the intersection down to flow and across to head.
A filled closed circulation loop has no net elevation lift around the complete circuit. An open transfer between water surfaces can have static lift and a pressure difference. The pump head guide explains the loss terms; fill pressure and suction pressure are separate checks.
Find an operating point with an original example
Synthetic inputs: fixed speed and water-like conditions; Q in m³/h and H in metres. These equations illustrate the method and do not describe a catalog pump:
H_pump = 30 − 0.02Q²H_system = 10 + 0.03Q²
The constants 30 and 10 are in metres; the coefficients 0.02 and 0.03 are in m/(m³/h)². The 10 m intercept is an assumed static requirement. A quadratic loss approximation is useful for this example; real systems need their own loss model.
| Q (m³/h) | Pump H (m) | System H (m) | Throttled system H (m) |
|---|---|---|---|
| 0 | 30 | 10 | 10 |
| 10 | 28 | 13 | 16 |
| 20 | 22 | 22 | 34 |
| 30 | 12 | 37 | 64 |
Set pump head equal to system head: 30 − 0.02Q² = 10 + 0.03Q². Thus 20 = 0.05Q², so Q = 20 m³/h and H = 22 m. Substitution into both equations gives 22 m.
What happens when a valve closes or speed changes?
For a valve change, keep the synthetic pump curve and increase the system loss coefficient to 0.06: H_throttled = 10 + 0.06Q². Now Q = √(20/0.08) = 15.81 m³/h, and H = 25 m. Flow decreases while pump head increases in this example. Efficiency and power cannot be inferred from these two head curves.
A speed change changes the pump curve. Use the manufacturer’s curve at the proposed speed, then find its intersection with the applicable system curve again. Grundfos describes variable-speed performance curves at minimum and maximum RPM. A lower speed does not guarantee a particular flow or energy saving, especially when static lift remains; respect pump and motor operating limits.
Operating point, duty point and BEP
The design duty is the requested Q and H; the operating point comes from the actual curve intersection. The best efficiency point (BEP) is the peak of the efficiency curve. It need not coincide with either. Read efficiency and power at the intersection using the manufacturer’s companion curves, as shown in Zoeller’s curve-reading guide.
Check the manufacturer’s permissible operating range, motor capacity and NPSHr at each expected operating condition. Compare NPSHr with NPSH available from the system using a documented margin. Our synthetic example supplies no efficiency, BEP, power or NPSHr data, so those results remain unknown.
Worksheet for your own pump
- Record model, impeller, speed, fluid and curve revision.
- Record the static boundary and losses at minimum, design and maximum expected flow.
- Mark intersections for relevant valve and speed states.
- At each flow, record manufacturer efficiency, power, NPSHr and operating-range limits; check the suction condition separately.
Estimate system head · Calculate NPSH available · Pump selection and standards
Sources checked 2026-10-08
- Grundfos — Pump curves: Q–H axes, efficiency and variable-speed curve conditions.
- Zoeller — How to Read Pump Curves: intersection and companion power/efficiency curves. No product data or graphic reproduced.
- Bell & Gossett — Cooling Tower Pumping and Piping, printed pages 2 and 15–16: closed/open boundaries and curve intersections.