How to read a centrifugal pump curve and choose the right pump
FZ INOX technical guide · October 2026 · with the real curves of the pumps we build in Thiene, Italy
In short. The H–Q curve of a centrifugal pump shows how much head (H, in metres) the pump delivers at each flow (Q, in m³/h). To choose the pump you work out the point your system needs, flow and head, and look for the curve that passes over it without going beyond the motor limit. In service the pump runs where its curve crosses the system curve: that is the duty point.
The axes: flow and head
On the horizontal axis is the flow Q, how many cubic metres per hour go through. On the vertical axis is the head H, in metres: the energy the pump gives to the liquid, expressed as the height of the liquid column it could lift.
Metres are used instead of bar because the head of a centrifugal pump does not change with the density of the liquid: the pressure does. With water there is a rule of thumb: 10 m of head is about 1 bar. That is why our FZ-CP pumps, which reach 45 m, give at most 4.5 bar at the discharge at 50 Hz.
The shape of the curve
The curve starts at the top left, from the shut-off head: the pressure the pump gives with the discharge valve closed. Then it goes down: the more liquid flows, the less head is left.
On our curves every line ends with an ×: that is the flow at which the motor reaches full load. Further right the motor would be overloaded, so the flow you need must stay before the ×.
Many manufacturers also show efficiency, absorbed power and NPSH required on the same chart. They are useful for energy use and suction: if you need them for your pump, ask us for the data sheet of the exact configuration.
The curve family
The same pump casing can take impellers of different diameters. Each diameter has its own curve: larger impeller, higher curve. All the curves of one model form the curve family.
The FZ-CP range has impellers from Ø105 to Ø180 mm and motors from 0.55 to 7.5 kW: 57 curves at 50 Hz in all. You will find them in the interactive tool on the FZ-CP page, and a model-by-model summary in the “Performance by model” table.
The system curve
The system has a curve too: it shows how much head is needed to push each flow through it. It has two parts:
- Static head: the height between the liquid level you draw from and the highest point you pump to, plus any pressure difference between the two tanks. It does not change with flow.
- Friction losses: friction in pipes, bends, valves, heat exchangers and filters. They grow with the square of the flow: twice the flow, four times the losses.
In the example below the system has 10 m of static head and 15 m of losses at 20 m³/h: at that flow it needs 25 m.
The duty point: an example on real curves
The pump runs where its curve crosses the system curve: there the head the pump gives equals the head the system asks for. For 20 m³/h at 25 m, among our 50 Hz curves:
| Pump | At 20 m³/h | Motor limit | Duty point on the system |
|---|---|---|---|
| FZ-CP30, Ø155 impeller | 25 m | 23 m³/h | 20 m³/h at 25 m: exactly what is needed |
| FZ-CP40, Ø160 impeller | 28 m | 35 m³/h | 21.5 m³/h at 27.4 m: more than needed |
The FZ-CP30 Ø155 is the right choice: it runs on the required point, with the × just to the right. The FZ-CP40 Ø160 is not wrong, but on the same system it pushes 21.5 m³/h instead of 20, with a bigger motor: you pay more energy for flow you do not need.
How to choose the pump, step by step
- The flow you need, in m³/h or l/h.
- The head: static height, plus the friction losses of the line at that flow, plus the pressure needed downstream.
- The curve that passes on the point, or just above it, with the × beyond your flow.
- Do not oversize: between two good curves, the one closer to the point uses less energy.
- The frequency of your mains: 50 or 60 Hz, because the curves change.
- The suction: a centrifugal pump must receive the liquid already full, with a positive head or slight pressure. Hot liquid or a long suction line reduce the margin against cavitation (the NPSH available). If you must draw from a lower level you need a self-priming pump, like the FZ-SP.
- The product: viscosity, temperature, particles. FZ-CP pumps have an open impeller, which lets fine solid particles through without clogging.
50 or 60 Hz: the affinity laws
At 60 Hz the motor runs faster: about 3,500 rpm instead of 2,900. The affinity laws say how performance changes with speed: flow grows with the speed, head with its square, power with its cube. From 2,900 to 3,500 rpm: flow +21%, head +45%, power +76%.
Our measured curves confirm it. The FZ-CP30 Ø155 at zero flow goes from 30 m at 50 Hz to 43 m at 60 Hz. But the 3 kW motor reaches full load at 9 m³/h instead of 23. That is why at 60 Hz the pump is chosen on the 60 Hz curves, which you find in the “50 vs 60 Hz” section of the tool on the product page.
Adjusting the duty point
- Discharge valve: closing it adds losses, the system curve rises and the flow drops. It is simple, but the extra energy is lost in the valve.
- Inverter: changing the motor speed moves the pump curve, following the affinity laws. It is the way that wastes the least energy.
- Impeller diameter: chosen when ordering, to put the curve where it is needed.
The product matters: viscosity and temperature
The curves are measured with water. For milk, whey, wine, beer and low-viscosity liquids they are a good basis. With more viscous products the pump gives less flow and less head and draws more power; with hot liquids the suction margin gets smaller. When you ask us for a pump, tell us what you pump, at what temperature and how dense or viscous it is: we choose it on those data.
Frequently asked questions
What is pump head?
It is the energy the pump gives to the liquid, expressed as the height of a column of liquid, in metres. It includes the height to overcome, the friction losses in the piping and any pressure required downstream.
How many bar is 10 metres of head?
With water about 1 bar: a 10 m water column is 0.98 bar. With a denser liquid the same head gives a higher pressure, in proportion to the density.
What does the × at the end of FZ INOX curves mean?
The flow at which the motor reaches full load. Beyond that point the motor would be overloaded: your flow must stay before the ×.
What happens if I choose a pump that is too big?
On your system it will deliver more flow than needed and use more energy. You can bring it back to the right point by throttling a valve on the discharge or slowing it down with an inverter, but it is better to choose it right.
Are the curves valid for milk?
The curves are measured with water. For milk and low-viscosity liquids such as wine, beer and whey they are a good basis. With viscous products the pump gives less flow and less head and draws more power: tell us the viscosity and we take it into account.
Can I use the same pump at 60 Hz?
The pump yes, but the performance changes: at 60 Hz the head rises by about 45% and the motor reaches full load much earlier. It must be chosen on the 60 Hz curves.
Got flow and head? Send us your system data: we pick the right curve among ours and build the pump in our workshop in Thiene, Italy.