Each centrifugal pump is associated with its own characteristic curve, which is the graphical representation of the pump's performance.

  • On the x-axis (horizontal axis), the flow rate Q is indicated, usually in m3/h. It represents the amount of fluid passing through each section of the centrifugal pump over a defined period. This quantity depends on the dimensional characteristics of the pump, the number of motor revolutions (thus the rotational speed of the impeller), and the characteristics of the fluid (density and viscosity depending on temperature). The flow rate influences all the performances of the centrifugal pump and is the first technical parameter to consider.
  • On the y-axis (vertical axis), the head H is indicated, usually in meters. It is calculated from the pressure difference between the outlet and the inlet of the centrifugal pump and represents how far the fluid can be pushed if it encounters resistance in its path like height, curves, or valves.

Let's define:
  • ΔZ the height difference between the downstream basin A and the upstream basin B;
  • PA and PB the pressures acting respectively on the free surface of the upstream basin A and on the free surface of the upstream basin B;
  • Îł = specific weight of the fluid (= density of the fluid*acceleration of gravity g);
  • ÎŁY the sum of distributed and localized losses inside the system.

In ideal conditions, with perfectly smooth conduits, without curves, valves, or filters so 𝛴𝑌 = 0 and with 𝑃𝐴 = 𝑃𝐵 = 𝑎𝑚𝑏𝑖𝑒𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒, we would have 𝐻 = ΔZ, thus the centrifugal pump transfers all the energy to overcome only the height.
In reality, the pump must overcome a greater amount than just the height difference as ideal conditions can never be achieved, so the head it needs to reach is
H = ΔZ + (PB - PA)Îł + Σ Y

Once the dimensions of the centrifugal pump (impeller and volute) and the rotational speed of the impeller (given by the number of motor revolutions) are established, the characteristic curve is unique and typical for each pump.


Knowing the specific weight of the fluid γ, it is also possible to calculate the theoretical power 𝑾, in Watts, required to move it:

𝑊 = 𝛾 ⋅ 𝑄 ⋅ 𝐻
The actual power 𝑾a absorbed by the motor is slightly greater as it is necessary to consider that there will always be losses due to friction and fluid dynamics within the pump itself, considered in the efficiency η. The power curve, always a function of the flow rate Q, then refers to the following formula
𝑊𝑎 = 𝑊/𝜂
It is intuitive to understand that as the flow rate increases, the required power will also increase, as shown in the graph below.

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