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

  • On the x-axis (horizontal axis) is the flow rate Q, usually in m3/h. It indicates the amount of fluid that passes through each section of the centrifugal pump over a defined period. This amount depends on the pump's dimensional characteristics, the motor's speed (i.e., the rotation speed of the impeller), and the fluid's characteristics (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) is the head H, 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 along its path, such as height, curves, or valves.

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

In ideal conditions, with perfectly smooth conduits, no curves, valves, or filters, thus 𝛴𝑌 = 0 and with 𝑃𝐴 = 𝑃𝐵 = ambient pressure, we would have 𝐻 = ΔZ, so the centrifugal pump imparts all its energy to overcome only the height.
In reality, the pump must overcome more than just the height difference as ideal conditions can never be achieved. Therefore, the head that needs to be reached is
H = ΔZ + (PB - PA)γ + ΣY

Once the dimensions of the centrifugal pump (impeller and volute) and the rotation speed of the impeller (given by the motor's RPM) are established, the characteristic curve is unique and typical for each pump.


Knowing the specific weight of the fluid γ, it's 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's necessary to consider that there will always be losses due to friction and fluid dynamics within the pump itself, which are taken into account in the efficiency η. The power curve, always as a function of the flow rate Q, refers to the following formula
𝑊𝑎 = 𝑊/η
It's 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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