Specific speed (Ns) is a non-dimensional design index which identifies the geometrical similarity of pumps. It is used to classify rotors according to their types and proportions. Pumps with the same Ns, but with different sizes, are considered geometrically similar, even with one pump being a multiple size of the other.
Specific speed is calculated at the point of best efficiency of the pump, with a maximum diameter rotor.
It is mathematically expressed by the following equation:
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Where:
n [rpm] |
Pump revolutions |
Q [m³/h] |
Pump Flow |
H [m] |
Manometric height per stage |
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For double rotors, the total flow should be divided by two in the calculation of the specific speed.
Specific speed identifies approximately the acceptable ratio between the diameter of the eye of the rotor (D1) and the maximum rotor diameter (D2) in a well designed impeller.
Ns: 500 a 5.000; D1/D2 > 1.5 – radial flow pump
Ns: 5.000 a 10.000; D1/D2 < 1.5 – mixed flow pump
Ns: 10.000 a 15.000; D1/D2 = 1.0 – axial flow pump
Specific speed is also used in sizing a new pump by expanding the scale of a smaller pump with the same specific speed. The performance and construction of the smaller pump are used to predict the performance and model the construction of the new pump.
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