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Pump affinity laws

The affinity laws for centrifugal pumps state that the H/Q curves at various rotational speeds can be derived from the following relationship as conventionally represented: Affinitaetsgesetz_Formel_1_neu

Q    Flow rate
H    Head
Y    Specific energy
n    Rotational speed

If PQ = ρ · g · Q · H (see Pump power output) and P = PQ / η (see Pump power input), and assuming a constant density (ρ) of the fluid handled and a constant pump efficiency (η), it can be said that:

Affinitaetsgesetz_Formel_2_neu

Equivalent points of the H/Q curves at various rotational speeds lie on parabolas whose vertices are in the origin of the QH and QY coordinate system. A characteristic of these points is the similarity of the velocity triangles.

As different Reynolds numbers (see Similarity conditions) are associated with different rotational speeds, a physical similarity between their respective friction effects cannot be achieved.

The affinity laws therefore strictly apply to frictionless, incompressible (no change in density at constant temperature and changing pressure), non-cavitating fluids handled only (see Cavitation). A change in rotational speed also results in a shift of the operating point.