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:
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:
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.
