PUMP AFFINITY LAWS
The the pump affinity laws, just like the fan laws, are laws which determine the change in the performance of the pump when the various parameters of the pump are changed.
These laws are only to be applied to the same pump, and for pumps which are geometrically similar and have identical pump curves.
Variables involved
Q - Flow rate (amount of water pushed)
p - Pressure built by the pump
N - Rotational Speed of the pump impeller
P - Power input to the pump
D - pump impeller diameter (or pump size in case of geometrically similar pump)
ρ - Density of water
"Flow in a pump is proportional to its diameter and impeller speed."
When diameter is constant
When the speed is constant
LAW
1
LAW
2
“Static Pressure produced by the pump varies as the square of the ratational speed or the impeller diameter.”
When diameter is constant
When the speed is constant
“The pumping power, P, decreases with the cube of impeller speed or diameter.”
When diameter is constant
When the speed is constant
LAW
3
CAUTION : Although the impeller size or diameter can be used to manipulate the various pump characteristics, care must be taken while doing so. Reducing the diameter (around 4-6 %) leads to increased space between the casing and the impeller and causes internal flow re-circulation and head loss, thus greatly reducing efficiency of the pump.