Page 59 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The Affinity Laws and the impeller diameter
~~ ~~
If the velocity should remain fixed, the flow, head and BHP will change
when the impeller diameter changes. With a change in the impeller
diameter (this is called an impeller ‘trim’), the affinity laws indicate:
rn The Flow changes directly proportional to the change in diameter,
Q a D.

rn The Head changes directly proportional with the square of the
change in the impeller diameter, H a D2.
The BHp changes directly proportional with the cube of the change
in the impeller diameter, BHp a D3.
In the form of Equations:

New Diameter D1
New Flow = Initial Flow x
Initial Diameter D2

New Diameter
New Head = Initial Head x
Initial Diameter

Initial RHp x ( New Diameter
New BHp =
Initial Diameter

What’s the practical application of these laws?
~ _ _ _ _ -
The pump curve, the H-Q curve, is in a descending profile. This means
that with an increase in flow ‘Q, gpm, the Head ‘H’, or pressure falls.
And if flow is reduced, the pressure rises. At times, in normal industrial
production, the flow must rise and fall, but the pressure or head must
remain a constant.
Many industrial processes experience seasonal rises and falls. This means
that flow varies. For example, we consume more cough syrup and
aspirin in the flu season and less of these products in other seasons.
(This chapter is written with the authors suffering this ailment.)
Normally we buy more ice cream in summer, and less in winter.
The pasteurization process for milk and ice cream requires heating the
milk to a specified temperature and pressure for a specified time to kill
all germs and bacteria in the milk. This pressure is constant although
the production of milk and ice cream goes up and down with
consumption and the seasonal changes. It is the same with the

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