Page 57 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

3. Brake Horsepower = BHP: Energy needed to pump a liquid.

4. Speed = Velocity = N: Shaft speed measured in revolutions per
minute (rpm).
Stated simply, the Affinity Laws indicate:

w Flow, Q, changes directly proportional to a change in velocity.
QaN.
w Head, H, changes directly proportional with the square of the
change in velocity, H a N2.
w Power, BHP, changes directly proportional with the cube of the
change in velocity, BHP a N3.

w a means ‘directly proportional to’.
In the form of equations:

r.p.m. New N1
New Flow = Initial Flow x Q1 = -
r.p.m. initial 42 N2

New Head = Initial Head x
r.p.m. initial

r.p.m. initial ) BHp2 =(E)
New BHp = Initial BHp x r.p.m.New 3 BHpl N1 3

With a small change in velocity (say 20 or 50 rpm), the pump’s
efficiency would not be affected. But with a velocity change of say twice
the speed, or one half the speed, you could expect a small change in the
pump’s efficiency of maybe 2 or 3%. The pump’s efficiency would tend
to increase by a couple of points at twice the speed, and the efficiency
would tend to decrease a couple of points at one half the speed. In a
practical situation, a process engineer or operator might double the
speed of the electric motor because the plant is undergoing an
expansion and needs twice the production. That engineer or operator
should know that:
2 x velocity = 2 x flow
= 2 x capacity

= 2 x gpm
= 2 x production

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