Agriculture Reference
In-Depth Information
12.4.4 Specific Speed
Two pumps are geometrically similar when the ratios of corresponding dimen-
sions in one pump are equal to the same ratios of the other pump. Specific speed
is a constant for any geometrically similar pump. It is an index number correlat-
ing pump flow, head, and speed at the optimum efficiency point which classifies
pump impellers with respect to their geometric similarity. Specific speed is usually
expressed as
N
√
Q
H
3
/
4
N
s
=
(12.4)
where
N
s
=
specific speed of the pump (rpm)
N
=
rotational speed of pump at optimum efficiency (rpm)
flow of pump at optimum efficiency (m
3
/s)
Q
=
H
=
head at optimum efficiency (m).
The specific speed is an index which is used when selecting impellers to meet dif-
ferent conditions of head, capacity, and speed. Knowing this index is very helpful
in the determination of the maximum permissible suction lift, or minimum suction
head, which is necessary to avoid cavitation under different capacities, heads, and
pump speeds. For a given head and capacity, suction lift is greater for a pump with
lower specific speed.
The calculation of specific speed allows for determination of the pump type
required for a given set of conditions to be determined. Usually high-head impellers
have low specific speeds and low-head impellers have high specific speeds.
There is often an advantage in using pumps with high specific speeds since, for
a given set of conditions, their operating speed is higher, and the pump is there-
fore smaller and less expensive. However, there is also some trade-off since pumps
operating at higher speeds will wear faster.
12.4.5 Affinity Laws
Effect of Change of Speed
Affinity laws state that for a given pump, the capacity will vary directly with
a change in speed, the head will vary as the square of speed, and the required
horsepower will vary as the cube of speed. Mathematically, affinity laws can be
expressed as
For flow:
Q
∞
N
N
2
For BHP: BHP
For head:
H
∞
(12.5)
N
3
∞
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