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Centrifugal Pumps

pressure, impellers are designed in various configurations. This allows
them to be selected to meet the governing criteria of the specific service
in which they will operate.

1.4.1 Specific speed
The form and proportions of impellers vary with a non-dimensional
design index that is referred to as Specific Speed. This is defined as the
revolutions per minute at which a geometrically similar impeller would
run if it were of such a size as to discharge one gallon per minute
against one foot of head. It has been found that the ratios of the major
dimensions of the impeller vary uniformly with specific speed.
Consequently it is useful to the pump designer in predicting the
proportions required when comparing impellers.

N~Q
Ns = Ho.75

When calculating the specific speed, it must be noted that all values
must be taken at the best efficiency point (BEP), at the maximum
impeller diameter and at the rated speed of the pump.
If an impeller has a low value of Specific Speed (say 500 to 1,000), it is
identified as a Radial Vane Design in which the liquid makes a 90 ~ turn
from the horizontal entrance flow to radially from the shaft. These
impellcrs generally deliver a relatively low flow rate against a high head,
but with a low efficiency. They arc used extensively in such services as
boiler feed pumps where the main function is to pressurize the water
before it goes into the boiler.
The high values of Specific Speed (5,000 and up) identify Mixed Flow
and Axial Flow Impellers where the flow path varies minimally from
being parallel to the axis of the shaft. These impellers generally deliver a

Specific Speed

,D

r- I'--

RadialVane Fran,~i,z Vane I~1 ;_._~d Flaw Axial Fkw
Figure 1.10: Specific speed chart