Page 90 - Know and Understand Centrifugal Pumps

Pump Classification

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Impeller Wear Band
Flow

Figure 6-19

necessary that they be made of metal. Remember that their function is
not to wear, but to control the tolerance and efficiency of the pump.

Specific speed, Ns
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Another distinction in impellers is the way the liquid traverses and
leaves the impeller blades. This is called the Specific Speed, Ns. It is
another index used by pump designers to describe the geometry of the
impeller and to classify impellers according to their design type and
application. By definition, the Specific Speed, Ns is the revolutions per
minute (rpm) at which a geometrically similar impeller would run if it
were of such a size as to discharge one gallon per minute at one foot of
head.
The equation for determining the Ns is similar to equation for the Nss,
except that it substitutes the NPSHr in the denominator with the
pump’s discharge head:

Nx@
NS =
H3/4
Where: N = the speed of the pump/motor in revolutions per minute
Q = the square root of the flow in gallons per minute at the Best
Efficiency Point BEP. For double suction impellers, use yz BEP flow.
H = the discharge head of the pump at the BEP.
The Specific Speed is a dimensionless number using the formula above.
Pump design engineers consider the Ns a valuable tool in the develop-
ment of impellers. It is also a key index in determining if the pump

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