Page 23 - Centrifugal Pumps 2E

2

Specific

Speed and

Modeling

Laws

Specific speed and suction specific speed are very useful parameters
for engineers involved in centrifugal pump design and/or application. For
the pump designer an intimate knowledge of the function of specific
speed is the only road to successful pump design. For the application or
product engineer specific speed provides a useful means of evaluating
various pump lines. For the user specific speed is a tool for use in com-
paring various pumps and selecting the most efficient and economical
pumping equipment for his plant applications.
A theoretical knowledge of pump design and extensive experience in
the application of pumps both indicate that the numerical values of spe-
cific speed are very critical. In fact, a detailed study of specific speed
will lead to the necessary design parameters for all types of pumps.

Definition of Pump Specific Speed and Suction Specific Speed

Pump specific speed (N s) as it is applied to centrifugal pumps is de-
fined in U.S. units as:

Specific speed is always calculated at the best efficiency point (BEP)
with maximum impeller diameter and single stage only. As specific speea
can be calculated in any consistent units, it is useful to convert the calculated
number to some other system of units. See Table 2-1. The suction specific
speed (N ss) is calculated by the same formula as pump specific speed (N s)

11