Specific Speed is a dimensionless design index number used to classify pumps by impeller type and proportion. It is defined as the speed in revolutions per minute that a geometrically similar pump would operate to deliver one unit of flow at one unit of head. This definition has no practical meaning and specific speed should be thought of only as an index number that reflects certain pump characteristics.

When the operating speed of a pump changes, its specific speed does not change. The specific speed of a pump is determined at the best efficiency point. The following formula is used to determine the specific speed number:

Where:
N = pump speed in rpm
Q = capacity in m³/sec at BEP
H = total head per stage in meters at BEP

Figure 2.7-1

The specific speed determines the general shape and class of the impeller as depicted in Figure 2.7-1. Low specific speeds (under 30) indicate radial impellers which are generally low flow, high head designs, and have impeller outlet to inlet diameter ratios of 2:1 or more. As the specific speed increases, the ratio of the impeller outlet diameter (D2) to the inlet diameter (D1) decreases. Axial flow pumps have specific speeds of 200 or more and are generally high flow, low head designs. Their outlet to inlet diameter ratio is 1:0.

Low specific speed pump impellers develop head principally through radial force. Francis vane and mixed flow impellers develop head with a combination of radial and axial forces. High specific speed impellers develop head with only axial forces. As the specific speed increases, the impeller changes the way it develops head from purely radial forces to purely axial forces.

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