Specific Speed

The net suction specific speed is a dimensionless number defined by centrifugal and axial pumps' inherent physical characteristics and operating pointcite web| url = http://www.gouldspumps.com/cpf_0008.html| title = NPSH and Suction Specific Speed - Goulds Pumps - ITT Corporation | accessdate = 2007-07-13| author = | publisher = ITT Corporation] . It is one of a number of factors used in selecting a pump for use and is mainly used to see if there will be problems with cavitation during the pumps operation on the suction sidecite web| url = http://www.mcnallyinstitute.com/09-html/09-12.html| title = Specific speed | accessdate = 2007-07-13| author = | publisher = McNally Institute] . The net suction specific speed of a pump will define the range of operation in which a pump will experience stable operationcite web| url = http://www.pumpingmachinery.com/pump_magazine/pump_articles/article_03/article_03.htm| title = Article #3: Suction Specific Speed (NSS) | accessdate = 2007-07-13| author = | publisher = Pumping Machinery] . The higher the net suction specific speed, then the smaller the range of stable operation. The envelope of stable operation is defined in terms of the best efficiency point of the pump.

The Net suction specific speed is defined ascite web| url = http://www.engineeringtoolbox.com/suction-speed-pumps-d_638.html| title = Specific Suction Speed for Pumps | accessdate = 2007-07-13| author = | publisher = Engineering Toolbox] :

mbox{Where:}\N_{ss} &= mbox{Net suction specific speed} \N &= mbox{rotational speed of pump in rpm} \Q &= mbox{flow of pump in US gallons per minute} \mbox{NPSH}_R &= mbox{NPSH required in feet at pumps best efficiency point} \

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References

Specific Speed and Pump Type Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions. It is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot head.

The understanding of this definition is of design engineering significance only, however, and specific speed should be thought of only as an index used to predict certain pump characteristics. The following formula is used to determine specific speed:

Where N = Pump speed in RPM Q = Capacity in gpm at the best efficiency point H = Total head per stage at the best efficiency point

The specific speed determines the general shape or class of the impeller as depicted in Fig. 3. As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, Di, decreases. This ratio becomes 1.0 for a true axial flow impeller.

Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. A higher specific speed indicates a pump design with head generation more by axial forces and less by centrifugal forces. An axial flow or propeller pump with a specific speed of 10,000 or greater generates it's head exclusively through axial forces.

Radial impellers are generally low flow high head designs whereas axial flow impellers are high flow low head designs.

Values of Specific Speed, Ns

ee also

* Pump
* Net Positive Suction Head (NPSH)
* Best Efficiency Point