Specific Speed

Specific speed N_s, is a dimensionless parameter used to characterize turbomachinery speed. Common commercial and industrial practice creates dimensioned results which are of equal value. One of the most valued uses of specific speed is the application of this parameter to pumps to define the suction specific speed, is a quasi non-dimensional number used to classify pump impellers as to their type and proportions. In Imperial units 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 of hydraulic head. In metric units flow may be in l/s or m³/s and head in m, and care must be taken to state the units used.

Performance is defined as the ratio of the pump or turbine against a reference pump or turbine, which divides the actual performance figure to provide a unitless figure of merit. The resulting figure would more descriptively be called the "ideal-reference-device-specific performance." This resulting unitless ratio may loosely be expressed as a "speed," only because the performance of the reference ideal pump is linearly dependent on its speed, so that the ratio of is also the increased speed the reference device would need to turn, in order to produce the performance, instead of its reference speed of "1 unit."

Specific speed is used in engineering design where it is thought of as an index used to predict desired pump or turbine characteristics; e.g., the general shape of a pump's impeller. Often it is used to predict the type of pump or turbine required for a design flow rate and head. Once the desired specific speed is known, basic dimensions of the unit's components can be easily calculated.

Several mathematical definitions of specific speed (all of them actually ideal-device-specific) have been created for different devices and applications.