Specific speed

Specific speed N_s, is used to characterize turbomachinery speed. Common commercial and industrial practice uses dimensioned versions which are equally useful. One of the most valued uses of specific speed is its application to pumps to define the suction specific speed -- a quasi non-dimensional numberused 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.