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Suction Specific Speed is an index number for a centrifugal pump which defines suction characteristic. Higher numerical values of Nss are connected with better suction capabilities.
Specific Suction Speed can be used to determine what pump geometry - radial, mixed flow or axial - to use for maximum efficiency and to prevent cavitation. Suction Specific Speed is commonly used to estimate pumps safe operating ranges.
Suction Specific Speed is dimensionless and can be expressed as
Nss = ω q1/2 / NPSHr3/4 (1)
where
Nss = Suction Specific Speed
ω = pump shaft rotational speed (rpm)
q = flow rate capacity (m3/h, l/s, m3/min, US gpm, British gpm) at the Best Efficiency Point (BEP)
NPSHr = Required Net Positive Suction Head for the pump at the best efficiency point (m, ft)
Specific Suction Speed can be compared with Specific Speed but instead of using the total head of the pump, the Required Net Positive Suction Head (NPSHr) is used.
As a rule of thumb the Specific Suction Speed should be below 8500 (calculated with US gpm) to avoid cavitation.
According to the definition for Specific Speed, pumps that develop a high flow and low head have relatively high specific speed values, while pumps that develop high heads at low flow rates have low specific speeds. Specific speed values range from a low of 500 to a high of about 10,000, with the more typical range being between 1,000 and 5,000.
