Suction Specific Speed is used to determine what pump geometry – radial, mixedflow or axial – to select for a stable and reliable operation with max efficiency without cavitation.

Nss can also be used to estimate safe operating ranges.

Suction Specific Speed can be compared with Specific Speed but instead of using the total head for the pump the Required Net Positive Suction Head (NPSHr) is used.

Nss have the same value for geometrically similar pumps. As a rule of thumb the Specific Suction Speed should be below 9000 (calculated with USgpm) to avoid cavitation and unstable and unreliable operations.

Empirical studies indicates that safe operating ranges from Best Efficiency Points (BEP) are more narrow at higher Suction Specific Speeds.

Take a look at this interesting paper TPS

By Simon Bradshaw, David Cowan,  and Thomas Liebner