Specific Speed Pump Calculator

Efficiency estimation: maximum achievable efficiency correlates with specific speed — values between 2,000-3,500 generally yield the highest efficiencies.

NPSH assessment: suction specific speed (related to N_s) indicates cavitation resistance.

In pump selection for new installations, the specific speed calculation determines the optimal impeller type for the duty point, with the choice between radial, mixed-flow, and axial-flow impellers driven by the head and flow requirements.

A 1,000 GPM, 200 ft head pump at 3,550 rpm has N_s = 3,550 × 1,000^0.5 / 200^0.75 = 3,550 × 31.6 / 53.2 = 2,110 US — a mixed-flow impeller with good efficiency.

A 10,000 GPM, 50 ft head circulating water pump at 600 rpm has N_s = 600 × 10,000^0.5 / 50^0.75 = 600 × 100 / 14.95 = 4,015 US — a mixed-flow to axial-flow impeller for high-flow service.

In pump retrofit and impeller trim applications, the existing pump's specific speed determines the acceptable range of impeller trim and speed change — trimming an impeller reduces Q and H but does not change N_s significantly (both Q and H change proportionally to D, the impeller diameter, and N_s depends on Q^0.5 and H^0.75 with Q changing as D and H changing as D², giving N_s unchanged).

Pump efficiency estimation at the design point uses the specific speed and the specific speed-efficiency correlation published by the Hydraulic Institute (HI 1.3.4) and various academic references (Worster, Stepanoff, etc.) — a 3,000 N_s pump at 100 GPM and 100 ft head will typically have a BEP efficiency of 82–86%, while a 7,000 N_s pump at the same conditions will have a BEP efficiency of 75–80%.

The suction specific speed (N_ss) is used in pump tender evaluation and in cavitation risk assessment for new pumps — the HI 9.6.7 standard recommends N_ss > 8,500 US (170 SI) for good cavitation resistance, and pumps with lower N_ss require special attention to the NPSHa margin, the suction piping design, or the use of a suction-specific impeller (inducer, double-suction first stage).

The affinity laws and the specific speed are related: changing the pump speed (N) while maintaining geometric similarity does not change N_s (because N_s is proportional to N, and both Q^0.5 and H^0.75 also change with N, leaving N_s unchanged in the geometrically similar scaling).

This is why N_s is a pure geometry index, independent of the operating speed, and is the primary classification parameter for centrifugal pump impellers.