Pump Suction Specific Speed (Nss)

Nss = N sqrt(Q) / NPSHr^(3/4) (rpm, gpm at BEP, required NPSH in ft) indexes how hard a pump works its suction. The Hydraulic Institute design guideline caps Nss near 8,500; above it (especially above ~11,000) a pump shows suction-recirculation damage and shorter life. A 1,750 rpm, 2,000 gpm pump with 25 ft NPSHr sits at a safe Nss 7,000, but on a tighter 16 ft NPSHr it jumps to 9,783 - above the limit, nothing changed but the suction margin. A screening index, not an NPSH-margin calc; the manufacturer's curves govern.

Nss = N sqrt(Q) / NPSHr^(3/4) (N rpm, Q gpm at BEP, NPSHr ft); guideline Nss < 8,500, caution 8,500-11,000, concern >= 11,000.

The US pump suction specific speed Nss = N sqrt(Q) / NPSHr^(3/4) and the Hydraulic Institute design guidelines, by name.

The suction-specific-speed definition and the ~8,500 / ~11,000 guidelines are public results in the standard Hydraulic Institute and pump references.

Estimate. AHJ and licensed professional govern.

Field names used by the API: n_rpm, q_gpm, npshr_ft, nss

• Suction specific speed Nss = N sqrt(Q) / NPSHr^(3/4) at BEP (US dimensional form); half the total flow for a double-suction impellerHydraulic Institute
• Advisory guidelines ~8,500 traditional design limit and ~11,000 reliability threshold are advisory practice, not a hard code limitHydraulic Institute / pump-reliability studies