Pump Suction Specific Speed Pump Suction Specific Speed
Pump Speed (N, rpm):
BEP Flow per Impeller Eye at Max Dia(Q,usgpm):
NPSH Required(HNPSHr, ft):
Suction Specific Speed(Nss):

Nss =
N * Q 0.5 / NPSHr0.75

1, As a rule of thumb, pump applications with Nss over 9,000 for water and 11,000 for hydrocarbons should be avoided and reviewed carefully as they are susceptible to cavitation and recirculation damage.

2, Often, a margin is added to the NPSH available. In case of over estimation of NPSH could lead to an oversize pump that is vulnerable to recirculation, cavitation and vibration resulting poor reliability.

3, Half the total flow for double suction impellers.

4, Specific speed is used by pump designers to help determine the required impeller geometry. The lower the specific speed, the more the impeller shape will approach true radial flow. The higher the specific speed, the more closely the impeller needs to approach true axial flow.

5, Specific speed is also useful in estimating maximum attainable pump efficiency. This is done from below Figure, which is published by the Hydraulic Institute. This figure is useful to help estimate brake horsepower and to help validate vendor's quoted efficiencies.

Ground Consulting & Technology Services Ltd.