Dimensionless specific speed Ωs classifies pump type and impeller geometry. Also computes US dimensional Ns and European Nq forms.
Turbomachinery · ClassificationCommon 50 Hz synchronous speeds: 750, 1000, 1500, 3000 rpm
Use BEP (best efficiency point) conditions for correct pump classification
Specific speed — three conventions:
Pump classification by specific speed:
| Ωs | Ns (US) | Nq (EU) | Type | Impeller |
|---|---|---|---|---|
| < 0.4 | < 1100 | < 21 | Centrifugal (very high head) | Narrow radial / multi-stage |
| 0.4–1.5 | 1100–4100 | 21–79 | Centrifugal (radial) | Radial impeller |
| 1.5–4.0 | 4100–10900 | 79–212 | Mixed flow | Mixed-flow impeller |
| > 4.0 | > 10900 | > 212 | Axial flow | Propeller / axial |
Where:
Specific speed is a shape number, not a dimensional speed. It determines the optimal impeller geometry at the best efficiency point. High Ωspumps deliver high flow at low head (axial), while low Ωs pumps deliver high head at low flow (radial). The affinity laws and pump similarity both rely on Ωs being conserved.