What is Specific Speed?
Specific speed (Ns) is a dimensionless or dimensional number that describes the shape and design characteristics of a turbine or pump. It is used to classify hydraulic machines and select the right type for a given head and flow rate.
Specific speed is defined as the speed at which a geometrically similar turbine would run if scaled to produce unit power under unit head, or a geometrically similar pump that would deliver unit discharge under unit head.
Specific Speed Formula for Turbines
Ns = N√P / H^(5/4)
Where:
- Ns = Specific speed
- N = Speed of the turbine (rpm)
- P = Power output (kW or hp)
- H = Net head (m)
In SI units (with P in kW and H in metres), typical Ns ranges:
- Pelton turbine: 4–70
- Francis turbine: 60–400
- Kaplan turbine: 300–900
- Propeller turbine: 500–1200
Specific Speed Formula for Pumps
Ns = N√Q / H^(3/4)
Where:
- Q = Discharge / flow rate (m³/s or litres/s)
- H = Total head developed (m)
Significance of Specific Speed in Turbine Selection
| Turbine Type | Specific Speed (Ns) | Best for | Head Range (m) |
|---|---|---|---|
| Pelton | 4–70 | High head, low flow | 200–2000+ |
| Francis | 60–400 | Medium head, medium flow | 40–600 |
| Kaplan | 300–900 | Low head, high flow | 2–40 |
Worked Example: Calculate Specific Speed
Problem: A Francis turbine runs at 300 rpm, produces 5000 kW under a net head of 80 m. Calculate its specific speed.
Solution:
Ns = N × √P / H^(5/4) Ns = 300 × √5000 / 80^(5/4) Ns = 300 × 70.71 / 170.86 Ns = 21213 / 170.86 Ns ≈ 124
This falls within the Francis turbine range (60–400), confirming the selection is appropriate.
Turbine Lab Experiments at Scientico India
Engineering colleges can study specific speed and turbine performance characteristics using:
All Scientico India turbine test rigs include CE declaration of conformity and ISO 9001:2015 certification. Get a CIF quote within 24 hours.
Scientico India manufactures this and the full range of Fluid Mechanics Lab equipment — CE-certified and supplied to engineering colleges worldwide.
