Francis Turbine Specific Speed & Cavitation Calculator
Calculate Francis turbine specific speed, runner inlet diameter, critical Thoma cavitation sigma, and maximum allowable setting height above tailwater per USBR.
Hydraulic Head & Flow Rate
Plant Elevation & Tailrace Setting
Specific Speed & Cavitation Sizing
| Shaft Power Output (P_shaft): | -- MW |
| Runner Inlet Diameter (D1): | -- m |
| Runner Discharge Diameter (D2): | -- m |
| Critical Thoma Cavitation Coeff (σ_c): | -- |
| Plant Cavitation Coefficient (σ_plant): | -- |
| Atmospheric Barometric Head (H_atm): | -- m |
| Max Safe Elevation Above Tailwater: | -- m |
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Frequently Asked Questions
What is a Francis turbine and why is it the most widely used hydro turbine?
The Francis turbine is a mixed-flow reaction machine invented by James B. Francis in 1848. Water enters radial-inward through a spiral casing and adjustable guide vanes (wicket gates), flows across curved runner blades, and discharges axially through a draft tube. It covers a vast range of operating heads (40 to 600 meters) and achieves peak efficiencies exceeding 94%.
What is Thoma's cavitation coefficient (sigma) and why is it vital for runner setting?
Developed by Dieter Thoma, the cavitation coefficient sigma = (H_atm - H_v - H_s) / H defines the ratio of local net static suction head to total turbine net head. If the plant sigma drops below the runner's critical sigma, local static pressure drops below water vapor pressure, forming millions of vapor bubbles that implode violently against the steel blades, eroding kilograms of stainless steel per month.
What is the function of the draft tube in a reaction turbine?
The draft tube is an expanding diffuser pipe connecting the runner discharge to the tailrace. It performs two critical functions: (1) It allows the turbine to be set above tailwater without losing head, and (2) Its diffusing geometry decelerates water exiting the runner, recovering dynamic kinetic energy (V²/2g) and converting it back into usable suction pressure drop across the runner.