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Francis Turbine

Francis Turbine

Made of

SteelCast IronBronzeWater

Brief description

A Francis turbine is a reaction water turbine that combines radial and axial flow concepts to extract energy from water under medium-to-high pressure heads. Water enters the turbine radially through a spiral casing (volute) and adjustable guide vanes, flows inward toward the center, and turns 90 degrees to discharge axially through a central draft tube. It is the most widely used water turbine in the world for large-scale hydroelectric power generation due to its exceptionally high efficiency (over 90%).

Use / Function

  • Primary use: Generating continuous medium-to-large scale hydroelectric power from rivers and dam reservoirs.
  • Secondary uses: Direct mechanical power for industrial complexes, water pumping operations, and energy storage in pumped-storage hydroelectric plants.
  • Scale: Medium industrial plants (hundreds of kilowatts) to mega-hydroelectric dams (hundreds of megawatts), working efficiently under water heads ranging from 10 to 600 meters.

Operating principle

Reaction Force and Energy Conversion:

  1. Spiral Casing (Volute): Pressurized water from the penstock enters a decreasing spiral casing that maintains constant water velocity and distributes the flow evenly around the outer circumference.
  2. Wicket Gates (Stay Vanes & Guide Vanes): Water passes through stationary stay vanes and adjustable wicket gates, which direct the fluid at an optimal angle onto the runner blades while controlling total water flow.
  3. Inward Radial Flow: As pressurized water moves inward through the curved runner blades, its static pressure drops and it imparts momentum to the runner via reaction forces.
  4. Axial Turn & Exit: The water is turned 90 degrees from radial to axial direction as it exits through the bottom of the runner.
  5. Draft Tube Pressure Recovery: The exiting water flows down a expanding tube (draft tube) beneath the turbine, which decelerates the water and recovers kinetic energy by creating a suction pressure head below the runner.

How to create it

1. Runner and Blade Fabrication

  1. Cast or weld curved hydrofoil blades made from Steel or Bronze to withstand hydrodynamic forces and cavitation.
  2. Secure the blades between a solid top crown and bottom band to form the central reaction runner.
  3. Balance the runner dynamically on a heavy steel shaft supported by lubricated bearings.

2. Guide Mechanism and Spiral Volute Assembly

  1. Construct a spiral casing (volute) out of Cast Iron or welded steel plates.
  2. Mount a ring of pivoting guide vanes (wicket gates) around the inner circumference of the casing, linked to a central control ring to adjust gate angles.

3. Draft Tube and Discharge Channel

  1. Install an expanding conical draft tube directly beneath the runner outlet, extending below the tailrace water level.
  2. Mount the turbine assembly inside a reinforced structure above the tailrace discharge channel.

Materials needed

  • Runner & Blades: High-strength Steel or corrosion-resistant Bronze.
  • Housing & Casing: Cast Iron or thick structural steel plate.
  • Working Fluid: High-pressure Water supplied via a penstock pipe from a dam or elevated reservoir.

Variants and improvements

  • Pumped-Storage Reversible Francis Turbine: Operates as a turbine-generator in peak power demand hours, and reverses direction as a motor-pump during off-peak hours to pump water back into an upper reservoir.
  • Variable-Speed Francis Turbine: Utilizes electronic power converters to maintain optimal efficiency across wide variations in reservoir head levels.
  • Micro-Francis Turbine: Compact cast-iron or stainless steel units engineered for small community micro-hydro projects.

Limits and risks

  • Cavitation: Localized low pressure at the exit of runner blades can form vapor bubbles that collapse violently, pitting metal surfaces and destroying blades over time.
  • Silt Erosion: High-velocity water containing sand or abrasive sediment causes severe wear on runner blades and wicket gate seals.
  • Complex Fabrication: Precise blade curvature and tight mechanical tolerances make manufacturing significantly more complex than simple impulse wheels.