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Spiral casing and draft tube location

Short answer

The spiral casing surrounds the stay vanes, guide vanes and runner at runner level; the draft tube starts below the runner and ends under tailwater. The casing distributes flow uniformly around the runner; the draft tube recovers kinetic energy and raises efficiency.

Specify with a neat sketch the location of a spiral casing and draft tube used in hydroelectric power generation. Mention their importance.

Sketch (vertical section through a vertical-shaft Francis unit)

  Penstock
     |
     v         generator
  +--+---+     ====|====
  | Valve|        shaft
  +--+---+         |
     |       stay ring/guide vanes
   ==+==============+====
  /   SPIRAL CASING   \   <- surrounds the runner
  \_____ runner _______/
          |   |
          |   |  DRAFT TUBE
          \   /   (diverging pipe)
           \ /
 TWL ~~~~~~~~V~~~~~~~~~~~~~~> tailrace

The spiral (scroll) casing is connected to the end of the penstock (through the inlet valve) and surrounds the stay vanes, guide vanes and runner at the runner level; the draft tube starts directly below the runner and ends under the tailwater.

Importance

  • Spiral casing: distributes the water uniformly all around the guide vanes and runner at a constant velocity (its area decreases along the flow path), so the flow enters the runner without shock and with low loss. It also withstands the water pressure and supports the turbine.
  • Draft tube: recovers the kinetic energy at the runner exit, allows the turbine to be set above tailwater, increases the effective head, and conveys the water to the tailrace.

✓ Answer checked by Claude Opus 5.5 (high effort)

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