- 2074 Ashwin · 2 marks
Functions of draft tube
Short answer
A draft tube lets the turbine be set above tailwater, converts exit velocity head into pressure, creates suction at the runner outlet to increase effective head, and discharges water smoothly into the tailrace while reducing losses.
What are the functions of a draft tube?
The draft tube is the diverging pipe from the runner outlet to the tailrace. Its functions are:
- Allows the turbine to be placed above tailwater (convenient, dry installation) without losing the head between runner exit and tailwater level.
- Converts kinetic energy into pressure energy: the gradually increasing area reduces velocity, recovering most of the exit velocity head that would otherwise be wasted.
- Creates suction at the runner outlet, so the pressure there is below atmospheric and the effective head on the runner is increased.
- Discharges water to the tailrace smoothly, submerged below tailwater, which keeps the tube full and sealed against air entry.
- Reduces exit losses and increases the overall efficiency of the turbine, especially for low-head, high-speed turbines.
✓ Answer checked by Claude Opus 5.5 (high effort)
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