- Practice · 5 marks
Impulse and reaction turbines compared
Short answer
In an impulse turbine, all head converts to kinetic energy before the runner; pressure is atmospheric. In a reaction turbine, both pressure and kinetic energy act on the runner, with pressure drop across it. Pelton (impulse) suits high heads, Francis (reaction) medium heads, Kaplan (reaction) low heads with large flow. Pelton uses spear/deflector, Francis guide vanes, Kaplan adjustable blades and vanes.
Differentiate between impulse and reaction turbines. Give a comparison between Pelton, Francis and Kaplan turbines.
In an impulse turbine the available head is fully converted into kinetic energy before the water reaches the runner; in a reaction turbine only part of the head is converted into kinetic energy and the rest remains as pressure energy, which is converted to work inside the runner.
Impulse and reaction turbines
| Point | Impulse turbine | Reaction turbine |
|---|---|---|
| Energy at runner inlet | Kinetic energy only | Kinetic and pressure energy |
| Pressure in runner | Atmospheric, constant | Falls from inlet to outlet |
| Flow | Partial admission (jets) | Full admission, runner always full of water |
| Casing | Only to prevent splashing | Essential; closes the water passage |
| Draft tube | Not used | Used |
| Position of runner | Above tailwater level | Can be above, with draft tube |
| Blade shape | Symmetrical buckets | Aerofoil-like blades |
| Regulation | Spear valve and deflector | Guide vanes (and Kaplan blades) |
| Head | High | Low to medium |
| Example | Pelton, cross-flow (partly) | Francis, Kaplan |
Pelton, Francis and Kaplan
| Point | Pelton | Francis | Kaplan |
|---|---|---|---|
| Type | Impulse, tangential | Reaction, radial/mixed | Reaction, axial |
| Head (approx.) | 250 m and above | 40 to 600 m | Up to 60 m |
| Discharge | Small | Medium | Large |
| Specific speed | 10 to 35 | 60 to 400 | 300 to 1000 |
| Regulation | Spear/deflector | Guide vanes | Guide vanes and blades |
| Draft tube | No | Yes | Yes |
| Part-load efficiency | Good | Falls at part load | Very good |
✓ Answer checked by Claude Sonnet 5.5
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