- Asked 2/18
- 2081 Chaitra · 4 marks
- 2080 Asoj · 5 marks
Impulse versus reaction turbines
Short answer
Impulse (Pelton) uses only kinetic energy of a jet at atmospheric pressure, runs in air, no draft tube, for high head, low Ns. Reaction (Francis) uses both pressure and kinetic energy, runner full of water, draft tube essential, for low-medium head, high Ns, cavitation risk.
Differentiate between impulse and reaction turbines with suitable examples.
An impulse turbine uses only the kinetic energy of a free jet at atmospheric pressure, while a reaction turbine uses both pressure and kinetic energy, with water flowing through the runner under pressure.
| Basis | Impulse turbine | Reaction turbine |
|---|---|---|
| Energy at runner inlet | Only kinetic (whole head converted to velocity in nozzle) | Partly pressure, partly kinetic |
| Pressure in runner | Atmospheric throughout | Decreases from inlet to outlet (above to below atmospheric) |
| Water supply | Free jet on a few buckets (partial admission) | Full admission, all around the runner |
| Casing | Only to prevent splashing; no hydraulic function | Watertight spiral casing essential |
| Runner | Must run in air, never submerged | Always full of water |
| Draft tube | Not used | Essential (recovers energy, allows setting above TWL) |
| Flow control | Spear/needle in nozzle | Movable guide vanes (and runner blades in Kaplan) |
| Head | High head, low discharge | Low to medium head, large discharge |
| Specific speed | Low (8.5–60 SI) | Medium to high (60–1000) |
| Part-load efficiency | Flat, good | Lower (except Kaplan) |
| Cavitation | Not a major problem | Serious problem; limits setting |
| Examples | Pelton, Turgo, Crossflow | Francis, Kaplan, propeller, bulb, Deriaz |
Nepalese examples:
- Impulse: Pelton turbines at Khimti (about 680 m head), Upper Tamakoshi (about 820 m head), Chilime and Kulekhani I.
- Reaction: Francis turbines at Kaligandaki A, Middle Marsyangdi and Kulekhani II; Kaplan turbines at Gandak and in low-head micro plants.
✓ Answer checked by Claude Opus 5.5 (high effort)
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