- 2072 Asoj · 6 marks
Pelton versus Francis performance characteristics
Short answer
Pelton is an impulse turbine for high heads with flat efficiency from 25–100% load, discharge constant with speed, runaway speed ~1.8–1.9 N. Francis is a reaction turbine for medium heads, peak efficiency ~92–95% but falls sharply below 50% load, discharge decreases with speed, runaway ~2–2.2 N, requires a draft tube.
Differentiate between Pelton turbine and Francis turbine with the help of their performance characteristics.
The Pelton turbine is a tangential-flow impulse turbine for high heads, while the Francis turbine is a mixed (radial-inward to axial) flow reaction turbine for medium heads. Their performance curves at constant head differ as follows.
Characteristic curves (constant head)
(a) eta vs % load (b) Q vs N (gate const.)
eta Q
^ ______ Pelton ^ ------------ Pelton
| / \_ |\
| / .--. Francis | \___ Francis
|/ / \ | \___
+-----------> % load +-----------> N
0 50 100 (decreases)
Comparison
| Basis | Pelton turbine | Francis turbine |
|---|---|---|
| Type | Impulse, tangential flow | Reaction, mixed flow |
| Head range | High (above about 250–300 m) | Medium (30–300 m and more) |
| Specific speed (SI, kW) | 8.5–60 | 60–300 |
| Peak efficiency | About 88–91% | About 92–95% |
| Efficiency at part load | Flat curve; stays high from 25–100% load | Peaked curve; falls quickly below 50–60% load |
| Discharge vs speed | nearly constant with speed (depends only on spear opening) | decreases as speed increases (slow/normal runners) |
| Power vs speed | Max power near ; zero at runaway | Max power at speed ratio about 0.6–0.9 |
| Runaway speed | About 1.8–1.9 | About 2–2.2 |
| Flow control | Spear (needle) + deflector | Guide vanes (wicket gates) |
| Draft tube | Not used | Used, essential |
| Cavitation | Negligible | Serious at part load and high setting |
| Sediment abrasion | Buckets and needles easily replaced | Runner/guide vanes erode, costly repair |
| Nepalese example | Khimti, Upper Tamakoshi | Kaligandaki A, Middle Marsyangdi |
Interpretation
- Because of its flat efficiency curve, a Pelton turbine is preferred where the flow varies widely (e.g. RoR plants in the dry season).
- The Francis turbine has a higher peak efficiency but needs to run near rated load; for large flow variation, several Francis units are installed and switched on/off.
✓ Answer checked by Claude Opus 5.5 (high effort)
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