--- title: "Pelton versus Francis performance characteristics" subject: "Hydro Power" chapter: 7 key: "semester_6/hydro_power:pelton-vs-francis-performance" source: https://www.bctengineer.com/q/semester_6/hydro_power/pelton-vs-francis-performance site: BCT Engineering --- # Pelton versus Francis performance characteristics Hydro Power · Chapter 7: Hydro-Electric Machines - Asked in: 2072 Asoj, 6 marks - Video: [Differences between / Pelton turbine / Francis turbine / Kaplan turbine / type of turbine / Turbine](https://www.youtube.com/watch?v=omITy0IZhPM) — Saral Mechanical - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/pelton-vs-francis-performance - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7 ## Question Differentiate between Pelton turbine and Francis turbine with the help of their 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. ## Answer 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) ```text (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 | $Q$ nearly constant with speed (depends only on spear opening) | $Q$ decreases as speed increases (slow/normal runners) | | Power vs speed | Max power near $u = 0.46V$; zero at runaway | Max power at speed ratio about 0.6–0.9 | | Runaway speed | About 1.8–1.9 $N$ | About 2–2.2 $N$ | | 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which turbine suits widely varying flow because of its flat efficiency curve? A. Pelton B. Francis C. Both equally D. Neither Answer: A. Pelton efficiency stays high from 25-100% load; Francis falls below 50-60%. (Key point: Part-load efficiency) 2. Which turbine has the higher peak efficiency? A. Pelton, about 92-95% B. Francis, about 92-95% C. They are equal D. Neither exceeds 80% Answer: B. Francis peaks at about 92-95%, Pelton about 88-91%, but Francis must run near rated load. (Key point: Peak efficiency) 3. At what speed does a Pelton turbine give maximum power at constant head? A. u = 0.46 V B. u = 2 V C. u = 0.1 V D. u = V Answer: A. Maximum power occurs near u = 0.46V and falls to zero at runaway. (Key point: Max power speed ratio) 4. What is done for a Francis plant with large flow variation? A. Run one large unit at 20% load B. Install several units and switch them on/off C. Replace with Kaplan automatically D. Use a draft tube only Answer: B. Several units keep each running near rated load for good efficiency. (Key point: Francis at varying flow) ## Related in Hydro Power - [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7 - [Pelton turbine design (2 m³/s, 500 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-design-500m) — Hydro Power, chapter 7, asked 2× - [Pelton turbine design (1.8 m³/s, 605 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-design-605m-1-8) — Hydro Power, chapter 7, asked 2× - [Pelton turbine design (3.75 m³/s, 380 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-design-3-75) — Hydro Power, chapter 7 - [Pelton turbine design (5 m³/s, 312.5 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-design-312-5m) — Hydro Power, chapter 7 - [Pelton turbine design (5 m³/s, 310 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-design-310m) — Hydro Power, chapter 7 ## Related in other subjects - [Pelton vs Francis performance; Francis setting](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-francis-performance-curves-setting) — Hydropower Engineering, chapter 7 - [Compressor and turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acompressor-turbine-performance-characteristics) — Turbo Machines, chapter 3 - [Conditions favouring Francis over Pelton](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-preferable-over-pelton) — Hydropower Engineering, chapter 7 - [Turbine setting; three single-jet Pelton wheels](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-setting-pelton-three-jets) — Hydropower Engineering, chapter 7