--- title: "Pelton vs Francis performance; Francis setting" subject: "Hydropower Engineering" chapter: 7 key: "semester_7/hydropower_engineering:pelton-francis-performance-curves-setting" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/pelton-francis-performance-curves-setting site: BCT Engineering --- # Pelton vs Francis performance; Francis setting Hydropower Engineering · Chapter 7: Hydro-electric Machines - Asked in: 2073 Shrawan, 2+2+2+2 marks - Video: [Francis Turbine lab characteristic's and efficiency |performance of Francis turbine| Eduscription](https://www.youtube.com/watch?v=fZMuMT0npfQ) — Edu Scription - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/pelton-francis-performance-curves-setting - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-7 ## Question Drawing efficiency curves, discuss the performance characteristics of Pelton and Francis turbines. What is the advantage of Pelton turbine over Francis? Write down the principle behind the setting of Francis turbine relative to the tail water level. ## Short answer Pelton efficiency stays high from 30–100% load, while Francis peaks near design load and drops below 50%. Pelton advantage: no cavitation, flat part-load efficiency, silt resistance. Francis setting: runner outlet elevation Hs = atmospheric head minus vapour head minus σc×net head to prevent cavitation; higher specific speed requires a lower setting. ## Answer #### Efficiency curves ```text Eff % 100| 90| __Pelton___ 80| / ___Francis_ 70| / / \ 60| / / \ 50| / / 40| / / +----+----+----+----+---- % load 25 50 75 100 ``` #### Performance characteristics - **Pelton:** efficiency is high (about 85 to 90 %) over a wide range of load (about 30 to 100 %), because load is regulated by changing the number of jets or nozzle spear position, so each jet works at its best speed ratio. - **Francis:** peak efficiency (about 92 to 95 %) occurs near the design load; at part load the guide vane opening alters the flow angle at runner inlet, causing shock and a fall of efficiency below about 50 % load. - Both curves are approximately parabolic in shape; head variation changes the best-efficiency speed, more strongly for Francis. #### Advantage of Pelton over Francis - Nearly flat efficiency curve at part load; suitable for plants with varying load or flow. - No cavitation, no draft tube, easy to inspect and repair buckets. - More tolerant of silt, since only buckets and nozzles need replacement and are easy to access. - Best for high heads where Francis would be too fast or heavily stressed. #### Principle of setting a Francis turbine The runner is set so that the pressure at the runner exit (draft tube inlet) never falls below the vapour pressure, to avoid cavitation. The maximum height of the runner outlet above tailwater is $$ H_s = H_{atm} - H_v - \sigma_c H $$ where $\sigma_c$ is the Thoma cavitation coefficient (a function of specific speed). Higher $N_s$ means a larger $\sigma_c$, so the turbine must be set lower, and at high elevation (lower $H_{atm}$) it must also be set lower; the setting may be below tailwater level. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does Pelton keep high efficiency at part load? A. Cavitation increases B. Runner turns slower C. Guide vanes change the angle D. Load is regulated by number of jets or spear position, so each jet works near best speed ratio Answer: D. Jet regulation keeps velocity triangles optimum. (Key point: Pelton flat efficiency) 2. Why does Francis efficiency fall at part load? A. Guide vane opening changes the flow angle at inlet, causing shock B. Jets reduce C. Draft tube removed D. Head increases Answer: A. Inlet angle mismatch causes shock loss. (Key point: Francis part load) 3. Which is an advantage of Pelton over Francis? A. Higher specific speed B. No cavitation or draft tube, and silt tolerance C. Lower head requirement D. Smaller runner at low head Answer: B. Only buckets and nozzles wear. (Key point: Pelton advantages) 4. What is the principle for setting a Francis turbine? A. Set at the highest possible position B. Set above tailwater by 20 m always C. Pressure at runner exit must not fall below vapour pressure; Hs = Hatm - Hv - sigma_c H D. Set by head loss only Answer: C. Cavitation limits height of runner outlet above tailwater. (Key point: Setting principle) ## Related in Hydropower Engineering - [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 - [Conditions favouring Francis over Pelton](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-preferable-over-pelton) — Hydropower Engineering, chapter 7 - [Pumped storage plant: power, turbine, setting, surge](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-turbine-surge-numerical) — Hydropower Engineering, chapter 7 - [Francis size and setting (150 m, 160 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-size-setting-150m-160cumec) — Hydropower Engineering, chapter 7 - [Francis diameter and setting (110 m, 140 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-diameter-110m-140cumec) — Hydropower Engineering, chapter 7 - [Francis overall efficiency with draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-efficiency-32mw) — Hydropower Engineering, chapter 7 ## Related in other subjects - [Pelton versus Francis performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-vs-francis-performance) — Hydro Power, 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 - [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7 - [Hydraulic turbine characteristics and water starting time](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Ahydraulic-turbine-water-starting) — Power Plant Equipment, chapter 2