--- title: "Impulse and reaction turbine performance characteristics" subject: "Hydro Power" chapter: 7 key: "semester_6/hydro_power:impulse-reaction-performance" source: https://www.bctengineer.com/q/semester_6/hydro_power/impulse-reaction-performance site: BCT Engineering --- # Impulse and reaction turbine performance characteristics Hydro Power · Chapter 7: Hydro-Electric Machines - Asked in: 2073 Bhadra, 4 marks - Video: [Characteristics of turbine](https://www.youtube.com/watch?v=mOhSrfJjJXM) — Dinesh Kumar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/impulse-reaction-performance - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7 ## Question Differentiate between impulse and reaction turbines with the help of their performance characteristics. ## Short answer Pelton efficiency remains high over a wide load range (30–100%), whereas Francis efficiency peaks near full load and drops sharply below 50–60%. Discharge in Pelton is independent of speed, while Francis discharge decreases with speed. Pelton suits variable flow; Francis has higher peak efficiency but requires near-rated operation. ## Answer **Performance characteristic curves** show how a turbine's discharge, power and efficiency vary with speed, load or gate opening. Impulse (Pelton) and reaction (Francis, Kaplan) turbines show clearly different curves. #### Main characteristics (constant head) ```text Efficiency vs % of full load eta ^ Kaplan ________ | Pelton /--------\_ | / / Francis \ | / / .---. \ | / / / \ propeller | / / / \ +-------------------------> % load 25 50 75 100 ``` | Basis | Impulse (Pelton) | Reaction (Francis/Kaplan) | |---|---|---| | Efficiency vs load | Flat curve; high efficiency (above about 85%) from 30–100% load | Francis peaked: efficiency drops sharply below about 50–60% load; Kaplan flat; propeller very peaked | | Discharge vs speed ($Q$–$N$) | $Q$ almost independent of speed (set by nozzle opening only) | $Q$ changes with speed: decreases with speed in Francis (slow), increases in Kaplan (fast) | | Power vs speed ($P$–$N$) | Parabolic, maximum near $u/V \approx 0.46$ | Parabolic, maximum at different speed ratio ($\phi \approx 0.6$–0.9 for Francis, up to 2 for Kaplan) | | Efficiency vs speed | Peaks sharply at optimum speed ratio, falls to zero at runaway | Similar, with broader peak in Kaplan | | Runaway speed | About 1.8–1.9 times normal | Francis about 2 times; Kaplan up to 2.5–3 times | | Part-load operation | Good (spear control) | Poorer; draft tube vortex and cavitation at part load | | Effect of cavitation | Negligible | Limits operating range and setting | #### Conclusions - Pelton turbines suit plants with **widely varying load and flow** at high heads because of their flat efficiency curve. - Francis turbines give the highest peak efficiency (about 93–95%) but should run near full load; several units are used when flow varies. - Kaplan turbines, with adjustable blades, keep high efficiency over a wide load range at low heads. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does Pelton efficiency vary with load compared with Francis? A. Pelton is peaked and Francis flat B. Pelton stays high over 30-100% load; Francis drops sharply below about 50-60% C. Both are identical D. Francis is flat and Pelton drops to zero Answer: B. Pelton has a flat curve, while Francis has a peaked curve. (Key point: Efficiency vs load) 2. How does discharge Q vary with speed N in a Pelton turbine? A. Q almost independent of speed (set by nozzle opening) B. Q increases linearly with N C. Q falls to zero at runaway D. Q doubles with N Answer: A. The jet discharge depends only on nozzle opening and head. (Key point: Discharge vs speed) 3. Which turbine has the highest runaway speed ratio? A. Pelton (1.8-1.9 N) B. Francis (about 2 N) C. Kaplan (up to 2.5-3 N) D. All the same Answer: C. Kaplan can run away to 2.5-3 times normal speed. (Key point: Runaway speed) 4. Which turbine keeps high efficiency over a wide load range at low heads? A. Francis B. Propeller (fixed blades) C. Pelton D. Kaplan with adjustable blades Answer: D. Kaplan's adjustable blades give a flat efficiency curve. (Key point: Operating choice) ## Related in Hydro Power - [Impulse versus reaction turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-vs-reaction-turbines) — Hydro Power, chapter 7, asked 2× - [Pelton versus Francis performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-vs-francis-performance) — Hydro Power, chapter 7 - [Cavitation in reaction turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acavitation-reaction-turbines) — Hydro Power, chapter 7 ## Related in other subjects - [Reaction and impulse turbine types and performance](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-types-reaction-impulse-suitability) — Hydropower Engineering, chapter 7 - [Characteristics of impulse and reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aimpulse-reaction-characteristics) — Energy Environment and Society, chapter 3 - [Impulse versus reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aimpulse-vs-reaction) — Power Plant Equipment, chapter 7, asked 2× - [Impulse and reaction turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-vs-reaction-turbine) — Fluid Machines, chapter 3