---
title: "Impulse versus reaction turbines"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:impulse-vs-reaction-turbines"
source: https://www.bctengineer.com/q/semester_6/hydro_power/impulse-vs-reaction-turbines
site: BCT Engineering
---

# Impulse versus reaction turbines

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2081 Chaitra, 4 marks; 2080 Asoj, 5 marks (asked 2 times)
- Video: [Impulse and Reaction turbine with animation](https://www.youtube.com/watch?v=dS3GpvIl6fc) — Wasique Ahmad
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/impulse-vs-reaction-turbines
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

Differentiate between impulse and reaction turbines with suitable examples.

## 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.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What energy does an impulse turbine runner use?
   A. Only kinetic energy of a free jet at atmospheric pressure
   B. Pressure and kinetic energy
   C. Only pressure energy
   D. Only potential energy
   Answer: A. The whole head is converted to jet velocity in the nozzle, so the runner gets only kinetic energy. (Key point: Energy at inlet)

2. How does pressure behave in the runner of a reaction turbine?
   A. Atmospheric throughout
   B. Constant above atmospheric
   C. Decreases from inlet to outlet
   D. Increases from inlet to outlet
   Answer: C. Water flows under pressure and the pressure drops along the runner, going below atmospheric at exit. (Key point: Pressure in runner)

3. Which turbine needs a draft tube?
   A. Pelton
   B. Francis
   C. Crossflow
   D. Turgo
   Answer: B. Reaction turbines such as Francis need a draft tube; impulse turbines run in air and do not. (Key point: Draft tube use)

4. Which pair is correctly classified?
   A. Pelton: reaction; Francis: impulse
   B. Pelton and Kaplan: impulse
   C. Pelton and Turgo: impulse; Francis and Kaplan: reaction
   D. Crossflow: reaction; Kaplan: impulse
   Answer: C. Pelton, Turgo and Crossflow are impulse; Francis, Kaplan, propeller and bulb are reaction turbines. (Key point: Examples)

## 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
- [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

- [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
- [Impulse and reaction steam turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-reaction-steam-turbine-types) — Fluid Machines, chapter 5
- [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