---
title: "Impulse and reaction turbines compared"
subject: "Fluid Machines"
chapter: 3
key: "semester_5/fluid_machines:impulse-vs-reaction-turbine"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/impulse-vs-reaction-turbine
site: BCT Engineering
---

# Impulse and reaction turbines compared

Fluid Machines · Chapter 3: Water Turbine

- Asked in: year not given, 5 marks
- Video: [Impulse Vs Reaction Turbine](https://www.youtube.com/watch?v=D2LSD-q-xMg) — Gear Institute Mechanical Engineering Videos and Physics by Gear Institute
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/impulse-vs-reaction-turbine
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3

## Question

Differentiate between impulse and reaction turbines. Give a comparison between Pelton, Francis and Kaplan turbines.

## Short answer

In an impulse turbine, all head converts to kinetic energy before the runner; pressure is atmospheric. In a reaction turbine, both pressure and kinetic energy act on the runner, with pressure drop across it. Pelton (impulse) suits high heads, Francis (reaction) medium heads, Kaplan (reaction) low heads with large flow. Pelton uses spear/deflector, Francis guide vanes, Kaplan adjustable blades and vanes.

## Answer

In an **impulse turbine** the available head is fully converted into kinetic energy before the water reaches the runner; in a **reaction turbine** only part of the head is converted into kinetic energy and the rest remains as pressure energy, which is converted to work inside the runner.

#### Impulse and reaction turbines

| Point | Impulse turbine | Reaction turbine |
|---|---|---|
| Energy at runner inlet | Kinetic energy only | Kinetic and pressure energy |
| Pressure in runner | Atmospheric, constant | Falls from inlet to outlet |
| Flow | Partial admission (jets) | Full admission, runner always full of water |
| Casing | Only to prevent splashing | Essential; closes the water passage |
| Draft tube | Not used | Used |
| Position of runner | Above tailwater level | Can be above, with draft tube |
| Blade shape | Symmetrical buckets | Aerofoil-like blades |
| Regulation | Spear valve and deflector | Guide vanes (and Kaplan blades) |
| Head | High | Low to medium |
| Example | Pelton, cross-flow (partly) | Francis, Kaplan |

#### Pelton, Francis and Kaplan

| Point | Pelton | Francis | Kaplan |
|---|---|---|---|
| Type | Impulse, tangential | Reaction, radial/mixed | Reaction, axial |
| Head (approx.) | 250 m and above | 40 to 600 m | Up to 60 m |
| Discharge | Small | Medium | Large |
| Specific speed | 10 to 35 | 60 to 400 | 300 to 1000 |
| Regulation | Spear/deflector | Guide vanes | Guide vanes and blades |
| Draft tube | No | Yes | Yes |
| Part-load efficiency | Good | Falls at part load | Very good |

## Check yourself

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

1. What is the difference in the energy of water at the runner inlet between impulse and reaction turbines?
   A. Impulse: kinetic only; reaction: kinetic plus pressure
   B. Impulse: pressure only; reaction: kinetic only
   C. Both have kinetic only
   D. Impulse: kinetic plus pressure; reaction: pressure only
   Answer: A. In an impulse turbine the head is fully converted to kinetic energy before the runner; in a reaction turbine part remains as pressure energy. (Key point: Energy at runner inlet)

2. Which statement is correct for the pressure in the runner?
   A. Impulse: falls through the runner; reaction: atmospheric
   B. Both fall through the runner
   C. Impulse: atmospheric and constant; reaction: falls from inlet to outlet
   D. Both are constant and atmospheric
   Answer: C. Impulse runners work at atmospheric pressure, while reaction runners are always full and the pressure drops across them. (Key point: Pressure and casing)

3. Which matching of turbine to typical conditions is correct?
   A. Pelton: low head, large discharge
   B. Kaplan: high head, small discharge
   C. Francis: highest head above 250 m only
   D. Pelton: high head, small discharge; Kaplan: low head, large discharge
   Answer: D. Pelton suits high heads and small discharge, Kaplan suits low heads and large discharge, with Francis in between. (Key point: Head and discharge ranges)

## Related in Fluid Machines

- [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
- [Compounding of impulse steam turbines](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Asteam-turbine-compounding) — Fluid Machines, chapter 5

## Related in other subjects

- [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×
- [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×
- [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
- [Number of Francis/Kaplan units (300 m³/s, 30 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-units-number-francis-kaplan-300cumec) — Hydropower Engineering, chapter 7