---
title: "Characteristics of impulse and reaction turbines"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:impulse-reaction-characteristics"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/impulse-reaction-characteristics
site: BCT Engineering
---

# Characteristics of impulse and reaction turbines

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: year not given
- Video: [Impulse Turbine Working | Reaction Turbine Working | Turbines | Types of Turbines | Telugu Lecture](https://www.youtube.com/watch?v=1pRLuYpna98) — Upendrakumar malla
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/impulse-reaction-characteristics
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

What are the characteristics of impulse turbines and of reaction turbines?

## Short answer

Impulse turbines (Pelton) convert all head to kinetic energy via a nozzle; runner at atmospheric pressure, no draft tube, for high heads (>50 m) and low flow. Reaction turbines (Francis, Kaplan) have submerged runner, pressure drop across blades, require draft tube, for low to medium heads and high flow, with cavitation risk.

## Answer

#### Characteristics of impulse turbines (Pelton, Turgo, cross-flow)
- Whole available head is converted into kinetic energy in the nozzle; a high-velocity free jet strikes the buckets.
- The runner works at atmospheric pressure; casing is not filled with water.
- Suitable for **high head (above ~50 m, up to 2000 m) and low discharge**; low specific speed.
- Flow is regulated by a spear valve (needle) in the nozzle; the jet deflector prevents overspeed.
- No draft tube is needed, and the runner is set above the tailwater level (so some head is lost).
- Efficiency curve is flat over a wide range of loads (85–92%).
- Simpler construction and easier maintenance; not subject to cavitation, but vulnerable to sand erosion of buckets.
- Examples: Pelton in Khimti, Upper Tamakoshi; cross-flow in many micro-hydro plants.

#### Characteristics of reaction turbines (Francis, Kaplan, propeller)
- Only part of the head becomes velocity in the guide vanes; the rest acts as pressure drop across the runner blades.
- The runner is fully submerged and the casing is full of water; pressure is above atmospheric at the inlet and below at the outlet.
- Suitable for **low to medium head and high discharge**; medium to high specific speed.
- Flow is regulated by wicket gates (guide vanes); Kaplan also adjusts blade pitch.
- A **draft tube** is needed to recover the kinetic energy and allow setting below the tailwater.
- Risk of **cavitation** and need for careful setting level.
- Higher rotational speed, compact for the same power; efficiency up to 90–95%.
- Examples: Francis in Kali Gandaki A and Marsyangdi; Kaplan for low heads.

| Point | Impulse | Reaction |
|---|---|---|
| Runner | Partly submerged, atmospheric pressure | Fully submerged, pressure change |
| Head | High | Low to medium |
| Specific speed | Low | Medium to high |
| Draft tube | No | Yes |

## Check yourself

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

1. In an impulse turbine, where is the available head converted to kinetic energy?
   A. Partly in guide vanes and partly in the runner
   B. In the draft tube
   C. Entirely in the nozzle, before the jet hits the runner
   D. In the runner blades by pressure drop
   Answer: C. The whole head becomes a high-velocity jet in the nozzle, and the runner works at atmospheric pressure. (Key point: Energy conversion in impulse)

2. Which application suits an impulse turbine?
   A. Low head and high discharge
   B. Medium head and very high discharge
   C. Low head and low discharge only
   D. High head and low discharge
   Answer: D. Impulse turbines such as Pelton suit high heads with low discharge and have low specific speed. (Key point: Head and discharge)

3. Which statement about reaction turbines is correct?
   A. A draft tube is needed and cavitation is a risk
   B. No draft tube is needed and cavitation cannot occur
   C. The runner works at atmospheric pressure in air
   D. Flow is regulated by a spear valve
   Answer: A. Reaction runners are submerged; the draft tube recovers kinetic energy, and low outlet pressure creates cavitation risk. (Key point: Draft tube and cavitation)

4. How is flow regulated in a Francis turbine compared with a Pelton turbine?
   A. Spear valve versus wicket gates
   B. Wicket gates (guide vanes) versus a spear valve in the nozzle
   C. Jet deflector versus draft tube
   D. Both use a spear valve
   Answer: B. Reaction turbines use wicket gates; impulse turbines use a needle spear valve. (Key point: Flow regulation)

## Related in Energy Environment and Society

- [Classification of water turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Awater-turbines-impulse-reaction) — Energy Environment and Society, chapter 3, asked 2×

## 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×
- [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