---
title: "Impulse versus reaction turbines"
subject: "Power Plant Equipment"
chapter: 7
key: "semester_7/power_plant_equipment:impulse-vs-reaction"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/impulse-vs-reaction
site: BCT Engineering
---

# Impulse versus reaction turbines

Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant

- Asked in: 2075 Chaitra, 5 marks; 2074 Chaitra, 5 marks (asked 2 times)
- 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 Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/impulse-vs-reaction
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7

## Question

Differentiate between the operation of impulse and reaction turbines?

## Short answer

In an impulse turbine, the entire stage pressure drop occurs in nozzles, so moving blades experience only impulse force while pressure stays constant. In a reaction turbine, pressure drops in both fixed and moving blades, generating impulse plus reaction forces. Moving blade passages are converging; leakage losses are larger but efficiency is higher.

## Answer

In an **impulse turbine** the whole pressure drop of a stage occurs in fixed nozzles, and the high-velocity jet only changes direction on the moving blades. In a **reaction turbine** the pressure drops in both the fixed and moving blades, so the moving blades are also pushed by the reaction of the steam accelerating through them.

| Point | Impulse turbine | Reaction turbine |
|---|---|---|
| Pressure drop | Whole drop occurs in the nozzles only | Drop occurs in both fixed and moving blades |
| Pressure on moving blades | Constant over the moving blades | Falls continuously across moving blades |
| Force on blades | Impulse of high-velocity jet only | Impulse + reaction of steam accelerating in the blade passage |
| Blade profile | Symmetrical, constant passage area | Aerofoil shape, converging passage |
| Fixed element | Nozzles | Fixed (guide) blades acting as nozzles |
| Steam admission | Partial or full (nozzle arcs) | Full admission all round |
| Blade velocity / speed | Very high speed per stage (needs compounding) | Lower speed, many stages |
| Number of stages | Fewer | More (for same pressure drop) |
| Efficiency | Lower | Higher |
| Size / cost | Smaller, cheaper for given power | Larger, costlier (more stages) |
| Tip leakage | Small (no pressure difference across blades) | Larger, needs close clearances and dummy piston |
| Example | De Laval, Curtis, Rateau | Parsons turbine |

**Degree of reaction** $R = \dfrac{\Delta h_{\text{moving}}}{\Delta h_{\text{stage}}}$ is 0 for a pure impulse stage and usually 0.5 for a Parsons (50% reaction) stage. Large modern turbines use impulse stages at the HP end and reaction stages in the IP/LP parts.

## Check yourself

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

1. Where does the pressure drop occur in an impulse turbine stage?
   A. Entirely in the fixed nozzles
   B. Equally in fixed and moving blades
   C. Entirely in moving blades
   D. In the condenser only
   Answer: A. In an impulse stage the whole pressure drop is in the nozzles; pressure is constant over the moving blades. (Key point: Pressure drop location)

2. The moving blades of a reaction turbine are driven by:
   A. Impulse of the jet only
   B. Impulse plus reaction of steam accelerating in the blade passage
   C. Gravity
   D. Reaction only, with no change of direction
   Answer: B. Pressure also falls across moving blades, so steam accelerates through them and gives reaction force. (Key point: Force on blades)

3. What is the degree of reaction of a pure impulse stage and a Parsons stage?
   A. 0 and 0.5
   B. 0.5 and 0
   C. 1 and 0.5
   D. 0 and 1
   Answer: A. R = enthalpy drop in moving blades / stage drop: zero for impulse and usually 0.5 for Parsons. (Key point: Degree of reaction)

## Related in Power Plant Equipment

- [Rankine efficiency and impulse versus reaction](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Arankine-efficiency-impulse-reaction) — Power Plant Equipment, chapter 7

## 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 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