---
title: "Compounding of impulse steam turbines"
subject: "Fluid Machines"
chapter: 5
key: "semester_5/fluid_machines:steam-turbine-compounding"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/steam-turbine-compounding
site: BCT Engineering
---

# Compounding of impulse steam turbines

Fluid Machines · Chapter 5: Steam turbine and Hydraulic machine

- Asked in: year not given, 8 marks
- Video: [Compounding Of Impulse Turbine](https://www.youtube.com/watch?v=FCyagN2-OBw) — LEARN AND GROW
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/steam-turbine-compounding
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-5

## Question

What is compounding of steam turbines? Why is it necessary? Explain the types of compounding with neat sketches showing the variation of pressure and velocity of steam.

## Short answer

In a single-stage impulse turbine, steam leaves at very high velocity causing excessive rotor speed and exit loss. Compounding divides the pressure or velocity drop into stages, limiting blade speed. Types: pressure compounding (Rateau) with pressure drop in nozzles only; velocity compounding (Curtis) with pressure drop in one nozzle and velocity drop in moving rows; and pressure-velocity combination.

## Answer

#### Need for compounding

In a simple impulse turbine (De Laval), the whole pressure drop occurs in one nozzle, so the steam leaves at a very high velocity (1000 m/s or more). For best efficiency the blade speed must be about half the steam velocity component, so the rotor speed is very high (20000 to 30000 rpm). This causes severe centrifugal stresses, large vibration and bearing problems, and the exit kinetic energy loss is large. The speed must also be reduced by gearing.

**Compounding** is the method of dividing the total pressure drop or velocity drop into several stages, so that the steam velocity and the blade speed in each stage are moderate. It gives lower rotor speed and better efficiency.

#### Types

**1. Pressure compounding (Rateau turbine)**

The turbine has several stages in series. Each stage has a nozzle (fixed row) and a row of moving blades. The total pressure drop is divided among the nozzles; in each stage the steam is expanded in the nozzle, and its velocity is absorbed fully in the moving blades.

```text
 P  \
     \___                     Pressure falls in nozzles
         \___                 only; constant over blades
  V   /\  /\  /\             Velocity rises in nozzle,
     /  \/  \/  \            falls in blades (saw tooth)
  [N1][B1][N2][B2][N3][B3]
```

Used in medium capacity turbines; each stage is separated by a diaphragm with nozzles.

**2. Velocity compounding (Curtis turbine)**

The whole pressure drop occurs in one nozzle. The high-velocity steam then passes through several rows of moving blades, with rows of fixed (guide) blades between them to redirect the steam to the next row. The velocity falls in each moving row.

```text
 P  \____________________      Pressure drops in nozzle only
                                 and stays constant after
 V  /\                          Velocity rises in nozzle
   /  \  /\  /\                 and falls in the moving rows,
  /    \/  \/  \                 slightly in fixed rows
  [N] [M1][F][M2]
```

It gives a large work output from one stage, but efficiency is lower than pressure compounding because friction in the blades is high. It is often used as the first (governing) stage of a large turbine.

**3. Pressure-velocity compounding**

This combines both. The total pressure drop is divided into several stages, and in each stage a velocity-compounded Curtis wheel (two or three rows of moving blades) is used. It gives a shorter turbine than pure pressure compounding for a given pressure drop.

#### Comparison

| Point | Pressure compounding | Velocity compounding | Pressure-velocity |
|---|---|---|---|
| Nozzle exit velocity | Moderate | Very high | Moderate-high |
| Rotor speed | Low | Medium | Low |
| Number of stages | Many | One (few rows) | Few |
| Efficiency | Highest | Lowest | Intermediate |
| Size | Long | Short | Intermediate |

## Check yourself

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

1. Why is compounding needed in an impulse turbine?
   A. A single stage gives a very high steam velocity, so rotor speed becomes very high with large stresses and exit loss
   B. To increase the steam pressure
   C. To make the steam wet
   D. To reduce the number of nozzles
   Answer: A. Best blade speed is about half the steam velocity, so a single large drop forces 20000 to 30000 rpm. (Key point: Need for compounding)

2. In pressure compounding (Rateau), where does the pressure fall?
   A. Over the moving blades only
   B. In the nozzles of each stage, and is constant over the blades
   C. In the fixed blades between moving rows
   D. Only in the first stage
   Answer: B. The total pressure drop is divided among the nozzle rows; blades are impulse type. (Key point: Pressure compounding)

3. What happens in velocity compounding (Curtis)?
   A. Pressure drops in each of many nozzles
   B. The whole pressure drop occurs in one nozzle, then steam passes through several moving rows with fixed guide blades between
   C. Steam expands in the moving blades
   D. Pressure rises in guide blades
   Answer: B. Velocity of the high-speed jet is absorbed in stages by successive moving rows. (Key point: Velocity compounding)

4. Which statement compares the types correctly?
   A. Velocity compounding has the highest efficiency
   B. Pressure compounding has the highest efficiency but is long; velocity compounding is short but least efficient
   C. Pressure compounding has the fewest stages
   D. All have equal efficiency
   Answer: B. Blade friction is high in velocity compounding, giving lower efficiency but a compact machine. (Key point: Comparison)

## 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
- [Impulse and reaction turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-vs-reaction-turbine) — Fluid Machines, chapter 3
- [Steam nozzles and critical pressure ratio](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Asteam-nozzle-flow-critical-pressure) — Fluid Machines, chapter 5
- [Convergent-divergent steam nozzle design](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Asteam-nozzle-numerical) — Fluid Machines, chapter 5
- [Single-stage impulse turbine blade work and efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ade-laval-turbine-velocity-diagram-numerical) — Fluid Machines, chapter 5

## Related in other subjects

- [Pressure variation after sudden valve closure](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Awater-hammer-pressure-time-valve-and-midpoint) — Hydraulics, chapter 4
- [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