---
title: "Kaplan turbine construction and working"
subject: "Fluid Machines"
chapter: 3
key: "semester_5/fluid_machines:kaplan-turbine-working"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/kaplan-turbine-working
site: BCT Engineering
---

# Kaplan turbine construction and working

Fluid Machines · Chapter 3: Water Turbine

- Asked in: year not given, 6 marks
- Video: [Kaplan Turbine Working and Design](https://www.youtube.com/watch?v=0p03UTgpnDU) — Sabin Civil Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/kaplan-turbine-working
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3

## Question

Explain the construction, working and advantages of a Kaplan turbine. How does it differ from a propeller turbine?

## Short answer

Kaplan turbine is an axial-flow reaction turbine with adjustable runner blades and guide vanes, used for low heads and large discharge. Water flows axially, and both blade and guide vane angles adjust with load, giving high efficiency over a wide range. It differs from a propeller turbine which has fixed blades, resulting in poor part-load efficiency.

## Answer

The **Kaplan turbine** is an **axial-flow reaction turbine** used for low heads (up to about 60 m) and very large discharge. Its runner looks like a ship's propeller, with blades that can be rotated about their own axes.

```text
        Spiral casing
        +---------------+
        |  guide vanes  |  wicket gates
        |  ===========  |
        |      ||       |   water turns 90 deg
        |      \/       |   and flows axially
        |   [HUB]       |
        |  / | | | \    |  <- adjustable blades
        +--------+------+
                 |
            Draft tube
```

#### Construction

- **Scroll casing and guide vanes** like a Francis turbine; water enters radially and is turned through 90 degrees by the vanes before reaching the runner (a space without vanes, between guide vanes and runner, helps the whirl to become free-vortex).
- **Runner:** a hub (boss) carrying 3 to 8 blades. The hub is streamlined and is about 0.35 to 0.6 of runner diameter.
- **Blade turning mechanism:** inside the hub, a servo-motor operated by the governor turns the blades.
- **Draft tube:** usually elbow type; very important for recovering the exit kinetic energy.

#### Working

Water flows axially through the runner blades. The guide vane opening and the blade angle are adjusted together by the governor with the load, so the water always enters the blades without shock. The pressure drops across the runner, and the axial change of whirl momentum gives torque. Exit is axial and without whirl at design point.

#### Advantages

- High efficiency (about 90 %) over a wide range of load, since both guide vane and blade angles adjust. The efficiency curve is flat.
- High specific speed, so a small turbine and generator, and a high rotational speed, suit low head.
- Suitable for sites with variable head and discharge.

#### Kaplan versus propeller turbine

| Point | Kaplan | Propeller |
|---|---|---|
| Runner blades | Adjustable | Fixed |
| Guide vanes | Adjustable | Adjustable or fixed |
| Part-load efficiency | High | Poor |
| Cost | Higher (blade mechanism) | Lower |
| Use | Variable load | Nearly constant load |

## Check yourself

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

1. The Kaplan turbine is:
   A. Cross-flow
   B. Axial-flow reaction turbine for low head and large discharge
   C. Impulse turbine
   D. Radial high head
   Answer: B. It has a propeller-like runner with adjustable blades. (Key point: Type)

2. The key difference from a propeller turbine is:
   A. Kaplan has adjustable runner blades; propeller has fixed
   B. Kaplan has no draft tube
   C. Propeller has higher part-load efficiency
   D. Kaplan is impulse
   Answer: A. Adjustable blades give high part-load efficiency; propeller is cheaper but poor at part load. (Key point: Kaplan vs propeller)

3. Kaplan efficiency curve is flat because:
   A. Speed is zero
   B. It has more blades
   C. Both guide vane and blade angles adjust with load
   D. It is a high head machine
   Answer: C. The governor adjusts both so water enters without shock. (Key point: Flat efficiency curve)

4. Kaplan has a high specific speed, which gives:
   A. Small turbine and generator, high speed for low head
   B. High head
   C. Low speed
   D. Large runner
   Answer: A. High Ns means higher speed and a smaller, cheaper machine. (Key point: Specific speed)

## Related in Fluid Machines

- [Francis turbine: construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-working-components) — Fluid Machines, chapter 3
- [Kaplan turbine discharge and blade angles](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-numerical) — Fluid Machines, chapter 3
- [Cross-flow (Banki-Mitchell) turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Across-flow-turbine-notes) — Fluid Machines, chapter 3
- [Specific speed of a turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aspecific-speed-turbine-derivation) — Fluid Machines, chapter 3

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- [Kaplan turbine suitability in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Akaplan-suitability-nepal) — Power Plant Design, chapter 3
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