---
title: "Why draft tube is used in Francis turbine"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:draft-tube-francis"
source: https://www.bctengineer.com/q/semester_6/hydro_power/draft-tube-francis
site: BCT Engineering
---

# Why draft tube is used in Francis turbine

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2071 Bhadra, 3 marks
- Video: [Draft Tube](https://www.youtube.com/watch?v=X3aypwGhMME) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/draft-tube-francis
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

Why draft tube is used in Francis turbine? Discuss.

## Short answer

A draft tube recovers exit kinetic energy by diverging flow, creates suction so the turbine can be set above tailwater without head loss, improving efficiency and enabling maintenance. Its height is limited by cavitation (Thoma's criterion).

## Answer

A **draft tube** is a gradually diverging tube connecting the runner exit of a Francis (reaction) turbine to the tailrace, with its outlet submerged below tailwater. It is used in a Francis turbine for the following reasons:

1. **Recovery of exit kinetic energy:** in a Francis turbine water leaves the runner with a considerable velocity (about 5–10 m/s). Without a draft tube this kinetic energy ($V^2/2g$) would be wasted in the tailrace. The diverging tube reduces the velocity and converts most of this energy into pressure energy, increasing the useful head.

2. **Creating suction (negative head) below the runner:** the column of water in the draft tube produces a pressure below atmospheric at the runner exit:
$$
\frac{p_1}{\rho g} = \frac{p_a}{\rho g} - H_s - \left(\frac{V_1^2 - V_2^2}{2g} - h_f\right)
$$
So the head between the runner and the tailwater ($H_s$) is not lost; the full head is utilised.

3. **Setting above tailwater:** the turbine can be placed above the tailwater level, making the powerhouse dry, easy to inspect and maintain, while still using the full available head.

4. **Higher efficiency and output:** overall efficiency improves by several per cent, which matters greatly for large Francis units.

5. **Safe discharge** of water to the tailrace at low velocity.

```text
   spiral casing + runner
       ===[  ]===
          \  /   <- inlet: high V, p < p_atm
           ||    Hs
 ~~~~~~~~~ || ~~~~~~~~ TWL
          /  \   diverging tube
         /____\  <- outlet: low V
```

The height $H_s$ is limited by cavitation (Thoma's criterion), since the inlet pressure must stay above vapour pressure.

## Check yourself

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

1. Why is a draft tube used on a Francis turbine?
   A. To increase exit velocity
   B. To reduce exit velocity and convert the otherwise wasted kinetic energy into pressure energy
   C. To control the flow
   D. To filter silt
   Answer: B. Water leaves the runner at 5-10 m/s; the diverging tube recovers much of V^2/2g. (Key point: Kinetic energy recovery)

2. How can a Francis runner be set above tailwater and still use the full head?
   A. The draft tube creates suction below the runner so Hs is not lost
   B. Tailwater is raised artificially
   C. The penstock is shortened
   D. The runner is made lighter
   Answer: A. Pressure at the runner exit is below atmospheric, so the head Hs between the runner and tailwater is regained. (Key point: Setting above tailwater)

3. What limits the height Hs of the runner above tailwater?
   A. Penstock diameter
   B. Cavitation (Thoma's criterion), since inlet pressure must stay above vapour pressure
   C. Generator speed
   D. Governor setting
   Answer: B. If Hs is too large, pressure at the draft tube inlet falls to vapour pressure and cavitation occurs. (Key point: Limit on Hs)

## Related in Hydro Power

- [Draft tube function and turbine selection criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-turbine-selection) — Hydro Power, chapter 7
- [Draft tube objectives and governors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-objectives-governors) — Hydro Power, chapter 7
- [Draft tube functions and inlet pressure](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-pressure-inlet) — Hydro Power, chapter 7, asked 2×
- [Conical draft tube inlet pressure and efficiency](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aconical-draft-tube-numerical) — Hydro Power, chapter 7
- [Powerhouse layout with Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apowerhouse-layout-francis) — Hydro Power, chapter 5
- [Purpose of governor and governing of Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agoverning-francis-turbine) — Hydro Power, chapter 7

## Related in other subjects

- [Francis overall efficiency with draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-efficiency-32mw) — Hydropower Engineering, chapter 7
- [Max height of straight draft tube (Francis)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-max-height-11760kw) — Hydropower Engineering, chapter 7
- [Francis turbine: construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-working-components) — Fluid Machines, chapter 3
- [Draft tube: purpose, setting and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adraft-tube-purpose-setting-equation) — Hydropower Engineering, chapter 7