---
title: "Draft tube: function and types"
subject: "Fluid Machines"
chapter: 3
key: "semester_5/fluid_machines:draft-tube-types-function"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/draft-tube-types-function
site: BCT Engineering
---

# Draft tube: function and types

Fluid Machines · Chapter 3: Water Turbine

- Asked in: year not given, 6 marks
- Video: [Draft tube,function of draft tube and types of draft tube](https://www.youtube.com/watch?v=YIEdO63BvgQ) — BEST MECHANICAL ENGINEERING
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/draft-tube-types-function
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3

## Question

What is a draft tube? Explain its function with an expression for the head gained and describe the types of draft tube.

## Short answer

A draft tube is a diverging tube at the runner exit of a reaction turbine, submerged in tailwater. It converts kinetic energy to pressure, allowing the runner to be placed above tailwater without head loss, and creates suction below atmospheric pressure. Types include conical (90% efficiency), simple elbow (60%), Moody spreading (85%), and elbow with rectangular outlet (85%).

## Answer

A **draft tube** is a gradually expanding tube fitted at the exit of the runner of a reaction turbine; its lower end is submerged in the tailwater.

#### Functions

1. The runner can be placed above the tailwater level for easy maintenance without losing the head between the runner and the tailrace.
2. It converts the large kinetic energy at the runner exit into pressure energy (by the diverging area), so the water is released to the tailrace at low velocity.
3. It creates a pressure below atmospheric at the runner exit. This increases the pressure drop across the runner and hence the effective head.

#### Head gain (theory)

Let $H_s$ be the height of the runner exit (section 1) above the tailwater surface, $V_1$ the velocity at the runner exit, $V_2$ the velocity at the draft tube outlet (section 2), and $h_f$ the loss in the tube. The outlet is submerged, so the pressure there is atmospheric plus the depth of submergence; this depth cancels against the extra elevation difference. Applying Bernoulli's equation between sections 1 and 2:

$$
\frac{p_1}{\rho g}+\frac{V_1^2}{2g}+H_s=\frac{p_{a}}{\rho g}+\frac{V_2^2}{2g}+h_f
$$

$$
\frac{p_1}{\rho g}=\frac{p_a}{\rho g}-H_s-\left(\frac{V_1^2-V_2^2}{2g}-h_f\right)
$$

So the pressure at runner exit is below atmospheric by $H_s$ plus the recovered kinetic energy $(V_1^2-V_2^2)/2g - h_f$. The efficiency of the draft tube is

$$
\eta_d=\frac{(V_1^2-V_2^2)/2g-h_f}{V_1^2/2g}
$$

Also $H_s$ must be limited (about 5 to 6 m) so that the pressure at the runner exit does not fall to the vapour pressure, which would cause cavitation.

#### Types

```text
 Conical      Simple elbow    Moody spreading   Elbow with
 (straight)                   (bell mouth)      varying section
   \  /          |  |           \      /          |   \
    \/           |  |_____       \    /           |    \_____
  (vertical)     \________       \__/ (hollow      \________
                 horizontal      center, flared)
```

| Type | Features | Efficiency |
|---|---|---|
| Conical (tapering) | Straight cone, angle 8 to 10 degrees to avoid separation | About 90 % |
| Simple elbowed | Elbow turns flow horizontal; used where excavation is limited | About 60 % |
| Moody spreading | Cone with solid central core or flared bell to the bottom; diffuser | About 85 % |
| Elbow with rectangular outlet | Varying section from circular to rectangular; for large Kaplan units | About 85 % |

## Check yourself

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

1. What is a draft tube?
   A. A converging tube before the runner
   B. A gradually expanding tube at the runner exit, with its lower end submerged in tailwater
   C. A pipe that supplies water to the nozzle
   D. A tube that bypasses the runner
   Answer: B. It is a diverging tube fitted at the exit of the runner of a reaction turbine and dips into tailwater. (Key point: Definition)

2. Why does a draft tube increase the effective head on the turbine?
   A. It adds extra water
   B. It makes the exit pressure below atmospheric, increasing the pressure drop across the runner
   C. It increases the exit velocity
   D. It raises the tailwater level
   Answer: B. By recovering kinetic energy and keeping the exit submerged, the pressure at runner exit falls below atmospheric and the pressure drop across the runner rises. (Key point: Pressure at runner exit)

3. Why must the height Hs of the runner exit above tailwater be limited to about 5 to 6 m?
   A. To avoid cavitation because the exit pressure would approach vapour pressure
   B. To avoid overspeeding
   C. To keep the draft tube cone angle small
   D. To prevent water hammer
   Answer: A. A larger Hs lowers the exit pressure toward vapour pressure, causing cavitation. (Key point: Limit on Hs)

4. Which draft tube type is used for large Kaplan units with a section changing from circular to rectangular?
   A. Conical
   B. Simple elbow
   C. Elbow with rectangular outlet
   D. Straight cylindrical
   Answer: C. The elbow tube with varying section ending in a rectangular outlet is used for large Kaplan units, with efficiency about 85 %. (Key point: Draft tube types)

## 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
- [Reciprocating pump working, slip and air vessel](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Areciprocating-pump-slip-air-vessel) — Fluid Machines, chapter 4

## Related in other subjects

- [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
- [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
- [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