---
title: "Functions of draft tube"
subject: "Hydropower Engineering"
chapter: 7
key: "semester_7/hydropower_engineering:draft-tube-functions"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/draft-tube-functions
site: BCT Engineering
---

# Functions of draft tube

Hydropower Engineering · Chapter 7: Hydro-electric Machines

- Asked in: 2074 Ashwin, 2 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 Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/draft-tube-functions
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-7

## Question

What are the functions of a draft tube?

## Short answer

A draft tube lets the turbine be set above tailwater, converts exit velocity head into pressure, creates suction at the runner outlet to increase effective head, and discharges water smoothly into the tailrace while reducing losses.

## Answer

The draft tube is the diverging pipe from the runner outlet to the tailrace. Its functions are:

1. **Allows the turbine to be placed above tailwater** (convenient, dry installation) without losing the head between runner exit and tailwater level.
2. **Converts kinetic energy into pressure energy:** the gradually increasing area reduces velocity, recovering most of the exit velocity head $V_1^2/2g$ that would otherwise be wasted.
3. **Creates suction at the runner outlet,** so the pressure there is below atmospheric and the effective head on the runner is increased.
4. **Discharges water to the tailrace** smoothly, submerged below tailwater, which keeps the tube full and sealed against air entry.
5. **Reduces exit losses** and increases the overall efficiency of the turbine, especially for low-head, high-speed turbines.

## Check yourself

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

1. What does the draft tube allow?
   A. Turbine placed above tailwater without losing the head between runner and tailwater
   B. Turbine placed far below the tailwater
   C. Removal of the spiral casing
   D. Increased tailwater level
   Answer: A. Suction recovers that head. (Key point: Setting above tailwater)

2. What energy conversion occurs?
   A. Pressure to kinetic energy
   B. Kinetic energy to pressure energy as area increases and velocity falls
   C. Heat to pressure
   D. Potential to electrical energy
   Answer: B. Diverging area slows flow and recovers pressure. (Key point: Energy conversion)

3. Why does it increase the effective head?
   A. It raises pressure at the outlet
   B. It adds pumping
   C. It creates pressure below atmospheric at the runner outlet
   D. It shortens the penstock
   Answer: C. Suction at the exit adds to the head across the runner. (Key point: Suction)

## Related in Hydropower Engineering

- [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
- [Spiral casing and draft tube location](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspiral-casing-draft-tube-location) — Hydropower Engineering, 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

## Related in other subjects

- [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×
- [Why draft tube is used in Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-francis) — Hydro Power, chapter 7
- [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