---
title: "Spiral casing and draft tube location"
subject: "Hydropower Engineering"
chapter: 7
key: "semester_7/hydropower_engineering:spiral-casing-draft-tube-location"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spiral-casing-draft-tube-location
site: BCT Engineering
---

# Spiral casing and draft tube location

Hydropower Engineering · Chapter 7: Hydro-electric Machines

- Asked in: 2079 Baishakh, 2+2 marks
- Video: [Hydro power project Draft tube and spiral cacing assembly](https://www.youtube.com/watch?v=P0hRqwDhx9g) — Ritun Brahma  practical
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spiral-casing-draft-tube-location
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-7

## Question

Specify with a neat sketch the location of a spiral casing and draft tube used in hydroelectric power generation. Mention their importance.

## Short answer

The spiral casing surrounds the stay vanes, guide vanes and runner at runner level; the draft tube starts below the runner and ends under tailwater. The casing distributes flow uniformly around the runner; the draft tube recovers kinetic energy and raises efficiency.

## Answer

#### Sketch (vertical section through a vertical-shaft Francis unit)
```text
  Penstock
     |
     v         generator
  +--+---+     ====|====
  | Valve|        shaft
  +--+---+         |
     |       stay ring/guide vanes
   ==+==============+====
  /   SPIRAL CASING   \   <- surrounds the runner
  \_____ runner _______/
          |   |
          |   |  DRAFT TUBE
          \   /   (diverging pipe)
           \ /
 TWL ~~~~~~~~V~~~~~~~~~~~~~~> tailrace
```

The **spiral (scroll) casing** is connected to the end of the penstock (through the inlet valve) and surrounds the stay vanes, guide vanes and runner at the runner level; the **draft tube** starts directly below the runner and ends under the tailwater.

#### Importance
- **Spiral casing:** distributes the water uniformly all around the guide vanes and runner at a constant velocity (its area decreases along the flow path), so the flow enters the runner without shock and with low loss. It also withstands the water pressure and supports the turbine.
- **Draft tube:** recovers the kinetic energy at the runner exit, allows the turbine to be set above tailwater, increases the effective head, and conveys the water to the tailrace.

## Check yourself

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

1. Where is the spiral casing located?
   A. Below the runner
   B. Within the tailrace
   C. Above the generator
   D. Around the guide vanes and runner, fed by the penstock
   Answer: D. It surrounds the runner and distributes water. (Key point: Spiral casing location)

2. Why does the casing area decrease along the flow?
   A. To keep velocity constant while discharge is fed to the runner, giving uniform shock-free entry
   B. To increase pressure to cavitation level
   C. To reduce the number of guide vanes
   D. To store silt
   Answer: A. Water leaves the casing progressively so area reduces. (Key point: Casing purpose)

3. Where is the draft tube?
   A. Above the spiral casing
   B. Directly below the runner, ending below tailwater
   C. Beside the generator
   D. Inside the penstock
   Answer: B. It connects runner outlet to tailrace. (Key point: Draft tube location)

4. What is a draft tube's importance?
   A. Distributes flow to guide vanes
   B. Supports the generator
   C. Recovers kinetic energy and allows setting above tailwater
   D. Regulates the load
   Answer: C. It recovers exit velocity head and increases effective head. (Key point: Draft tube importance)

## Related in Hydropower Engineering

- [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
- [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
- [Functions of draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adraft-tube-functions) — 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
- [Peaking RoR: max power and wet/dry energy ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apeaking-ror-reservoir-250m-q25) — Hydropower Engineering, chapter 3

## Related in other subjects

- [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 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×
- [Draft tube objectives and governors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-objectives-governors) — Hydro Power, chapter 7