---
title: "Why vertical shaft arrangement is preferred"
subject: "Hydropower Engineering"
chapter: 8
key: "semester_7/hydropower_engineering:powerhouse-vertical-shaft-turbine-generator"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/powerhouse-vertical-shaft-turbine-generator
site: BCT Engineering
---

# Why vertical shaft arrangement is preferred

Hydropower Engineering · Chapter 8: Powerhouse

- Asked in: 2076 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/powerhouse-vertical-shaft-turbine-generator
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-8

## Question

Why is a vertical shaft arrangement preferred while laying turbine and generator in a powerhouse? Explain briefly.

## Short answer

Vertical shaft arrangement places turbine below generator on the same shaft, saving floor area, improving draft tube hydraulics and submergence, keeping generator dry, and allowing one thrust bearing and easy vertical lifting. It suits large low-speed units; disadvantages are taller building and costly thrust bearing, so horizontal shaft is used for small units and Pelton/bulb turbines.

## Answer

A vertical-shaft arrangement places the turbine below and the generator directly above it on the same vertical shaft. It is preferred, especially for medium and large Francis and Kaplan units, because:

1. **Compact plan area:** the units need less floor space, so the powerhouse is shorter and narrower, reducing excavation and concrete cost.
2. **Better hydraulics:** the spiral casing and draft tube are symmetric about the shaft, and a smooth vertical draft tube gives less loss and more uniform flow.
3. **Better setting:** the turbine can be kept low relative to tailwater, giving larger submergence and reducing the risk of cavitation.
4. **Generator kept dry, single thrust bearing:** the generator sits above the flood level, safe from water and damp, and the weight of the rotating parts is carried by a single thrust bearing.
5. **Easy erection and maintenance:** the generator and turbine can be lifted vertically by the overhead crane; the runner can be removed through the generator stator or through the draft tube access.
6. **Suitable for large sizes and low speeds:** large-diameter, low-speed generators suit a vertical shaft, with less bending in the shaft.
7. **Less wear on bearings and shaft:** the rotor weight causes no sideways load or sag on the shaft.

The disadvantages are a taller building and the higher cost of the thrust bearing, but these are accepted for large units. A horizontal shaft is generally used only for small units, Pelton wheels and bulb turbines.

## Check yourself

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

1. In a vertical-shaft arrangement, the generator is placed:
   A. Beside the turbine on a horizontal shaft
   B. Directly above the turbine on the same shaft
   C. Below the draft tube
   D. In a separate building
   Answer: B. The turbine is below and the generator directly above on the same vertical shaft. (Key point: Arrangement)

2. Why is the powerhouse cheaper with a vertical shaft?
   A. It needs less floor space, so it is shorter and narrower, reducing excavation and concrete
   B. It needs no crane
   C. It is shorter in height
   D. It needs no draft tube
   Answer: A. Compact plan area reduces excavation and concrete cost. (Key point: Plan area advantage)

3. How does a vertical shaft help against cavitation?
   A. The turbine can be set low relative to tailwater, giving larger submergence
   B. It raises the turbine above tailwater
   C. It reduces the head
   D. It removes the need for a draft tube
   Answer: A. A low setting gives greater submergence, reducing cavitation risk. (Key point: Cavitation and setting)

4. Horizontal shafts are generally used for:
   A. Large Francis units
   B. Large Kaplan units
   C. Small units, Pelton wheels and bulb turbines
   D. All low-speed generators
   Answer: C. Vertical shafts suit medium and large Francis and Kaplan units; horizontal is for small units, Pelton and bulb turbines. (Key point: Where horizontal shaft is used)

## Related in Hydropower Engineering

- [Powerhouse layout with vertical axis Francis turbine](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-layout-vertical-axis-francis) — Hydropower Engineering, chapter 8, asked 3×
- [Powerhouse general arrangement and dimensions](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-arrangement-and-dimensions) — Hydropower Engineering, chapter 8, asked 3×
- [Powerhouse plan and sections (Francis, 10 MW)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-plan-section-francis-10mw) — Hydropower Engineering, chapter 8

## Related in other subjects

- [Powerhouse layout with Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apowerhouse-layout-francis) — Hydro Power, chapter 5
- [Two-shaft gas turbine, pressure ratio 7:1](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-two-shaft-7-1) — Power Plant Equipment, chapter 6, asked 2×
- [Gas turbine engine components and arrangement](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-engine-schematic-components) — Turbo Machines, chapter 3
- [Transient response of turbine-generator with governor](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agovernor-transient-response) — Power Plant Equipment, chapter 2, asked 3×