---
title: "Criteria for turbine selection"
subject: "Power Plant Design"
chapter: 3
key: "semester_8/power_plant_design:turbine-selection-criteria"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/turbine-selection-criteria
site: BCT Engineering
---

# Criteria for turbine selection

Power Plant Design · Chapter 3: Hydro Power Plant Design

- Asked in: 2081 Shrawan, 4 marks; 2074 Bhadra, 5 marks (asked 2 times)
- Video: [Turbine||Classification of turbine||Selection of hydro turbine](https://www.youtube.com/watch?v=zJgxe7ddikM) — PK Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/turbine-selection-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3

## Question

Mention major factors governing during appropriate turbine selection.

## Short answer

Key factors: net head (high head Pelton, medium Francis, low Kaplan), discharge variation, specific speed, rotational speed, part-load efficiency, cavitation risk and setting level, sediment content (favors Pelton in silty rivers), cost, and maintenance.

## Answer

The turbine type is selected mainly from the **head** and **discharge** of the site, then refined by speed, efficiency and site conditions.

#### Major governing factors
1. **Net head:** the most important factor. High head (above ~300 m) suits Pelton; medium head (30–300 m) Francis; low head (below ~40 m) Kaplan/propeller.
2. **Discharge and its variation:** large discharge at low head favours Kaplan; small discharge at high head favours Pelton. A river with widely varying flow needs a turbine with a flat part-load efficiency curve.
3. **Specific speed:** $N_s = N\sqrt{P}/H^{5/4}$ combines head, power and speed into one number that fixes the runner type.
4. **Rotational speed:** higher speed means a smaller, cheaper generator, but is limited by cavitation and runaway speed.
5. **Part-load efficiency:** Pelton and Kaplan keep high efficiency at part load; Francis efficiency falls quickly below about 40–50% load.
6. **Cavitation and setting level:** reaction turbines must be set low enough (Thoma's $\sigma$), which adds excavation cost.
7. **Sediment in water:** silty Himalayan rivers wear Francis runners quickly; Pelton is easier to repair.
8. **Cost, size and transport:** runner size, powerhouse size and access road limits.
9. **Overall efficiency and maintenance** needs, and availability of spares and local expertise.

## Check yourself

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

1. Which matching of head to turbine is right?
   A. High head - Kaplan
   B. High head (above ~300 m) - Pelton; medium - Francis; low - Kaplan
   C. Low head - Pelton
   D. Medium head - Kaplan
   Answer: B. Net head is the most important factor in selecting the turbine. (Key point: Head ranges)

2. Which turbine's efficiency falls quickly below about 40-50% load?
   A. Pelton
   B. Kaplan
   C. Francis
   D. Cross-flow
   Answer: C. Francis has a poor part-load curve; Pelton and Kaplan stay high. (Key point: Part-load efficiency)

3. Why is Pelton favoured in very silty rivers?
   A. Francis runners wear quickly and Pelton is easier to repair
   B. Pelton has no runner
   C. Pelton needs no head
   D. Sediment improves Pelton efficiency
   Answer: A. Sediment erodes Francis runners; Pelton buckets are easier to repair. (Key point: Sediment)

4. Cavitation affects reaction turbines by requiring:
   A. A higher setting above tailwater
   B. A low enough setting level (Thoma's sigma), adding excavation cost
   C. No draft tube
   D. A larger transformer
   Answer: B. They must be set low to avoid cavitation, which costs more excavation. (Key point: Cavitation)

## Close variants set in other exams

- [Turbine selection method and governing factors](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-selection-method-factors) — Power Plant Design, chapter 3, asked 3×

## Related in Power Plant Design

- [Hydropower site selection factors](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asite-selection-discharge-factors) — Power Plant Design, chapter 3

## 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
- [Major sources and selection factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asources-and-selection-factors) — Water Supply Engineering, chapter 2, asked 2×
- [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
- [Factors governing illumination level](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aillumination-factors-lighting-schemes) — Industrial Power Distribution and Illumination, chapter 7