---
title: "Scientific method of turbine selection"
subject: "Power Plant Design"
chapter: 3
key: "semester_8/power_plant_design:turbine-selection-scientific"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/turbine-selection-scientific
site: BCT Engineering
---

# Scientific method of turbine selection

Power Plant Design · Chapter 3: Hydro Power Plant Design

- Asked in: 2079 Shrawan, 4 marks; 2077 Chaitra, 4 marks (asked 2 times)
- Video: [Turbine Selection, Specific Speed, and Setting: Optimizing Hydropower Performance ⚙️💧#pelton#francis](https://www.youtube.com/watch?v=054RAD8TB80) — Hydropower
- 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-scientific
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3

## Question

What is scientific method of turbine selection?

## Short answer

Compute net head and design discharge, unit power, choose synchronous speed, then specific speed Ns = N sqrt(P) / H^(5/4) (N in rpm, P in kW, H in m). Select turbine type from Ns range: Pelton 10–35, Francis 60–300, Kaplan 300–1000. Verify with manufacturer's chart and cavitation limits.

## Answer

The **scientific method of turbine selection** chooses the turbine type from the **specific speed** ($N_s$) that the site's head, discharge and unit speed demand, and then checks it against the head range, efficiency curve and cavitation limits, rather than choosing by habit.

#### Steps
1. **Find net head and design discharge** per unit from the flow duration curve (e.g. $Q_{45}$), after deducting riparian release and head losses.
2. **Find unit power:**
$$
P = \eta_t \rho g Q H_{net}\ \text{(W)}
$$
3. **Choose synchronous speed** for the generator: $N = \dfrac{120 f}{p}$ (e.g. 500, 600, 750, 1000 rpm at 50 Hz).
4. **Compute specific speed:**
$$
N_s = \frac{N\sqrt{P}}{H^{5/4}}\quad (N\ \text{in rpm},\ P\ \text{in kW},\ H\ \text{in m})
$$
5. **Pick the turbine type** whose range contains $N_s$:

| Turbine | $N_s$ (metric, kW) | Usual head |
|---|---|---|
| Pelton | 10–35 per jet | 50–1300 m |
| Francis | 60–300 | 10–350 m |
| Kaplan / propeller | 300–1000 | 2–40 m |

6. **Check** with the manufacturer's head–discharge application chart, part-load efficiency, and cavitation (Thoma's $\sigma$ and setting level).
7. Adjust number of units or speed if $N_s$ falls in an overlap zone, and compare cost.

## Check yourself

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

1. The scientific method chooses the turbine type from:
   A. Habit and past practice
   B. Specific speed from head, discharge and unit speed, then checks against ranges and cavitation
   C. Colour of the water
   D. The price of the runner only
   Answer: B. Ns is computed and compared with the ranges for each type. (Key point: Basis of method)

2. Specific speed is calculated as:
   A. N sqrt(P) / H^(5/4)
   B. N P / H^2
   C. N sqrt(H) / P^(5/4)
   D. P sqrt(N) / H
   Answer: A. Ns = N sqrt(P)/H^(5/4), N in rpm, P in kW, H in m. (Key point: Specific speed formula)

3. An Ns of about 150 indicates:
   A. Pelton
   B. Francis
   C. Kaplan
   D. Water wheel
   Answer: B. Francis ranges 60-300; Pelton is 10-35 per jet and Kaplan 300-1000. (Key point: Ns ranges)

4. Generator synchronous speed is chosen using:
   A. N = 120 f / p
   B. N = f p / 120
   C. N = 60 / f
   D. N = p / f
   Answer: A. At 50 Hz it gives standard speeds such as 500, 600, 750 and 1000 rpm. (Key point: Synchronous speed)

## Related in Power Plant Design

- [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×
- [Criteria for turbine selection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-selection-criteria) — Power Plant Design, chapter 3, asked 3×

## Related in other subjects

- [Job analysis and scientific selection](https://www.bctengineer.com/question.md?key=semester_7%2Forganization_and_management%3Ajob-analysis-selection) — Organization and Management, 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