---
title: "Selection criteria of hydraulic turbines"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:turbine-selection-criteria"
source: https://www.bctengineer.com/q/semester_6/hydro_power/turbine-selection-criteria
site: BCT Engineering
---

# Selection criteria of hydraulic turbines

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2081 Asoj, 6 marks; 2074 Bhadra, 4 marks (asked 2 times)
- Video: [Lecture 06 (Part 02) - HYDRAULIC MACHINES - Selection of Different Turbines](https://www.youtube.com/watch?v=y683s_TDBM4) — Farhan Kazi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/turbine-selection-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

Discuss the selection criteria of hydraulic turbines for a hydroelectric powerplant.

## Short answer

Selection by net head: Pelton >300m, Francis 30-300m, Kaplan <40m; specific speed Ns = N√P/H^5/4 (higher Ns gives smaller machine); discharge variation; synchronous speed N=120f/p; cavitation limit σ≥σc; sediment; tailwater variation; cost; number of units.

## Answer

Selection of a hydraulic turbine means choosing the **type** (Pelton, Turgo, Crossflow, Francis, Kaplan/propeller, bulb), **number of units**, **speed** and **setting** that give the highest efficiency and lowest cost for the site.

#### 1. Net head (main criterion)
| Head range | Suitable turbine |
|---|---|
| Above about 300 m | Pelton (impulse) |
| 30–300 m (up to about 600 m) | Francis |
| 2–40 m (up to about 70 m) | Kaplan / propeller / bulb |
| Small hydro, 5–200 m | Crossflow, Turgo |

#### 2. Specific speed
$$
N_s = \frac{N\sqrt{P}}{H^{5/4}}
$$
($N$ rpm, $P$ kW, $H$ m) relates head, power and speed and is the scientific basis of selection:

| $N_s$ (SI, kW) | Turbine |
|---|---|
| 8.5–30 | Pelton, single jet |
| 30–60 | Pelton, multi-jet / Turgo |
| 60–300 | Francis (slow to fast) |
| 300–1000 | Kaplan / propeller |

A higher $N_s$ gives a smaller, cheaper, faster machine and generator, so the highest $N_s$ allowed by cavitation is chosen.

#### 3. Discharge and its variation (part-load efficiency)
- Pelton and Kaplan have flat efficiency curves and work well at part load (wide flow variation, e.g. RoR plants in dry season).
- Francis and propeller turbines have peaked curves; efficiency falls sharply below 50–60% flow. Several units are used when flow varies widely.

#### 4. Operating speed
Speed should be a **synchronous speed** $N = \dfrac{120f}{P}$ (50 Hz); higher speed reduces generator cost.

#### 5. Cavitation and turbine setting
Reaction turbines must satisfy Thoma's criterion $\sigma = \dfrac{H_a - H_v - H_s}{H} \ge \sigma_c$. High-$N_s$ turbines need deep setting below tailwater, increasing excavation cost.

#### 6. Sediment and abrasion
Nepalese rivers carry hard quartz silt; Pelton turbines are easier to repair (buckets/needles replaceable) than Francis runners, so this influences selection at medium-high heads.

#### 7. Tailwater level variation
Impulse turbines lose the head between runner and tailwater, so they suit high heads; reaction turbines with draft tubes use the full head and suit low and medium heads with varying tailwater.

#### 8. Other factors
Number of units, overall cost (turbine, generator, powerhouse), runaway speed, ease of maintenance, transport limits in hilly areas, local manufacturing and experience.

## Check yourself

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

1. Which turbine suits a net head above about 300 m?
   A. Kaplan
   B. Francis
   C. Pelton
   D. Propeller
   Answer: C. Pelton impulse turbines are used for heads above about 300 m; Francis for 30-300 m and Kaplan for low heads. (Key point: Head ranges)

2. Which statement about specific speed Ns = N sqrt(P)/H^(5/4) is correct?
   A. Low Ns gives a cheaper, faster machine
   B. A higher Ns gives a smaller, cheaper, faster machine, so the highest Ns allowed by cavitation is chosen
   C. Kaplan has the lowest Ns
   D. Pelton has Ns of 300-1000
   Answer: B. Higher Ns means smaller turbine and generator, limited only by cavitation. (Key point: Specific speed)

3. For a RoR plant with widely varying flow, which turbine has a flat efficiency curve?
   A. Pelton or Kaplan
   B. Francis only
   C. Propeller only
   D. None of these
   Answer: A. Pelton and Kaplan keep high efficiency at part load, while Francis and propeller fall sharply below 50-60% flow. (Key point: Part-load behaviour)

4. What is the effect of selecting a high-Ns reaction turbine on setting?
   A. It can be set far above tailwater
   B. It needs deeper setting below tailwater, increasing excavation cost
   C. Setting does not matter
   D. It becomes an impulse turbine
   Answer: B. High-Ns turbines have a larger critical Thoma coefficient, so they need deep setting. (Key point: Cavitation and setting)

## Related in Hydro Power

- [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
- [Purpose of governing hydraulic turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aturbine-governing-purpose) — Hydro Power, chapter 7, asked 3×

## Related in other subjects

- [Hydroelectric schemes by hydraulic characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Ahydroelectric-scheme-types-hydraulic) — Hydropower Engineering, chapter 2
- [Hydraulic design of tunnel; alignment criteria](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-hydraulic-design-alignment-selection) — Hydropower Engineering, chapter 6
- [Selection criteria for ADC](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc-selection-criteria) — Instrumentation II, chapter 4, asked 3×
- [Criteria for site selection](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asite-selection-criteria) — Building Technology, chapter 1