---
title: "Draft tube function and turbine selection criteria"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:draft-tube-turbine-selection"
source: https://www.bctengineer.com/q/semester_6/hydro_power/draft-tube-turbine-selection
site: BCT Engineering
---

# Draft tube function and turbine selection criteria

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2078 Chaitra, 2+6 marks
- 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/draft-tube-turbine-selection
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

Write down the function of a draft tube. Describe the selection criteria of hydraulic turbines for hydroelectric power plants.

## Short answer

A draft tube recovers exit velocity energy, creates suction allowing turbine above tailwater, and discharges water safely. Selection criteria: head (>300m Pelton, 30-300m Francis, <40m Kaplan), specific speed, part-load efficiency, synchronous speed, cavitation limit, sediment, tailwater variation, cost, number of units.

## Answer

#### Function of a draft tube
A **draft tube** is a gradually expanding tube between the runner exit of a reaction turbine and the tailrace, with its outlet submerged. Its functions:
1. Converts the high exit kinetic energy into pressure energy (energy recovery), raising efficiency.
2. Produces a negative head (suction) at the runner exit, so the turbine can be set above tailwater without loss of head.
3. Permits the turbine to be installed above tailwater for easy inspection and maintenance.
4. Discharges water to the tailrace at low velocity.

#### Selection criteria of hydraulic turbines
**1. Net head** – the primary guide:

| Head | Turbine |
|---|---|
| Above ~300 m | Pelton |
| 30–300 m (to ~600 m) | Francis |
| 2–40 m (to ~70 m) | Kaplan / propeller / bulb |
| Small hydro, 5–200 m | Crossflow, Turgo |

**2. Specific speed** $N_s = \dfrac{N\sqrt{P}}{H^{5/4}}$ (SI: rpm, kW, m):

| $N_s$ | Turbine |
|---|---|
| 8.5–30 | Pelton, single jet |
| 30–60 | Pelton multi-jet, Turgo |
| 60–300 | Francis |
| 300–1000 | Kaplan / propeller |

The highest permissible $N_s$ is chosen because it gives a smaller and cheaper turbine and generator.

**3. Discharge and part-load operation:** for widely varying flow (RoR plants), Pelton or Kaplan turbines (flat efficiency curves) or several Francis units are chosen; Francis and propeller turbines lose efficiency below about 50–60% load.

**4. Speed:** must be a synchronous speed $N = 120f/p$ ($p$ = number of generator poles) for direct coupling; a high speed reduces generator size.

**5. Cavitation and setting:** Thoma's criterion $\sigma = \dfrac{H_a - H_v - H_s}{H} \ge \sigma_c$ fixes the suction head. High-$N_s$ machines need a deeper setting and more excavation.

**6. Sediment and abrasion:** in silt-laden Nepalese rivers, turbines whose parts are easy to repair or replace (Pelton buckets, needles) are preferred at medium–high heads; coatings are used for Francis runners.

**7. Tailwater variation:** reaction turbines with draft tubes use the full head even with varying tailwater; impulse turbines lose the setting height.

**8. Number of units:** depends on load pattern, flow variation, transport size limits, and reliability (at least two units).

**9. Cost and maintenance:** combined cost of turbine, generator, powerhouse and excavation; availability of spare parts, local expertise and manufacturer.

## Check yourself

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

1. Which is a function of the draft tube?
   A. Convert exit kinetic energy to pressure energy and give suction at the runner exit
   B. Increase exit kinetic energy
   C. Control discharge
   D. Reduce the net head
   Answer: A. It recovers energy and creates a negative head at runner exit. (Key point: Draft tube function)

2. For a net head of 100 m, which turbine is the usual choice?
   A. Pelton
   B. Francis
   C. Kaplan
   D. Propeller
   Answer: B. Francis suits heads from 30 to 300 m. (Key point: Head-based selection)

3. Which turbine has Ns of 300-1000 (SI)?
   A. Pelton single jet
   B. Pelton multi-jet
   C. Francis
   D. Kaplan / propeller
   Answer: D. Kaplan and propeller are high-Ns low-head turbines. (Key point: Specific speed range)

4. Why are Pelton turbines preferred at medium-high heads in silty Nepalese rivers?
   A. They are immune to wear
   B. Their buckets and needles are easy to repair or replace
   C. They need no maintenance
   D. They use draft tubes
   Answer: B. Abrasion is inevitable, so easily replaceable parts are preferred. (Key point: Sediment consideration)

## Related in Hydro Power

- [Selection criteria of hydraulic turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aturbine-selection-criteria) — Hydro Power, chapter 7, asked 2×
- [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 objectives and governors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-objectives-governors) — 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×
- [Conical draft tube inlet pressure and efficiency](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aconical-draft-tube-numerical) — 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

- [Draft tube: function and types](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Adraft-tube-types-function) — Fluid Machines, chapter 3
- [Classification of hydroelectric plants](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ahydroelectric-plant-classification) — Fluid Machines, chapter 2
- [Spiral casing and draft tube location](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspiral-casing-draft-tube-location) — Hydropower Engineering, chapter 7
- [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×