---
title: "Pelton wheel: components and working"
subject: "Fluid Machines"
chapter: 3
key: "semester_5/fluid_machines:pelton-wheel-working-components"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/pelton-wheel-working-components
site: BCT Engineering
---

# Pelton wheel: components and working

Fluid Machines · Chapter 3: Water Turbine

- Asked in: year not given, 8 marks
- Video: [Pelton Wheel](https://www.youtube.com/watch?v=8tROcNSgT04) — TutorialsPoint
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/pelton-wheel-working-components
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3

## Question

With a neat sketch, explain the construction and working of a Pelton wheel. State the functions of its main components and how the jet is regulated.

## Short answer

A Pelton wheel is a tangential impulse turbine for high heads. A nozzle with a spear valve converts head into a high‑speed jet that strikes twin‑cup buckets, splitter dividing the flow. Momentum change from ~165° deflection rotates the runner. A deflector diverts the jet during load rejection, and governing uses the spear and deflector. Speed ratio ~0.46, efficiency 85‑90%.

## Answer

The **Pelton wheel** is a tangential-flow **impulse turbine** for high heads (above about 250 m) and small discharge. All the available head is converted into kinetic energy in the nozzle, and the jet at atmospheric pressure strikes buckets on the wheel rim.

```text
        Penstock
           |
        [Spear valve]---> nozzle
              \  jet
               \             _
   Casing       \     ()  ()   ) buckets
                 ===>(  WHEEL  )
                      ()  ()
                    ______________
                    Tailrace (open)
   Deflector plate hinged at nozzle exit
```

#### Main components

1. **Penstock:** brings water from the reservoir to the nozzle.
2. **Nozzle with spear (needle) valve:** the nozzle forms a high-velocity jet. A conical spear moved axially by the governor changes the annular exit area, hence the discharge, while the jet velocity stays almost the same.
3. **Runner and buckets:** a disc carries double-hemispherical (twin cup) buckets on its rim. A splitter ridge in the middle divides the jet into two halves that leave from the sides. Each bucket has a notch (cut-out) so that the jet enters it freely and the following bucket does not cut off the jet too early. Buckets are of cast iron, bronze or stainless steel.
4. **Casing:** a cover that prevents splashing and guides the water down to the tailrace. It is open to the atmosphere (the wheel works at atmospheric pressure) and does not run full of water.
5. **Brake nozzle:** a small jet applied to the back of the buckets to stop the wheel quickly after shutdown.
6. **Deflector (jet deflector):** when the load drops suddenly, it deflects the jet away from the buckets without closing the spear quickly, which avoids water hammer in the penstock.

#### Working

1. Water of net head $H$ flows through the penstock to the nozzle; the pressure head turns into jet velocity $V_1 = C_v\sqrt{2gH}$, $C_v\approx0.97$ to $0.99$.
2. The jet strikes the splitter at the centre of the bucket and is divided into two streams, which turn through about $165^\circ$ (not $180^\circ$, so that the water leaving a bucket does not hit the next bucket).
3. The change of momentum of the jet produces a tangential force on the buckets; the wheel rotates and drives the generator shaft.
4. Water leaves with low velocity and falls into the tailrace. The wheel is placed above the tailrace level, so a draft tube is not used.

Maximum efficiency occurs when the bucket speed is about 0.45 to 0.46 of the jet speed ($u\approx\tfrac12 V_1$ for a frictionless 180° bucket). Overall efficiency is 85 to 90 %.

Governing is by moving the spear (change of discharge) and, for sudden load rejection, the deflector.

## Check yourself

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

1. The Pelton wheel is:
   A. Tangential-flow impulse turbine for high heads
   B. Mixed-flow turbine
   C. Inward reaction turbine
   D. Reaction axial turbine
   Answer: A. All head is converted to kinetic energy in the nozzle; the jet at atmospheric pressure hits buckets. (Key point: Type of turbine)

2. Discharge is regulated by:
   A. Changing the casing
   B. Changing bucket number
   C. Brake nozzle
   D. Moving the spear valve in the nozzle, changing exit area
   Answer: D. The spear changes the annular exit area, hence discharge, with jet velocity almost constant. (Key point: Discharge regulation)

3. The deflector is used for:
   A. Splitting the jet
   B. Increasing velocity
   C. Stopping the wheel
   D. Sudden load drop, deflecting the jet to avoid water hammer
   Answer: D. It diverts the jet without closing the spear quickly, avoiding water hammer. (Key point: Deflector)

4. The jet is turned through about 165 degrees rather than 180 because:
   A. Water leaving a bucket must not hit the next bucket
   B. Of draft tube
   C. Efficiency is higher at 165
   D. The splitter is wider
   Answer: A. At 180 degrees the leaving water would strike the following bucket. (Key point: Jet turning angle)

5. A draft tube is not used in a Pelton wheel because:
   A. It is axial
   B. The wheel is above tailrace level and works at atmospheric pressure
   C. The pressure is high
   D. The wheel is submerged
   Answer: B. The wheel is placed above the tailrace; water falls freely. (Key point: No draft tube)

## Related in Fluid Machines

- [Pelton wheel power with penstock losses](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apelton-wheel-numerical-penstock) — Fluid Machines, chapter 3
- [Design of Pelton wheel for given power and head](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apelton-wheel-design-numerical) — Fluid Machines, chapter 3
- [Francis turbine: construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-working-components) — Fluid Machines, chapter 3
- [Kaplan turbine construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-working) — Fluid Machines, chapter 3
- [Force of jet on flat plate, stationary and moving](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ajet-on-flat-plate-numerical) — Fluid Machines, chapter 1
- [Jet on series of curved vanes: power and efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aseries-of-moving-vanes-numerical) — Fluid Machines, chapter 1

## Related in other subjects

- [Multi-jet Pelton wheel design (200 m, 4 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-multijet-200m-4cumec) — Hydropower Engineering, chapter 7
- [Pelton wheel 70 kW: flow rate and diameter](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-wheel-70kw-100m-400rpm) — Hydropower Engineering, chapter 7
- [Pelton wheel design (310 m, 5 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-design-310m-5cumec-specific-speed) — Hydropower Engineering, chapter 7
- [Number of jets of Pelton wheel (300 m, 1.5 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-jets-300m-1-5cumec) — Hydropower Engineering, chapter 7