---
title: "Cavitation effects and prevention in reaction turbines"
subject: "Hydropower Engineering"
chapter: 7
key: "semester_7/hydropower_engineering:cavitation-effects-prevention"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/cavitation-effects-prevention
site: BCT Engineering
---

# Cavitation effects and prevention in reaction turbines

Hydropower Engineering · Chapter 7: Hydro-electric Machines

- Asked in: 2082 Bhadra, 2+2 marks
- Video: [Cavitation and Cavitation in Turbine](https://www.youtube.com/watch?v=xkHFgUCx59c) — BEST MECHANICAL ENGINEERING
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/cavitation-effects-prevention
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-7

## Question

What are the possible effects of cavitation in reaction turbines? Also, write down preventive measures of cavitation.

## Short answer

Cavitation causes pitting and erosion of runner and draft tube, efficiency and power loss, noise, vibration, and reduced life. Prevent by setting the runner low, choosing lower specific speed, using a well-designed draft tube, cavitation-resistant materials, avoiding poor operating zones, and aeration.

## Answer

Cavitation is the formation and collapse of vapour bubbles in flowing water where local pressure falls to the vapour pressure. In reaction turbines it occurs mainly at the runner outlet (back of the blades) and in the draft tube inlet, where pressure is lowest.

#### Effects
- **Pitting and erosion** of runner blades, draft tube cone and guide vanes by the high-pressure impact of collapsing bubbles.
- **Loss of efficiency and power** because flow passages are blocked by vapour and the blade profile is damaged.
- **Noise and vibration**, which can damage bearings and cause fatigue failure.
- **Reduced life**, with frequent repair (welding and grinding) and plant outage.
- **Unstable operation** and surging in the draft tube at part load.

#### Preventive measures
1. Set the runner low enough: keep the turbine setting $H_s \le H_{atm} - H_v - \sigma H$, i.e. closer to or below tailwater level.
2. Use a suitable specific speed; lower $N_s$ (a smaller $\sigma_c$) for high heads.
3. Use a well-designed draft tube with a gradual divergence (included angle 8 to 10 degrees) to recover pressure.
4. Use cavitation-resistant materials: stainless steel (13Cr-4Ni) runners, stellite or stainless overlay welding.
5. Maintain smooth, polished blade surfaces and correct blade profile, avoid sharp edges.
6. Avoid operation at very low part load or overload; admit air through the draft tube or runner centre (aeration) to cushion bubble collapse.
7. Keep silt out of the water (silt makes erosion worse), and carry out regular inspection and repair.

## Check yourself

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

1. What is cavitation?
   A. Formation and collapse of vapour bubbles where local pressure falls to vapour pressure
   B. Air entrainment from the tailrace
   C. Boiling due to high temperature of water
   D. Shock from sudden gate closure
   Answer: A. Pressure at vapour pressure forms bubbles that collapse. (Key point: Cause)

2. Where does cavitation mainly occur in reaction turbines?
   A. Spiral casing inlet
   B. Runner outlet back of blades and draft tube inlet, where pressure is lowest
   C. Penstock bends
   D. Guide vane pivot only
   Answer: B. Pressure is lowest at runner exit. (Key point: Location)

3. Which is an effect of cavitation?
   A. Increase in efficiency
   B. Higher head
   C. Pitting, loss of efficiency, noise and vibration
   D. Reduced silt
   Answer: C. Collapsing bubbles pit surfaces and disturb flow. (Key point: Effects)

4. Which setting measure prevents cavitation?
   A. Raise the runner far above tailwater
   B. Increase specific speed
   C. Use a narrow draft tube
   D. Set runner lower, Hs <= Hatm - Hv - sigma H
   Answer: D. Lower setting raises exit pressure. (Key point: Prevention by setting)

## Related in Hydropower Engineering

- [Reaction and impulse turbine types and performance](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-types-reaction-impulse-suitability) — Hydropower Engineering, chapter 7

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