---
title: "Cavitation in reaction turbines"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:cavitation-reaction-turbines"
source: https://www.bctengineer.com/q/semester_6/hydro_power/cavitation-reaction-turbines
site: BCT Engineering
---

# Cavitation in reaction turbines

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2075 Bhadra, 1+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_6/hydro_power/cavitation-reaction-turbines
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

What is cavitation? What are its effects and how can it be avoided in reaction turbines?

## Short answer

Cavitation is formation and collapse of vapor bubbles where pressure falls to vapor pressure, causing pitting, efficiency loss, noise and vibration. Avoid by setting runner according to Thoma’s criterion (H_s ≤ H_a−H_v−σ_c H), using cavitation-resistant materials, smooth blade profiles, avoiding part-load, and admitting air into the draft tube.

## Answer

**Cavitation** is the formation of vapour bubbles in flowing water at points where the local absolute pressure falls to the **vapour pressure**, and their sudden **collapse** when they are carried to regions of higher pressure. In reaction turbines it occurs mainly at the **runner exit and draft tube inlet**, where the pressure is lowest.

#### Effects of cavitation
1. **Pitting and erosion** of runner blades, guide vanes and draft tube liner (metal surface becomes rough and spongy); repeated repair is needed.
2. **Drop in efficiency and output** as bubbles disturb the flow and reduce effective flow area.
3. **Noise and vibration** of the machine, leading to fatigue of parts.
4. **Sudden fall in discharge and power** at severe cavitation; in Nepal, cavitation combined with silt abrasion damages runners very quickly.

#### Avoiding cavitation in reaction turbines
1. **Proper setting (suction head) by Thoma's criterion:**
$$
\sigma = \frac{H_a - H_v - H_s}{H} \ge \sigma_c
\;\Rightarrow\; H_s \le H_a - H_v - \sigma_c H
$$
where $H_a$ = atmospheric pressure head, $H_v$ = vapour pressure head, $H_s$ = height of runner above tailwater, $H$ = net head, $\sigma_c$ = critical Thoma coefficient. Setting the runner lower (even below tailwater) raises the exit pressure.
2. **Selecting a suitable specific speed:** high-$N_s$ turbines have higher $\sigma_c$, so they need lower setting.
3. **Cavitation-resistant materials:** stainless steel (13Cr-4Ni), welded overlays, or coatings on runner blades.
4. **Smooth, well-designed blade profiles** without sharp edges; polished surfaces.
5. **Avoid prolonged part-load or overload operation**, which causes vortices in the draft tube.
6. **Admit air** into the draft tube (air admission valves) to cushion the collapse of bubbles at part load.

## Check yourself

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

1. Where does cavitation mainly occur in a reaction turbine?
   A. At the penstock inlet
   B. At the runner exit and draft tube inlet, where pressure is lowest
   C. At the spiral casing
   D. At the tailrace
   Answer: B. Pressure is lowest at the runner exit and draft tube inlet. (Key point: Where cavitation occurs)

2. Which is an effect of cavitation?
   A. Increased efficiency
   B. Pitting of runner blades, noise, vibration and drop in efficiency
   C. Reduced vibration
   D. Smoother surfaces
   Answer: B. Bubble collapse erodes surfaces and reduces output. (Key point: Effects)

3. According to Thoma's criterion, the permissible setting satisfies:
   A. Hs <= Ha - Hv - sigma_c H
   B. Hs >= Ha + Hv
   C. Hs = H
   D. Hs <= sigma_c H only
   Answer: A. sigma = (Ha - Hv - Hs)/H must be at least sigma_c, giving Hs <= Ha - Hv - sigma_c H. (Key point: Thoma's criterion)

4. Which measure helps avoid cavitation?
   A. Setting the runner higher above tailwater
   B. Admitting air into the draft tube at part load
   C. Running at prolonged overload
   D. Using sharp blade edges
   Answer: B. Air admission cushions bubble collapse; lower setting, resistant materials and smooth blades also help. (Key point: Avoiding cavitation)

## Related in Hydro Power

- [Impulse versus reaction turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-vs-reaction-turbines) — Hydro Power, chapter 7, asked 2×
- [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7

## Related in other subjects

- [Cavitation effects and prevention in reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Acavitation-effects-prevention) — Hydropower Engineering, chapter 7
- [Impulse versus reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aimpulse-vs-reaction) — Power Plant Equipment, chapter 7, asked 2×
- [Characteristics of impulse and reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aimpulse-reaction-characteristics) — Energy Environment and Society, chapter 3
- [Impulse and reaction turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-vs-reaction-turbine) — Fluid Machines, chapter 3