Cavitation effects and prevention in reaction turbines

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.