Cavitation in reaction turbines

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.