Max height of straight draft tube (Francis)

Data

$H = 25$ m, $P = 11{,}760$ kW, $N = 120$ rpm, $H_{atm} = 10$ m, $H_v = 0.20$ m.

Specific speed

$$N_s = \frac{N\sqrt{P}}{H^{5/4}} = \frac{120\sqrt{11760}}{25^{1.25}} = 232.8$$

Thoma cavitation coefficient

$$\sigma_c = 0.0432\left(\frac{N_s}{100}\right)^2 = 0.0432 \times (2.328)^2 = 0.2341$$

Maximum height (setting) of the draft tube

The maximum height of the runner outlet / draft tube inlet above the tailwater level is

$$H_s = H_{atm} - H_v - \sigma_c H = 10 - 0.20 - 0.2341 \times 25 = 3.95\ \text{m}$$

Answer: the straight draft tube may be set at most about 3.95 m above the tailwater level (a lower setting is safer).