Selection criteria of hydraulic turbines

Selection of a hydraulic turbine means choosing the type (Pelton, Turgo, Crossflow, Francis, Kaplan/propeller, bulb), number of units, speed and setting that give the highest efficiency and lowest cost for the site.

1. Net head (main criterion)

2. Specific speed

$$N_s = \frac{N\sqrt{P}}{H^{5/4}}$$

($N$ rpm, $P$ kW, $H$ m) relates head, power and speed and is the scientific basis of selection:

A higher $N_s$ gives a smaller, cheaper, faster machine and generator, so the highest $N_s$ allowed by cavitation is chosen.

3. Discharge and its variation (part-load efficiency)

• Pelton and Kaplan have flat efficiency curves and work well at part load (wide flow variation, e.g. RoR plants in dry season).
• Francis and propeller turbines have peaked curves; efficiency falls sharply below 50–60% flow. Several units are used when flow varies widely.

4. Operating speed

Speed should be a synchronous speed $N = \dfrac{120f}{P}$ (50 Hz); higher speed reduces generator cost.

5. Cavitation and turbine setting

Reaction turbines must satisfy Thoma's criterion $\sigma = \dfrac{H_a - H_v - H_s}{H} \ge \sigma_c$. High-$N_s$ turbines need deep setting below tailwater, increasing excavation cost.

6. Sediment and abrasion

Nepalese rivers carry hard quartz silt; Pelton turbines are easier to repair (buckets/needles replaceable) than Francis runners, so this influences selection at medium-high heads.

7. Tailwater level variation

Impulse turbines lose the head between runner and tailwater, so they suit high heads; reaction turbines with draft tubes use the full head and suit low and medium heads with varying tailwater.

8. Other factors

Number of units, overall cost (turbine, generator, powerhouse), runaway speed, ease of maintenance, transport limits in hilly areas, local manufacturing and experience.