Characteristics of hydro turbines

Hydraulic turbines are of two main kinds: impulse turbines, in which the whole pressure head is turned into a free jet before it strikes the runner (Pelton, Turgo, crossflow), and reaction turbines, in which the runner is full of water under pressure and both pressure and kinetic energy act on it (Francis, Kaplan, propeller).

Pelton turbine (impulse, tangential flow)

• Head: high, about 50–1300 m; small discharge.
• Specific speed: about 10–35 per jet (up to ~60 with multiple jets).
• Water from one or more nozzles strikes double-cup buckets at atmospheric pressure; flow controlled by a spear (needle); a deflector cuts the jet on load rejection.
• Flat efficiency curve: high efficiency from about 20% to 100% load. Maximum efficiency about 90–92%.
• No draft tube; no cavitation in the runner; easy to inspect and repair, tolerant of sediment.

Francis turbine (reaction, mixed flow)

• Head: medium, about 10–350 m (most common 30–300 m); medium discharge.
• Specific speed: about 60–300.
• Spiral casing, stay vanes, adjustable guide vanes, runner (radial inflow, axial outflow) and draft tube.
• Highest peak efficiency (about 93–95%) but falls at part load below about 40–50%; vibration and draft tube surges at low load.
• Sensitive to cavitation and sediment erosion; runner setting fixed by Thoma's $\sigma$.

Kaplan turbine (reaction, axial flow)

• Head: low, about 2–40 m; large discharge.
• Specific speed: about 300–1000.
• Adjustable runner blades and guide vanes (double regulation) keep efficiency high over a wide load and head range.
• Large size, high speed relative to head; high cavitation risk, so deep setting.
• Propeller turbine: same with fixed blades; cheap but poor part-load efficiency.

Small-hydro turbines

• Crossflow (Michell–Banki): head 3–250 m, simple, cheap, flat efficiency (about 75–85%); used in Nepalese micro-hydro.
• Turgo: impulse, head 50–250 m, higher speed than Pelton for the same head.

Summary