Reaction and impulse turbine types and performance

Turbines are classified by the action of water on the runner: impulse turbines (jet at atmospheric pressure) and reaction turbines (runner fully submerged, with the pressure dropping across the runner).

Impulse turbines

• Pelton wheel: one to six jets strike double-cup buckets on a wheel. Suited to high heads (mainly above about 300 m, and used down to about 100 m), low discharge, $N_s$ about 4 to 70. Mounted above tailwater.
• Turgo: inclined jet, runs at a higher speed; medium heads (50 to 250 m).
• Cross-flow (Banki-Michell): low cost, for small hydro, heads of 5 to 100 m and small discharges.

Reaction turbines

• Francis: radial inlet and axial outlet, with a spiral casing, guide vanes and draft tube. Medium heads (about 30 to 500 m), $N_s$ about 60 to 400. The most widely used type.
• Kaplan: axial flow with adjustable runner blades and guide vanes (double regulated). Low heads (up to about 70 m), large discharges, $N_s$ about 300 to 1000.
• Propeller: like the Kaplan but with fixed blades; suited to a nearly constant load.
• Bulb / Straflo: horizontal axial turbines with the generator in a bulb or at the rim; very low heads (below 20 m), for run-of-river and tidal schemes.

Suitability and performance

Major performance characteristics

• Efficiency vs load: Pelton and Kaplan turbines keep their efficiency above about 85 % from 25 to 100 % load; the Francis has a peak near 80 to 90 % load and falls at light load; the propeller falls sharply.
• Specific speed: increases from Pelton to Francis to Kaplan, and decides the type for a given head.
• Runaway speed: about 1.8 to 1.9 $N$ for Pelton, 2 to 2.2 $N$ for Francis, 2.5 to 3 $N$ for Kaplan.
• Cavitation: a risk for reaction turbines, controlled by the Thoma coefficient and the setting; absent in Pelton wheels.
• Typical peak efficiency: 90 % for Pelton, 92 to 95 % for Francis and 92 to 94 % for Kaplan.