Characteristics of impulse and reaction turbines

Characteristics of impulse turbines (Pelton, Turgo, cross-flow)

• Whole available head is converted into kinetic energy in the nozzle; a high-velocity free jet strikes the buckets.
• The runner works at atmospheric pressure; casing is not filled with water.
• Suitable for high head (above ~50 m, up to 2000 m) and low discharge; low specific speed.
• Flow is regulated by a spear valve (needle) in the nozzle; the jet deflector prevents overspeed.
• No draft tube is needed, and the runner is set above the tailwater level (so some head is lost).
• Efficiency curve is flat over a wide range of loads (85–92%).
• Simpler construction and easier maintenance; not subject to cavitation, but vulnerable to sand erosion of buckets.
• Examples: Pelton in Khimti, Upper Tamakoshi; cross-flow in many micro-hydro plants.

Characteristics of reaction turbines (Francis, Kaplan, propeller)

• Only part of the head becomes velocity in the guide vanes; the rest acts as pressure drop across the runner blades.
• The runner is fully submerged and the casing is full of water; pressure is above atmospheric at the inlet and below at the outlet.
• Suitable for low to medium head and high discharge; medium to high specific speed.
• Flow is regulated by wicket gates (guide vanes); Kaplan also adjusts blade pitch.
• A draft tube is needed to recover the kinetic energy and allow setting below the tailwater.
• Risk of cavitation and need for careful setting level.
• Higher rotational speed, compact for the same power; efficiency up to 90–95%.
• Examples: Francis in Kali Gandaki A and Marsyangdi; Kaplan for low heads.