Skip to main content

Pelton vs Francis performance; Francis setting

Short answer

Pelton efficiency stays high from 30–100% load, while Francis peaks near design load and drops below 50%. Pelton advantage: no cavitation, flat part-load efficiency, silt resistance. Francis setting: runner outlet elevation Hs = atmospheric head minus vapour head minus σc×net head to prevent cavitation; higher specific speed requires a lower setting.

Drawing efficiency curves, discuss the performance characteristics of Pelton and Francis turbines. What is the advantage of Pelton turbine over Francis? Write down the principle behind the setting of Francis turbine relative to the tail water level.

Efficiency curves

 Eff %
 100|
  90|        __Pelton___
  80|      /      ___Francis_
  70|     /     /        \
  60|    /    /            \
  50|   /   /
  40|  / /
    +----+----+----+----+---- % load
        25   50   75  100

Performance characteristics

  • Pelton: efficiency is high (about 85 to 90 %) over a wide range of load (about 30 to 100 %), because load is regulated by changing the number of jets or nozzle spear position, so each jet works at its best speed ratio.
  • Francis: peak efficiency (about 92 to 95 %) occurs near the design load; at part load the guide vane opening alters the flow angle at runner inlet, causing shock and a fall of efficiency below about 50 % load.
  • Both curves are approximately parabolic in shape; head variation changes the best-efficiency speed, more strongly for Francis.

Advantage of Pelton over Francis

  • Nearly flat efficiency curve at part load; suitable for plants with varying load or flow.
  • No cavitation, no draft tube, easy to inspect and repair buckets.
  • More tolerant of silt, since only buckets and nozzles need replacement and are easy to access.
  • Best for high heads where Francis would be too fast or heavily stressed.

Principle of setting a Francis turbine

The runner is set so that the pressure at the runner exit (draft tube inlet) never falls below the vapour pressure, to avoid cavitation. The maximum height of the runner outlet above tailwater is

Hs=Hatm−Hv−σcHH_s = H_{atm} - H_v - \sigma_c H

where σc\sigma_c is the Thoma cavitation coefficient (a function of specific speed). Higher NsN_s means a larger σc\sigma_c, so the turbine must be set lower, and at high elevation (lower HatmH_{atm}) it must also be set lower; the setting may be below tailwater level.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Hydropower Engineering chapter 7 questions →