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Impulse and reaction turbine performance characteristics

Short answer

Pelton efficiency remains high over a wide load range (30–100%), whereas Francis efficiency peaks near full load and drops sharply below 50–60%. Discharge in Pelton is independent of speed, while Francis discharge decreases with speed. Pelton suits variable flow; Francis has higher peak efficiency but requires near-rated operation.

Differentiate between impulse and reaction turbines with the help of their performance characteristics.

Performance characteristic curves show how a turbine's discharge, power and efficiency vary with speed, load or gate opening. Impulse (Pelton) and reaction (Francis, Kaplan) turbines show clearly different curves.

Main characteristics (constant head)

 Efficiency vs % of full load
 eta
  ^   Kaplan  ________
  |   Pelton /--------\_
  |       / /  Francis \
  |      / /  .---.     \
  |     / /  /     \  propeller
  |    / /  /       \
  +-------------------------> % load
     25   50   75  100
BasisImpulse (Pelton)Reaction (Francis/Kaplan)
Efficiency vs loadFlat curve; high efficiency (above about 85%) from 30–100% loadFrancis peaked: efficiency drops sharply below about 50–60% load; Kaplan flat; propeller very peaked
Discharge vs speed (QQ–NN)QQ almost independent of speed (set by nozzle opening only)QQ changes with speed: decreases with speed in Francis (slow), increases in Kaplan (fast)
Power vs speed (PP–NN)Parabolic, maximum near u/V≈0.46u/V \approx 0.46Parabolic, maximum at different speed ratio (ϕ≈0.6\phi \approx 0.6–0.9 for Francis, up to 2 for Kaplan)
Efficiency vs speedPeaks sharply at optimum speed ratio, falls to zero at runawaySimilar, with broader peak in Kaplan
Runaway speedAbout 1.8–1.9 times normalFrancis about 2 times; Kaplan up to 2.5–3 times
Part-load operationGood (spear control)Poorer; draft tube vortex and cavitation at part load
Effect of cavitationNegligibleLimits operating range and setting

Conclusions

  • Pelton turbines suit plants with widely varying load and flow at high heads because of their flat efficiency curve.
  • Francis turbines give the highest peak efficiency (about 93–95%) but should run near full load; several units are used when flow varies.
  • Kaplan turbines, with adjustable blades, keep high efficiency over a wide load range at low heads.

✓ Answer checked by Claude Opus 5.5 (high effort)

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