Power Plant Engineering

1.18.1 IMPULSE AND REACTION TURBINES
Hydraulic turbines are required to transform fluid energy into usable mechanical energy as effi-
ciently as possible. Further depending on the site, the available fluid energy may vary in its quantum of
Water
Inlet
Non Return
Valve
Converging
Cones
Diverging
Cone
Discharge to
Hot Well
Exhaust
Steam
Fig. 1.15

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POWER PLANT ENGINEERING
potential and kinetic energy. Accordingly a suitable type of turbine needs to be selected to perform the
required job.
Based upon the basic operating principle, water turbines are categorized into impulse and
reaction turbines depending on whether the pressure head available is fully or partially converted into
kinetic energy in the nozzle.
Bearing
Blades
Water from
Reservoir
Fixed
Nozzle
Bearing
Bearing
Moving
Nozzles
Water from
Reservoir
Bearing
Fig. 1.16. Impulse Turbine.
Fig. 1.17. Reaction Turbine.
Impulse Turbine wherein the available hydraulic energy is first converted into kinetic energy by
means of an efficient nozzle. The high velocity jet issuing from the nozzle then strikes a series of
suitably shaped buckets fixed around the rim of a wheel (Fig. 1.16). The buckets change the direction of
jet without changing its pressure. The resulting change in momentum sets buckets and wheel into rotary
motion and thus mechanical energy is made available at the turbine shaft. The fluid jet leaves the runner
with a reduced energy. An impulse turbine operates under atmospheric pressure, there is no change of
static pressure across the turbine runner and the unit is often referred to as a free jet turbine. Important
impulse turbines are: Pelton wheel, Turgo-impulse wheel, Girad turbine, Banki turbine and Jonval tur-
bine etc., Pelton wheel is predominantly used at present.