Figure: Kaplan turbine
The water from the scroll casing flows over the guide
vanes. It is deflected through 90° between guide vanes and runner.
Then, it flows axially into the runner. The blades are shaped such that water
flows axially in the runner. The force exerted on the blades causes the runner
shaft to rotate. This rotation is transmitted to the generator which is couple
to the runner shaft. After passing through the runner, the water enters the
tailrace through a draft tube.
of Kaplan turbine
A Kaplan turbine has adjustments in both guide vanes and
runner vanes. Generally, double regulators are provided in a Kaplan turbine.
The governor regulates the guide blade opening as well as the runner vane
angles simultaneously. The adjustment of guide vanes is similar to that of a
Francis turbine. The runner vanes are regulated by a separate Servomotor. The
control valves for both the runner and guide vanes are interconnected to ensure
a define runner vane angle for the given vane opening.
Figure: Governing of Kaplan turbine.
The runner vane angles may be adjusted while the turbine
is in motion. The piston rod of the servomotor (of the runner vanes) pass
through the hollow turbine shaft as shown in figure. The movement of the piston
is transmitted to the runner of the piston is transmitted to the runner vane by
a small crank connected to cross head. The servomotor acts as the coupling
between the turbines shaft and the generator shaft. Oil from the governor is
admitted to the upper or lower side of the servomotor piston through pipes.
This will reduce or increase the blade angle. The governor actuates
simultaneously both the guide vane and the runner vane. Thus for all loads, the
turbine is able to maintain high efficiency.
Compare Impulse and reaction turbines
S.No Impulse turbine
The machine is suitable for high installation. (H=100 +
2. Nature of input energy to the runner:
The nozzle converts
the entire hydraulic energy
into kinetic energy before water
strikes the runner.
3. Method of energy transfer:
The buckets of
the runner are
so shaped that they extract
almost all the kinetic energy of the
4. Operating pressure:
The turbine works
under atmospheric pressure. Which
is the difference
between the inlet and exit points of the runner.
5. Admission of water to the wheel:
Only a few buckets
comprising a part of the wheel are
exposed to the water jet.
They are essential
low discharge turbines.
7. Speed of operation:
The speed are
8. Size : These are generally small size.
9. Casing: It prevents splashing of water. It has no hydraulic function to serve.
setting: The head
between the wheel and race is lost.
efficiency: The highest
efficiency (=88%) is less than that of
12. Part load operation: From about 20% to
100% of design
output, the efficiency
remains nearly the
same. Hence the
machine is ideal
for generating small
loads over long
periods of time.
These machine are
not susceptible to cavitation.
14. Civil engineering works: Civil works like
excavation and concreting
are much simpler and economical.
S.No Reaction turbine
1. Reaction turbine The machines can be used for medium
heads (H=50 to 500 m) and low heads (less than 50 m)
2. The head is usually inadequate to produce high velocity
jet. Hence water
is supplied to the
runner in the forms of
both pressure and kinetic energy.
3. The wicket gates accelerate the flow a little and
direct the water
to runner vanes
to which energies of water are
4. The runner works is a closed system under the action of
5. The entire circumference of the wheel receives
water and all
passages between the
runner blades are always full of water.
6. Since power is a product of head and weight of the rate
of flow, these turbines consume large quantities of
water in order
to develop a
reasonable power under a relatively low head.
7. Although the specific speeds of these turbines is high,
their actual running
speeds are comparatively low.
8. The turbines sizes is much larger than impulse
wheels, in order
to accommodate heavy discharge.
9. The spiral casing has an important role to play;
it distributes water
under the available pressure uniformly around the
periphery of the runner.
10. The draft tube ensures that the head of water below
tail race level is not lost.
maximum efficiency (=95%)
of design output is higher than
that of impulse wheels.
12. With the exception
of a Kaplan
turbine, all reaction turbines
give poor part
load performance i.e., appreciably low efficiency at less than design
blades and draft
tube invariably undergo cavitation
14. Civil works are more expensive on account of spiral
casing and draft tube.