Power Plant Engineering

HYDRO-ELECTRIC POWER PLANTS 363

As per the action of water on the prime-mover, they are classified as impulse turbine and reaction
turbine. In impulse type turbine, the pressure energy of the water is converted into kinetic energy when
passed through the nozzle and forms the high velocity jet of water. The formed water jet is used for
driving the wheel.
In case of reaction turbine, the water pressure combined with the velocity works on the runner.
The power in this turbine is developed from the combined action of pressure and velocity of water that
completely fills the runner and water passage.
The casing of the impulse turbine operates at atmospheric: pressure whereas the casing of the
reaction turbine operates under high pressure. The pressure acts on the rotor and vacuum underneath it.
This is why the easing of reaction turbine is made completely leak proof.
The details of few turbines which are commonly used in hydro-electric power plants are given
below.
Pelton Turbine. Figure 11.20 shows the layout of the Pelton turbine. This was discovered by
Pelton in 1880.This is a special type of axial flow impulse turbine generally mounted on horizontal
shaft, as mentioned earlier A number of buckets are mounted round the periphery of the wheel as shown
in Fig. 11.20. The water is directed towards the wheel through a nozzle or nozzles. The flow of water
through the nozzle is generally controlled by special regulating system. The water jet after impinging on
the buckets is deflected through an angle of 160° and flows axially in both directions thus avoiding the
axial thrust on the wheel. The hydraulic efficiency of Peltan wheel lies between 85 to 95%. Now-a-days,
Pelton wheels are used for very high heads upto 2000 meters.

Arrangement of jets. In most of the Pelton wheel plants, single jet with horizontal shaft is used.
The number of the jets adopted depends upon the specific speed required.

Nozzle

Deflector

Spear Jet

Horizontal

Shaft

Brake Nozzle

Flow from

main pipe

Fig. 11.20

Any impulse turbine achieves its maximum efficiency when the velocity of the bucket at the

center line of the jet is slightly under half the jet velocity. Hence, for maximum speed of rotation, the

mean diameter of the runner should be as small as possible. There is a limit to the size of the jet which

can be applied to any impulse turbine runner without seriously reducing the efficiency. In early twenties,

a normal ratio of D/d was about 10 : I. In a modern Turgo impulse turbine, it is reduced upto 4.5 to I. The

basic advantage of Turgo impulse turbine is that a much larger jet could be applied to a runner of a given

mean diameter. The jet of pelton turbine strikes the splitter edge of the bucket, bifurcates and is dis-

charged at either side.