Power Plant Engineering

196 POWER PLANT ENGINEERING

tum and therefore a force. This constitutes the driving force of the turbine. This arrangement is shown.
It should be realized that the blade obtains no motive force from the static pressure of the steam or from
any impact of the jet, because the blade in designed such that the steam jet will glide on and off the blade
without any tendency to strike it.
As shown in Fig. 6.2, when the blade is locked the jet enters and leaves with equal velocity, and
thus develops maximum force if we neglect friction in the blades. Since the blade velocity is zero, no
mechanical work is done. As the blade is allowed to speed up, the leaving velocity of jet from the blade
reduces, which reduces the force. Due to blade velocity the work will be done and maximum work is
done when the blade speed is just half of the steam speed. In this case, the steam velocity from the blade
is near about zero i.e. it is trail of inert steam since all the kinetic energy of steam is converted into work.
The force and work done become zero when the blade speed is equal to the steam speed. From the above
discussion, it follows that a steam turbine should have a row of nozzles, a row of moving blades fixed to
the rotor, and the casing (cylinder). A row of nozzles and a raw of moving blades constitutes a stage of
turbine.

Nozzle

Entering velocity

= C 1

Blade velocity

u = 0

F

Blade

Leavingvelocity = C locked

2

()b1

C 1

C 2

()b2

u = C /4 1

u = C /2 1

C 1

C= 0 2

()b3

F

Fig. 6.2. Action of Jet on Blade.

6.2 Classification of Steam Turbine

Steam turbine may be classified as follows: -

(A) On the Basis of Principle of Operation :

(i) Impulse turbine

(a) Simple, (b) Velocity stage, (c) Pressure stage, (d) combination of (b) and (c).