2. Steam Turbines
“Steam Turbine is a prime-mover in which
Pressure energy of steam is transformed
into Kinetic energy, and later in its turn is
transformed into the mechanical energy
of rotation of turbine shaft”
3. Classification based on
Principle of Action
Pressure energy of Steam is converted into Kinetic
Impulse action of high velocity jet of steam, due to
change in its direction is used to rotate the turbine shaft.
Reaction force due to expansion of high pressure steam
when it passes through a set of moving and fixed blades
is used to rotate the turbine shaft.
Due to expansion of steam, pressure drop occurs
continuously over both fixed and moving blades.
This pressure difference exerts a thrust on the blades.
The resulting reaction force imparts rotary motion.
5. Impulse turbine - Working
High pressure steam from boiler is
supplied to fixed nozzles.
Nozzle – Pressure falls from boiler
pressure to condenser pressure
Reduction in pressure increases velocity.
High velocity steam impinges on moving
Causes change in momentum
force on blades.
Pressure remains constant when steam
flows through blades.
Eg: De Lavel Turbine
7. Disadvantages of Impulse Turbine
The velocity of Rotor is too high for
The velocity of steam leaving the
turbine is considerably high and hence
there is a loss in Kinetic Energy
These problems can be overcome by
expanding the steam in different
This is known as Compounding.
High pressure steam directly supplied to
turbine blades with out nozzles.
Steam expands(diameter increases) as it
flows through fixed and moving blades
Continuous drop of pressure.
Produces reaction on blades
Reaction causes rotor to rotate.
Propulsive force causing rotation of turbine
is the reaction force. Hence called reaction
Eg: Parson’s Turbine
11. Compounding of Impulse Turbine
The extreme high speed of Impulse Turbine of
the order of 30,000rpm, cannot be directly used
for practical purpose. To reduce the speed more
than one set of blades are used. This is called
There are three types of compounding
Pressure – Velocity Compounding
Compounding .. 2
Velocity of steam
absorbed in stages
Moving and fixed
Entire pressure drop
takes place in
Kinetic energy of
work in 2 stages in
Compounding .. 3
Velocity reduced to
velocity in the 1st
row of moving
Fixed blade direct
steam to 2nd set of
reduced in 2nd set of
Eg: Curtis Turbine
Pressure energy of
steam absorbed in
Expansion of steam
takes place in more
than one set of
Nozzles followed by
set of moving blades
Pressure energy of
steam converted into
kinetic energy in
20. Pressure Velocity
Combination of pressure
compounding and velocity
In a 2 stage pressure velocity
compounded turbine – total drop in
steam pressure carried out in 2
Velocity obtained in each stage is