Steam Turbine

 Steam turbine convert a part of the energy of the steam evidenced by

high temperature and pressure into mechanical power-in turn electrical
power

 The steam from the boiler is expanded in a nozzle, resulting in the emission
of a high velocity jet. This jet of steam impinges on the moving vanes or
blades, mounted on a shaft. Here it undergoes a change of direction of
motion which gives rise to a change in momentum and therefore a force.

 The motive power in a steam turbine is obtained by the rate of change in

momentum of a high velocity jet of steam impinging on a curved blade
which is free to rotate.

 The conversion of energy in the blades takes place by impulse, reaction

or impulse reaction principle.

 Steam turbines are available in a few kW(as prime mover) to 1500

MW Impulse turbine are used for capacity up to
 Steam, Gas and Hydraulic Turbines
The working substance differs for different types of turbines.
 Steam turbines are axial flow machines (radial steam turbines are
rarely used) whereas gas turbines and hydraulic turbines of both axial
and radial flow type are used based on applications.
 The pressure of working medium used in steam turbines is very high,
whereas the temperature of working medium used is gas turbine is high
comparatively.
 The pressure and temperature of working medium in hydraulic turbines
is lower than steam turbines.
 Steam turbines of 1300 MW single units are available whereas
largest gas turbines unit is 530 MWand 815 MW
 Steam Turbine Classification
Steam turbines can be classified in several different ways:
1. By action of steam
• Impulse or reaction.
2. By steam supply and exhaust conditions
• Condensing, or Non-condensing (back pressure),
3. By Pressure
• Low pressure (Up to 2 ata), Medium pressure (2 to 40 ata), High pressure (40
to 170 ata), Very high pressure (170 to 220 ata), Supercritical (Above 225
ata),
4. By number of stages :
• Single stage or Multistage ( LP, IP, HP)
5. By direction of steam flow:
• Axial flow, Radial flow or Tangential flow
6. According to use in industry : Stationary or mobile (non stationary)
7. By method of governing
Throttle governed or nozzle governed or by pass governed
8. By method of compounding: Velocity compounded, pressure compounded or
 Steam Turbine Stage

 A turbine stage consists of stationary stator row (guide

vanes
or nozzle ring) and rotating rotor row.
 In the guide vanes high pressure, high temperature steam
is expanded resulting in high
 The guide vanes direct the flow to the rotor velocity
 The blades at an appropriate angle.
 In the rotor, the flow direction is changed and
kinetic energy of the working fluid is absorbed by
the rotor shaft producing mechanical energy
 Types of Steam Turbine
Impulse turbine Reaction turbine
 Continue..
Impulse turbine Reaction turbine

• Process of complete • Pressure drop with

expansion of steam takes generation of kinetic
place in stationary nozzle energy takes place in the
and the velocity energy is moving as well as fixed
converted into mechanical blades progressively.
work on the turbine work
turbine blades.
Types of Steam Turbine
 Impulse Reaction Turbine
 Modern turbines are neither purely impulse
or reaction but a combination of both.
 Pressure drop is effected partly in
nozzles and partly in moving blades which
are so designed that expansion of steam
takes place in them.
 Flow Through Steam Turbine Stage
Distance through turbine(reaction
Distance through turbine(impuls turbine)
turbine)
 Compounding of Steam Turbines
 This is done to reduce the rotational speed of the impulse turbine to
practical limits.
 Compounding is achieved by using more than one set of nozzles,
blades rotors in a series keyed to a common shaft; so that either the
steam pressure or the jet velocity is absorbed by the turbine in
stages.

Three main types of compounded impulse turbines are:

• a. Pressure compounded
• b. Velocity compounded
• c. Pressure and velocity compounded impulse turbines.
 Pressure
compounding
Involves splitting up of the whole
pressure drop
into a series of smaller pressure
drops across several stages of
impulse
Turbine.
Velocity drop is achieved through many moving rows of blades instead of a
single row of moving blades. It consists of a nozzle or a set of nozzles and
rows of moving blades attached to the rotor or the wheel and rows of fixed
blades attached to the casing
Pressure velocity compounding
 Comparison between Impulse &
Reaction Turbine
Impulse turbine Reaction turbine
 An impulse turbine has fixed nozzles that  Reaction turbine makes use of the reaction
orient the steam flow into high speed jets. force produced as the steam accelerates
 Blade profile is symmetrical as no through the nozzles formed by the rotor
pressure drop takes place in the rotor  Blades have aerofoil profile (convergent p
blades drop occurs partly in the rotor
 Suitable for efficiently absorbing Blades passage) since pressure
the high velocity and high  Efficient at the lower pressure stages
pressure
 Fine blade tip clearances are necessary
 Steam pressure is constant across the
due
blades and therefore fine tip clearances are
not necessary to the pressure leakages
 Efficiency is not maintained in the lower 
pressure stages (high velocity cannot be Inefficient at the high pressure stages
achieved in steam for the lower pressure
due to the pressure leakages around
stages)
the blade tips
 Fine tip clearances can cause damage
to
the tips of the blades
 Losses in Steam Turbine
 Profile loss: Due to formation of boundary layer on blade surfaces. Profile loss is a
boundary layer phenomenon and therefore subject to factors that influence boundary layer
development. These factors are Reynolds number, surface roughness, exit Mach number and
trailing edge thickness.
 Secondary loss: Due to friction on the casing wall and on the blade root and tip. It
is a boundary layer effect and dependent upon the same considerations as those of profile
loss.
 Tip leakage loss: Due to steam passing through the small clearances required
between the moving tip and casing or between the moving blade tip and rotating shaft. The
extend of leakage depends on the whether the turbine is impulse or reaction. Due to pressure
drop in moving blades of reaction turbine they are more
prone to leakages.
 Disc windage loss: Due to surface friction created on the discs of an impulse
turbine as the disc rotates in steam atmosphere. The result is the forfeiture of shaft power for
an increase in kinetic energy and heat energy of steam
 continue
 Lacing wire loss: Due to passage blockage created by the presence of lacing wires
in long blade of LP Stages.
 Wetness loss: Due to moisture entrained in the low pressure steam at the exit of LP turbine.
The loss is a combination of two effects; firstly, reduction in efficiency due to absorption of
energy by the water droplets and secondly, erosion of final moving blades leading edges.
 Annulus loss: Due to significant amount of diffusion between adjacent stages
or where wall cavities occur between the fixed and moving blades. The extent of
loss is greatly reduced at high annulus area ratios (inlet/outlet) if the expansion of
the steam is controlled by a flared casing wall.
 Leaving loss: Due to kinetic energy available at the steam leaving from the last
stage of LP turbine. In practice steam does slow down after leaving the last blade,
but through the conversion of its kinetic energy to flow friction losses.
 Partial admission loss: Due to partial filling of steam, flow between the blades is
considerably accelerated causing a loss in power.
 TURBINE
• FEATURES OF TURBINES
We shall consider steam as the working
fluid Single stage or Multistage
Axial or Radial turbines
Atmospheric discharge or discharge below atmosphere in
condenser Impulse/and Reaction turbine
• Impulse Turbines
Impulse turbines (single-rotor or multi-rotor) are simple stages of the
turbines. Here the impulse blades are attached to the shaft. Impulse blades
can be recognized by their shape. They are usually symmetrical and have
entrance and exit angles respectively, around 20 ° . Because they are
usually used in the entrance high-pressure stages of a steam turbine, when
the specific volume of steam is low and requires much smaller flow than
at lower pressures, the impulse blades are short and have constant cross
sections
 IMPULSE TURBINE

Blade efficiency or Diagram efficiency or

Utilization factor is given by
 TO BE CONTINUE

The maximum value of blade efficiency

For equiangular blades

If the friction over blade surface is

neglected
 Compounding in Impulse
• Turbine
If high velocity of steam is allowed to flow through one row of moving
blades, it produces a rotor speed of about 30000 rpm which is too high
for practical use.
• It is therefore essential to incorporate some improvements for practical
use and also to achieve high performance. This is possible by making use
of more than one set of nozzles, and rotors, in a series, keyed to the shaft
so that either the steam pressure or the jet velocity is absorbed by the
turbine in stages. This is called compounding. Two types of compounding
can be accomplished: (a) velocity compounding and (b) pressure
compounding
Velocity And Pressure
Compounding
Stage Efficiency and Reheat factor
The Thermodynamic effect on the turbine efficiency can be understood
by considering a number of stages between two stages as shown in
Figure
 Velocity Triangles
 The three velocity vectors namely, blade speed, absolute velocity
and relative velocity in relation to the rotor are used to form a
triangle called velocity triangle.
 Velocity triangles are used to illustrate the flow in the bladings of
turbo machinery.
 Changes in the flow direction and velocity are easy to understand
with the help of the velocity triangles.
 Note that the velocity triangles are drawn for the inlet and outlet of
the rotor at certain radii.
Steam Turbine Blade Terminology
 Velocity triangle

Inlet Velocity Triangles Outlet Velocity Triangles

Combined velocity triangle
 Work Done – Impulse Steam Turbine
If the blade is symmetrical then β1 = β2 and neglecting frictional effects of the
blades on the steam, W1 = W2.
In actual case, the relative velocity is reduced by friction and expressed by a blade
velocity coefficient k.
Thus k = W2/W1
From Euler’s equation, work done by the steam is given by;
Wt = U(Vw1 ± Vw2) (1)
Since Vw2 is in the negative r direction, the work done per unit mass flow is given by,
Wt = U(Vw1+Vw2) (2)

If Va1 ≠ Va2, there will an axial thrust in the flow direction. Assume that Va is constant then,
Wt = UVa (tanα1+ tanα2) (3)
W UV (t β + t β ) (4)

Wt = UVa tanβ1+ tanβ2) Equation (4) is often referred to as the diagram work per unit mass flow and
hence the diagram efficiency is defined as
Work Done – Impulse Steam Turbine
 Degree of reaction
 Degree of reaction is a parameter that describes the relation
between the energy transfer due to the static pressure
change and the energy transfer due to dynamic pressure
change.
 Degree of reaction is defined as the ratio of static
pressure drop in the rotor to the static pressure drop in the
stage. It is also defined as the ratio of static enthalpy drop in
the rotor to the static enthalpy drop in the stage
Degree of reaction
Zero reaction stage
Let us first discuss the special
case of zero reaction. According
to the definition
of reaction, When Λ = 0, equation
(upper) reveals that h1 = h2 and
equation (lower) that
β1 = β2.
Fifty percent reaction stage
From equation (16) for Λ = 0.5
α1
= β2 and the velocity diagram is
symmetrical Because of
symmetrical. symmetry, it is also
clear that α2 = β1. For Λ=1/2,
the enthalpy drop in the nozzle
row equals the enthalpy drop in
the rotor.
h0 - h1 = h1 - h2
Blade Height in Axial Flow
turbine
The continuity equation m = ρAV may be used to find the blade
height ‘h’. The annular area of flow = πDh. Thus the mass
flow rate through an axial flow turbine is

Blade height will increase in the direction of flow in a

turbine and decrease in the direction of flow in a
compressor.
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