Steam Turbine
A steam turbine is prime mover which uses steam as its working
fluid.
Ø It operates by performing two functions :
i) a part or whole of the pressure energy of steam is transformed
into kinetic energy by means of expansion through suitable
passages such as nozzles and
ii) then the kinetic energy and the remaining portion of the
pressure energy of steam, if any, are converted into
mechanical work with the help of moving blades fitted on the
wheel.
Ø Therefore, a steam turbine may be defined as a form of heat engine
in which the pressure energy of steam is transformed into kinetic
energy by means of expansion through nozzles or similar devices
and then the kinetic energy and the remaining portion of pressure
energy are converted into mechanical work with the help of moving
blades.
Ø Basically, a steam turbine is an assemblage of nozzles and blades
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Turbine Details
A steam turbine mainly
consists of
(a) Nozzles,
(b) Blades or buckets,
(c) Wheel or rotor,
(d) Casing or cylinder
(e) Diaphragms, and
(f) Glands
The blades, called moving blades,
are fitted over the circumference of
the wheel which is again mounted
over a shaft. The wheel is covered
with a casing or cylinder. The
nozzles and fixed blades are fitted
with the cylinder.
W-Wheel; S-Shaft; M-Moving blades;
L-Fixed blade/nozzle; C-Casing;
D-Diaphragm; G-Gland; N-Nozzle
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Nozzles:
A nozzle is a steady flow device and is nothing but a passage of
varying cross section for the flow of steam in order to increase its
velocity by expansion with decrease of pressure. Its main function
is to convert the available enthalpy into kinetic energy by
producing a jet of steam at a high velocity.
The section of a nozzle may be round, square, or rectangular.
They are used in impulse turbines and fitted with the casing or
with diaphragms.
Blades:
Turbine blades also called buckets may be classified according to
its shape as impulse blades and reaction blades. The blades of
both the groups may be of moving type or of stationary type.
Moving blades are fixed on the rim of the wheel or rotor and
stationery or guide or fixed blades are fitted with the casing.
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Wheel and rotors:
A turbine wheel in its simplest form is like a flat disc mounted on a
shaft. Moving blades are fitted over the rim of the wheel in the form of
a ring. The rotor consists of several discs.
Diaphragms:
The diaphragm is in the form of a disc which is fitted inside the casing
cylinder. It serves the purpose of separating walls between the
different stages of the turbine and carries nozzles and fixed blades. It
must be strong enough to withstand the high temperature and the
pressure difference of working fluid.
Glands:
Glands are required to prevent the
(a) leakage of working fluid from the cylinder to the outside if its
pressure is above the atmosphere,
(b) leakage of air from outside to the cylinder if the inside pressure
is less than the atmosphere,
(c) leakage of working fluid from one stage to the other.
Glands are fitted in the place where the shaft enters into the cylinder
and the passage between the rotor and the diaphragms.
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Depending on the types of blades and methods of energy transfer
from fluid to rotor wheel, turbines may be of two types:
1. Impulse Turbine, 2. Reaction Turbine
1. Impulse Turbine
In an impulse turbine, the steam
taken from the boiler first comes
to the steam chest and then it
passes through nozzles and
impacts on the moving blades.
Due to the impulse of steam over
the moving blades, the wheel
rotates and so the power is
available from the shaft.
Diagram
Section
Fig. 3: Simple impulse turbine
W-Wheel; S-Shaft; M-Moving blades;
C-Casing; G-Gland; N-Nozzle
As the steam expands through the
nozzle, the velocity and the
volume of steam are increased
with decrease in pressure
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2. Reaction Turbine
Ø Constructed by using rows of fixed and moving blades
Ø Fixed blades act as nozzles
Ø Moving blades move as a result of change of momentum of steam and
also as a result of expansion.
Fig. Two stages of reaction turbine
Fig. Multi-stage reaction turbine
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Impulse and Reaction Turbine Principles and Differences
Impulse Turbines
Ø In an impulse turbine steam flows through the
nozzle and impinges in moving blades.
Ø The steam hits on the blades with Kinetic energy.
Ø The steam pressure remains constant.
Ø Their relative velocity remains constant.
Ø Blades are symmetrical.
Ø The number of stages required is less for the same
power developed.
Impulse
Reaction turbines:
Ø Steam flows initially from the guide mechanism and
then from moving blades.
Ø Steam glides over moving blades.
Ø Steam pressure is reduced during its flow.
Ø Relative velocity is increased.
Ø Blades are not symmetrical.
Ø The number of stages required is more for the same
power developed.
Reaction
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Compounded Impulse Turbines (IT) may be classified as:
(a) Pressure compounded IT (Rateau stage)
(b) Velocity compounded IT (Curtis stage)
(c) Pressure-Velocity compounded IT
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Compounded Impulse Turbines (IT) may be classified as:
(a) Pressure compounded IT (Rateau stage)
The pressure compounded Impulse turbine is
also called as Rateau turbine, after its inventor.
This is used to solve the problem of high blade
velocity in the single-stage impulse turbine.
It consists of alternate rings of nozzles and
turbine blades. The nozzles are fitted to the
casing and the blades are keyed to the turbine
shaft.
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In this type of compounding the steam is expanded in a number of stages,
instead of just one (nozzle) in the velocity compounding. It is done by the fixed
blades which act as nozzles.
The steam expands equally in all rows of fixed blade. The steam coming from
the boiler is fed to the first set of fixed blades i.e. the nozzle ring.
The steam is partially expanded in the nozzle ring. Hence, there is a partial
decrease in pressure of the incoming steam. This leads to an increase in the
velocity of the steam. Therefore the pressure decreases and velocity increases
partially in the nozzle.
This is then passed over the set of moving blades. As the steam flows over the
moving blades nearly all its velocity is absorbed. However, the pressure
remains constant during this process.
After this it is passed into the nozzle ring and is again partially expanded. Then
it is fed into the next set of moving blades, and this process is repeated until
the condenser pressure is reached.
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(b) Velocity compounded IT (Curtis stage)
The velocity compounded Impulse turbine was
first proposed by C G Curtis to solve the problem
of single stage Impulse turbine for use of high
pressure and temperature steam.
The rings of moving blades are separated by
rings of fixed blades. The moving blades are
keyed to the turbine shaft and the fixed blades are
fixed to the casing.
The high pressure steam coming from the boiler
is expanded in the nozzle first. The Nozzle
converts the pressure energy of the steam into
kinetic energy.
It is interesting to note that the total enthalpy
drop and hence the pressure drop occurs in the
nozzle. Hence, the pressure thereafter remains
constant.
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This high velocity steam is directed on to the first set (ring) of moving blades.
As the steam flows over the blades, due the shape of the blades, it imparts
some of its momentum to the blades and loses some velocity.
Only a part of the high kinetic energy is absorbed by these blades. The
remainder is exhausted on to the next ring of fixed blade. The function of the
fixed blades is to redirect the steam leaving from the first ring of moving
blades to the second ring of moving blades.
There is no change in the velocity of the steam as it passes through the fixed
blades. The steam then enters the next ring of moving blades; this process is
repeated until practically all the energy of the steam has been absorbed.
A schematic diagram of the Curtis stage impulse turbine, with two rings of
moving blades one ring of fixed blades is shown in figure. The figure also
shows the changes in the pressure and the absolute steam velocity as it
passes through the stages.
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(c) Pressure-velocity compounded IT
It is a combination of the above two
types of compounding.
The total pressure drop of the steam is
divided into a number of stages. Each
stage consists of rings of fixed and
moving blades.
Each set of rings of moving blades is
separated by a single ring of fixed
blades.
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In each stage there is one ring of fixed blades and 3-4 rings of moving
blades.
Each stage acts as a velocity compounded impulse turbine.
The fixed blades act as nozzles.
The steam coming from the boiler is passed to the first ring of fixed blades,
where it gets partially expanded.
The pressure partially decreases and the velocity rises correspondingly.
The velocity is absorbed by the following rings of moving blades until it
reaches the next ring of fixed blades and the whole process is repeated
once again.
This process is shown diagrammatically in the figure.
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Combination turbine:
Ø Early turbines were either impulse or reaction types
Ø Next popular arrangements were used to be Curtis stage
(VCIT) or Rateau stage (PCIT)
Ø More common arrangements now-a-days are
Curtis stage + reaction stages
EXAMPLE: 15 stage turbine
2-row curtis stage
N MB FB MB
14 reaction stages
FB MB FB MB …… FB MB
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STEAM TURBINE BLADES