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Lecture 2
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Steam Turbine (Part II)
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Steam Turbine: Velocity Diagram for
Impulse Blade
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Steam Turbine: Velocity Diagram
Explanation The steam jet issuing from the nozzle with absolute velocity V1, falls on the moving
blade at an angle with the plane of rotation of moving blades
Relative velocity U1of steam with respect to the moving blade is the vector sum of
steam absolute velocity and reverse of blade velocity
Assuming no frictional resistance to the flow of steam by the moving blades, the
relative velocity at blade outlet will be equal to the relative velocity at blade inlet,i.e., U1= U2
If friction is taken into account, U2will be < U1
As the steam glides over the moving blade, its relative velocity U2at blade exit
follows its outlet angle 2
Absolute velocity at blade outlet V2is the vector sum of relative velocity U2andblade velocity u
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Steam Turbine: Velocity Diagram
Explanation (contd...) The change in momentum of the steam in the direction of blade motion
=(Vu1-Vu2) * (Mass of steam)
Vu1and Vu2denote absolute tangential velocity at inlet and outlet of the moving
blade
This change in momentum provides the required force in the direction of blade
motion
The component of absolute velocity Vf1in axial direction called velocity of flow, is
responsible for the flow of steam in axial direction
The multiplication of the mass of steam and change in the velocity of flow (Vf1
Vf2) is responsible for the axial thrust on the blade
Assuming no friction, U1= U2, and if the blade inlet and outlet angle 1and 2areequal, then Vf1= Vf2. making axial thrust zero. In actual design, angle (2) is made
slightly larger than angle (1) to account for friction between steam and blade.
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Steam Turbine: Velocity Vector
Diagram for Impulse Blade (With
Friction)
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= (Vf1- Vf2)
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Steam Turbine
Impulse Reaction Turbine
In impulse reaction turbine, the moving blades of a reaction stage are shaped so as
to create a nozzle effect in the space between the blades
Thus, pressure gets reduced in both fixed and moving blades
Work is done by the impulse effect due to the change of direction of the high
velocity steam plus a reaction effect due to the expansion of steam through the
moving blades
The relative velocity U2 at the blade outlet is therefore larger than U1 in the
velocity diagram for reaction turbine
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Steam Turbine: Reaction Turbine
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Steam Turbine
Pressure, Velocity or Combination Compounding in Turbine
The turbine speed shall become very high if the steam kinetic energy is absorbed
in single stage, i.e., turbine having single row of nozzles and moving blades
It may lead to the failure of blades due to the centrifugal force
Therefore, division of steam energy is done in multiple steps to keep the velocity
of steam and turbine within practical values
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Steam Turbine
Pressure, Velocity or Combination Compounding in Turbine (contd)
Following are the various types of compounding:
In pressure compounding turbine, pressure drop takes place equally in in each
stage in the nozzle part, the velocity gained at nozzles is subsequently
absorbed in the next moving blades (refer to the figure in the next slide)
In velocity compounding, the full pressure drop occurs in the first nozzle ring
and the pressure remains constant in the moving and fixed blades. The total
velocity gained at the first nozzle is absorbed in each moving blades.
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Steam Turbine
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Example of Pressure Compounding
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Steam Turbine
Pressure, Velocity or Combination Compounding in Turbine (contd)
Pressure and velocity compounding is a combination of both the previous
methods. The merits of the pressure and velocity compounding are as follows:
Less stages are necessary because of bigger pressure drop between the stages
For a given pressure drop, the turbine will be shorter
It may be noted that the diameter of the turbine is increased at each stage to
allow for the increasing volume of steam
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Steam Turbine
Moisture Formation: Causes and Effects on Turbine
As steam passes through the turbine, its heat energy is consumed for providing
kinetic energy
As this occurs, part of the steam gets converted into minute water droplets
These droplets are carried along with steam strike against the moving blades
This phenomenon leads to erosion of the turbine blades
As water droplets do not move as fast as steam, the back side of the blades are
continuously in water droplets
This results in retarding force against the moving blades and loss of turbine
efficiency
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Steam Turbine
Design Features for Addressing Adverse Impact of Moisture in Turbine
Modern turbine incorporates various design features for moisture removal and
against erosion
Wet steam with greater than 10% moisture cannot be tolerated
In case of nuclear plants, with saturated steam entry at the turbine inlet, the
steam reaches 10% moisture content before extraction of its all useful energy
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Steam Turbine
Design Features for Addressing Adverse Impact of Moisture in Turbine (contd)
The turbine unit is spilt into two turbines namely High Pressure (HP) turbine which
receives steam directly from steam generator and Low Pressure (LP) turbine
connected to condenser for exhausting its steam in it
HP turbine is designed so as to provide not more than 10% wet steam at its
exhaust/into a moisture separator
With almost complete removal of moisture from the moisture separator, steam is
superheated in a reheater prior to admission in LP turbine
This reheated steam improves overall turbine efficiency
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Steam Turbine
Simplified Turbine Unit
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Simplified Diagram of NPP Turbine Unit
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Steam Turbine
Axial Thrust and its Management
When steam enters at one end of the turbine and after its expansion, exhausts
through the other, such turbine are called single flow as steam flows in one
direction
Modern reaction turbines contain blading which allows substantial pressure drop
across the moving blading due to steam flow
These pressure drops with large steam flow tend to push the blade wheels from
high pressure side to low pressure side making it difficult to manage such thrust
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Steam Turbine
Axial Thrust and its Management (contd)
To balance out such forces on rotor, the following provisions are used either alone
or in combination:
1. Thrust bearing
2. Double flow turbine
3. Use of only impulse blading
4. Dummy piston
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Steam Turbine
Thrust Bearing
This is the most common method for dealing with axial thrust
The thrust bearing collar rotates with the shaft and stationary shoes on either side
of the shaft absorb the thrust
Oil is supplied between the shoes and thrust collar to lubricate and cool the
bearing
Though thrust bearing is used in all turbines, it may not suffice except for small
turbines
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Steam Turbine
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Sketch of a Thrust
Bearing