Turbine Part II

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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