Turbine Part 3

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    Lecture 3

    Steam Turbine (Part III)

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    Steam Turbine

    Axial Thrust Management by Double Flow Turbine Design

    In large turbines, managing large axial thrust on rotor is not feasible by

    thrust bearing alone

    It is therefore common to build double flow HP and LP turbines having

    reaction blading

    The axial thrust roduced at one end cancels out nearly same axial thrust

    develoed in oosite direction due to design

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    Steam Turbine

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    Steam Turbine

    Management of axial thrust by the use of:

    1 Im!ulse bla"ing

    The imulse stage has no ressure dro across the moving blades

    The axial thrust is therefore eliminated

    # Dummy !iston

    "ot commonly used in nuclear installations

    Seen in some single flow HP turbine at conventional ower stations

    Here the axial thrust on the blade wheels is balanced by the dummy

    iston

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    Steam Turbine

    Labyrinth packing to limit:Steam leakage

    Past balancing

    Piston

    Balancing

    piston

    Steam pressure

    balance example

    HP steam inlet

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    Steam Turbine

    Tan"em $om!oun"ing

    The steam exands roughly $%% times from the inlet to the exhaust of the

    turbine

    Therefore in large nuclear ower stations, it may not be feasible to

    accommodate large increase in the volume of the steam in only one LPturbine

    "ormally one HP turbine exhausts its steam into two or three LP turbines

    with the double flow design

    In one of the $#% &'e Indian "PP, one double flow HP sulies steam totwo double flow LP turbines

    The arrangement of all the turbines in the turbine unit on a common shaft is

    (nown as a tandem comounded turbine unit

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    Steam Turbine* Tandem +omounding

    Illustration of Tan"em $om!oun"ing

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    Steam Turbine

    Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren"

    "early all turbines of resent age are imulse reaction turbine -commonly

    called as reaction turbine.

    It is established that the efficiency of the turbine deends on enthaly dro

    er stage/ 0or reaction stage, this efficiency is efficiency of imulse stage,rovided that enthaly dro er stage is (et small/

    2fficiency also deends on the ratio between blade velocity and steam

    velocity/ Imulse stage is the most efficient when this ratio is about one half,

    while reaction stage is the most efficient when this ratio is nearly one/

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    Steam Turbine

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    Steam Turbine

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    Steam Turbine

    Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren" ('ont")

    In view of large turbines, re5uiring high steam velocities and therefore high

    velocity ratios, the advantage of reaction stage are obvious excet with

    following limitations

    This efficiency is offset by the ossible wastage of energy due to assing ofsteam through the clearance saces between the moving blades and the

    casing from ustream side of moving blade to downstream without doing

    useful wor( in case of reaction turbine

    0or a given ercentage of moisture in the steam assing through a stage,

    reaction turbines suffer a loss in efficiency almost twice as great as inimulse turbine

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    Steam Turbine

    Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren" ('ont")

    6s the reaction turbines have ressure dro across moving blades, the

    cumulative force of axial thrust of moving blades in HP turbine will be so

    large that the thrust bearing would be extremely large and costly/ Seeing

    this, HP turbine are designed as reaction turbine as double flow turbine to

    be considered with imulse stages/

    Thus, a reaction stage should be located in the low ressure region of

    turbine

    There is a general rule to use a greater ercentage of imulse on the HP

    end and greater ercentage of reaction on the LP end

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    Steam Turbine

    ffe't of xhaust Pressure $hange on Turbine x!ansion

    6s noted from the 7an(ine cycle, lowering the bac( ressure on turbine

    from P! to P3 will increase the wor( done by the area shown hatched

    2xhaust ressure deends on the temerature of the available cooling water

    &oreover wor( done er unit dro in ressure of steam is more at lower

    ressures

    2xansion to lower ressure results in increased wetness fraction, which

    causes erosion of last stage blades

    Increased secific volume at lower exhaust ressure demand greater heightof blades in the last stages of turbine to rovide larger area for the flow of

    increased volume of steam

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    Steam Turbine

    ffe't of xhaust Pressure on Turbine x!ansion

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    Steam Turbine

    The &eheat $y'le

    Steam at a given initial temerature is artially exanded and leads to wor( on

    turbines

    This artially exanded steam at some stages of turbine -rocess e8f. is again

    heated

    This is done first by bleed steam and then by live steam u to its original

    temerature

    Process figure indicates temerature rise and figure on slide 3# for reheating

    arrangement

    The reheated steam is then exanded in the remaining stages of the turbine

    -rocess g8h. before being condensed

    The reheat cycle incororates an imrovement in thermal efficiency and reduces

    moisture content at exhaust

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    Steam Turbine

    The &eheat $y'le

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    Steam Turbine

    &egenerati%e Fee" *eating

    Steam is extracted at intermediate stages

    This is to heat feedwater in feedwater heaters before sending to boiler

    Higher thermal and cycle efficiency due to such bleed steam releasing all its heat in

    feedwater

    It is universally used in central ower generating station

    6 small loss of wor( is exected from the bleed steam not exanding in the turbine

    This loss is outweighed by*

    9ain in cycle efficiency

    :uantity of exhaust steam is less

    Si;e of condenser decreases

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    Steam Turbine

    &egenerati%e Fee" *eating

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    Steam Turbine

    ffe't of Fee" *eating

    It may be seen that feed heating has raised the feedwater temerature from

    T) to T1 in T8S diagram

    So, the boiler needs to increase the temerature from T1 to T! before

    steam roduction begins

    Thus, the contribution of heat addition from boiler side becomes less,

    leading to cycle efficiency imrovement

    0eedwater tyically enters a steam generator of "PP at around 1$

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    Steam Turbine

    ffe't of Fee" *eating ('ont")

    +ycle efficiency is also better as the extraction steam leaving the feed

    heater and the feedwater leaving the same feed heater is tyically $

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    Steam Turbine

    ffe't of Fee" *eating