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    TVA COMBINED CYCLE HRSGsAEP BRO - 2010

    David G Nesbitt, Principal Engineer

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    The amount of information yourgrandparents absorbed in their

    lifetime is equal to what we absorb

    in one day

    From Legendary Football Coach BillCurry

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    Acknowledgments:

    Barry Dooley, Structural Integrity Associates

    Bob Anderson, Competitive Power ResourcesMatt Dowling, Structural Integrity Associates

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    A heat recovery steam generator or HRSG is anenergy recovery heat exchanger that recovers heat

    from a hot gas stream. It produces steam that can

    be used in a process or used to drive a steam

    turbine. A common application for an HRSG is in acombined-cycle power station, where hot exhaust

    from a gas turbine is fed to an HRSG to generate

    steam which in turn drives a steam turbine. This

    combination produces electricity more efficientlythan either the gas turbine or steam turbine alone.

    http://en.wikipedia.org/wiki/Energy_recoveryhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Combined_cyclehttp://en.wikipedia.org/wiki/Gas_turbinehttp://en.wikipedia.org/wiki/Gas_turbinehttp://en.wikipedia.org/wiki/Combined_cyclehttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Steam_turbinehttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Energy_recovery
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    What makes Combined Cycle plants

    so desirable?

    Cheaper to build

    Current low price

    of natural gas

    GREEN !!

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    Comparison of fossil fuel overall thermal efficiencies:

    Coal = 35%

    Natural Gas = 45%

    Oil = 38%

    CC / HRSGs = 58% (approaching 60%)

    Gas home heating = >90%

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    TVAs Combined Cycle HRSGs

    Caledonia, Steens MS Nominal Capacity = 834MW

    (purchased already completed)

    Lagoon Creek, Brownsville TN - Nominal Capacity =

    582MW (just commissioned)

    South Haven, Southaven MS - Nominal Capacity =

    834MW (purchased already completed)

    John Sevier

    Under construction due to North Carolina lawsuit

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    http://upload.wikimedia.org/wikipedia/en/f/f3/HRSGPoster.jpghttp://upload.wikimedia.org/wikipedia/en/f/f3/HRSGPoster.jpg
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    Flow diagram for an HRSG more complicated

    than you thought?

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    The primary components in a triple pressure

    HRSG arranged according to their positionrelative to gas flow are:High pressure secondary superheater

    Secondary Reheater

    Primary ReheaterHigh Pressure Superheater

    High Pressure Evaporator

    Intermediate Pressure Primary Superheater

    Low Pressure Primary Superheater

    High Pressure Secondary EconomizerIntermediate Pressure Evaporator

    HP / LP Primary Economizer

    Intermediate Pressure Evaporator

    Feedwater Heater

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    According to SIA, 70% of tube failures in

    HRSGs are mechanisms in the water side ofthe tubes

    Under Deposit Corrosion in HP evaporators

    Thermal Fatigue in Economizers & SH/RH (header to tubeconnections)

    Creep Fatigue in SH/RH (header to tube connections)

    FAC in LP components (40% of failures)

    Corrosion Fatigue in LP components

    Hydrogen Damage / Acid Phosphate Corrosion / Caustic Gouging

    Pitting

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    What makes HRSGs susceptible to tube failures?

    Frequent and rapid startups and shutdowns

    Thin wall tubes connected to thick wall vessels in

    the gas stream

    T91 material

    Design deficiencies

    Operational, Maintenance, and Water Chemistryissues

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    Why T91?

    T91 is 9% chrome with Vanadium, Niobium ,

    controlled Nitrogen

    Much lighter (40% less wall thickness than T22)

    Higher creep characteristics

    Reduces support structure costs (60% less weight

    than other materials)

    This makes initial cost of HRSG lower

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    What makes T91 bad:

    Much thinner tube material welded to thicker wall

    headers in the gas stream they change temperature at

    different rates during startup/shutdown.

    T91 MUST! be post weld heat treated even if you just arc

    strike it

    What to look for in T91

    Hardness (Must remove surface due to de-carburization)

    should be 195 200 HRB

    Chemical Analysis

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    Design Conditions that increase the propensity for HRSGtube leaks:

    Condensate drains not adequately sized and clear

    Condensate drains not adequately sloped and combined

    with different pressure sources

    Condensate drains are manual and are not controlled to

    open before and during the purge cycle

    OEM has not supplied startup/shutdown ramp rates

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    Design Conditions that increase the propensity for HRSG

    tube leaks (continued):

    Attemperators are too close to superheater

    Harps are rigid structures intolerant of large

    differential temperatures

    Use of pressure and temperature indications to ensurecondensate is clear versus the use of condensate

    detection drain pots

    Attemperators do not have low drain points

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    Operational Conditions that increase the propensity for

    HRSG tube leaks:

    OEM required ramp rates are not in Operating procedure

    and are not followed

    Lack of operating instrumentation and data not stored in

    PI system

    Lack of training to Operators

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    Maintenance Conditions that increase the propensity for

    HRSG tube leaks:

    Regular boiler inspections are not performed

    Regular attemperator inspections not performed

    FAC inspections not performed

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    Water Chemistry conditions that increase the propensity

    for HRSG tube leaks:

    Operating outside the Rule of 2 and 5

    That is:

    Iron (Fe) < 2 ppb in Feed water

    Iron (Fe) < 5 ppb in drums

    Operating on chemistry outside AVT(R) with PH < 9

    What to look for:

    Ruggedness of redness in Drums

    Shiny black is bad

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    What makes working on HRSGs so difficult:Most tubes are finned

    Close proximity of tube to tube and component to

    component spacing

    Typical spacing between modules = 2-6

    Typical CL spacing between tubes front to rear = 4

    (remember the tubes are finned) and 12 tubes deepTypical CL spacing between tubes side to side = 4

    (remember the tubes are finned)

    Inability to use UT for inspections due to finned tubing

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    Next Steps:

    Integrate HRSGs in existing fossil boiler tube failure

    reduction program

    Develop Boiler Books for all units (Basic drawings,

    materials with tube sizes, etc)

    Perform design assessment of existing units

    Educate boiler inspectors on configuration and materials

    of HRSGs (versus using an outside contractor)

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    Next Steps (cont):

    Have SIA perform operational reviews of existing units

    Provide training to plant employees

    Provide the EPRI BTFR manual to plant sites

    Start tracking tube failures and inspections in AWARE

    Participate in the EPRI HRSG program

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    Conclusion:

    CC / HRSGs are here and we have to deal with them!

    We must get aggressive and review operational

    procedures and assess the design, maintenance, and

    operation to prevent future failures.

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    Photos of TVA John Sevier under construction

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