1TVA COMBINED CYCLE HRSGsAEP BRO - 2010

David G Nesbitt, Principal Engineer

2 The amount of information your grandparents absorbed in their lifetime is equal to what we absorb in one day

From Legendary Football Coach Bill Curry

3Acknowledgments:

Barry Dooley, Structural Integrity AssociatesBob Anderson, Competitive Power ResourcesMatt Dowling, Structural Integrity Associates

4A heat recovery steam generator or HRSG is an energy 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 a combined-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 efficiently than either the gas turbine or steam turbine alone.

5What makes Combined Cycle plants so desirable?

Cheaper to build

Current low priceof natural gas

GREEN !!

6Comparison of fossil fuel overall thermal efficiencies:

Coal = 35%

Natural Gas = 45%

Oil = 38%

CC / HRSGs = 58% (approaching 60%)

Gas home heating = >90%

7TVAs 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 SevierUnder construction due to North Carolina lawsuit

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9Flow 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 position relative to gas flow are:High pressure secondary superheaterSecondary ReheaterPrimary ReheaterHigh Pressure SuperheaterHigh Pressure EvaporatorIntermediate Pressure Primary SuperheaterLow Pressure Primary SuperheaterHigh Pressure Secondary EconomizerIntermediate Pressure EvaporatorHP / LP Primary EconomizerIntermediate Pressure EvaporatorFeedwater Heater

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According to SIA, 70% of tube failures in HRSGs are mechanisms in the water side of the tubes

Under Deposit Corrosion in HP evaporators

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

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 Chemistry issues

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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 HRSG tube 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 ensure condensate 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 5That 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 DrumsShiny 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 = 2-6Typical 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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