tva hrsg presentation
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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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