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TRANSCRIPT
ACC Corrosion/FAC A Perspective as Introduction
to Session 4: Chemistry and Corrosion
Third Annual ACC Users Group San Francisco, California
19th and 20th September 2011
Barry Dooley
Copyright Structural Integrity Associates, Inc. 2011
Corrosion/FAC in ACC and The Consequences • High concentrations of iron around the cycle
- Boiler/HRSG deposits - Boiler/HRSG Tube Failures - Steam Turbine Deposits
• Need for Iron Removal Processes - Condensate Polishing and/or Filters
• Limitations around the cycle - Condensate polishing • Overall an ACC “controls” the unit cycle chemistry - International Guidelines didn’t consider ACC or
two-phase flow up to 2010 (IAPWS Volatile Guidance)
Copyright Structural Integrity Associates, Inc. 2011
And we have an ACC Corrosion Index to Categorize Corrosion
and Track Improvments
DHACI
(Dooley, Howell, Air-cooled Condenser, Corrosion Index)
We Know what the Corrosion Looks Like
DHACI 3 DHACI 4
DHACI 3
And what Holes at Tube Entries Look Like
DHACI 5
Inside diameter surface of an ACC Tube The black areas are where the Fe3O4 is Precipitated Locally
Shiny white metal
Copyright Structural Integrity Associates, Inc. 2009
ID Oxide Scale
Oxide free area with preferential grain boundary attack
Corroded (FAC?) ID Surface of ACC Tube
Copyright Structural Integrity Associates, Inc. 2009
Copyright Structural Integrity Associates, Inc. 2009
6 inch section of ACC tube and detail of the surface showing black deposits and white “bare” metal areas
Second Tube Showed the Same Features
But do we Understand the Environment and the Corrosion
Mechanism?
Solutions are already being applied
Increase bulk pH up to 9.8 Increase local pH (amines)
Coatings (epoxy), Sleeves, Inserts Materials Designs?
To Understand the Corrosion Here we need to Understand the Environment in the PTZ
The PTZ Environment in the LP Steam Turbine is Completely Understood
Generation of the ACC Environment
Heterogeneous droplet Nucleation and
Liquid Films on ST Blades (Droplets and Liquid Films in the ACC don’t contain any
oxygen until during shutdown)
Copyright Structural Integrity Associates, Inc. 2010
2000
2200
2400
2600
2800
3000
3200
3400
3600
3800
5.5 6.5 7.5 8.5Entropy (kJ/kg/K)
Ent
halp
y (k
J/kg
)
500
400
300
200
100
600
°C
200100 50 20 10 5
21 0.5 0.2 0.1 0.05
0.950.90
0.850.80bar
bar
x
A B
C
The Liquid in ACC Upper Ducts
0
5
10
15
20
25
30
35
valu
es
date
Chloride
duct cpp
Courtesy Setsweke Phala, Eskom
The Liquid in ACC Upper Ducts
0
2
4
6
8
10
12
valu
es
date
pH
duct cpp
Courtesy Setsweke Phala, Eskom
So is the ACC Corrosion Mechanism Low Temperature FAC?
Dependent on Removing the Saturation of Fe3O4 at the Surface and
Precipitating it Adjacently
Two-phase FAC in Flowing Turbulent Flow at ACC Tube Entries (Removes the Saturation of Fe3O4)
Dooley, PPChem 2008
Typical Microscopic Appearance of FAC is Missing
Or is the ACC Corrosion Mechanism Something Other than FAC?
and if so
Why does the Dooley/Aspden Relationship Work ?
Dooley/Aspden pH Versus Iron Relationship
Iron Levels Measured at Condensate Pump Discharge and Using Only Ammonia for pH Control Iron solubility data extracted from Sturla, 1973
Dooley, Aspden, Howell & DuPreez, PPChem 2009
In Summary • Some aspects relate to Low Temperature FAC
– Adjacent black and white areas in severe turbulent areas – Increasing pH reduces damage
• Some aspects don’t (normal FAC scalloped appearance) • Environment is known and has been measured
– Two-phase mixture formed in PTZ of ST – Concentrating liquids (Higher in chloride/sulphate, organics ) – Lower in pH
• Clearly more tubes need to be analyzed • Clearly we need to provide solutions which address the
mechanism
Copyright Structural Integrity Associates, Inc. 2011