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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)


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


ID Oxide Scale

Oxide free area with preferential grain boundary attack

Corroded (FAC?) ID Surface of ACC Tube



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)


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


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