acc come in many sizes acc...

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ACC Corrosion/FACIntroduction

for Session on Chemistry and Corrosion

Fifth Annual ACC Users Group

Las Vegas, Nevada

15th and 16th October 2013

Barry Dooley

ACC Come in Many Sizes

Courtesy of Eskom.But the FAC / Corrosion damage is the same worldwide with all chemistries

Corrosion/FAC in ACC and The Consequences

• High concentrations of iron around the cycle

- Boiler/HRSG deposits (including aluminum)

- Boiler/HRSG Tube Failures (overheating and TF)

- Steam Turbine Deposits (including aluminum)

• Need for Iron Removal Processes

- Condensate Polishing and/or Filters

• Limitations around the cycle

- Condensate polishing (may have to change mode)

• Overall an ACC “controls” the unit cycle chemistry

- International Guidelines didn’t consider ACC or two-phase flow up to 2008 (IAPWS Volatile Guidance)

There is an ACC Corrosion Index to Categorize Corrosion

and Track Improvments

DHACIDHACIDHACIDHACI

((DDooley, ooley, HHowell, owell, AAirir--cooled Condenser, cooled Condenser,

CCorrosion orrosion IIndex) ndex)

((DDooley, ooley, HHowell, owell, AAirir--cooled Condenser, cooled Condenser,

CCorrosion orrosion IIndex) ndex)

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We know what the Corrosion Looks Like

DHACI 3DHACI 4

DHACI 3

Further Inspections in Australia750 MW Supercritical on OT at pH 9. GEA ~4,000 hrs.

DHACI 4

Concentration of Two-phase FAC beneath Supports

Source: Richardson and Joy, ACCUG 2011

Inspections in China650 MW Supercritical with Shuang Liang ACC. 15 Months.

ACC Duct Work not PassivatedDHACI 3

Concentration of Two-phase FAC beneath Supports

Inspection in India show the same Features 150 MW Fluidized Bed Unit with GEI ACC. 2000 hours.

DHACI 2

DHACI B

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And what Holes at Tube Entries Look Like

DHACI 5

Do we Fully Understand the Environment and the Corrosion

Mechanism?

Solutions are already being appliedIncrease bulk pH up to 9 8Increase bulk pH up to 9.8

Increase local pH (amines including filming)

Filters (average and absolute) and condensate polishers

Coatings (epoxy), Sleeves, Inserts

Alternate Materials to CS

Designs

The PTZ Environment in the LP Steam Turbine is Completely Understood

Generation of the ACC Environment

Heterogeneous droplet Nucleation andLiquid Films on ST Blades

3400

3600

3800

500

600

°C

200100

50 20 10 5bar

(Droplets and Liquid Films in the ACC vary from

0.1 – 50 microns and don’t contain any oxygen until during

shutdown)

2000

2200

2400

2600

2800

3000

3200

3400

5.5 6.5 7.5 8.5

Entropy (kJ/kg/K)

En

tha

lpy

(k

J/k

g)

400

300

200

100

21 0.5 0.2 0.1 0.05

0.950.90

0.85

0.80bar

x

AB

C

Droplets <0.1µm

<100µm

Generation of the ACC Environment is in the PTZ

L-0 and L-1 Blades

Liquid Films <120µm

High ChargeDensity to Exhaust

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The Liquid in ACC Upper Ducts (Lower pH and higher chlorides)

10

15

20

25

30

35 Chlorideduct cpp

10

12 pH

0

5

Courtesy Setsweke Phala, Eskom

0

2

4

6

8

duct cpp

Inside diameter surface of an ACC TubeThe black areas are where the Fe3O4 is Precipitated Locally

Shiny white metal

Copyright Structural Integrity Associates, Inc. 2013

ID Oxide Scale

Corroded (FAC?) ID Surface of ACC Tube

Oxide free area with preferential grain boundary attack


50

60

70

80

90

100

Iron in Condensate (ug/L)

Operation at pH 9.0

Operation at pH 9.5

Operation at pH 9.8

Dooley/Aspden pH/Fe Relationship

0

10

20

30

40

6/Aug/2007 14/Nov/2007 22/Feb/2008 1/Jun/2008 9/Sep/2008 18/Dec/2008 28/Mar/2009 6/Jul/2009 14/Oct/2009 22/Jan/2010 2/May/2010


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Damage takes time to repair (ex. 2 Years with pH 9.8)

DHACI 2Cross member

LDI not “repaired” by pH


DHACI 4DHACI 2

Thoughts on “corrosion” in air-cooled condensers Based on work conducted in US, Australia, China, India, Ireland and South Africa

Increasing pH to 9.8 will gradually eliminate the damage at the tube entries and iron levels will reduce to 5 - 10 ppb Documentedlevels will reduce to 5 - 10 ppb. Documented by DHACI

Damage on cross members is not “repaired” by increasing pH. Is this LDI caused by the larger droplets leaving the PTZ?


Summary from 2012 ACCUG• Some aspects relate to (LT Two-phase) FAC

– Adjacent black and white areas in severe turbulent areas

– Increasing pH reduces damage

• Some aspects don’t (normal FAC scalloped appearance and white areas on cross members is probably LDI)

• Environment is known and has been measured (one plant)

– Two-phase mixture formed in PTZ of STTwo phase mixture formed in PTZ of ST

– Concentrating liquids (Higher in chloride/sulphate, organics )

– Lower in pH

• Clearly more tubes need to be analyzed and we need to provide solutions which address the mechanism

• Latest results from a number of plants indicate increased Al levels in turbine and in drum deposits

Deposits in HP Exhaust (18% Al2O3 and 10% CuO)

Source: Joy, ACCUG 2012

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IAPWS Technical Guidance Documents for Plants with ACC

• Volatile treatments for the steam-water circuits of fossil and combined cycle / HRSG power plants (July 2010). This document includes the basis for AVT and OT for all plants. Only use oxidizing treatments in combined cycle/HRSG plants. Will need close to pH 9.8 to provide protection to ACC.

• Phosphate and NaOH treatments for the steam-water circuits of fossil and combined cycle / HRSG power plants (Oct 2011). This document includes the basis for selecting the optimum boiler/HSRG evaporator water treatment for (phosphate and NaOH treatments) for all drum plants(phosphate and NaOH treatments) for all drum plants.

• Steam Purity for Turbine Operation (Sept 2013). This document covers a wide range of turbines (fossil, nuclear, industrial, geothermal, etc) and failure mechanisms. Includes allowances for CO2 and some amines as long as the composition is known.

• Corrosion Product Sampling and Analysis (Dec 2013). This document covers the optimum procedures for iron and copper (will add aluminum in 2014).

Freely available and downloadable on IAPWS websitewww.IAPWS.org

Questions for ACCUG DiscussionMonitoring Iron and Aluminum

• Any new experience on monitoring total iron?

• Has any user identified aluminum in boiler/HRSG deposits or on the steam turbine

• Does any user have aluminum monitoring experience and methods

ACC Tube Inspections

• Any experience of selecting and removing an ACC tube for inspection –(assume the removed tube ends are just capped?)

• Has any user conducted inspections using video probes down ACC tubes – observation and interpretation? Given that high pH reduces FAC – it is still occurring albeit at a reduced rate – hence how to determine tube life and are there areas in a ACC tube where the damaged (under high pH) might be focussed.

• Are spare ACC tubes available from ACC suppliers?

Questions for ACCUG DiscussionACC Tube Inspections Continued

• Have any members inspected new ACC tubes before use – particularly interested in the tubes with aluminized external coatings.

• Have members considered removing a complete ACC finned cooler tube to investigate tube condition with respect to chemistry and residual and on-going FAC damage after moving to a high pH regime. If a tube has been removed - what was the criteria for selection between a normal tube or a dephlegmator tubenormal tube or a dephlegmator tube.

Questions for ACCUG Discussion• How do organisations monitor sub-cooling and subsequent poor ACC

operation with respect to removal of non-condensibles. What techniques are used to "burp" the ACC to stop the sub-cooling.

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Questions for ACCUG DiscussionCondensate Polishing and Filters

• What types of condensate filters are used (average or absolute size?)

• What type of condensate polishers are used:

- powdex - deep bed mixed beds - separate cation and anion beds

- hydrogen cycle polishers - ammonia cycle polishing

• For ACC units with condensate polishers and condensate filters :

- what are the high temperature trip points for the filters and polishers -what are the high temperature trip points for the filters and polishers how often do polishers and filters trip on high condensate temperatures.

• What has been the experience with polisher resin life - cation and anion. Is organic carbon monitoring conducted ?

Questions for ACCUG DiscussionCondensate Polishing and Filters (continued)

• With ACC with condensate polishers - do organizations monitor condensate and feedwater chloride and sulfate levels by ion chromatography? What levels are found and do measurements comply with Guidelines? Are polishers operating with acceptable chloride and sulphate levels across hydrogen to ammonia changeover and across the temperature range that the polisher may experience with daily ambient temperatures variations?ambient temperatures variations?