2-Boiler Corrosion Issues-By Harmen Bouwman-SHELL
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Transcript of 2-Boiler Corrosion Issues-By Harmen Bouwman-SHELL
Shell Global Solutions
Boiler Corrosion IssuesBoiler Corrosion Issues
Harmen BouwmanHarmen Bouwman
Steam system a thin layer of magnetite Steam system … a thin layer of magnetite …
HP t
Attemporator Condensate
HP steam
Steam drumSuperheater
TurbineSuperheater
SteamDeaeratorEvaporatorEconomiser
Condensate StackPolishing
Mud tap
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Mud tap
2
Magnetite formationMagnetite formation
Steel is o idi ed b (p re) waterSteel is oxidized by (pure) water
At high temperature
⇒ layer of magnetite = Fe3O4
Protective barrier against further oxidationg
Black
Thin with little reduction of thermal conductivityThin with little reduction of thermal conductivity
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Magnetite formation – reactionsMagnetite formation – reactionsat elevated temperature - passivation:p p
3 Fe + 4 H2O ⇒ Fe3O4 + 4 H2, protective !l / dat low/moderate temperature:
2 Fe + O2 + 2 H2O ⇒ 2 Fe(OH)2 ⇔ FeO
2 Fe(OH)2 + ½ O2 + H2O ⇒ 2 Fe(OH)3 ⇔ Fe2O3
⇒ ensure proper formation & prevent damage !
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Magnetite – DamageMagnetite – Damage
Damage to protective film byDamage to protective film by
oxygenyg
high alkalinity
idiacidity
corrosion fatigue/mechanical stress
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Improper magnetiteImproper magnetite
Visible after1 dayambient exposureambient exposureas brown stains
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A boiler should A boiler should …
generate steam in a clean circuitcircuit
controlled nucleate boiling
efficient heat transfer
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A boiler might A boiler might …
generate steam in a circuit with heavy deposits (waterwith heavy deposits (water side)
l b bbllarge steam bubbles
less efficient heat transferless efficient heat transfer
higher tube temperatures
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Scales and depositsScales and deposits
scale:l ti l l tirelatively selective
precipitation, hard glassy t i lmaterial
deposit:deposit:precipitated elsewhere, transported and depositedtransported and deposited
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Impact of scales and deposits 2Impact of scales and deposits 2
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Failure causesFailure causes
mechanical nature 81% f ilmechanical nature 81%
• long term overheating 15 %
failure causes
long term overheatingcorrosionoverheating 15 %
• short termh ti 66 %
g15%
corrosion19%
overheating 66 %
corrosion 19%f h h d h t tof which due to
feed water 37 %
short term overheating
66%
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Three major problems in BoilersThree major problems in BoilersDepositsC iCorrosionCarry over
In order to avoid these problems BFW and BW must comply to certain specifications, depending on the pressure and the design of the boilerthe design of the boiler.
BFW/BW impurities are usually controlled by a combination ofExternal treatment, i.e. purificationInternal treatment, i.e. counter measures to reduce or eliminate bj i bl ff f iobjectionable effects of contaminants
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l l dTypical Boiler Feed Water IOW’s
Oxygen < 20 ppbConductivity < 0.2 μS/cm
C diti d ith l til lk li h h tConditioned with volatile alkalies or phosphates pH between 9.5 and 11
Non-volatile : Sodium-phosphate & Causticp pVolatile : Ammonia, Morpholine & Hydrazine
Iron < 50 ppbSilica < 20 ppbSilica < 20 ppb
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Boiler Feed Water
Quality requirements for BFW, BW and steam are specified in different countries by internationally known standardization committees and/or organizations e.g.:
• British Standards Institute (BSI)• British Standards Institute (BSI),• Vereinigung der Technische Überwachungs Vereine (VdTUV ) < 68 bar• Vereinigung Grosskraftswerk Betreiber (VGB) > 64 bar Vereinigung Grosskraftswerk Betreiber (VGB) 64 bar
Shell Global Solutions uses the latest recommendations published by EN 12952.
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BFW spec: EN 12952BFW spec: EN 12952 1212BFW spec: EN 12952BFW spec: EN 12952--1212
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Main Boiler Corrosion IssuespHOxygen CorrosionOxygen CorrosionCaustic corrosionCaustic CrackingFlow Assisted CorrosionErosion/Corrosion-ErosionChelant CorrosionChelant CorrosionScalingCorrosion fatiguegOverheating & Stress RuptureCreepFire-Side CorrosionFlue Gas Corrosion (cold-end corrosion)Condensate Corrosion
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Condensate Corrosion
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Causes of corrosionCauses of corrosionpoor pH control oxygen pitting
poor chemicalfeed control
downtime
inadequateblowdowncontrol
poor boileroxygenin-leakage
mechanicalde-aeratorperformance
downtimecorrosion
pfeedwaterquality
condensatepoor external
in leakage
scavenger underfeed
downtimecorrosion
boilercorrosion
contamination treatment
poor boilerfeedwater concentratingfeedwaterquality
condensatecontamination
poorexternal poor chemical
feed/control
concentratingmechanism stressed
area
deposition stress corrosion cracking
treatmentinadequateblowdowncontrol
feed/control embrittling watercharacteristics
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deposition stress corrosion cracking
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Corrosion vs pHCorrosion vs. pH
Corrosion rateCorrosion rate
Safe range8.5 12.7
4 6 8 10 12pH
at 25ºC
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pHpHThe pH is an important factor influencing either scale formation and
i t d i f th t corrosion tendencies of the water. Water with a low pH will result in corrosion of the feed water lines, boiler materials and heat exchangers (Acid corrosion)materials and heat exchangers (Acid corrosion).A high pH may lead to excessive scaling.
Boiler feed water : pH controlled to around 9 - IOWBoiler water : pH between 9.5 and 11- IOW
pH Control:Non-volatile - Sodium phosphate & Caustic for BFW and BWNon volatile Sodium phosphate & Caustic for BFW and BWVolatile - Ammonia, Morpholine, Hydrazine for BFW mainly
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Oxygen corrosionOxygen corrosion
High Oxygen levels destroy the protective Magnetite (Fe O ) by the High Oxygen levels destroy the protective Magnetite (Fe3O4) by the formation of soft Fe2O3.
Localized corrosion (pitting) occurs in oxygen corrosion cells.(p g) yg
Oxygen corrosion can occur throughout the system.Oxygen corrosion is influenced by [O2] pH and temperatureOxygen corrosion is influenced by [O2], pH and temperature
General strategy/approach to mitigate: gy pp gDecrease Oxygen levels by mechanical deaeration and chemical dosing (e.g. Hydrazine, Hydroquinone) – IOWO l l h ld b k t b l 20 b IOWOxygen levels should be kept below 20 ppb - IOW
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Oxygen corrosionOxygen corrosion
Attack in FW and economizer sections where magnetite is damagedAttack in FW and economizer sections where magnetite is damagedUnder depositsBy thermal cyclingBy thermal cycling
Deposit + porous Fe-oxides
F F + e++ +++ -→
O + 2 H O + 4e 4 OH2 2- -→
O2OH-
Cathode Anode
Fe Fe + e++ +++→OH
Cathode Anode
Fe Fe + 2e+ + -→
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Oxygen corrosionToo much oxygen (> 20 ppb dissolved)
Reaction on magnetite:4 Fe3O4 + O2 6 Fe2O3
Reaction on bare surface:4 Fe + 2 n H2O + 3 O2 2 (Fe2O3.nH2O)
⇒ Hematite - not protective
⇒ Pitting⇒ Pitting
Fe O th dOOFe3O4
Fe
cathode
anodeFen+
O2O2
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Oxygen pitting corrosionOxygen pitting corrosionCorrosion product
presentpresentremoved
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Oxygen corrosionOxygen corrosionOxygen corrosionOxygen corrosion
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Oxygen removalMechanical de-aeration
Chemical oxygen scavengingChemical oxygen scavenging
Sulphite ⇒ sulphate
d ( )Hydrazine (N2H4)⇒ N2, H2O ⇒ s pports passivation⇒ supports passivation ⇒ NH3 (neutralisation of condensate)
C b h d idCarbohydrazides⇒ hydrazine, CO2
O hOthers
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High alkalinity attackHigh alkalinity attackCaustic attack / Gouging
Local concentration of caustic under deposits and in crevices causing a high pH.g g pDissolution of protective magnetite layer and the steel below as ferrates.
Caustic embrittlement♦ form of stress corrosion cracking, along grain boundaries in
metal♦ high alkalinity in crevice / deposit
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Caustic CorrosionCaustic Corrosion
U ll f d i hi h b ilUsually found in high pressure boilersProblem mainly caused by depositsLocalized corrosionAlso known as crater attack or caustic gougingAlso known as crater attack or caustic gouging
ConditionsConditionsPresence of caustic soda in the boiler waterPresence of a concentrating mechanism
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Caustic CorrosionCaustic corrosion can occur in the presence of
Porous metal oxide depositsPorous metal oxide depositsOperation above rated capacityExcessive localized heat inputExcessive localized heat inputLocalized pressure differentialsRestrictions in the steam generating tubesRestrictions in the steam generating tubes
MitigationLimit the presence of “free” causticCoordinated phosphate treatmentsCongruent sodium phosphatePhosphate-low hydroxide
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Caustic CorrosionCaustic CorrosionSteam
Boiler waterPorous Iron Deposit
Na+ local increase of pH
MagnetiteOH-local increase of pH
Caustic attacks iron SteelSteel
Fe + 4OH FeO + 2H O2+ -2
2-2→ 2 2
→4OHFe - 2FeO + 2H O2-
2 FeO22- +O 3 4 +
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Caustic CorrosionCaustic Corrosion
Removal of magnetite film at high pHRemoval of magnetite film at high pHFe3O4 + 4 NaOH ⇒
2 NaFeO2 + Na2FeO2 + 2 H2O,2 NaFeO2 + Na2FeO2 + 2 H2O,soluble products
Attack of bare steel at high pHAttack of bare steel at high pHFe + 2 NaOH ⇒ Na2FeO2 + H2,
soluble productsp
Attack of bare steel at moderately high pH3 Fe + 4 H2O ⇒ Fe3O4 + 4 H2,3 e 2O ⇒ e3O4 2,
porous oxide layer
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Co-ordinated phosphate effectCo-ordinated phosphate effectBuffer: HPO4
2- + OH- ↔ PO43- + H2O
Ph h hid i i k
Boiler water
Phosphate hide-out is a risk
SteamBoiler water
Porous Iron DepositHPO4
2-OH-
Magnetite4 OH-
Na+
Phospate pneutralises OH Steel- No attack
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Caustic Corrosion
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Caustic gougingCaustic gouging
ref: The NALCO Guide to Boiler Failure Analysis (ISBN 0-07-045873-1)
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Caustic cracking / embrittlementCaustic cracking / embrittlementC diti i dConditions required
Concentrating mechanismMetal under high stressesMetal under high stressesTemperature (rare below 150 ºC)
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Flow Assisted CorrosionThinning corrosion associated with high purity, low oxygen condensate
(or boiler feed water) caused by the relative movement of a fluid against ( ) y gthe metal surface.
Metal loss results from the dissolution of the protective oxide film by localized turbulencelocalized turbulence
Type of Corrosion-Erosion
Feed water velocities above 2.1 m/s can remove oxide filmsDamage is aggravated by local turbulence Low oxygen concentration destabilizes the magnetite layer pH < 9.3 (note: pH or feed water controlled between 8.5 – 9.3)Maximum damage inMaximum damage in o one-phase flow @140 deg Co two-phase flow @ 180 deg C
C i t b t 0 1 10 /Shell Global Solutions
Corrosion rates between 0.1 – 10 mm/year35
Flow Assisted CorrosionFlow Assisted CorrosionToo low oxygen concentrations with too high liquid velocities oo o o yge co ce t at o s t too g qu d e oc t escan lead to Flow Assisted Corrosion
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Flow Assisted CorrosionFlow Assisted CorrosionFeed water line inside a boiler drum Feed water contained <5 ppb O<5 ppb O2.
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Flow Assisted CorrosionFlow Assisted CorrosionMitigationg
Modification of the water chemistry(usually minimum changes allowed)(usually minimum changes allowed)
Materials selection:
- Resistance improves with small additions of Cr, Cu and Mo
- Use of 1.25 Cr or 2.25 Cr steels
- Use of 12Cr stainless steels provides best protection
Ch i l i h hi h iChange material at areas with high corrosion rates
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Carry-OveryCarry-over is any contaminant in the solid-, liquid- or vapour form, which leaves the boiler with the steam due to incomplete separation of the steam in the steam drum. It is related to steam quality.Carry-over may lead to superheater failures, sticking govenor valves in turbines, erosion of turbine parts, reduced turbine efficiency.
Causes for carry-over:yMechanical
Pressure or level surges inadequate design overloading Pressure or level surges, inadequate design, overloading, high drum level operation
ChemicalChemicalFoaming, formation of bubbles (accumulation of solids), vapour
carry over (silica)Shell Global Solutions
carry-over (silica)39
Cracking due to deposits from carry-over in superheaterCracking due to deposits from carry over in superheater
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Erosion - Corrosion/ErosionCombined action of corrosion and erosionM t l l t d d Metal loss rates depend on
Velocity and concentration of impacting mediumSize and hardness of impacting particlesHardness/corrosion resistance of material.Hardness/corrosion resistance of material.
Factors increasing corrosivity of the environment ( temperature, pH) can increase susceptibility to metal losscan increase susceptibility to metal loss.Prevention
D i i t dif h / t / t i l l tiDesign improvements – modify shape/geometry/materials selection
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Erosion - Corrosion/Erosion
Major locations for ErosionSoot BlowersSteam cutting from adjacent tube failuresSteam cutting from adjacent tube failuresFly-Ash Erosion in eco, superheater, reheater & rooftubing(higher sensitivity for lower temperatures: harder particles)(higher sensitivity for lower temperatures: harder particles)
Erosion on water side is comparatively rarep yLocations with increased turbulence(internal surface discontinuities & tube ends: Corrosion-Erosion)(internal surface discontinuities & tube ends: Corrosion Erosion)
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Corrosion/Erosion: tube endCorrosion/Erosion: tube end
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Chelant CorrosionChelant Corrosion
Only when Feed Water is chelant controlledOnly when Feed Water is chelant controlledAttacks the protective magnatite layer of the boiler (drum, economizer tubes lines)economizer, tubes, lines)Mechanism
Fe3O4 + Fe + 8H+ 4Chelant 4Fe(II)Chelant + 4H2OMitigation
Close control of Chelant levelsControl of dissolved Oxygen levelsControl of dissolved Oxygen levelsReduce fluid velocity (turbulent flow)
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Scaling Scaling Silica
The presence of Silica can promote the formation of Sodium-iron-silicates. The solubility of Silica is dependent of the lk l f halkalinity of the water.
The higher the alkalinity, the higher the solubility.For high pressure steam boilers the Silica content in the BFWFor high pressure steam boilers, the Silica content in the BFW should be limited to 20 ppb.Cycles of concentration raises the Silica concentration to yppm levels e.g. 60 cycles x 20 ppb > 1.2 ppm
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ScalingScalingIron
Can form a scale in boiler tubes, particular in the presence of Silica thru the formation of Sodium iron silicateIron levels should be limited to 50 ppb (commonly required by boiler manufacturers)(co o y equ ed by bo e a u actu e s)
CopperppThe presence of copper is thought to accelerate on-going corrosion processesprocessesRecovered condensate (unpolished) is the main source of copper
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ScalingScaling
Mud drum scalesMud drum scales
Riser tubeRiser tube
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Corrosion fatigue/mechanical stressCorrosion fatigue/mechanical stressDamage to protective Fe3O43 4
mechanical damage (hammer testing, etc.):
tirusting
tube vibration or thermal cycling :
cyclic stress with repetitive breakage/formation of Fe3O4corrosion fatigue cracking
water hammer at de-aerator inlet
i i f h t d t t ith ld k tmixing of hot condensate steam with cold make-up water
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De aerator crackingDe-aerator crackingCracking has been observed in de-aerator vesselsg
The root cause of failure is corrosion fatigue
Si th k ti ht it ll l bSince the cracks are very tight, it can usually only be found with (wet) fluorescent magnetic particle examinationexamination
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Overheating & Stress RuptureOverheating & Stress Rupture
Localized overheating (short term/long term) causes permanentLocalized overheating (short term/long term) causes permanent deformation (bulging) and eventual failure/rupture at normal operating stress levelsp g
Critical factors: Temperture & Time (stress)Loss in thickness reduces the time to failureAvoid overheating
Burner managementPrevent scalingPrevent scaling
Main sources of fouling in boiler tubes are silica, iron and copper
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Creep
Time dependent deformation of stressed components under load at high temperaturesCritical factors: Material, Load/Stress and TemperatureBeyond threshold temperatures damage/cracking/failure is a concernBeyond threshold temperatures damage/cracking/failure is a concernMitigation: Operate equipment within design limits, Inspect & Monitor
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Creep damageCreep damage
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Failure from creep voidsFailure from creep voidsMembrane tube from boilerOverheated at 750 °C after voids had developed
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CreepCreepAllowable stress
SS 321time dependent
CS LAtime dependentcreep strength
time independent800H
time independentstrength
200 400 600 800
T t i °C
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Temperature in °C54
Creep Threshold TemperaturesCreep – Threshold Temperatures
Carbon Steel : 410 ºCCarbon Steel : 410 CC - ½Mo : 490 ºC1¼C ½M 500 ºC1¼Cr - ½Mo : 500 ºC2¼Cr - 1Mo : 480 ºC5Cr - ½Mo : 470 ºC9Cr – 1Mo : 515 ºC9Cr 1Mo : 515 C304H : 580 ºC347H 595 ºC347H : 595 ºC
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Fire-side CorrosionOxidation
Firing with excess of Oxygen (1 2 %)Firing with excess of Oxygen (1-2 %)
Oxide scale (protective)
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Fire-side CorrosionOil-Ash Corrosion
Metal temperature range: 593 – 816 CPresence of Vanadium and Sodium Compounds in Fuel
MechanismFormation of V2O5 & Na2O by oxidation (in flame)Ash particles stick to the metal surface (Na2O as binder)V2O5 + Na2O forms a liquid (eutectic)Liquid fluxes the protective oxide resulting in rapid corrosion
MitigationMetal temperature < 593 ºCAdditive: Mg-compounds, when fuels with very low quantities of V and Na can not
be specified
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Flue Gas Corrosion (Cold-End Corrosion)Flue Gas Corrosion (Cold-End Corrosion)Sulfuric acid corrosion if
Steel temperature is below 150°C 1)
ANDAND
Fuel contains sulphur♦ natural gas: [S] > 20 ppmv ( 30 mg/Nm3)♦ natural gas: [S] > 20 ppmv (~30 mg/Nm3)
♦ oil: [S] > ~ 0.1 %w
1) or the actual acid dew point of the flue gas, calculated on the basis of the SO3 concentration, estimated from the radiant cell temperature and the excess air.
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Condensate corrosion – COCondensate corrosion – CO2
CO2 is formed by the dissociation 2 yof carbonates in the boiler
NaHCO3 ⇒ NaOH + CO2
Mechanism:
CO2 + H2O ⇒ H+ + HCO3-
low pH at carbon steel
large patches of attack with steep edges
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Condensate corrosion – OCondensate corrosion – O2
O iO2 corrosion sources:
Oxygen ingress
DDe-aerator
Mechanism:Mechanism:
Oxygen pitting Fe
Worse: CO2 & O2 combined
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Condensate corrosion – NHCondensate corrosion – NH3
Sources:Sources:NH3 from boiler
M h iMechanism:Cu & alloys Stress Corrosion Cracking
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Prevention Condensate CorrosionPrevention Condensate CorrosionReduce …
carbonates BFW
oxygen ingressoxygen ingress
vacuum systems
vents
condensate pumps / traps / valvesp p / p /
intermittently operated systems
ammoniaammonia
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Condensate TreatmentCondensate TreatmentDe-oiling (oil from turbines)g
Oil removed by skimming tanks, clarifiers and filters. Alternatively by coalescers and absorbentsy y
Condensate PolishingOil-free condensate is treated in a condensate
polishing treater (ion exchangers)
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Once a boilerOnce a boiler ...
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Thank you for your attentionThank you for your attention
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