Tube Maintenance

Korea Electric Power Research Institute

Boiler Tube Maintenance

ContentsContents

§§ 1. Tube design consideration1. Tube design consideration

§§ 2. Tube Failure Mechanism2. Tube Failure Mechanism

§§ 3. Repair and replacement of boiler tube3. Repair and replacement of boiler tube

1. Tube design consideration1. Tube design consideration

1.1 Stress calculation1.1 Stress calculation

4.2 Tours4.2 Tours

4.3 Tube wastage and wall thinning4.3 Tube wastage and wall thinning


§§ Minimum tube wall thicknessMinimum tube wall thickness-- The principal stress in a tubular cross sectionThe principal stress in a tubular cross section-- t = ( P*D)/(2S+P) + 0.005D + et = ( P*D)/(2S+P) + 0.005D + e

P = design pressureP = design pressureD = outside tube diameterD = outside tube diameterS = B & PV Code allowable stressS = B & PV Code allowable stresse = rolling allowancee = rolling allowance


- ASME B&PV Section 1- The equation of tube thickness

KOR

USA

UK

GER

JPN

ISO


Decision of allowable stress Comparison of allowable stress

1.2 Tours1.2 Tours

§§ Calculate the hoop stress in a tube bendCalculate the hoop stress in a tube bend-- Torus is a doughnut shapeTorus is a doughnut shape-- Tube bend is half a torusTube bend is half a torus-- SH, RH, Eco elementSH, RH, Eco element

Sketch of half a torus, representative of a tube

bend

1.2 Tours1.2 Tours

§§ The hoop stress (D>5W)The hoop stress (D>5W)

-- The max hoop stress is on the inside of the bendThe max hoop stress is on the inside of the bend

-- The wall thickness : Intrados > Extrados*(1+0.6)The wall thickness : Intrados > Extrados*(1+0.6)

-- Hoop stress in a cylinder > in a torusHoop stress in a cylinder > in a torus

-- Tube fail in the cylinder rather than in the bend Tube fail in the cylinder rather than in the bend


§§ During normal boiler operationDuring normal boiler operation-- Wall thinning occursWall thinning occurs-- Due to fly ash, soot blowers, corrosionDue to fly ash, soot blowers, corrosion-- In most case the metal loss canIn most case the metal loss can’’t be preventedt be prevented


§§ The minimum wall thickness for replacementThe minimum wall thickness for replacement-- P : Actually operating PressureP : Actually operating Pressure-- RiRi : Inside radius: Inside radius-- S : Allowable stress for materialS : Allowable stress for material

Tubes equal to or greater than 85% t

Monitor thickness Furnace support Tubes Economizer stringer support Tubes

Tubes less than 85% t Restore tube wall thickness or replace tube


Monitor thickness Economizer, Furnace Wall and other water cooled Tubes



Monitor thickness Super-heater, Reheater and other steam cooled Tubes


Source : Babcock &Wilcox, cited in G.G Stephenson and J.W. Prince

u Guidelines for Tube Repair/Replacement

Location Actual wall thickness Course of Action

2. Tube Failure Mechanism2. Tube Failure Mechanism

2.1 Creep2.1 Creep

2.2 Fatigue2.2 Fatigue

2.3 Corrosion2.3 Corrosion

2.4 Crevice Corrosion2.4 Crevice Corrosion

2.5 Pitting2.5 Pitting

2.6 Corrosion Fatigue2.6 Corrosion Fatigue

2.7 Stress Corrosion Cracking2.7 Stress Corrosion Cracking

4.1 Creep4.1 Creep

§§ Three elements of creepThree elements of creep-- Elevated temperatureElevated temperature-- StressStress-- TimeTime

§§ Creep Creep -- Metal undergoes permanent plastic Metal undergoes permanent plastic deformation deformation

§§ Creep ProcedureCreep Procedure-- Microstructure damage Microstructure damage →→ Void Void →→ CrackCrack-- Longitudinal cracks within steam side scaleLongitudinal cracks within steam side scale

4.1 Creep4.1 Creep

§§ Creep curveCreep curve

-- Primary Creep : Transient creepPrimary Creep : Transient creep

-- Secondary Creep : Constant Creep rateSecondary Creep : Constant Creep rate

-- Tertiary Creep : Rapidly increasing creep rate Tertiary Creep : Rapidly increasing creep rate

Fig8Creep Void & Crack

Boiler Tube Creep Damage - Crack

Figure 9. Two super-heater tubes for a utility boiler, failed creep


§§ A tensile load is alternately applied and removedA tensile load is alternately applied and removed

§§ Failure occur at a level much less than stress of staticFailure occur at a level much less than stress of static

§§ Endurance limit (Fatigue strength) Endurance limit (Fatigue strength) -- For stress level below a For stress level below a certain limit, the number of cycles to failure is nearly infinitcertain limit, the number of cycles to failure is nearly infinitee

Figure10 A schematic presentation of a fatigue curve for mild steel


§§ Endurance limit can be affected by surface condition.Endurance limit can be affected by surface condition.

§§ Notches Notches

§§ Corrosion causes the notches and magnifies their effect.Corrosion causes the notches and magnifies their effect.

§§ Fatigue may be also be caused by temperature gradient.Fatigue may be also be caused by temperature gradient.


§§ Fatigue fracture surfaceFatigue fracture surface

beach mark Striation

Case : Fatigue CrackCase : Fatigue Crack

§§ Unit : Unit : IlsanIlsan #5#5

§§ Date : 2000. 7Date : 2000. 7

§§ Location : HP 2Location : HP 2’’ry economizer inlet header stub tube ry economizer inlet header stub tube

#1#1--bb

§§ State : The same cracks takes place five times at the State : The same cracks takes place five times at the same place.same place.

§§ Cause : The elevated temperature of tube because Cause : The elevated temperature of tube because of blocking of blocking tube internal.tube internal.

§§ CounterCounter--measure measure -- Eliminating foreign material inside of tubeEliminating foreign material inside of tube-- Plugging the damaged tube Plugging the damaged tube

Fig22. Stub tube crack(2000.7.9) Fig23. Tube Crack(2000.7.23)

4.3 Corrosion4.3 Corrosion

§§ An electrochemical processAn electrochemical process

§§ Anode Anode -- Oxidation Oxidation -- Dissolve Dissolve -- Ion, ElectronIon, Electron

§§ Cathode Cathode -- Reduction Reduction -- Consuming electronConsuming electron

§§ Corrosion Occur ConditionCorrosion Occur Condition

-- Potential differencePotential difference

-- Electron moveElectron move

-- ElectrolyteElectrolyte

4.4 Crevice Corrosion4.4 Crevice Corrosion

§§ The Difference of oxygen concentration generate The Difference of oxygen concentration generate the crevice corrosion.the crevice corrosion.

§§ The dissolution of metal The dissolution of metal -- M M →→ M+ + eM+ + e--

-- OO22 + 3H+ 3H22O +4eO +4e-- →→ 4OH4OH--

Fig 12.Crevice Corrosion


§§ Localized attackLocalized attack

§§ In the pit the metal dissolveIn the pit the metal dissolve

§§ Outside pit the reduction reaction of oxygen take Outside pit the reduction reaction of oxygen take place place

Fig 13. Pitting Mechanism


§§ Pitting CasePitting Case

Low alloy re-heater tube


§§ The failure by repeated or cyclic stresses in a The failure by repeated or cyclic stresses in a corrosive environmentcorrosive environment

§§ Corrosion fatigue has major cracks and their Corrosion fatigue has major cracks and their branchesbranches

§§ Cracks are typically transCracks are typically trans--granulargranular

§§ Fatigue crack has nearly one major crackFatigue crack has nearly one major crack

§§ Oxide wedge forms once, fatigue crack occursOxide wedge forms once, fatigue crack occurs

§§ The volume occupied by the oxide, oxide wedgeThe volume occupied by the oxide, oxide wedge

§§ The oxide wedge adds stress to the crack tipThe oxide wedge adds stress to the crack tip

§§ This cycle repeats and crack moves This cycle repeats and crack moves furherfurher

Corrosion Fatigue


Case : Corrosion fatigue crackCase : Corrosion fatigue crack

§§ Unit : Unit : SeoinchunSeoinchun #1 #1 -- 88

§§ Date : Since operationDate : Since operation

§§ Location : HPHT & HPIT Location : HPHT & HPIT SuperheaterSuperheater TubeTube

§§ State : The crack has been taken place since State : The crack has been taken place since operation.(157times)operation.(157times)

§§ Causes : Causes : -- Straight tube configuration between headersStraight tube configuration between headers-- The separated upper header as partition plateThe separated upper header as partition plate-- The upwards expanding configurationThe upwards expanding configuration-- Austenite stainless steel being used Austenite stainless steel being used

§§ Counter measuresCounter measures-- Complete separation of upper headersComplete separation of upper headers

Fig24.

Super-heater Tube Configuration.

Fig25. #1 HPHT #3p-A Crack Fig26. #1 HPIT 6A-17 Crack

4.7 Stress corrosion 4.7 Stress corrosion crackingcracking

§§ The failure of a metal stressed in tension below the The failure of a metal stressed in tension below the yield point and exposed to a specific corroding yield point and exposed to a specific corroding environmentenvironment

§§ Pressure force, residual, effects of cold workPressure force, residual, effects of cold work§§ Carbon steels fail in concentrated hydroxide solution Carbon steels fail in concentrated hydroxide solution §§ Stainless steels fail in chloride solutionStainless steels fail in chloride solution§§ Crack usually interCrack usually inter--granular but transgranular but trans--granular under granular under

certain conditions. certain conditions.

Fig 15.Inter-granular Stress-

corrosion cracking , ferritic steel

2.8 Hydrogen damage2.8 Hydrogen damage

§§ Under heavy scale deposit Under heavy scale deposit

§§ In the highestIn the highest--heatheat--release regionsrelease regions

§§ Local upsets to the uniform fluid flowLocal upsets to the uniform fluid flow

§§ Either strongly basic or acidic conditionEither strongly basic or acidic condition

§§ Iron reacts with hydrochloric acid, to form iron chloride and Iron reacts with hydrochloric acid, to form iron chloride and hydrogenhydrogen

Fe + 2HCl = FeCl2 + 2HFe + 2HCl = FeCl2 + 2H

§§ Some hydrogen diffuses into the Some hydrogen diffuses into the tseeltseel

§§ 4H + Fe3C = CH4 + 3Fe4H + Fe3C = CH4 + 3Fe

§§ Methane collect at grain boundary, the steel is weakenMethane collect at grain boundary, the steel is weaken

§§ Thick edged, low ductility failuresThick edged, low ductility failures

2.8 Hydrogen damage2.8 Hydrogen damage

Total decarburization and intergranular cracks(magnification 500× etched)

Total decarburization and intergranular cracks(Magnification 500× etched

Boiler explosion in Nigiria

Case : Case : Hydrogen damageHydrogen damage

- 220MW X 6호기 : 1320MW- Steam : 125㎏/㎠ 541/541℃- Maunfacture : HITACHI- Completion : 1986 - 1987

Unit Date Restoration

#4 1992년 --월 일 2002년 복구

#6 1990년 --월 일 2002년 복구

#6 2006년 03월 14일 미복구

#3 2007년 01월 30일 미복구

Egbin Power Plant History

#6 Boiler damage

Buckstay deform Corner Tube damage

#6 보일러 Tube Damage

Evaporator 27Point 30m ruptureBrittle, Window type

Tube internal corrosion

Typical hydrogen damage

5. 5. Repair and Replacement of Boiler TubesRepair and Replacement of Boiler Tubes

5.1 General Strategies for Damaged Tubes5.1 General Strategies for Damaged Tubes

5.2 Pre5.2 Pre--Repair : Confirm materials to be Repair : Confirm materials to be repairedrepaired

5.3 Specific Repair Procedure5.3 Specific Repair Procedure

5.1 General Strategies for Damaged Tubes5.1 General Strategies for Damaged Tubes

§§ Five maintenanceFive maintenance--controlcontrol--label activities.label activities.-- PrePre--repair inspectionrepair inspection-- Removal of the failed tube sectionRemoval of the failed tube section-- Repair/Replacement of the failed tubeRepair/Replacement of the failed tube-- PostPost--repair inspection and testsrepair inspection and tests-- Future preventive/control actionsFuture preventive/control actions

§§ The following historical information should be The following historical information should be considered in making the decision about repairconsidered in making the decision about repair

-- Was the original material selection in the design appropriate?Was the original material selection in the design appropriate?-- Did all the original materials meet specified requirement?Did all the original materials meet specified requirement?-- Did the operating conditions contribute to the failure in any Did the operating conditions contribute to the failure in any way : creep, fatigue, erosion, shock or thermal loading?way : creep, fatigue, erosion, shock or thermal loading?-- Are there any unusual metallurgical characteristics in the Are there any unusual metallurgical characteristics in the defective or suspect area?defective or suspect area?-- Has the problem occurred in any other units at the same site Has the problem occurred in any other units at the same site or within the industry?or within the industry?

Furnace support Tubes Economizer stringer support Tubes


Monitor thickness


Economizer, Furnace Wall and other water cooled Tubes


Monitor thickness


Super-heater, Reheater and other steam cooled Tubes


Monitor thickness


Source : Babcock &Wilcox, cited in G.G Stephenson and J.W. Prince

u Guidelines for Tube Repair/Replacement

Location Actual wall thickness Course of Action

5.2 Pre5.2 Pre--Repair : Confirm materials to be repairedRepair : Confirm materials to be repaired

§§ It is critical to confirm the materials to be replaced prior to It is critical to confirm the materials to be replaced prior to developing an optimized procedure.developing an optimized procedure.

§§ The specific grade of material is required as well as the The specific grade of material is required as well as the specification number : For example ASME SAspecification number : For example ASME SA--213 T22213 T22

§§ Upgrading materials is, for several key BTF mechanisms, the Upgrading materials is, for several key BTF mechanisms, the optimal strategy.optimal strategy.

5.3 Specific Repair Procedures5.3 Specific Repair Procedures

§§ General CommentsGeneral Comments

-- Every utility should use a qualified welder for all repairs.Every utility should use a qualified welder for all repairs.-- Use gas tungsten arc welding for root passesUse gas tungsten arc welding for root passes-- Do not use backing rings for tubing in the water touched Do not use backing rings for tubing in the water touched circuits.circuits.-- Some semiSome semi--automatic welding processes have been used inautomatic welding processes have been used in

limited applications.limited applications.-- Slag removal is required prior to performing maintenance Slag removal is required prior to performing maintenance work.work.-- Access is critical.Access is critical.

� � Repair Techniques and their ApplicationRepair Techniques and their Application

Method ApplicabilityTube section replacement - The preferred repair method

- Consists of replacing tube section with one ofsame material and dimensions.

Grind out and re-weld - Applicable for small defects such as pin-holeleaks.

- Vee-preparation and re weld.Pad welding - Should only be used in an emergency as repeat

failures are nearly always guaranteed. Should bereplaced at next major outage.

- Involves mechanically closing the rupture tooriginal contour, veeing a weld groove and re-welding.

Window welding - May be required if access all around the tube islimited. Should be used only as a temporarymeasure with full replacement at next outage.

Boiler tube build-up - For restoring thinned tubes to original thickness.ssSource : G.G. Stephenson and J.W. Prince

Welding sequence for replacing a defective section using the window welding technique. Source :: G.G. Stephenson and J.W. Prince

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Tube Maintenance