boiler tube leakages

7/29/2019 boiler tube leakages

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BOILERTUBELEAKAGE

ANALYSIS,SYMPTOMS,CAUSES

SHIVAJI CHOUDHURY


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Cause of boiler tube leakages

1.Caustic Attack 2.Oxygen Pitting 3.Hydrogen Damage

4.Acid Attack 5.Stress Corrosion Cracking

6.Waterside Corrosion Fatigue 7.Superheater Fireside Ash Corrosion

8.High-temperature Oxidation9.Waterwall Fireside Corrosion10.Fireside Corrosion Fatigue11.Short-term Overheat

12.Long-term Overheat13.Graphitization

14.Dissimilar Metal Weld (DMW) Failure15.Erosion


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1.Caustic Attack

Symptoms: Localized wall loss on the insidediameter (ID) surface of the tube, resulting inincreased stress and strain in the tube wall.

Causes:Caustic attack occurs when there is

excessive deposition on ID tube surfaces. This leadsto diminished cooling water flow in contact with thetube, which in turn causes local under-deposit boilingand concentration of boiler water chemicals. Ifcombined with boiler water chemistry upsets of highpH, it results in a caustic condition which corrosivelyattacks and


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1.1.Caustic Attack


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2.Oxygen Pitting

Symptoms: Aggressive localized corrosion and loss of tubewall, most prevalent near economizer feedwater inlet onoperating boilers. Flooded or non-drainable surfaces aremost susceptible during outage periods.

Causes: Oxygen pitting occurs with the presence ofexcessive oxygen in boiler water. It can occur duringoperation as a result of in-leakage of air at pumps, orfailure in operation of preboiler water treatment equipment.

This also may occur during extended out-of-service periods,such as outages and storage, if proper procedures are notfollowed in lay-up. Non-drainable locations of boiler circuits,

such as superheater loops, sagging horizontal superheaterand reheater tubes, and supply lines, are especiallysusceptible.


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2.1.Oxygen Pitting


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3.Hydrogen Damage

Symptoms: Intergranular micro-cracking. Loss ofductility or embrittlement of the tube material leadingto brittle catastrophic rupture.

Causes: Hydrogen damage is most commonlyassociated with excessive deposition on ID tubesurfaces, coupled with a boiler water low Ph xcursion.Water chemistry is upset, such as what can occurfrom condenser leaks, particularly with salt watercooling medium, and leads to acidic (low pH)contaminants that can be concentrated in the deposit.

Under-deposit corrosion releases atomic hydrogenwhich migrates in to the tube wall metal, reacts withcarbon in the steel (decarburization) and causesintergranular separation.


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3.1.Hydrogen Damage


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5.Stress Corrosion Cracking

Symptoms: Failures from SCC are characterized by a thickwall, brittle-type crack. May be found at locations of higherexternal stresses, such as near attachments.

Causes: SCC most commonly is associated with austenitic

(stainless steel) superheater materials and can lead toeither transgranular or intergranular crack propagation inthe tube wall. It occurs where a combination of high-tensilestresses and a corrosive fluid are present.

The damage results from cracks that propagate from theID. The source of corrosive fluid may be carryover into thesuperheater from the steam drum or from contamination

during boiler acid cleaning if the superheater is not properlyprotected.


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4.Acid Attack

Symptoms: Corrosive attack of theinternal tube metal surfaces, resulting in

anirregular pitted or, in extreme cases, aswiss cheese appearance of the tube ID.

Causes: Acid attack most commonly isassociated with poor control of process

during boiler chemical cleanings and/orinadequate post-cleaning passivation ofresidual acid.


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4.1.Acid Attack


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5.1.Stress Corrosion Cracking


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6.Waterside Corrosion Fatigue

Symptoms: ID initiated, wide transgranular cracks whichtypically occur adjacent to external attachments.

Causes: Tube damage occurs due to the combination ofthermal fatigue and corrosion. Corrosion fatigue is

influenced by boiler design, water chemistry, boiler wateroxygen content and boiler operation. A combination ofthese effects leads to the breakdown of the protectivemagnetite on the ID surface of the boiler tube. The loss ofthis protective scale exposes tube to corrosion.

The locations of attachments and external weldments, suchas buckstay attachments, seal plates and scallop bars, are

most susceptible. The problem is most likely to progress during boiler start-up

cycles.


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6.1.Waterside Corrosion Fatigue

Corrosion fatigueon tube ID adjacentto attachment

7 S h t Fi id A h


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7.Superheater Fireside AshCorrosion

Symptoms: External tube wall loss and increasing tubestrain. Tubes commonly have a pock-marked appearancewhen scale and corrosion products are removed.

Causes: Fireside ash corrosion is a function of the ash

characteristics of the fuel and boiler design. It usually isassociated with coal firing, but also can occur for certaintypes of oil firing.

Ash characteristics are considered in the boiler design whenestablishing the size, geometry and materials used in theboiler.

Combustion gas and metal temperatures in the convectionpasses are important considerations. Damage occurs whencertain coal ash constituents remain in a molten state onthe superheater tube surfaces. This molten ash can behighly corrosive.


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7.1.Superheater Fireside AshCorrosion


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8.High-temperature Oxidation

Similar in appearance and often confusedwith fireside ash corrosion, high-temperature oxidation can occur locally in

areas that have the highest outside surfacetemperature relative to the oxidation limitof the tube material.

Determining the actual root cause betweenthe mechanisms of ash corrosion or high-temperature oxidation is best done by tubeanalysis and evaluation of both ID and ODscale and deposits.


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8.1.High-temperature Oxidation

Surface appearance ofmetal showing firesidecoal ash corrosion


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9.Waterwall Fireside Corrosion

Symptoms: External tube metal loss (wastage)leading to thinning and increasing tube strain.

Causes: Corrosion occurs on external surfaces of

waterwall tubes when the combustion processproduces a reducing atmosphere ubstoichiometric).

For fossil fuel boilers, corrosion in the burner zoneusually is associated with coal firing. Boilers havingmaladjusted burners or operating with staged air

zones to control combustion can be more susceptibleto larger local regions possessing a reducingatmosphere, resulting in increased corrosion rates.


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9.1.Waterwall FiresideCorrosion

Fireside corrosionof studded tube


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10.Fireside Corrosion Fatigue

Symptoms: Tubes develop a series of cracks that initiate on theoutside diameter (OD) surface and propagate into the tube wall.Since the damage develops over longer periods, tube surfacestend to develop appearances described as elephant hide,

alligator hide or craze cracking. Most commonly seen as a seriesof circumferential cracks. Usually found on furnace wall tubes of

coal-fired once-through boiler designs, but also has occurred ontubes in drum-type boilers.

Causes: Damage initiation and propagation result from corrosionin combination with thermal fatigue. Tube OD surfaces experiencethermal fatigue stress cycles which can occur from normalshedding of slag, sootblowing or from cyclic operation of the boiler.Thermal cycling, in addition to subjecting the material to cyclictress, can initiate cracking of the less elastic external tube scalesand expose the tube base material to repeated corrosion.


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10.1.Fireside Corrosion Fatigue

Transverse viewof surface crack


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11.Short-term Overheat

Symptoms: Failure results in a ductile rupture of the tubemetal and is normally characterized by the classic fish mouthopening in the tube where the fracture surface is a thin edge.

Causes: Short-term overheat failures are most commonduring boiler start up. Failures result when the tube metaltemperature is extremely elevated from a lack of cooling steamor water flow.

A typical example is when superheater tubes have not clearedof condensation during boiler start-up, obstructing steam flow.

Tube metal temperatures reach combustion gas temperaturesof 850 deg C or greater which lead to tube failure.


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11.1.Short-term Overheat

Thin edge fish mouthrapture


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12.Long-term Overheat

Symptoms: The failed tube has minimal swelling and alongitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scalebuild-up and secondary cracking.

Causes: Long-term overheat occurs over a period ofmonths or years. Superheater and reheat superheatertubes commonly fail after many years of service, as a resultof creep. During normal operation, alloy superheater tubeswill experience increasing temperature and strain over thelife of the tube until the creep life is expended.

Furnace water wall tubes also can fail from long-term

overheat. In the case of water wall tubes, the tubetemperature increases abnormally, most commonly fromwaterside problems such as deposits, scale or restrictedflow.


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12.1.Long-term Overheat

View of tube OD at failure (creep failure)


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13.Graphitization

Symptoms: Failure is brittle with a thick edgefracture.

Causes: Long-term operation at relatively high metal

temperatures can result in damage in carbon steels ofhigher carbon content, or carbon-molybdenum steel,and result in a unique degradation of the material in amanner referred to as graphitization. These materials,if exposed to excessive temperature, will experiencedissolution of the iron carbide in the steel andformation of graphite nodules, resulting in a loss ofstrength and eventual failure.


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13.1.Graphitization


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14.Dissimilar Metal Weld(DMW) Failure

Symptoms: Failure is preceded by little or no warning oftube degradation. Material fails at the ferritic side of theweld, along the weld fusion line. A failure tends to becatastrophic in that the entire tube will fail across thecircumference of the tube section.

Causes: DMW describes the butt weld where an autenitic(stainless steel) material joins a ferritic alloy, such asSA213T22, material. Failures at DMW locations occur on theferritic side of the butt weld.

These failures are attributed to several factors: highstresses at the austenitic to ferritic interface due to

differences in expansion properties of the two materials,excessive external loading stresses and thermal cycling,and creep of the ferritic material.

As a consequence, failures are a function of operatingtemperature .


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14.Dissimilar Metal Weld(DMW) Failure


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15.Erosion

Symptoms: Tube experiences metal loss from the ODof the tube. Damage will be oriented on the impactside of the tube. Ultimate failure results from rupturedue to increasing strain as tube material erodes away.

Causes: Erosion of tube surfaces occurs fromimpingement on the external surfaces. The erosionmedium can be any abrasive in the combustion gasflow stream, but most commonly is associated withimpingement of fly ash or soot blowing steam.

In cases where soot blower steam is the primary

cause, the erosion may be accompanied by thermalfatigue.


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15.1.Erosion

Erosion ontube OD


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boiler tube leakages