Fallas Boiler (Babcock and Wilcox)

8/12/2019 Fallas Boiler (Babcock and Wilcox)

1/8

a McDermott company

Boiler Tube Analysis

Reduce FutureBoiler Tube Failures


2/82

Look to the Industry Leader for Comprehensive Tube Assessment

Boiler tube fai lures conti nue to be the leadi ng cause of forced outages in fossi l-fi red boilers. To get your boiler back online an d red uce or eliminate future forced outages due to tub e failure, it is extremely important to d etermineand correct the root cause. Experience shows that a comprehensive assessment is the most effective methodof determining the ro ot cause of a failure.

With mo re th an 125 years experience d esigning, fabr icating, b uilding a nd servicing boilers, Babcock &Wilcoxis a recognized industry leader and a logical source for tube failure evaluation.

A tube failure is usually a symptom of other problems. In addition to evaluating the failure itself, youshould investigate all aspects of boiler operation leading to the failure to fully understand the cause.Babcock & Wilcox can assist you in this full-scope investigation with our experienced field service engineershelping to gather a ll the pertinent information. In many cases, the field investigation can isolate the ro otcause tha t led to the tube fa ilure.

World-class Research Center

For tube sample analyses, B&W candraw on the experience and capabilitiesof o ur world-class ISO certified AllianceResearch Center (ARC), Alliance, Ohio.We have metallurgical an d chemicalengineering expertise to complementour knowledge of boiler design andoperat ion. At the ARC, we analyzehundreds of samples every year a ndprovide a full range of failure analysesand material evaluation services.

Materials can be examined atmag nifi catio ns as great as 100,000Xusing our Scanning ElectronMicroscope (SEM). In conjunctionwith the SEM, our electron probemicroanalysis capability allows analysisof chemical elements on the tube

surfaces which a ids in root causeevaluation. D eposits, whether o nthe water side or ga s side of the tube,also can be an alyzed at the ARCusing X-ray diffraction and massspectroscopy tech niq ues.

ScanningElectron Microscope

A chemist tests deposits removed from boiler tubesusing X-ray dif fraction.

Al liance Research Center


3/83

Caustic Attack

Symptoms:Localized wall loss on the inside diameter (ID) surface of thetube, resulting in increased stress and strain in the tub e wall.

Causes:Caustic attack occurs when th ere is excessive d eposition on ID tubesurfaces. This leads to diminished co oling water f low in con tact with thetube, which in turn causes local under-deposit boiling and concentration

of bo iler water chemicals. If combined with boiler water chemistry upsetsof h igh pH , it results in a caustic cond ition which co rrosively attacks andbreaks down protective magn etite.

Oxygen Pitting

Symptoms:Aggressive localized corrosion and loss of tube wall, mostprevalent near economizer feedwater inlet on operating boilers. Floodedor n on-drain able surfaces are most susceptible during outage periods.

Causes:Oxygen pitting occurs with the presence of excessive oxygen inboiler water. It can occur durin g opera tion as a result of in-leakage of airat pumps, or failure in operation of preboiler water treatment equipment.

This also may occur during extended out-of-service periods, such as outagesand storage, if proper proced ures are not fo llowed in lay-up. Non-drain ablelocations of bo iler circuits, such as superheater loo ps, sagging horizonta lsuperheater and reheater tubes, and supply lines, are especially susceptible.

More gen eralized oxidation of tubes during idle periods is sometimesreferred to as out -of-servi ce corrosion. Wetted surfaces are subject tooxidation as the water reacts with the iron to form iron oxide.

When corrosive ash is present, moisture on tube surfaces from cond ensationor water washing can react with elements in the ash to form acids that lead toa much more a ggressive attack on metal surfaces.

Hydrogen Damage

Symptoms:Intergran ular micro -cracking. Loss of d uctility or embrittlementof the tube material leading to brittle catastrophic rupture.

Causes:Hydrogen damage is most commonly associated with excessivedeposition o n ID tube surfaces, coupled with a boiler water low pHexcursion. Water chemistry is upset, such as what can occur from condenserleaks, particularly with salt water cooling medium, and leads to acidic(low pH) contaminants that can be concentrated in the deposit.Un der-deposit corrosion releases atomic hydrogen which migrates intothe tube wall metal, reacts with carbo n in the steel (decarburization)and causes intergranular separation.

Causti c attackat backing ring

Oxygen pi tti ngon tube ID

Bri ttle fai lu re dueto hydrogen damage

Finding the Root Cause is Critical

Have you ever repaired a tube leak and put the boiler back in service, only to be forced off-line byanother leak? Identifying and correcting the root cause is essential. Shown on the following pagesare some of th e failure mech an isms found in fossil boiler tubes. When you see tubes in your b oilerlike those illustrated, take advantage of B&Ws tube expertise to help you d etermine a nd eliminatethe root cause of the problem. Better yet, let us assist you in putting together a complete conditionassessment program to help you find tube problems before failures occur.


4/8

Acid Attack

Symptoms:Corro sive attack of the intern al tube metal surfaces, resulting in anirregular pitted o r, in extreme cases, a swiss cheese a ppearance of th e tube ID .

Causes:Acid attack most commonly is associated with poor control of processduring boiler chemical cleanings and /or inadeq uate post-cleaning passivationof residual acid.

Stress CorrosionCracking (SCC)

Symptoms:Failures from SCC are ch aracterized by a thick wall,brittle-type crack. May be found at locations of higher external stresses,such as near attachments.

Causes:SCC mo st comm only is associated with austenitic (stainless steel)superheater materials and can lead to either transgranular or intergranularcrack propagation in th e tube wall. It occurs where a combination ofhigh-tensile stresses and a corrosive fluid are present. The damage resultsfrom cracks that propagate from th e ID. The source of corrosive fluid m ay becarryover into the superheater from the steam drum or from contamina tion

during b oiler acid cleaning if the superheater is not pro perly protected.

Waterside Corrosion Fatigue

Symptoms:ID in itiated, wide tran sgranular cracks which typically occuradjacent to external attachments.

Causes:Tube dam age occurs due to the combina tion of therm al fatigue andcorrosion. Corro sion fatigue is infl uenced by boiler design, water ch emistry,boiler water oxygen content and boiler operation. A combination of theseeffects leads to the breakdown of the protective magnetite on the ID surfaceof th e bo iler tube. The lo ss of this protective scale exposes tube to corr osion.The location s of attachm ents and external weldmen ts, such a s buckstayattachm ents, seal plates and scallop ba rs, are most susceptible. The p roblemis most likely to progress during boiler start-up cycles.

Superheater Fireside Ash Corrosion

Symptoms:External tube wall loss and increasing tube strain. Tubescommon ly have a pock-marked appeara nce when scale an d corrosionproducts are removed.

Causes:Fireside ash corrosion is a function of the ash characteristics ofthe fuel and boiler design. It usually is associated with coal firing, but alsocan occur for certain types of oil firing. Ash characteristics are consideredin th e boiler d esign when establishing the size, geometry an d materialsused in the boiler. Combustion gas and metal temperatures in the convectionpasses are important considerations. Dama ge occurs when certain coal ashconstituents remain in a molten state on the superheater tube surfaces.This molten ash can be highly corrosive.

High-temperature Oxidation

Similar in appearance and often confused with fireside ash corrosion,high-temperature oxidation can occur locally in areas that have the highestoutside surface temperature relative to the oxidation limit of the tube material.Determining the actual root cause between the mechanisms of ash corrosionor h igh-temperature o xidation is best do ne by tube a nalysis and evaluation ofboth ID and OD scale and deposits.

1.561"0.063"

1.583"0.087" 1.573"

0.055"

1.556"0.136"

0.160"

0.175"0.183"

0.116"

4

Tube Wall

Nickel Platin g

Tube Inside Diameter

Pitted appearance ofinternal tube caused

from acid attack

SEM photo of stresscorrosion cracking

Corrosion fati gueon tube ID adj acent

to attachment

Sectional photo oftube wi th severe wal lloss from fireside ash

corrosion

Surface appearance ofmetal showing fireside

coal ash corrosion


5/85

Waterwall Fireside Corrosion

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

Causes:Corro sion o ccurs on extern al surfaces of waterwall tubes when thecombustion process produ ces a reducing atmosphere (substoichiometric).This is commo n in the lo wer furnace of pr ocess recovery boilers in the pulpand paper ind ustry. For conventional fo ssil fuel boilers, corrosion in th eburner zone usually is associated with coal fi ring. Bo ilers having malad justed

burners or operating with staged air zones to control combustion can bemore susceptible to larger local region s possessing a reducing atmosphere,resulting in increased corro sion ra tes.

Fireside Corrosion Fatigue

Symptoms:Tubes develop a series of cra cks tha t initiat e on t he o utsidediameter (OD ) surface and propagate into the tube wall. Since the damagedevelops over longer periods, tube surfaces tend to d evelop appeara ncesdescribed as elephant hide, alligator hide or craze cracking. Mostcommo nly seen a s a series of circumferen tial cracks. Usually found o nfurnace wall tubes of coal-fired o nce-throug h bo iler designs, but also ha soccurred on tubes in d rum-type boilers.

Causes:Damage initiation and propagation result from corrosion in

combination with thermal fatigue. Tube OD surfaces experience thermalfatigue stress cycles which can occur from norm al sheddin g of slag,sootblowing or fro m cyclic operatio n o f the boiler. Therma l cycling, inadd ition to subjecting the material to cyclic stress, can initiate crackingof th e less elastic external tube scales and expose the tube ba se materialto repeated corrosion.

Short-term Overheat

Symptoms:Failure results in a ductile rupture of th e tube metal a nd isnormally characterized by the classic fish mouth opening in the tubewhere the fracture surface is a thin edge.

Causes:Short-term overheat failures are most common during boiler startup. Failures result when the tube meta l temperature is extremely elevatedfrom a lack of coo ling steam or water fl ow. A typical example is whensuperheater tub es have not cleared of cond ensation dur ing boiler start-up,obstructing steam f low. Tube meta l temperatures reach combustion g astemperatures of 1600 F or greater which lead to tube fa ilure.

Long-term OverheatSymptoms:The failed tube has minimal swelling and a long itudinal split thatis nar row when com pared to short-term overheat . Tube meta l often h as heavyexternal scale build-up and secondary cracking.

Causes:Long -term overheat occurs over a period of mo nths or years.Superheater and reheat superheater tubes commonly fail after many yearsof service, as a result of creep. During n ormal op eration, alloy superheatertubes will experience increasing temperatu re and strain o ver the life ofthe tube un til the creep life is expended . Furnace water wall tubes also canfail from long-term overheat. In the ca se of water wall tubes, the tub etemperature in creases abno rmally, most common ly from waterside problemssuch a s deposits, scale or restricted fl ow. In the ca se of either superheateror water wall tubes, eventual failure is by creep rupture.

Fireside corrosionof studded tu be

Craze crackingof OD sur face

Transverse vi ewof sur face crack

Thin-edged fishmouth rupture

View of tube ODat fail ure(creep fai lu re)


6/8

Graphitization

Symptoms:Failure is brittle with a thick edge fra cture.

Causes:Long-term operation at relatively high metal temperatures can resultin dam age in carbon steels of higher carbo n conten t, or carbon-molybdenumsteel, and result in a unique degradation of the material in a manner referredto as graphitization. These materials, if exposed to excessive temperature,will experience dissolution of th e iron carbide in the steel and form ation ofgraphite n odules, resulting in a loss of strength and eventual failure.

Dissimilar Metal Weld(DMW) Failure

Symptoms:Failure is preceded by little or no warning of tube degradation.Material fails at the ferritic side of th e weld, along th e weld fusion line. A failuretends to be catastrophic in that th e entire tube will fail across the circumferenceof the tube section.

Causes:DMW describes the butt weld where an autenitic (stainless steel)mat erial joins a fe rritic a lloy, such a s SA213T22, materia l. Failures at D MWlocations occur on the ferritic side of th e butt weld. These failures areattributed to several factor s: high stresses at the austenitic to ferritic inter facedue to d ifferences in expansion p roperties of the two materials, excessiveexternal loading stresses and thermal cycling, and creep of the ferriticmaterial. As a consequence, failures are a function of operating temperaturesand unit design.

Erosion

Symptoms:Tube experiences metal loss from the OD of the tube. Dam agewill be oriented on th e impact side of the tu be. Ultimate fa ilure results fromrupture due to increasing strain as tube material erod es away.

Causes:Erosion of tube surfaces occurs from impingement on the externalsurfaces. The erosion med ium can b e any abra sive in the co mbustion gasfl ow stream, but most common ly is associated with imping ement o f fly ashor soot blo wing steam. In cases where soot blower steam is the primar y cause,the erosion may be accompanied by thermal fatigue.

Mechanical FatigueSymptoms:Dam age m ost often results in a n OD initiated cra ck. Tends tobe localized to th e area o f high stress or con straint.

Causes:Fatigue is the result of cyclical stresses in the component. Distinctfrom th ermal fatigue effects, mechanical fatigue dam age is associated withexternally applied stresses. Stresses may be associated with vibration dueto fl ue ga s flow o r sootblowers (high -frequen cy low-amplitude stresses),or th ey may be associated with boiler cycling (low-freq uency high-amplitudestress mechanism). Fatigue fa ilure most often occurs at areas of con straint,such as tube penetra tions, welds, attach ments or supports.

6

Failu re ofcarbon steel tube

at butt weld

Photomicrographshowing

graphitization

DMW fail ure whereferr i tic material hascompletely separated,leaving the DMW

Erosion ontube OD

Mechanical fatiguefail ure at

an attachment

Photomicrographshowing DMWcreep voids at

ferr it ic interface


7/87

We can help you d etect, ana lyze andcorrect the problems leading to tube

failures. B&W has developed a line oftube inspection services to aid you inevaluating your boilers tubing. Thesepatented techniques have a proventrack record of success in helping youidentify tubes that may lead to failures.

NOTIS

Nondestr uctive Oxide Thi cknessInspection Servi ce

An ultrasonic ND E test coupledto a computer model enables thecalculation of remaining creep lifefor steam cooled superheater and

reheater tubes.

FHyNES

Furnace wal l H ydrogen damageNondestr uctive Examinati on Servi ce

This ultrasonic test utilizes multipletransducers to scan tubes forattenuation due to hydrogen damage.

A chemist measures the corrosion rate of a materi al usingalternating-current impedance.

Fast-Scanni ng T hickness(FST-Gage)detectswall loss and in ternal tube damage.

MANTIS

Modular Automated Nondestructive

Thi ckness Inspection Servi ceThe marriage of electronically acquiredultrasonic th ickness measurements withcomputer-aided da ta managemen tand ana lysis makes evaluation of wallthickness straightforward and effi cient.

FST-GAGETM

Fast-Scanni ng Thi ckness Gage

Our newest testing service usestechnology developed by B&Won aproject sponsored by the ElectricPower Research I nstitute (EPRI).

The FST-GAGE is an EMAT-based(ElectroMagnetic Acoustic Transducer),nond estructive examination techniquethat en ables rapid scanning of boilertubes to detect wall loss and internaltube damage.

Cleaning Your Boiler

When the elimination of tube damage

requires chemical cleaning, B&Wswater chemistry and d eposit removalexpertise can help. The chemistrysection at th e Alliance ResearchCenter can perform cleaning tests todetermine the best chemicals andmethods to clean your boiler tubeswithout h arming the tubes themselves.Our water chemistry field specialistthen can pr ovide th e expert, on -siteproject management capability toensure a successful cleaning.

Corrosion Assessment

If the conditions leading to corrosivedamage a re not clear, then thecorrosion section of the ARC can becalled in to h elp. Extensive labora toryfacilities are available to simulateboiler cond itions and identify causativecond itions. These simulations canbe performed for bo th fireside andsteamside applications.

Put B&Ws Team to Work for You


8/8

a McDermott companya McDermott company

For more information, or a complete listing of our sales and service offices worldwide, call 1 -80 0-BABCOCK (22 2-2625)in North America. Outside North America, call (33 0) 75 3-4511 or fax (33 0) 86 0-1886 (Barberton, Ohio, USA). Or access ourWeb site at http://www.babcock.com.

Power ing the Wor ld Th rough Teamw ork and Innova t i onSM

The information contained herein is provided for general information purposes only and is not intended or

to b e construed as a warranty, an offer, or any representation of cont ractual or other legal responsibility.

E101-3153 3MX7F The B ab coc k &Wilcox C ompa ny. All rights rese rved.

NOTIS , FHyNES , a nd MANTIS are tradema rksand Powering the World Through Teamwork andInnovationis a se rvice ma rk of The Ba bco ck&Wilcox Company.

FS T-G AGETM is a licensed EPRI product.

Canada:Cambridge, OntarioEdmonton, AlbertaHalifax (Dartmouth), Nova ScotiaMontrea l, QuebecSa int J ohn, New Brunswick

Vancouver (Richmond), British ColumbiaCzech Republic: PragueEgypt: CairoEngland:LondonIndia:PuneIndonesia: JakartaMexico: Mexico CityPeoples Republic of China: Beijing

Taiwan: Ta ipeiTurkey:Anka raUnited States of America:

Atlanta, Ge orgiaBarberton, OhioCha rlotte, North Ca rolina

Cherry Hill, New J erseyChicago (Downers Grove), IllinoisCincinnati, OhioDenver (Sherida n), ColoradoHous ton, Texa sKansas City, MissouriSan Francisco (Vacaville), California

B&Ws goal is to meet your evolving needs inhelping to keep your steam prod uction on-line an davailable when you need it. Throug h inn ovationand teamwork, we are committed to delivering thelatest inspection technologies, as well as providingfull-scope d esign, m anufacturin g, con struction a ndservice expertise. When needed, B&Ws researchcenter can support in-depth analyses and testingto determine the root cause of your problem.These capabilities include expertise in manydiverse disciplines, including:

Analytical metallurgy Chemical cleaning and preservation

Chemical engineering Chemistry Combustion Corrosion Dynamics and fl ow-induced vibration Fluid mechanics and heat transfer Fuels and fuels analysis Materials performance and mechanical testing Measurements Mechanical d esign Nondestructive methods and diagnostics Pollution control Quality control

Stress analysis/finite element analysis Water chemistry Welding or oth er manufacturin g technolog y

B&W is Committed to Helping YouKeep YourPlant On-line

Documents

Fallas Boiler (Babcock and Wilcox)