Boiler Life Assesment 11

PEER-REVIEWED MAINTENANCE

Boilers and other installations operat-ing at high pressures and tempera-tures are designed for a finite life [1].Thislimitation is due to in-service degradation(i.e., damage that occurs throughout itsyears of operation). For a chemical recov-ery boiler, the typical, ongoing damagemechanisms that will limit the units lifeare summarized TTaabbllee II.

The design life of a boiler often is notopenly specified.Many designers and spe-cialists consider 25 to 30 years to be a rea-sonable estimate for the life of water tubeboilers and for the power plants of whichthey are part [2]. Practical experience,however, shows that the real durability ofsuch installations, in most cases, is muchgreater than this estimation.This is espe-cially true with the advent of new materi-als and improved operational and mainte-nance practices.Thus, a life of 40 years ormore becomes a technically reasonableexpectation for a recovery boiler,which isobviously very desirable from the eco-nomic point of view.

To achieve this life extension, a carefulengineering evaluation is carried out todetermine any necessary repairs andreplacements required.The boilers oper-ations, maintenance and inspection histo-ry, and its current condition, are consid-ered in projecting its remaining life.

Brazilian regulations (the NR13 safetystandard) specify that every boiler shouldundergo a structural integrity evaluationand remaining life assessment upon com-

BASIC PREMISES FORSTRUCTURAL INTEGRITY

EVALUATION (SIE)Basic Requirements

The objective of a structural integrityevaluation (SIE) is to enable a safe con-tinuation of the units operating life, gen-erally beyond the period first intended.Of course, there is not a single formulafor conducting such investigations.However, some basic requirements mustbe taken into consideration in these pro-cedures.

Annual inspections do not fullyaddress such requirements.An SIE differsfrom routine inspections in the followingways:

It investigates time-dependant degradation mechanisms that occurin the mid- and long-term, such asfatigue, creep, metallurgical degradation, and water chemistryeffects.These mechanisms must beclearly differentiated from those acting faster or those more relatedto incidents or to accidental and circumstantial factors.

It investigates in greater depth theintegrity of parts that do not usuallyreceive such attention during periodic inspections due to lack oftime and difficulty in access: headers,drums, attemperators, main steamlines, structural elements, etc. (seeFFiiggss.. 11--33).

pleting 25 years in service [3].The NR13,however, is a general requirement for alltypes of steam generators and does notprovide technical guidance. This paperprovides additional information and rec-ommendations for conducting integrityassessments, specifically focusing onchemical recovery boilers, with the aimto assist pulp mills to fulfill this importanttechnical and legal requirement.

TIMING FOR STARTINGEVALUATIONS

Under normal conditions, integrity evalu-ations and remaining life assessmentsshould occur:

On completion of 80% of the designlife, or

On completion of 25 years (the legaltime period in Brazil, which under nocircumstances should be exceeded)[4].

Evaluations should occur sooner in thefollowing cases:

Operation above nominal capacity Excessive cycling (stop and start)

and/or variation of load History of stops and starts faster than

those anticipated by the design Known evidence of accumulated

damages History of accumulated damages on

similar units [5].

Guidelines for integrity evaluation and remaining life assessment of recovery

boilers CENIBRAs experience FLVIO A. PAOLIELLO

ABSTRACT: The need for integrity evaluations and remaining life assessments of boilers arises fromtechnical, economic, and legal reasons. The pulp and paper industry has less experience and tradition in usingthis engineering discipline when compared to other industries such as oil, petrochemical, and power gener-ation. The pertinent Brazilian legal standard (NR-13) is concise in establishing this requirement, without pro-viding details of applicable technical procedures. Furthermore, recovery boilers are a special type of steamgenerator, being very specific as to their inherent in-service degradation mechanisms and inspection needs.

In view of the above scenario, pulp and paper mill engineers often have doubts and encounter diverse inter-pretations of the official regulations when they need to carry out integrity evaluations of quarter-century oldrecovery boilers. This paper relates CENIBRAs recent experience in evaluating its recovery boiler No. 1.

AApppplliiccaattiioonn:: This paper may serve as a guide for other pulp mills and help answer questions about theprocess of integrity evaluation and remaining life assessment of aged recovery boilers.

VOL. 4: NO. 2 TTAAPPPPII JJOOUURRNNAALL 17

18 TTAAPPPPII JJOOUURRNNAALL FEBRUARY 2005

MAINTENANCE

It evaluates the integrity of each boiler component individually, todetermine the susceptibility of eachone to the corresponding degradation mechanisms, indicatingthose whose useful life will expirebefore the others.

It evaluates metallurgical aspects,mostly in high temperature parts, toidentify nonvisual damage mechanisms that may not be indicated by changes in wall thickness. Examples of microstructuraldegradation include spheroidizing,graphitization, precipitation of carbides, and microvoids from creep.These are illustrated in FFiiggss.. 44--77.The main concern with corrosion inrecovery boilers may divert the engineers attention from this point.

It evaluates and documents the kinetics of damage evolution,quantifying corrosion rates, wear,crack growth progress, etc.

It focuses attention on thesteam and water side,investigating the units historyin relation to possible waterside contamination (e.g.,with liquor) and evaluatingpossible consequences.

It investigates possible alterations in behavior ofstructural elements, such asincreases in hysteresis of elastic supports of steam piping.

It takes operations above ratedcapacity and excessive cyclinginto consideration.

SIE should encompass morethan the legally required evalua-

tion of the boiler itself. Common senseand best practices dictate that SIEsshould extend to peripheral and ancillaryelements closely associated with the boil-er,especially those subjected to high tem-peratures, such as the main steam line,headers, and accumulators, up to the tur-bines inlet flange. Additionally, the millshould evaluate the integrity of the deaer-ator, feed water tank, feed water line, dis-solving tank, and other support systems(see FFiigguurree 88).

SIE conceptual approachesMills vary in their approach to conduct-ing SIEs, their objectives, and the criteriathey may use to assess in-service dam-ages and degradation. The two mainapproaches are as follows:

TThhee PPrroojjeecctt CCooddee aapppprrooaacchhTheevaluation of used and degraded equip-ment, exclusively from a design codespoint of view (American Society ofMechanical Engineers, for example) isthe simplest and most traditionalapproach. It applies the same design andmanufacturing criteria for evaluating theboilers condition and damages resultingfrom service.Through testing and repairsand replacements, this approach aims torestore the boilers condition and reliabil-ity to as good as new.

TThhee FFiittnneessss--ffoorr--SSeerrvviiccee aapppprrooaacchhThis newer approach evaluates theequipments fitness-for-service, in spite ofaccumulated damages, in its degradedcondition.Analytical techniques to estab-lish the possibility to continue operatingwith existing defects are applied (takinginto consideration the flaws dimensions,shape, localization, evolution, kinetics,

I. Factors that limit the life of chemical recovery boiler components.

HHiigghh MMiiccrroo--DDaammaaggee TTeemmpp.. SSttrruuccttuurraall

MMeecchhaanniissmmss CCoorrrroossiioonn FFaattiigguuee EErroossiioonn OOxxiiddaattiioonn CCrreeeepp DDeeggrraaddaattiioonn Furnace

Drums Economizer Boiler bank

Superheaters High temperature

headersNon-refrigerated parts

exposed to flue gasMain steam line

Structures Dissolving tank

Deaerator

1. Inspection and non-destructivetesting of lower waterwall header.Investigation as to ligament cracking.

2. Steam drum without its insulationto enable detailed examination andtesting.

3. Access provided for inspection ofelements in the interior of penthouseand other cold chambers in therecovery boiler.


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etc.) The evaluation may include stressanalyses, calculations through fracturemechanics, and use of API 579 andBS:7910 standards. Among other bene-fits, this approach seeks to minimizerepairs and replacements, without com-promising reliability.

The second approach, already consoli-dated in the power generation, oil, andchemistry industries, has also gainedincreasing recognition and acceptance inother industries. Both philosophies men-tioned, and others, are acceptable fromthe technical point of view and recog-nized by legislation. It is up to the boilerowner to evaluate it and other approach-es in terms of cost, time, availability oftechnical resources, company culture,and other factors.

PROCEDURES FOR EVALUATING STRUCTURAL

INTEGRITY AND REMAINING LIFE

Integrity evaluations involve complexengineering work that must be carriedout by extremely specialized and quali-fied professionals. While specific proce-dures may vary, the evaluation is usuallyconducted in three distinct phases:

Phase 1: Preliminary EvaluationOObbjjeeccttiivveess Define physical limits, or the boiler

island, within which analyses will becarried out.

Collect information on the equipment (scrutinize the units operational, maintenance, and inspection history, and its design data).

Identify in-service degradation mechanisms taking place in the boiler.

Identify the boilers critical parts or zones, and which damage mechanisms affect them.

Analyze the collected information anddetermine whether available data aresufficient to establish the units structural condition.

Plan inspection and additional non-destructive testing (NDT), as needed.

Phase 2: Non-destructiveTesting

OObbjjeeccttiivvee To supplement data collected in Phase

1 (which in extremely rare cases aresufficient per se), verify through additional inspections and NDTs the

presence of accumulated in-servicedamage, defining its extent andresponsible mechanisms.

TTaabbllee IIII lists resources that are usuallyapplicable in this phase, with examples,some of which are illustrated in FFiiggss.. 99--1122 [6].

Phase 3: complementary analysis

GGooaallss Analyze data collected in the

previous phases.

4. Micro-structural gradient of ferric material subjected to high temperature (typically superheaters). These figures aremerely illustrative.

5. Structure showing microvoids fromcreep. Illustrative figure.

6. Preparation for field metallographyin the main steam line.

7. Preparation for field metallographyin a high pressure steam header.

8. Preparation for inspection and non-destructive testing in the deaeratortank saddle. Simultaneously, the ves-sel itself had been tested internally bywet fluorescent magnetic particles.


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Confirm the hypotheses formulatedduring Phase 2, using complementarytests to samples taken from the equipment, double-checks, actualstress analysis, etc.

Reach final evaluation of the structural condition and the remaining life.

A final report must then be issued,containing all the information regardingthe evaluation: the current structuralcondition and remaining life of each boil-er component; recommended proce-dures for future similar evaluations; pos-sible recommendations for repairs orreplacements, re-rating of components,possible process changes; calculations;and all applicable planning.

RECOMMENDATIONS FORNEW BOILERS

The procedures addressed in this paperare mainly intended for owners of olderboilers, especially those approaching the

II. Non-destructive tests that may be used as part of system integrity evaluations of recovery boilers.

NNoonn--DDeessttrruuccttiivvee TTeesstt

Dye penetrant

Magnetic particle

Ultrasonic thickness surveys

Ultrasonic for flaw detection

Field metallography (direct or replication)

Dimensional examinations

Hardness readings

Internal Rotary Inspection System(I.R.I.S.), near drum inspection

Endoscope inspection

Destructive testing

Digital radiography

Load analysis in pipelines supports

UUssuuaall LLooccaattiioonnss

Composite tube furnaces, structures,membranes, spacers, welded attachments

Weld seams of drums and headers

Tubing, headers, drums, deaerator

Weld seams of drums and headersand other thick-walled elements

High temperature tubing and headers,furnace tubing

High temperature tubing and headers

High temperature tubing and headers

Boiler bank

Attemperators (sometimes also calleda de-super heater), economizer, head-ers, downcomers

As necessary

Superheaters, lower bends

Live steam piping

NNootteewwoorrtthhyy SSIIEE AApppplliiccaattiioonnss

Investigation of cracking on furnaceopenings

Investigation of ligament cracking inheaders and drums

Remaining life estimations in thelight of corrosion rates

Investigation of creep, micro-structural degradation, localized over-heating, surveillance of knownflaws, etc.

Investigation of creep

Complementary investigation ofmicrostructural degradation

Investigation of near mud drum corrosion [7]

Investigation of changes in the support reactions of the piping [8]

11. Equipment for I.R.I.S. testing, beingused to inspect tubes of the boilerbank. The corrosion of tubes in theregion near the lower drum (known asnear mud drum corrosion) in a recov-ery boiler is a typical concern.

9. Non-destructive testing (magneticparticles) in progress in the waterdrum of a recovery boiler.

10. Preparation for non-destructivetesting (ultrasonic and magnetic par-ticles) of a crossover pipe welding.


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legally established time for starting theintegrity evaluation. However, the discus-sion and suggestions also may be usefulto those dealing with new boilers, andeven for projects in the early stages ofspecification and procurement for afuture unit.

The following suggestions may behelpful to owners of newer boilers, orthose specifying or procuring a futureunit:

Require that the vendor who is offering a new boiler provide recommendations or guidelines forfuture SIE of the equipment beingsupplied.

Keep samples of all materials of construction of a newly erected boiler, as a reference for future comparison as to micro-structuraldegradation and creep.

Conduct dimensional examinationsof the high temperature tubes beforethe initial start-up as a reference forfuture comparison as to creep.

Consider installing instrumentationto monitor the temperature of thehottest and most critical parts of thefuture boiler.

Perform an early start of the SIEprogram, collecting data of interestand recording relevant informationthrough the life of the boiler fromthe beginning of its operations.

FINAL CONSIDERATIONSAn integrity evaluation is not just anextensive program of inspections andtesting, or life-time projections, althoughelements of those are markedly present.It is rather an ongoing, multi-disciplinaryprogram that should be established fromthe beginning of the boiler life cycle. Itmust be customized for each individualplant, taking into consideration its actualstatus as to accumulated damage. Thus,the boiler owner should be cautiousabout vendors offering a predetermined,standardized program of inspections.The integrity evaluation also presumes aclose participation and co-operation ofthe boiler owner and the professionalscommissioned to do the evaluation.

The following are some of the benefitsafforded by integrity evaluations andremaining life assessment programs:

Establishment of a base line of theboilers structural condition

Increased safety for personnel andfacilities

Increased availability of the boiler atmaturity

Economic flexibility, allowing themill more time to explore replacement and repair options, andpossibly postpone investments

Good will through a positive imageof the mill, associated with safety andproper engineering practices

Training for the mills engineers Legal compliance.

AAuutthhoorrss NNoottee:: the information and rec-ommendations contained in this paperreflect the authors best knowledge andbelief at the time of writing. However,the text should serve only as a generalreference to the reader. No warrantiesare offered or responsibilities taken thatthe information provided is perfectlyadequate or sufficient for the purpose inquestion. TJ

LITERATURE CITED1. Singer, J.G., Combustion, Fossil

Power. ABB CombustionEngineering, Windsor,Connecticut, 1991.

2. French, D.N., MicrostructuralDegradation. The National Boardof Boiler and Pressure VesselInspectors web site, ClassicArticles, available online atwww.nationalboard.org/classics.

3. Manual Tcnico de Caldeiras eVasos de Presso, Ministrio doTrabalho, Braslia, 1996.

4. Procedimento Geral de Avaliaode Vida Restante de ComponentesEstticos Submetidos a AltaTemperatura. Instituto deSoldadura e Qualidade, Lisboa.

5. Prtica Recomendada para Anlisede Danos Acumulados em Serviopor Equipamentos Pressurizadosque Operam a Alta Temperatura,Recomendao Geral paraInspeo de Equipamentos queOperam em TemperaturasElevadas, Associao Brasileira deCincias Mecnicas, Rio deJaneiro, 1992.

6. Babcock & Wilcox ConditionAssessment Services. Available atwww.babcock.com.

7. Adams, T.N., Frederick, W.J., Grace,T.M., et al, Kraft Recovery Boilers,TAPPI Press, Atlanta, 1997.

8. Caldeira, R. B., Relatrio deAvaliao de Integridade e VidaRemanescente de Caldeira deRecuperao 1, CENIBRA. Institutode Soldadura e Qualidade - ISQBrasil, Belo Horizonte, 2002.

This paper was originally published in theJune, 2004 issue of O Papel, the officialpublication of the Brazilian TechnicalAssociation of Pulp and Paper (ABTCP). It isbeing reprinted by TAPPI JOURNALthrough a cooperative agreement betweenTAPPI and ABTCP.

This paper is also published on TAPPIsweb site and

summarized in the February Solutions!for People, Processes and Paper

magazine (Vol. 88 No. 2).

12. Ultrasonic thickness survey on astudded tube of a recovery furnace.

13. CENIBRAs recovery boiler No. 1.This unit had its first start-up in 1977,and underwent an integrity evalua-tion program in 2002, according tothe guidelines described in this paper.


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INSIGHTS FROM THE AUTHORSFacing the need to conduct an integrity evaluation andremaining life assessment program to a recovery boil-er in the mill I work for, I soon realized there was limit-ed information available about this subject within theBrazilian pulp industry, in spite of its importance. Also,I found diverse interpretations (by previous users andinspection services suppliers) of the official require-ments, and no written guidelines with a specific focuson recovery boilers. That prompted me to write this arti-cle.

Brazilian official standards require that boilers under-go an integrity evaluation at 25 years. But they give notechnical guidance and do not highlight the specialneeds and risks associated with recovery boilers. Thispaper is therefore intended to help provide mill engi-neers with some complementary support when theycarry out such programs.

The most difficult aspect of preparing for an integrityevaluation, and in writing this paper, is the lack of ref-erences and successful past experiences in Brazilianmills. I searched the procedures used in other indus-

tries, and by pulp mills abroad, then adapted them toour reality.

Through my research, I was particularly impressedby the wide array of available engineering disciplinesand testing techniques that now can help engineersevaluate the conditions of a used recovery boiler, giv-ing an excellent level of confidence in the results ofsuch evaluations.

Some Brazilian mills are systematizing and improv-ing their procedures in evaluating the integrity of agedrecovery boilers. This paper, while not intended to be acomplete or definitive guide, could be a first steptowards this improvement.

With time, more recommendations and aspects per-taining specifically to recovery boilerin-service evaluations should be addedto the contents of this paper, making itmore and more comprehensive. Flvio A. Paoliello

Paoliello is a maintenance engineer withCENIBRA, Belo Oriente, MG, Brazil. EmailPaoliello at [email protected]. Paoliello

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Boiler Life Assesment 11

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