190kW固定管板式換熱器設(shè)計(jì)6

摘要在工程中，將至少有兩種溫度不同的流體參與換熱，以一定的傳熱方式在流體間相互傳遞熱量的設(shè)備稱為熱交換熱器。換熱器種類繁多，應(yīng)用也極為普遍，尤其是管殼式換熱器，是目前工業(yè)生產(chǎn)中應(yīng)用最為廣泛的換熱器類型。本研究對(duì)象是總負(fù)荷190kW固定管板式換熱器。通過查閱各種相關(guān)文獻(xiàn)及設(shè)計(jì)標(biāo)準(zhǔn)、規(guī)范，對(duì)換熱器的發(fā)展現(xiàn)狀和趨勢(shì)、發(fā)展歷史、分類特點(diǎn)等方面進(jìn)行了解，并對(duì)換熱器設(shè)計(jì)過程進(jìn)行研究，按照換熱器設(shè)計(jì)步驟對(duì)換熱器做工藝計(jì)算、強(qiáng)度計(jì)算并進(jìn)行校核，再通過得出的結(jié)構(gòu)尺寸繪制CAD圖紙。關(guān)鍵詞：換熱器；固定管板式換熱器；工藝計(jì)算；強(qiáng)度計(jì)算Designof190kWfixed-tube-plateheatexchangerAbstractInengineering,wecalledtheheatexchangerthatthereareatleasttwokindsoffluidswithdifferenttemperaturesparticipatinginheatexchange,andtheequipmentthattransfersheatbetweenthefluidsinacertain.Thispapermainlyfocusesonthedesignoffixedtube-sheetheatexchangerwithatotalloadof190kw.Thisarticlewillusevariousrelatedliteraturesanddesignspecificationsasdesignstandardstounderstandthedevelopmentstatusandtrends,developmenthistory,classificationcharacteristicsofheatexchangers,etc.,andstudytheheatexchangerdesignprocess,andexchangetheminaccordancewiththeheatexchangerdesignsteps.Theheaterperformsthermalcalculation,structuralcalculationandcheck,andthendrawsCADdrawingsbasedontheobtainedstructuraldimensions.Keywords:Heatexchanger;fixedtube-plateheatexchanger;thermalcalculation;structuralcalculation目錄1前言 ………附錄D文獻(xiàn)翻譯原文Reviewofcommonfailuresinheatexchangers–PartI:MechanicalandelevatedtemperaturefailuresMuradAlia,AnwarUl-Hamidb,LuaiM.Alhemsb,AamerSaeedcABSTRACTThisstudyprovidesanoverviewofthecommonmodesandcausesoffailureofheatexchangertubes.Failureinvestigationsofheatexchangersreportedintheliteraturearecompiledinthisreviewbasedonthemechanismsoffailure.Alargemajorityofheatexchangercomponentsfailbywayoffatigue,creep,corrosion,oxidation,andhydrogenattack.Mostcommoncausesoffailureincludefouling,scaling,saltdeposition,welddefects,andvibration.E?ectiverecommendationstopreventsuchfailuresformanimportantpartofthisstudy.Itissuggestedthatsuitablematerialsselection,appropriatetubesdesign,effectivecontroloftheconstitutionoftheworking?uidandoperatingconditionsanduseofskilledworkforcecanprolongservicelifetimeofheatexchangers.Corrosion-relatedfailuresofheatexchangersarepresentedinarelatedarticle(PartII).Keywords:HeatexchangersFailureanalysisReviewFatigueCreepFoulingVibration1.Introduction1.1.HeatexchangerHeatexchangerisaheatmanagementsystemandakeycomponentinseveralindustries.Heatexchangersaretypicallyusedtotransferheatfromonemediumtoanothere?cientlywhilethetransferringmediumcouldbegas,steam,vapors,andvariouskindsofotherliquids.Heatexchangerscanbefoundintubularandplateforms,wheretubulartypeisusedforlarger?uidsystemsthantheplatetype.Materialsselectionforheatexchangersiscarriedoutbasedonseveralimportantconsiderationsincludingcorrosionresistance,thermalconductivity,materialstrength,andcost.Perhaps,thegreatestchallengeistoachievehighreliabilityinservice,becauseheatexchangersareexposedtoharshenvironmentsandconsiderablevariationinoperationalparameters.Heatexchangertubeshavewidespreadapplicationinautomotiveandaeronauticalindustriesaswellassteampowerplants,ammoniaplants,styreneplants,heatpipes,cooledcondensers,industrialcoolingsystems,waterpowerplants,o?shoreplatforms,desulfurizationunits,thermalequipment,fertilizerplants,ethanolvaporizers,gascompressors,nuclearpowerplants,lubricationoilcoolers,petrochemicalplants,coolingwaterunits,sulfurrecoveryunits,hydrocrackerunits,andpreheatertubes.Failurescouldoccurduetodefectsintroducedintopipesandtubingsduringthestagesofmanufacturing,handling,testing,shipment,andstoageorduringstart-up,shutdownandnormaloperationsoftheheatexchanger.Latentsurfaceorsubsurfaceimperfectionsproducedduringmanufacturingoperationscaninducefailureduringservice.However,mostfailuresoccurduetotheaggressiveenvironmentexistinginheatexchangersduringservice.Commonmodesoffailureincludefatigue,creep,corrosion,oxidationandhydrogenattack.Causesoffailurecomprisefouling,scaling,saltdeposition,welddefectsandvibrationthatcouldbebroughtaboutbyinappropriatematerialsselectionortubedesign,non-adherencetorecommendedoperatingconditionsand/orhumanerror.Thesefailuresappearintheformofcracksandleaksduetothee?ectofstressesinthecomponents,blockageduetothedepositsformedattheinnertubesurfaceand/ormaterialremovalduetocorrosionorhigh?owvelocitiesinthetubes.Avarietyofnondestructivetechniques(NDT)areusedtodetectfailuresinheatexchangersincludingvisualinspection,eddycurrent,hydrostatic,magneticparticle,ultrasonic,x-radiography,colorpenetrantandthermography[1,2].Eachofthesetechniqueshasitsstrengthsandlimitations.Topreventfailuresinheatexchangers,arangeofrecommendationsareputforwardwhichusuallyincludereviseddesignofboilercomponents,improvedmaterialsselection,re-evaluationofcorrosionprotection,anduseofe?ectivewatertreatmentandchemicalcleaningstrategies[3].Fig.1.Schematicofashellandtubeheatexchangerwithverticalba?es[3].1.2.CharacteristicsofshellandtubeheatexchangersHeatexchangersareusedtotransferheate?cientlybetweentwofluids,withorwithoutseparationofasolidwall.Theheatcouldbetransferredbetweentwofluidsthroughevaporation,condensationorevenwithoutphasechange.Whenatemperaturedi?erenceiscreated,energyintheformofheatistransferredandthisheatcanbesensibleorlatentheat.Themostcommontypeofheatexchangerusedinoilandpetrochemicalindustriesisshellandtubeheatexchanger[4].Themainpartsofacommonshellandtubeheatexchangerareshell,tubes,ba?es,andtube-sheets.Thebasicfunctionofthetubesistotransferheatfromthefluidontheshellsidetothefluidinsidethetubeandviceversa.AschematicdiagramofatypicalshellandtubeheatexchangerispresentedinFig.1.Scalingresistance,wallthicknessandthermalconductivityofthetubematerialareconsideredessentialdesignparametersforheatexchangers.Theinternalpressureoftubesprovidestheminimumresistancetoexternalloadings,thereforemostly,tubeswithhighyieldstrengthandultimatetensilestrengthwhichallowhighpermissiblestressesarenotusedintheindustry.Duringthemanu-facturingofheatexchangers,highsti?nessanddentingresistanceofthetubesisensured.Sti?nessdependsonelasticmodulus,diameter,andthicknessoftubeswhiledentingresistancedependsonthesamefactorsplusmaterialyieldstrength.Finnedtubingrequireshighlyductilematerialtomakesuretubescanbeproperlyextruded.Tominimizefailures,thetubingmaterialanddesignmustpossessadequateresistancetoscaling,overheating,weldingirregularities,corrosionfatigueandoxygenattackinworkingornon-workingcondition.Optimizedoperationalconditions,e?cientdesignandresistancetoenvironmentaldegradationareusedtocombatfailuresinservice.Thepurposeofthisworkistoreviewthepossiblemodesandcausesoffailureinheatexchangertubes.Thistypeofstudywillhelpplantoperatorstorecognizeandunderstandtubefailuresandadoptrecommendationstominimizedowntime,unexpectedshutdown,materialwasteandcostofproduction.2.Modesoffailure2.1.FatigueSimultaneousactionofacorrosiveenvironmentandcyclicstressescaninducefailurebycorrosionfatigue.Repetitiveloadappliedtotheheatexchangerintheformofthermalandmechanicalstressesresultsintubefailureduetocracking.Corrosionfatigueoccursinmetalsundertheactionofdynamicstressesinanycorrosiveenvironmentwhilestresscorrosioncrackingtakesplaceunderstaticstressesinaspecificchemicalenvironment.Materialssusceptibletocorrosionarealsopronetocorrosionfatigue.Thetwomaincausesoffatiguearethermaloverloadingandmechanicaloverstressing.Inthefirstcase,hightemperatureisresponsibleandinthesecondcase,highmechanicalstressesarethecauseoffailure.Incaseofhighmechanicalstresses,residualstressesareproducedinmaterialsduringbendingandrollingbutcanbeminimizedbypost-weldheattreatment.Microstructuretransformationandporositycouldalsoleadtofatigue.Fig.2.Circumferentialcracksandbulgedregion\h[5].Thermalfatigueisdestructivetoheatexchangersandcouldquicklycausefailure[5,6].Failureofabayonettypeboilertubesoperatinginarichenvironmentofhydrogen,carbonmonoxideandnitrogenwasinvestigated[5].ThetubesmadeofASTMA213gradeT-11operatedinamediumthatconsistedofwaterinsidethetubesandprocessgasattheshellsideinanammoniaplant.Theprocessgaswasusedat960°Cwhilesteamwasgeneratedat306°Cand1500psigpressureintheheatexchanger.Theinvestigationrevealedthattherootcauseoffailureoftubeswasthermalfatiguethatwascausedbyoscillationintemperaturebecauseofpoorwatercirculation.Thermalstressesweredevelopedresultinginfatiguethatwasfollowedbywaterleakage.Duringtheexamination,circumferentialcracksacrossthetubeaxiswerefound,thusconfirmingthermalfatiguefailureoftubesasdepictedinFig.2.Inadditiontothis,waterleakagecausedinsu?cientcoolingandariseintemperatureresultedinbulgingandformationoflongitudinalcracksasshowninFig.2.Thiskindoffailurecanbeavoidedbyregularinspectiontodetectanysignsofthermalfatigueandperiodicreplacementofa?ectedtubes[5].Azevedoetal.[6]alsoreportedthataverticalheatexchangermadeofASTMA178-gradeAsteeloperatinginawasteheatrecoveryplantfailedduetothermalfatigue.Thetubeshandledthermalfluidmediumwhilethetubesinletandoutlettemperatureswereat200°Cand300°C,respectively.InFig.3(a),wedgecracksinitiatedattheexternalsurfaceanda3-mmthicklayerofFe3O4-Fe2O3-Fe7S8(Hematite,Magnetite,Ironsulfide)wasobservedattheinternalsurfaceofserpentine.Inadditiontothis,theinternaloxidelayercrackedformingawedgecrackinthecrackgrowthdirectionasdepictedinFig.3(b).Thisoxidelayercausedariseintemperatureandthusthermalfatiguewasacceleratedwhichresultedinthefailureofthetubes.Thermalfatigueduetoariseintemperatureorlocalizedoverheatingcausedtransversecrackingintheserpentine.Overheatingcausedreductioninmaterialstrengthandfatiguelimitleadingtocracking.Hardnessnumberwassignificantlylowerinthezonea?ectedbycrackingcomparedtotheuna?ectedregionsofthetube.Thiskindoffailurecanbeavoidedbyoptimizingheatexchangeroperationandeliminatingtheformationofoxidelayersbycontrollingthermalfluidstability[6].Fig.3.(a)Wedgecracksinitiatedattheexternalsurfaceinthepresenceofthickinternaloxidelayer.(b)Internaloxidelayer(40μminthickness)crackedandformedwedgecracks\h[6].Fig.4.(aandb)SEMimagesofcracksatinternalsurfaceofthetubeand(c)Opticalimageoftransgranularcracksformedatinternalsurfaceofthetube\h[8]Inanotherstudy[7],inashellandtubeheatexchangermadeof304stainlesssteel,defectswerefoundinthetubetotube-sheetweldedjoints.Thesedefectsmightoriginatefromtheresonanceoftubeorvariationinthetemperatureandpressureoftheinsidefluidinthepresenceofcyclicloading,whichcanleadtotheinitiationofcracks.Heatexchangerweldedjointfailedduetofatiguefractureasobservedfromfatiguestriations.Thetubewasimproperlyweldedandtherefore,inadequateexpansionresultedincrackinitiation.Itwasconcludedthatadherencetostandardweldingpracticesandcontrolofexpansioncouldhavepreventedthistypeoffailure[7].Corrosionfatiguereducestheservicelifetimeofheatexchangers[8].Abrassheatexchangertubeusedincoolingapplicationsoperatinginamediumcontainingcoolingwaterontubesideandlubeoilontheshellsidewasexamined.Thetubesu?eredleakageduringserviceanduponexamination,manycrackswereobservedattheinternalsurface.Cross-sectionsofthea?ectedtubeswereexaminedusingscanningelectronmicroscopyasdepictedinFig.4(a)and(b).Itwasfoundthattheinternalsurfacehadsu?eredseverecorrosionattackandrepresentedthemainsourceofinitiationofcracks.Fromthechemicalinvestigation,itwasinferredthattubeareasa?ectedbyseverecorrosionweredepletedinZnascomparedtotherestoftheuna?ectedareas.Itwasconcludedthatbrasstubesu?ereddezincificationthatcausedcorrosionfatiguefailureinitiatingun-branchedmultipleparallelcracksaswellastransgranularcracksasdepictedinFig.4(c).Anumberofrecommendationsforthisfailurepreventionwereputforwardincludingtheuseofadmiraltybrass,arsenicadditiontocontrollingdezincification,andusageofinhibitedaluminumbrassorcupro-nickelsin-hibitedalloyssuitableforaggressiveenvironments[8].Criticalcomponentsofheatexchangersincludeboilerwatertubes,superheater,andreformertubes.Creepistheexposureofthesecomponentstoanaggressiveenvironmentthatcausesadditionalstrainovertime.Theprimaryfactorsthatdeterminetherateofdeformationincludetime,temperature,andstresseventuallyleadingtocreepruptureofcomponents.Thetubesexposedtooperatingconditonsthatarepronetocreepexperienceshorterlifetime[9].Approximately30%ofallfailuresofheatexchangercomponentsareduetocreeprupture.Creepdamagecanculminateintheformofintergranular,transgranularorrupturefailuresinthea?ectedregions.Awaterboilertubemadeof2.25Cr1Mosteelwasexaminedforcreeprupturefailure.Anoxidelayer(0.25mminthickness)interferedwiththecoolingactioninthesystemthatledtoanincreaseinthetemperatureofthetube.Riseintemperaturewasduetothedualactionoftheoxidelayerandfluegasthatwasusedat920°C.Evidenceoftheformationofintergranularferrite,trans-granularferrite,andintergranularausteniteeventuallyleadingtocreeprupturewasfound.Martensiteformationwasobservedattheedgeofthefracturedtube.Increaseintemperaturecausedaggravatedthinningthatledtoadecreaseintubethickness,whichdepictedthefracturemechanism.Adequatewatercirculationtokeeptheboilertubecleanwasdeemednecessarytoavoidsuchfailure[10].Inanotherstudy,pipelineofsuperheaterthatformspartofthemainheatexchangerassemblywasexaminedforitsfailure.Thesuperheatersteeltubesweremadeofdi?erentsteelgradesincluding15Mo3,13MOCr44and10CrMo910steels.Theaveragesteamvelocityandflowratethroughthepipeobservedwere9.1m/sand893th?1respectively.Anoxidelayerwasfoundaroundthecurvedtubesthathadfailed.Theoxidelayerwasresponsibleforanincreaseinthetemperatureofthetubesthatledtoitsfailurebycreeprupturecracking.Similaroxidelayerwasfoundattheouterwallofthesteeltubesthatweremuchthinner.Wallthinningwasobservedandchangeintubegeometrypointedtowardrupturebeingthefracturemechanism.Hardnessofthefailedcomponentalsodecreasedduetodynamicrecrystallizationintheneighboringweldseam.Itwassuggestedthat13Mo3steelshouldbereplacedwithamaterialwithhigherheatresistance[11].Shellofaheatexchangermadeofheat-resistantalloy(Incoloy800H)failedafter16yearsofservice[12].Carefulexaminationrevealedthatweldjointsintheshellsu?eredfromcreepfailure.Thisisbecausetheshellwasexposedtosuperheatedsteamatatemperatureof720°Cgivingrisetocreepmicrovoidsandcracks.Thechemicalcompositionoftheweldjointswasfoundtobedi?erentfromtherequiredspecification.Timelyinspectionoftheheatexchangerbaseandweldmaterialsforlong-termcreepexposureisessentialtoavoidsuchfailure[12].AsecondarysuperheatermadeofASMESA-213Gr.T-22andoperatinginthermalpowerplantfailedduringservice.Partialblockage,formationofathickadherentoxidelayeratthetubeinternalsurfaceandpresenceofhigh-temperaturefluegasresultedinpoorsteamflowwhichraisedthetubetemperatureandsubsequentlycreepdamageoccurred[13].AsimilarfailureofconvectiontubemadeofASTMA335gradeP91wasobserved[14].Thetubewasoperatinginpressurizedsteam(12.1MPa)with359°Cand492°Cinletandanoutlettemperatureoffluegas,respectively.AnoxidedepositcomposedofC,MOx,Al,Na,S,ClandFewasobservedattheinnerandoutersidesofthetubewhichledtopoorheattransfer.Eventually,long-termexposureofthetubetohightemperaturecausedruptureandbulging.Thiskindoffailurecanbeavoidedbyproperroutinemaintenance.2.3.OxidationHeatexchangersaresusceptibletooxidationduetomateriala?nity,hightemperatureandoxidizingenvironment[15].FailureofsuperheatertubesmadeofSA213-T22usedinamediumoffluegaswithparticlesofaluminaandsilicawasinvestigated[15].Stereoscopicanalysisofthefailedregionrevealedthatoxidelayerformednearaholeatthetubesurfacehadcracked.Afterex-amination,itwasinferredthatregionalongthetubelengthandawayfromtheholewasfullycoveredwithanoxidelayer.Thislayeratthetubesurfacewasformedduetohigha?nityofsteeltoreactwithoxygen.Oxidationkineticsishighatelevatedtemperaturesandinoxidizingenvironments.Exposureofoutsidewallofthetubetoaharshenvironmentresultedinoxidation.Waterorsteamformedadherentoxidelayersattheinternalsideofthetube,thereforenooxidationwasobservedthere.Inthepresentcaseoffailure,severelylocalizedoxidationoccurredbecauseofhightemperatureatthea?ectedregionandhighvelocityofaluminaandsilicaparticlesinthewayofcoalnozzle.Topreventthisfailure,possiblerecommendationsincludedchangesindesign,bandagingstainlesssteel,changingoperationconditionsandreducingthemaximumtemperatureofa?ectedregion[15].Inanelectricalpowerplant,superheaterboilertubesmadeofSA213-T12steeloperatinginwatermediumwith92barpressurewereinvestigated.Investigationconfirmedthefailureofboilertubesduetolocalizedoverheatingbecauseofheatfluxattackcausedbygasoroilburners.Duetooverheating,steamrapidlyformedinthetubesthatcausedsteamblanketingandtemperatureofthetubeswasraisedto700°C.Theanalysisofphysicalstructureshowedthatanoxidelayerhadformedattheinternalsurfaceofthetubesduetooverheating.Theoxidewasidentifiedasmagnetite(Fe3O4).Theoxidemeasuredattheheatedzonearoundthefissureareaofthea?ectedtubeswas0.44mminthickness.Riseintemperaturealteredthemicrostructureofthetubeatthefracturesite,asshowninFig.5,whereferritematrixwithcarbideparticlescanbeobserved.Theoverheatingresultedinphasetransformationofpearlitemicrostructuretospherodizedcarbideparticles.Overheatingalteredtheoriginaltubematerialpropertiesfromtheactualdesignvalues.Toavoidsuchfailuresinthefuture,itwasrecommendedthatoverheatingneedstobecontrolledalongsidetheamountsofsiliconandsulfurinthemediumwithwatertreatmentprograms[16].TubesmadeofFe-basedUSNN08810alloyusedat850°Cdevelopedlongitudinalcracksafteronethirdofitsexpectedservicelifetime.Thefailuremodewasintergranularcrackingduetohigh-temperaturecarburizationandthecauseoffailurewasdeterminedtobeoverheatingduetodecokingoperations.Itwasrecommendedtokeepaclosecontroloftemperaturetoavoidoverheating.UseofanalternativealloysuchasNi-basedUNSN07214wasalsosuggestedasareplacement[17].Fig.5.Spherodizedcarbideparticlesinthemicrostructure\h[16].2.4.HydrogenattackAheatexchangerinapetroleumrefinerywasalmost40yearsinservicebeforeitsu?eredcatastrophicfailure[18].Theexplosionkilledsevenpeopleinthesurroundings.Causeoffailurewasfoundtobehightemperaturehydrogenattack(HTHA).Thiskindoffailureoccursincarbonsteelwhenexposedtohydrogenatatemperatureabove205°C.Atthistemperature,molecularhydrogenconvertstonascenthydrogenandthendi?usesintocarbonsteelsurface.Afterdi?usion,itproducesmethaneuponreactingwithfreecarbonandmetalcarbides.Methanegeneratedbeneaththesurfaceisunabletoescape,thereforeitcreatesvoidsthatleadtohighinternalpressure.Thesevoidsbuildupwithtimeandtoreleasetheseinternalpressure,cracksforminthematerial.Highfailureriskiscloselyassociatedwiththeincreaseintemperatureandpressure.Highappliedorresidualstressesalsoelevatetheriskofhydrogenattackathightemperatures[18].3.Causesoffailure3.1.Fouling/scaling/saltdepositionWheninsolubleparticulates(e.g.,polymericdeposits,biologicalgrowth,etc.)presentinthecirculatingmediumsticktotheinnersurfaceofheatexchangertubesandresultinthereductionoftubecross-sectionalarea,theflowpathmaybesubsequentlyblocked.Thisphenomenonistermedasfoulingwhichcanbecleanedrelativelyeasilybymechanicalmeanssuchashydro-blastingorscrubbingwithbrush.Theaccumulationofdirtandvariousotherparticlesattheheatexchangertubesurfacesproducedepositlayerswhichreducetheheattransferandincreasethefluidtemperatureresultinginreducedperformance.Lowfluidvelocityincreasesthepossibilityofaccumulationofparticlesatthetubesurface.Duetohighsensitivityoftheprocessinheatexchanger,eventhecleancyclingwaterwithlittleriskoffoulingcouldbedangerousifusedinexcess[19].Alargeproportionofheatexchangertubefailuresareattributedtofoulingwhichmostlyoccursduetopoorheatexchangerdesignandoperationatanimproperflowratethantheintendeddesign.Thefluidmediumonthegassideisnormallyunclean.Toreducefouling,allcomponentsoftheheatexchangermustbeproperlycleaned.Heatexchangersurfacesmustbemadeinternallysmooth,asitisobservedthattexturedsurfacesaremorevulnerabletofoulingascomparedtothesmoothones.Foulinghassignificantcommercial,environmentalandoperationalsafetyimpactonindustrialoperations.Inonestudy[20],dynamicmeshmodelwasdevelopedtoinvestigatethee?ectoffoulingwithandwithoutremovalmechanismwhereprocesseswithdi?erentparticlesizeandvelocityoffluegasweresimulated.Foulingmassexhibitedlinearandasymptoticbehaviorswithandwithoutremovalmechanism,respectively.Scalingmassrapidlyreducedwithparticlediameter.Inadditiontothis,aftertheidentificationofmanifoldparticlesinjectedfromtheinlet,smallerparticlesexhibitedhigherfoulingrateandhighflowvelocityresultedinlowergrowthofthedeposit.Forasmallparticlediameterandlowfluegasvelocity,thermale?ectivenessnumberdecreasedbyapproximately10%after15min.Thisisanindicationthatheatexchangersneedtobedesignedwithanaimtoexhibithighheattransferandlowfoulingrate[20].Scalingoccurswhenamineralfilmsuchcalciumsulfateorcalciumcarbonatedepositsattheinnersurfaceofthetube.Asthetemperatureincreases,solubilityofthesesaltsinwaterdecreases,resultinginanincreaseddepositionatthetubesurface.Scalingdecreasestheheattransfersignificantly.Scalesaretenaciousandcannotberemovedbymechanicalmeans.Acidcleaningisnormallyemployedforthispurpose[21].Acomparativestudywascarriedoutforplainandenhancedheatexchangertubes.ArtificialhardwaterthatcontainedCaCO3wascirculatedthroughthetubes.Scalewasformedthatresultedinadecreaseinheattransfer.Heatexchangerwithenhanceddesignperformedbetterintermsofheattransferatthesamefluidvelocityunderseverescalingconditions[22].HighpHcancauseanunstablerateofheattransferwhilehightemperaturecauseslowersysteme?ciency.HighpHenhancesthekineticsofprecipitationformationandmakesiteasiertolosecontroloverheattransferwhilehightemperaturefacilitatesthescaleformationbysupersaturation[3].Ammoniumchloride-basedunder-depositscorrosionisfrequentlyreportedbyoperatorsandengineersinoverheatedequipment[23,24].Saltdepositioncanleadtoenergylossanddamage.Zhengetal.[23]reportedthattubesofaircoolerfailedduetounder-depositcorrosionofNH4Clsalts.Thermodynamicinvestigationsrevealedthatcorrosiona?ectedregionwascloselyrelatedtothecrystallizationtemperatureofNH4Clsaltsandwaterdewpointtemperature.NumericalinvestigationshoweddepositionofNH4Clsaltsattheuppersurfaceofthetubes.Themaximumsaltsdepositionwasfoundtobe65–73cmfromthetubeinletforthevelocityofmulti-phaseflowrangedfrom350to500cm/sec.Intheexperimentalinvestigation,thecorrosionresistanceof316Lstainlesssteelwasfoundtobebetterthancarbonsteelatdi?erentlevelsoftemperatureandpressurewhichindicatedthatcorrosionissuewasmoreseriousintubebundlesthantubeliners[23].Washingwithwatercanbeusedtoremovethesalts.Duringoperation,fluidtem-peratureinsidethetubecanbekeptbelowthewaterdewpointtoavoidNH4Clcrystallization.3.2.WelddefectsWeldingjointresultsinaninhomogeneousmicrostructurefromweldmetaltoheata?ectedzonetouna?ectedbasemetal.Inaddition,roughness,voids,porosity,inclusions,discontinuitiesandshrinkagecrackscouldresultfromwelding[2].Adherencetoproperweldingpracticeiskeytoproducingahomogeneousweldstructurethatleadstoimprovedandlong-lastingoperationofheatexchangers.Inappropriateselectionofbaseandweldmetals,improperheattreatmentandpresenceofmechanicalstressescouldleadtofailure[25–28].Inappropriatematerialselectionresultedinthefailureoftube-sheetweldofE-715heatexchanger.SEManalysis,metallographicexaminationandelectrochemicalcorrosioninvestigationconcludedthatbasemetalhadhighercorrosionpotentialthantheweldinserviceenvironment,whichledtothecorrosionofweldandeventualperforation.Inadditiontothis,thetubeenvironmentconsistedofhydrocarbon,wherehighcarboncontentsledtocarburizationofthetubesurface[26].Inanotherpaper,Oteguietal.[27]e