BOILER TUBE LEAKAGE
Failure data Tube Tube failures occur in all parts of the boiler including economisers, waterwaterwalls, re-heaters and super-heaters. super-heaters. A distribution (in %) of tube failure in the dierent boiler parts is i s given below !aterwall tubing "#% $uperheater tubing #% &eheater tubing '% conomiser tubing '#% *+clone burner tubing % There are twent+ two () reasons for tube tube failures in the boiler. boiler. nowledge, nspection, monitoring and good operating and maintenance practice ma+ reduce tube failures and increase the service life. There are si/ ma0or groups into which all tube failures can be categories and these si/ groups further divided in to primar+ t+pes. All high pressure boilers commissioned and put into operation go through a stabilisation period, during which some serious problems occur, including tube failures. *lassi1cation of tube failures Tube Tube failures are classi1ed classi1ed as in-service failures in boilers. The si/ ma0or causes are ' $tress rupture Fatigue !ater ater side side corr corros osio ion n " rosion Fire side side corros corrosion ion (*alled (*alled also also as 2igh 2igh temperat temperature ure *orro *orrosion) sion) 3 4ac5 4ac5 of 6ual 6ualit it+ + con contr trol ol.. 7 These lead to to twent+-t twent+-two wo primar+ primar+ causes causes that can can cause cause a tube tube failure failure in a high pressure boiler 8 $hor $hortt ter term m over overhe heat atin ing g fail failur ure e 9 4ong 4ong term term overheat overheating ing failure failure (called (called also as creep creep failures) failures) '# :issimilar :issimilar metal metal weld weld failure failure '' Fatigue caused caused b+ vibration vibration ' Thermal Thermal fatigue due to temperatur temperature e ;uctuation ;uctuation ' *orrosion *orrosion fatigue fatigue failures failures '" *austic *austic corrosion corrosion inside inside the tube tube ' 2+drogen 2+drogen damage damage in water wall interna internall surface '3 Tube internal internal pitting pitting '7 Fl+ ash erosi erosion on '8 Falling slag erosi erosion on '9 $oot $oot blower blower erosio erosion n # *oal particle particle erosion erosion ' 4ow temperatur temperature e ;ue gas corrosion corrosion Fire side side water wall corrosi corrosion on *oal *oal ash corr corrosi osion on "
3 *hemical e/cursion damage 7 =aterial defect and weld defects 8 9 T+pical >roblems?:amages, nspection Techni6ue and &ecommended Actions '. *orrosion *orrosion in li6uid metals is applicable to metals and allo+s processing, metals production, li6uid metal coolants in nuclear and solar power generation, other nuclear breeding applications, heat sin5s in automotive and aircraft valves, and bra@ing operations. *orrosion damage to containment materials is usuall+ the concern. Again, practical design and performance data are e/tremel+ limited. n materials selection several possible corrosion mechanisms need to be considered. *orrosion reactions can occur b+ a simple dissolution mechanism, whereb+ the containment material dissolves in the melt without an+ impurit+ eects. =aterial dissolved in a hot @one ma+ be re-deposited in a colder area, possibl+ compounding the corrosion problem b+ additional plugging and bloc5ages where deposition has ta5en place. . Flow Assisted *orrosion (FA*) or rosion-*orrosion Flow-assisted corrosion is o/ide la+er on a metal surface dissolves in f ast ;owing ;uid. nderl+ing metal get corrodes to re-create the o/ide, and thus the metal loss continuousl+. rosion corrosion is the cumulative damage induced b+ electrochemical corrosion reactions and mechanical eects from relative motion between the electrol+te and the corroding surface. rosion corrosion is de1ned as accelerated degradation in the presence of this relative motion. The motion is usuall+ one of high velocit+, with mechanical wear and abrasion eects. Brooves, gullies, rounded edges, and waves on the surface usuall+ indicating directionalit+ characteri@e this form of damage. rosion corrosion is found in s+stems such as piping (especiall+ bends, elbows, and 0oints), valves, pumps, no@@les, heat e/changers, turbine blades, baCes, and mills. mpingement and cavitation are special forms of erosion corrosion. # .' nspection techni6ues ? guidelines '. For FA*, internal visual inspection mostl+ we using bore scope or video probe . For 6uanti1cation of FA* using T thic5ness measurements, especiall+ on bends. .
&emedial Actions
=aterials selection pla+s an important role in minimi@ing erosion corrosion damage. *aution is in order when predicting erosion corrosion behavior on the basis of hardness. 2igh hardness in a material does not necessaril+ guarantee a high degree of resistance to erosion corrosion.
:esign features are also particularl+ important. t is generall+ desirable to reduce the ;uid velocit+ and promote laminar ;owD increased pipe diameters are useful in this conte/t. &ough surfaces are generall+ undesirable. :esigns creating turbulence, ;ow restrictions, and obstructions are undesirable. Abrupt changes in ;ow direction should be avoided. Tan5 inlet pipes should be directed awa+ from the tan5 walls and toward the center. !elded and ;anged pipe sections should alwa+s be carefull+ aligned. mpingement plates of baCes designed to bear the brunt of the damage should be easil+ replaceable. The thic5ness of vulnerable areas should be increased. &eplaceable ferrules, with a tapered end, can be inserted into the inlet side of heat e/changer tubes to prevent damage to the actual tubes. *athodic protection and the application of protective coatings ma+ also reduce the rate of attac5. . Acid dew point corrosion The acid dew point of a ;ue gas is the temperature at a given pressure at which an+ gaseous acid in the ;ue gas will initiate to concentrate into li6uid acid. n man+ industrial combustion processes, the ;ue gas is cooled b+ the recover+ of heat from the hot ;ue gases before the+ are produce to the environment from the last stage ;ue has stac5. t is ver+ important not to cool the ;ue gas below its acid dew point because the resulting li6uid acid strong from the ;ue gas can cause *orrosion problems for the e6uipment used in caring, cooling and emitting the ;ue gas. .'
nspection techni6ues ? guidelines
'. /ternal visual inspection to detect sulphuric acid attac5. . Annual visual inspection of 1nned tubes for deposits and fouling. ltrasonic thic5ness measurements ma+ be used (after removal of 1ns on straights) to determine the e/tent of tube thinning due to acid dew point corrosion. . &emedial Actions '. To prevent acid dew point corrosion, ma5e sure that the operating temperature of the tubes is higher than the dew point. pgrading to a higher allo+ed material should be considered for both the 1ns and tube, using a stainless steel if re6uired. . To reduce the possibilit+ of acid dew point corrosion, switch to a fuel with lesser sulphur content. ' ". >itting >itting corrosion is a locali@ed form of corrosion b+ which cavities, or Eholes, are produced in the material. >itting is considered to be more dangerous than uniform corrosion damage because it is more diGcult to detect, predict, and design against. *orrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering s+stem. >itting corrosion, which, for e/ample, is almost a common denominator of all t+pes of locali@ed
corrosion attac5, ma+ assume dierent shapes, as illustrated in Fig. . >itting corrosion can produce pits with their mouth open (uncovered) or covered with a semi permeable membrane of corrosion products. >its can be either hemispherical or cup-shaped. ".'
nspection techni6ues ? guidelines
'. nternal visual inspection using bore scope?video probe for pitting. . t is also possible to use dd+ *urrent Testing for detecting thinned areas due to pitting. ". &emedial Actions =aterial selection pla+s an important role in minimi@ing the ris5 of pitting corrosion. Benerall+ spea5ing, the ris5 of pitting corrosion is increased under stagnant conditions, where corrosive microenvironments are established on the surface. :r+ing and ventilation can prevent this accumulation of stagnant electrol+te at the bottom of pipes, tubes, tan5s, and so forth. Agitation can also prevent the buildup of local highl+ corrosive conditions. The use of cathodic protection can be considered for pitting corrosion, but anodic protection is generall+ unsuitable.
t is the t+pe of creep, but at temperatures above the design temperature and because of this, the damage is often referred to as overheating. t is either long-term or short-term. .' 4ong-term overheating 4ong-term overheating due to operation for long time periods at slightl+ elevated temperature from design temperature. . $hort-term overheating t is due to high heat ;u/ during low ;ow at start-up or due to operation of duct burners. .
nspection techni6ues ? guidelines
'. To 5now overheating damage 1rst we carr+ out visual inspection and ltrasonic Testing thic5ness chec5. . !ith 1nned tubing, the swelling or bulging associated with short-term overheating ma+ not be visible due to the presence of the 1ns. &eview damage b+ tube sampling. . Thic5er o/ide and wall thinning are related with long-term overheating. :etection of damage normall+ re6uires the local removal of 1ns to allow access to the tube surface, which allows ultrasonic inspection for wall thinning and internal tube o/ide measurement. Alternativel+, measurements ma+ be made on the bare tubes close to headers. ". &eplication procedure can also be used to chec5 for micro-structural degradation of the material. . nternal bloc5age of tubes can be found via using video-probes, endoscopes and bore-scopes. ." &emedial Action '. t is important to recogni@e the causes of overheating and to introduce
modi1cations to the plant or its operation in order to prevent further damage. . For 1red 2&$Bs, determine if heav+ duct 1ring has contributed to high local temperature. . An+ ;ame impingement on tubes should be reduced as much as achievable. ". :etermine if ;ow bloc5age or ;ow mal-distribution has occurred b+ installing temporar+ thermocouples at strategic points. 3. $teamside o/idation damage Browth of thic5 steam side o/ide la+ers due to long-term overheating, or simpl+ due to high application temperature in the case of T9'. $pallation of o/ide could create a problem. 3.' nspection techni6ues ? guidelines '. To monitor the integrit+ of steam side o/ide usuall+ we are using borescope or video probe. . An indication of an o/idation problem will be noted when scale collected in bottom headers. 3. &emedial Actions The on-set of o/ide e/foliation is a root for increased maintenance consideration to tubes and other components where reliable operation could be in;uenced b+ the build-up of spalled o/ide. 7. /ternal corrosion t is an o/idation reaction between o/+gen and the iron in the steel of high temperature tubes. This is an important problem in units where duct 1ring is used. 7.'. nspection techni6ues ? guidelines '. To avoid e/ternal corrosion, visual inspection is ver+ reliable techni6ue. This inspection should be carr+ out once in a wee5 to minimise the e/ternal corrosion. . ltrasonic thic5ness measurements can also be used to monitor loss of the weight. 7. &emedial Actions To reduce e/ternal corrosion, add more corrosion-resistant materials such as a stainless steel. Hoth the high-densit+ pol+eth+lene and pol+prop+lene three-la+er coating s+stems have low water permeation characteristics for better isolation from the ad0oining sea water. 8. $tress *orrosion *rac5ing ($**) t is the crac5ing induced from the combined in;uence of tensile stress and a corrosive medium. The impact of $** on a material seems to fall between dr+ crac5ing and the fatigue threshold of that material. The re6uired tensile stresses ma+ be in the form of directl+ applied stresses or residual stresses. *old deformation and forming, welding, heat treatment, machining, and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated.
The residual stresses set up as a result of welding operations tend to approach the +ield strength. suall+, most of the surface remains un-attac5ed, but with 1ne crac5s penetrating into the material. n the microstructure, these crac5s can have an inter-granular or a trans-granular morpholog+. =acroscopicall+, $** fractures have a brittle appearance. $** i s classi1ed as a catastrophic form of corrosion because the detection of such 1ne crac5s can be ver+ diGcult and the damage not easil+ predicted. 2+drogen embrittlement is sometimes classi1ed separatel+ from $**. t refers to the embrittlement and resulting increased crac5ing ris5 due to upta5e of h+drogen into the materialIs structure.
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8.' nspection techni6ues ? guidelines To con1rm the general condition, Hore scope? video probe inspection will be good procedure. ltrasonic testing is the most reliable test in some aspects but if crac5 is ver+ tight it ma+ not be a good and reliable method. To detect micro crac5, Acoustic emission monitoring is ver+ important techni6ue. =agnetic particle inspection or dd+ *urrent probes if crac5s are relativel+ large and e/tended through-wall. =etallurgical anal+sis will loo5 for secondar+ un-corroded crac5s attached to the 1rst corroded crac5 and also corrosion products on the crac5s surface. 8. &emedial Actions The use of materials e/hibiting a high degree of resistance to $** is a fundamental measure. =odi1cation of the environment (removal of the critical species, corrosion inhibitor additions) is a further important means of control. n principle, reduced tensile stress levels are a means of controlling $**. n the boiler water, control the level of chlorides and anal+se the material potential for chemical contamination. n order to reduce residual stresses, develop proper speci1cations for manufacturing processes of bending, welding and heat treatment. *ontrol of environment and stress is also important to avoid $**. 9 *austic Bouging (*austic Attac5) t is waterside corrosion that results from the build-up of interior deposits and the local concentration of Ja<2 to high p2 levels, results in a caustic conditions which corrosivel+ attac5 and brea5down the protective magnetite- la+er to a localised wall loss on the surface and hence an increase of the stress and strain through the tube wall.
9.' nspection techni6ues ? guidelines Kisual nspection for internal part of the tube while using video probes, bore scopes and radiograph+ techni6ues. 9 9. &emedial Actions
"# &emoval L replacement of high ;u/ areas where tube damage was recogni@ed "' =inimise entrance of corrosion deposits in the materials " &egular boiler cleaning using eective chemicals for removal of heav+ deposits " *ontrol water chemistr+ "" liminate weld bac5ing ring inside the tube " '# *orrosion Fatigue *orrosion fatigue is fatigue in a corrosive environment. t is the mechanical degradation of a metal under the action of corrosion and mechanical or thermal stresses. =ost of the engineering structures e/perience some t+pe of alternating stress and are e/posed to harmful environments during the service. The environment pla+s an important role in the fatigue of high strength structural materials li5e steels, aluminum allo+s and titanium all o+s. '#.' nspection techni6ues ? guidelines For good operations visual inspection techni6ue using video probes, bore scopes or using angle beam ultrasonic testing with custom transducer for the location of interest of tube. :etailed inspection using video borescopes, angle probe T and destructive testing revealed that corrosion fatigue crac5ing. "3 /ternall+ &adiographic testing and T shear wave inspection can also be used to 1nd out the corrosion fatigue surface and subsurface crac5ing.
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'#. &emedial Actions To avoid corrosion fatigue, e/amination and inspection which chec5s for fatigue and corrosion fatigue of tubes. *orrosive environment and water chemistr+ re6uires to control =onitoring of temperature and strain is also an important factor. '' <-4oad *orrosion *orrosion with visible rust on tube la+er due to o-load wet form and accelerated b+ h+groscopic deposition. t happens when the temperature is below the dew point, especiall+ when the tube surface has some fouling. <-4oad *orrosion mostl+ depends on the environment. 2igh humidit+ environment mostl+ has such issues. ''.' nspection techni6ues ? guidelines Kisual nspection pla+ an important rule to 1nd out the outside surface rust visible corrosion and <-load corrosion is measured b+ ultrasonic thic5ness measurement techni6ue. ''. &emedial Actions nclosure should 5eep dr+ when o load. *lean and dr+ the tube la+er if deposition is severe ' Fireside corrosion A high-temperature phenomenon in coal and oil-1red boilers, that manifests as e/ternal tube wall loss leading to thinning and ultimatel+ failure b+ overload. !hile increasing steam temperatures can greatl+ increase the c+cle eGcienc+ of power plants, such increases in temperature can adversel+ aect 1reside corrosion. n coal-1red boilers,
severe 1reside wastage is t+picall+ caused b+ a deposit-induced li6uid phase corrosion mechanism referred to as coal ash corrosion. *orrosion rates generall+ increase ver+ rapidl+ with temperature up to about 7M* ('#MF), after which the wastage rates tend to drop-o, thereb+ producing a bell-shaped curve. At temperatures e/pected for advanced steam c+cles, aggressive 1reside deposits containing moderate to high concentrations of al5ali sulfates and al5ali chlorides, are e/pected to cause rapid metal wastage, especiall+ in units burning high sulfur coal. Thus, coal ash corrosion is a widespread problem for super heater and re-heater tubes, especiall+ where high sulfur, high al5ali, and high chlorine coals are used, and is a critical problem that needs to be resolved before advanced ultra-supercritical boilers can be deplo+ed. *oal ash corrosion is caused b+ the formation of molten al5ali iron trisulfates on super heater and re-heater tube surfaces '.' nspection techni6ues ? guidelines Jo steel is protected, however, the higher the chromium content of the allo+, generall+ the more resistant to attac5. For Fireside corrosion inspection visual e/amination is carried out to 1nd out the defect. For wall thic5ness measurement, T probes are used for stainless steels or areas where erosion has removed surface o/ide. !hen 1reside corrosion pitting is pronounced, grinding of the tube wall to almost bottom the worst pit ma+ be necessar+ prior to the wall thic5ness measurements. '. &emedial Actions ' For avoid 1reside corrosion, ad0ust the burner alignment. f this problems occurs continuousl+ that change operating conditions of the tube. *orrosion resistant coating is use to minimise this defect. " Additives such as calcium o/ide and magnesium o/ide are added in the fuel to raise the melting temperature of the ash. Fre6uent cleaning of the tubes b+ soot blowers 3 ' Braphitisation This mostl+ aects carbon and carbon-mol+bdenum steels, in which the carbide phase is transformed into graphite after long-service e/posure to a temperature range of "#-7##o*. *hrome-=ol+bdenum steels containing '?% or more chromium are normall+ considered to be resistant to graphitisation. '.' nspection techni6ues ? guidelines For graphitisation hardness testing and replication process are sued for suspected areas '. &emedial Actions =inor graphitisation ma+ be cured b+ solution heat treatment.
7 '" :issimilar =etal !eld (:=!) Failures A crac5ing that ta5es place at weld fusion line or nearer in butt welds where austenitic stainless steel material 0oins in a ferritic allo+. t is caused b+ high stresses at the austenitic to ferritic interface caused b+ the dierences in e/pansion properties. Also carbon migration occurs from the lower allo+ to the higher allo+ resulting in a low strength area ad0acent to the weld fusion boundar+ '".' nspection techni6ues ? guidelines Two inspection techni6ues are performed for detecting dissimilar metal weld crac5ing defect. These are 4i6uid >enetrant testing (>T) and &adiographic testing (&T).
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'". &emedial Actions For good operation, control applied stress and temperature in weld areas. &elocate weld to lower temperature?stress areas to reduce?eliminate future damages. se alternative 1ller metal such as nic5el 1ller metal for repair?replacement to improve good performance. se alternative weld preparation?geometries that improve weld life. =aintain hangars, supports and spacing. ' 2+drogen damage 2+drogen damage is caused b+ a corrosive reaction between the steam of the boiler and steel, and its reaction is given below Fe N 2< O Fe< N 2 n other words, 2+drogen damage is a general term which refers to mechanical damage of a metal caused b+ the presence of h+drogen. 2+drogen damage is classi1ed into four distinct t+pesD 2+drogen blistering 2+drogen embrittlement :ecarburisation 2+drogen attac5 2+drogen reacts with carbon in steels that is called decarburisation and also bonding combines to form h+drogen molecules resulting in brittleness of the material and loss of its strength and thus brittle failure occurs. 4ow p2 of water chemistr+, improper chemical cleaning and concentration of corrosive contaminants are the main causes. '.' nspection techni6ues ? guidelines $everal techni6ues are used to 1nd out the damage mechanism, to 1nd out the micro structural state and mechanical characteristics of the metallic material. nner side tube visual inspection e/amination is carried out while using video probes techni6ue, borescopes techni6ue and nternal &otational nspection $+stems (&$) techni6ue. /ternal e/amination is also re6uired and mostl+ used radiograph+ techni6ue. !all thic5nesses measured are important techni6ue and this measurement is carried out at several locations along the length of the tube. *hemical anal+sis of tube material is also chec5ed b+ ;ame spectroscop+. =echanical test li5e tensile test is also carried out at room temperature on the samples get from the tube 1reside in the area of fracture. =icroscop+ techni6ue is used to investigate the =etallographic condition of the damaged @ones.
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'. &emedial Actions sing clean steel li5e 5illed steel. se eective chemicals to clean the boiler for removal of heav+ deposits. =aintain the correct water chemistr+ with good p2. 4oo5 for condenser lea5age to minimise the damage.
