Reliability Issues With HRSG

8/16/2019 Reliability Issues With HRSG

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Reliability Issues of HRSG


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Relative frequency of the dominant damage in HRSG


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Low Cycle Thermo-mechanical Fatigue

Cold end gas-side corrosion

e will focus on!

Flow "ccelerated Corrosion


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Some Techno-Commercial issuesaffecting HRSG Relia#ilities

The two $aramount concerns of most $urchasers ofCCGT installations are low installed cost and highfuel efficiency

High efficiency has driven ra$id increases in GTe%haust flow and tem$erature im$osed on HRSGs

G& CCGT 'odel &%haust Tem$erature

(oC)

Gas Flow

(T*H)

+,. (*") 4/+ 440

.1. (&") +,0 .22

3.. (&) +4, .4/4

3,+. (F") 02/ 1,./


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The two $aramount concerns of most $urchasers ofCCGT installations are low installed cost and highfuel efficiency

High efficiency has driven ra$id increases in GTe%haust flow and tem$erature im$osed on HRSGs

Com$ared to highly rated large GTs HRSGs are

$erceived as low-ris5 equi$ment

Low-cost HRSG designs which 6ust meet therequirements of s$ecifications and #oiler design code


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Hori7ontal HRSGs are constructed with tu#es arrangedvertically in 8har$s9:

These har$s are rigid structures requiring that ad6acent

tu#es remain at similar tem$eratures to avoid severe

thermal-mechanical fatigue damage and $rematurefailure9

HRSGs o$erating at high $ressure are having

sufficiently thic5 walled drum su$erheater andsometimes reheater outlet headers that require careful

management of heat u$ and cool down rates to avoidinternal crac5ing

Since com#ustion tur#ine ram$ rates directly affectHRSG com$onent tem$erature ram$ rates the $ush tora$id CT start u$s results in greater ram$ rates in HRSG

hot section com$onents


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Damage Mechanism: Thermo-mechanical Low Cycle Fatigue


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Location:

HP Superheater and outlet manifold HP Steam Drum and !aporator

circuit conomi"er



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#easons:

#apid increase in $T %haust

temperatureduring starts from any condition Header thic&ness Header to tu'e connection geometry

(uench cooling due to impropercondensate drain

)!er spraying of attemperator* faultyattemperation spray


HP Superheater and outlet manifold


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;ifferential e%$ansion am$lifies drain lateraldis$lacement

atch for #end tu#es and deflection ofsu$erheater floor $i$e $enetrations duringstartu$ and shutdown


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*recaution! Care to #e ta5en #oth for ram$ u$ and

coasting down rate &T' to #e used for 3F" machines




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*recaution!

Hold the GT load at a nominal #loc5load until steam flow is esta#lished in

the su$erheater

control the GT loading rate to ensure that the

tem$erature difference #etween the steamtem$erature and the average header wall

tem$erature remains at or #elow the initial

difference on first admission of steam




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*recaution!

=nloading the GT (and using &T' onF">3F"s) until outlet steam tem$erature isa#out +2oC a#ove the $revailing H* saturationtem$erature then holding at that load for fewminutes to let the header?s through-wall

tem$erature gradient equali7e #efore shuttingdown the GT




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Condensate quenching $recaution!

Low-$oint drains are o$en during startu$ to$urge water>condensate $rior to admittingsteam to tu#e $anels

;rains are o$en sufficiently early in the

startu$ $rocess to ensure that all condensateactually will #e drained $rior to steamadmission




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Condensate quenching $recaution!

*ro$er si7ing of drains is critical

Locate #lowdown tan5s #elow header drainlocations

chec5 to see that H* @* and L* drains arenot com#ined in a common collection $i$e

u$stream of the #lowdown tan59




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Aver s$raying of attem$erationB reasons!

Ty$ical causes include lea5ing s$ray-watersu$$ly valves hunting $oor $i$ing

arrangements overs$ray and a$rimary>secondary su$erheater surfacearrangement that is incom$ati#le with a

giventur#ine?s $erformance at startu$ or low load9




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Aver s$raying of attem$erationB reasons!

*oor atomi7ation of s$ray water #ecause of$ro#e>no77le damage or $artial $lugging

@m$ro$er $i$ing designin $articular aninsufficient straight run of $i$e u$streamand>or downstream of the attem$erator

"n arrangement of su$erheater surface thatallows overs$ray to occur at startu$ or lowload #ecause all the water cannot #eeva$orated9




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Aver s$raying of attem$erationB *recautions!

first things to investigate is s$ray valve#ehaviour at steady load Chec5 thermocou$le reading u$stream and

downstream when s$ray valve is closed $revent s$ray down #elow ,2oC of su$erheat

at the attem$erator outlet @f too much surface is installed remove fins

and>or gas #affles or use tu#e shields toreduce heat transfer




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reasons!

@n $anel-ty$e economi7ers water goes u$

and down in each $anel or har$ Return-#end economi7ers usually have

alternating u$flow and downflow tu#es in thesame row9 @n some cases water flows u$ one

row down the ne%ta ser$entinearrangement a tem$erature differential #etween tu#e rows

of from ,2 to .22 deg F can cause thermalshoc5 which contri#utes to fatigue failure


conomi"er:


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conomi"er:

&conomi7er tu#e lea5 at weld caused #y thermal shoc5


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reasons!

;uring startu$ o$erators e%$ect drum level to

swell and the feedwater control valve isclosedBno water is flowing through the #oiler9 ;uringthis time economi7er $anels 8soa5: to

tem$eratures higher than normal9

hen H* drum level finally starts to dro$ thefeedwater control valve o$ens and 8cold:

water8shoc5s: the economi7er9


conomi"er:


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*recautions!

Tric5le-feed water through the H* economi7er

as soon as drum $ressure #egins to increase "ssure $ositive feedwater flow thoughout the

startu$ $eriod #low down as needed to control drum level


conomi"er:


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Damage Mechanism: Flow +cceleratedCorrosion

H* &conomi7er draintu#e

Feed *i$e

Feed *i$e


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Flow-accelerated corrosion (F"C) is a well-5nown damage mechanism that affects car#onsteel com$onents carrying water or two-$haseflow9 Caused #y the mechanically-assistedchemical dissolution of the $rotective o%ide and#ase metal9


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Formation of 'agnetite!hen car#on steel is e%$osed to o%ygen-freewater the following reaction occurs!

Fe D 1H1A Fe,1D

D 1AH-

DH1 Fe(AH) 1 D H1 (.)

This reaction is then followed #y the Schi5orrreaction where $reci$itated ferrous hydro%ide isconverted into magnetite!,Fe(AH)1 Fe, ,A4 D 1H1A D H1 (1)


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Dissolution of Magnatite


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@t has lead to failures or severe wall thinning in!

&conomi7er>$reheater tu#es at inlet headers9

&conomi7er>$reheater tu#e #ends in regions where

steaming occurs9Eertical L* eva$orator tu#es on Hori7ontal HRSGs

es$ecially in the #ends near the outlet headers

L* eva$orator inlet headers which have a tortuous

fluid

entry $ath and where orifices are installed9

L* riser tu#es>$i$es to the L* drum9

L* eva$orator transition headers9


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;amage 'echanism! Flow "cceleratedCorrosion

F"C is a mass-transfer $rocess in which the$rotective o%ide (mostly magnetite) is removedfrom the steel surface #y flowing water9'aterial wear rate de$ends on

(.) Steel com$osition tem$erature flow velocity and tur#ulence(1) ater and water-dro$let $H and(,) The concentrations of #oth o%ygen and o%ygen scavenger9


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Factors affecting F"C

Tem$erature

$H

A1 concentration

'ass flow rate

Geometry

uality of fluid

"lloys of construction


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FLA "SS@ST&; CARRAS@A

Greatest potential for FAC occurs around 150 ºC

&ffect of tem$erature on material wearrate of various metallurgies

Cr content more than 1% nearly reduces thechance of FAC to 0


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$H has significant effect on materialwear rate of car#on steel

early forty (42) fold reduction #etween$H /90 and 394


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FAC minimized ao!e "0 pp #$

FAC increases eponentially elo& "0 pp #$

;issolved o%ygen has direct im$act


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&ffect of Eelocity


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Geometry affects location of FAC

Location


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*recaution!For HRSGs with all-ferrous systems thefeedwater chemistry should #e "ET(A) toavoid single-$hase F"C in the feedwater and

L* eva$orator circuit9

The #asic idea of "ET is to minimi7e corrosionand F"C #y using deaerated high $urity waterwith elevated $H9 The $H elevation should #eachieved #y the addition of ammonia.


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&ffect of Tem$erature and "mmonia oniron dissolution


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$H correction with "ET

"ET(A) A%idi7ing "ll Eolatile TreatmentI

@n o%idi7ing all volatile treatment the use ofreducing agent is eliminated9

For units o$erating with "ET(A) the achieva#leiron levels can #e around .$$# or less9

ith "ET(A) or AT the $rotective cover layer

$ores #ecome $lugged with ferric o%ide hydrate(FeAAH)

=sed for all ferrous metallurgy


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"ET(A) A%idi7ing "ll Eolatile TreatmentI

The requirements for "ET(A)!

&levated $H of 391 J 390 Cation conductivity of less than 2912 KS>cm

'inimum air in-lea5age to ensure less than.2 $$# dissolved o%ygen at C*;

o additional reducing agent


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AT (A%ygenated Treatment)

@n A%ygenated treatment o%ygen and ammoniaare added to feedwater


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*recaution!Tur#ulences should #e minimi7ed #y $ro$erdesign

For new re$lacement and for new unitsmaterial of construction may #e changed to*.. or *11

Regular ins$ection of susce$ti#le com$onents#y ultrasonic (=T) e%amination needs to #eunderta5en to $revent any catastro$hicfailure


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*recaution!'onitoring of iron concentration around thesteam cycle is also usefulB elevatedconcentrations may indicate ongoing damage

in a s$ecific su#system


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Damage Mechanism: Cold ndCorrosion


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CARRAS@A '&CH"@S'

S + A1 → SA1 1H1S + ,A1 → 1SA1 D 1H1A

1SA1 + A1 → 1SA, (E1A+ Fe1A, cataly7es reaction) SA1 D A "tomic A%ygenI → SA, SA, + H1A → H1SA4

"cid Gas Ea$our condenses when the come in

contact with low tem$erature metal of theCondensate *re-Heater L* &conomi7er or L*&va$orator as tem$erature falls #elow "cid ;ew*oint


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;amage is in evidence where first-$ass tu#es

intersect with inlet header and metal is coolestBthere?s no dew $oint corrosion of second-$asstu#es


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Damage Mechanism: Cold ndCorrosion"cid ;ew *oint Tem$erature is a factor SA,

and va$our content in the flue gas


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*recaution!"naly7e Flue Gas sam$le ta5en from cold sideof HRSG for SA, content and calculate "cid

;ew *oint tem$erature

&nsure water tem$erature at Feed ater inleta#ove "cid ;ew *oint tem$erature

Regular ins$ection of susce$ti#le com$onents#y ultrasonic (=T) e%amination needs to #eunderta5en to $revent any catastro$hicfailure


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Failures Case Studies


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Case .!

.12 ' Ail Fired


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The tu#e shows #listers at four locations (A; M/2 to /1 mm) in the s$ool $iece #etween twoweld 6oints9


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" circumferential crac5 is o#served near thefusion line of one of the #utt 6oint9 Heavyde$osits are o#served on the @; surface of the

tu#e9


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all thic5ness at 2o was measured as .9+mm

'icro e%amination of Transverse sections ofthe tu#e at two out of four #listers revealo%ides on the #lister edges and @; surface of

the tu#e


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A%ides filled rounded $its are o#served onthe @; surface9

Co$$er coloured co$$er rich $hasesegregation is o#served near the #lister

edges and @; surface


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o significant deformation > decar#urisationof the ad6oining grains of the $it edges and#lister edges are o#served9 The microstructure consists of $olygonalgrains of ferrite and $earlite


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Transverse section of the weld 6oint wherethe circumferential crac5 was o#served

reveals segregation of co$$er rich $hasenear the @; surface9

Several discontinuous grain #oundary crac5swith decar#urisation of the ad6oining grainsty$ical of Hydrogen em#rittlement crac5s areo#served in the s$ool $iece *' H"N andweld metal of the #utt 6oint


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Ane more transverse section of the weld 6oint at a#out ./2O from the circumferentialcrac5 was also micro e%amined9 'ismatch#etween the tu#e mem#ers lac5 of sidewallfusion incom$lete root $enetration and slagare o#served


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Conclusion!

'icro e%amination reveals $resence ofco$$er coloured co$$er rich $hase on the @;surface and #lister edges of the tu#e9 "lsodamage due to Hydrogen em#rittlement iso#served in the *' weld and H"N regions ofthe weld 6oint9

The #listers o#served on the A; surface isattri#uted to waterside corrosion may #edue to condenser lea5age9

The circumferential crac5 o#served near the

fusion line of the #utt 6oint is attri#utedHydrogen em#rittlement caused #ywaterside corrosion9

Swa# analysis shows $resence of somechloride and sul$hur with a $H value of 9/9


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Case 1!

.12 ' Ail Fired


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The middle tu#e has #urst o$ened with thic5li$s


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;imensional 'easurement (mm)!()*e l!at, -)t%e /ameter (!k,e%%

0°1$0° 90°270° 0° 90° 1$0° 270'ear ale l4 #.0$ 6.12 R,g %e!t, a 7#.15 7$.73 3.97 5.60 5.7$ 5.67

Flattening Test! Test 'ethod "ST' ",2" flattening s$ecimen was ta5en near the

failed region9 The flattening test showedcrac5s on @; surface indicative of hydrogenem#rittlement9


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The middle tu#e was transverse sectionedand micro e%amined at the failed region9 The

failed li$ shows a lot of o%ide filleddiscontinuous crac5s starting from @;surface

;ecar#uri7ation is o#served near the crac5edges throughout this region9


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• A$$osite to the failed region themicrostructure consists of $olygonal grainsof ferrite and $earlite

An a ring section a littleaway from the failedregion a similar structure is o#served


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Conclusion!Several o%ide filled discontinuous crac5s areo#served on the @; surface of the tu#e9 Theflattening test result indicates hydrogenem#rittlement9

The failure of the tu#e is attri#uted tohydrogen em#ritlement9


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Case ,!

.12 ' Ail Fired


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The tu#e shows #ulging (A;M0.9+ mm) and#urst o$ened li5e a fish mouth


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;imensional 'easurement (mm)!()*e l!at, -)t%e /ameter (!k,e%%

0°1$0° 90°270° 0° 90° 1$0° 270'ear ale l4 3.03 #.27 R,g %e!t, 59.21 61.$1 3.35 3.6$ 3.93 3.$2lttle aa


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Transverse section of the #urst li$ revealso%idation of the edges9Several o%ides filled rounded $its are

o#served on the edges of the li$ as wells onthe @; surface of the tu#e9o significant deformation > decar#urisationof the ad6oining grains of the $it edges areo#served9

The microstructure consists of $olygonalgrains of ferrite and $earlite9


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A$$osite to the #urst and a ring section littleaway from the #urst also show similar ty$eof microstructure9 A%ides filled $its areo#served in #oth the sections e%amined9


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Conclusion!

'icro e%amination indicates several rounded$its filled with o%ides on the @; surface ofthe tu#e9

The failure is attri#uted to watersidecorrosion9

However ta5e the case of a similar failure


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Tu#e shows #ulging (A;M /,9+ mm) and #ursto$ened with thin li$s9 o significant amount ofde$osits is o#served on @; and A; surface9

However ta5e the case of a similar failurein the same


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Some rounded $its filled with o%ides are

o#served on the @; surface tu#e in all thethree sections e%amined9


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Transverse section of the #urst li$ shows o%idation ofthe edges and the structure consists of #ainite9

A$$osite to the #urst the structure shows $olygonalgrains of ferrite and $earlite9


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" ring section little away from the #urst showstransformed ferrite and $earlite along the a%is of

#urst and $olygonal grains of ferrite and $earliteo$$osite to the #urst a%is9

"#ove o#servations suggests that the tu#ehas #een overheated to a#ove "C,tem$erature for the steel for a short $eriod

of time9


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Case 4!

,2 '


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Tu#e Location


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Failed Tu#e

Failed


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Failed Tu#e

;amage at .1 A Cloc5 $osition


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Failed Tu#e

;e$osits near damage


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;uring discussion it was noted !

all three #oilers are o$erated at nearly 1+P overloading conditions

Charcoal is used as start u$ fuel

Recently drum level maintained at / J .1P lower

than normal level to avoid mechanical carry over(which was o#served ta5ing $lace)

Residual *hos$hate was also maintained at 1-, $$minstead of 0-/ $$m as required for the same reason

Fuel ;istri#ution $lates were found in damagedcondition9


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Case +!

.12 ' Tangential Fired *F


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Tu#e .

The tu#e has #urst o$en with thic5 li$s9 A%ide scalingis seen on the @; and A; surfaces


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Tu#e 1

• The tu#e has #urst o$en with thic5 li$s9 Satellitecrac5ing is o#served on the failed region9 A%ide

scaling is o#served on @; surface


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Tu#e ,

• The tu#e has #urst o$en u$ with thin li$s andcrac5ed over a length of tu#e9 Severe e%ternal

metal wastage is o#served on the failed $ortion


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Location Auter ;iameter Thic5ness

2Q-./2Q 3Q-12Q 2Q 32Q ./2Q 12Q

.9Failed li$ - - ,9+1 - 49,2 -

.9Ring section

a little

away

,/94, 429/, ,943 4943 ,93. 49.+

19Failed li$ - - 190 - 49.2 -

19Ring sectiona littleaway

,/932 429+1 .92 ,9/0 ,904 ,9++

,9Failed li$ - - .93, - ,934 -

,9Ring sectiona littleaway

,092 ,932 19. ,9/3 ,931 4913


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Tu#e o9.! (e t)*e a% tra,%:er%e %e!t,e a,m!r exam,e. (e ale l4 %% ;,tergra,)lar

!ra!k% a, !a:te% rg,at,g rm -/ %)ra!e.

- l l


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-44%te t te ale l4 te %tr)!t)re %%4lg,al gra,% errte a, !m4letel%4er%e a, 4artl %4er%e !ar*e%.-, a r,g %e!t, aa rm te !ra!k tem!r%tr)!t)re %% 4lg,al errte gra,% a,!m4letel %4er%e a, 4artl %4er%e!ar*e%.

T # 1 ( t * t t


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Tu#e o91! (e t)*e a% tra,%:er%e %e!t,e a,m!r exam,e. (e *)r%t l4 %% !ree4 !a:te%a, ,ter l,ke gra, *),ar !ra!k% lle txe%

(e %tr)!t)re at te l4 a, 44%te t te l4 %% a


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(e %tr)!t)re at te l4 a, 44%te t te l4 %% a%tr)!t)re 4lg,al errte gra,% a, tra,%rme *a,te.-, a r,g %e!t, a rm te *)r%t te m!r%t)!t)re

!,%%t% 4lg,al gra,% errte a, tra,%rme *a,te, te ax% al)re. -44%te t t% ax% te %tr)!t)re!,%%t% 4lg,al gra,% errte a, tem4ere *a,te.

T # , ( l l l l t


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Tu#e o9,! (e ale l4 %% 4lg,al gra,% errtea, %4er%e a, 4artl %4er%e !ar*e%. -44%te tte ale l4 te %ame m!r%tr)!t)re % *%er:e. -, a r,g

%e!t, a lttle aa rm te ale reg, errte a, , %t)%4er%e *a,te are *%er:e


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1. (e al)re te t)*e '.1 % attr*)te t l,g term:ereat,g t *el AC1 tem4erat)re te materal

te %teel.2. re%e,!e !ree4 !a:te% a, ,ter l,ke gra,*),ar !ra!k% ,ear te *)r%t l4 ,!ate :ereat,gt *el AC1 tem4erat)re r te %teel r l,g tme ,()*e '.2 )r,g %er:!e. He:er te m!r%tr)!t)re

,!ate% tat te ,al al)re te t)*e % attr*)te t%rt term :ereat,g t a*:e AC1 a, *el AC3tem4erat)re te materal te %teel.3. ;, ()*e '.3 te al)re % attr*)te t exter,almetal a%tage.


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Reliability Issues With HRSG