Failure of Urea Strippers and RepairExperience

During annual turnaround of our urea plant, cracks were noted in CS multilayers oftwo strippers. An in-depth analysis is presented of the failures and corrective actions

taken to repair the strippers and prevent such failures in the future.

H.P. Pota, R.V. Nesari, S.K. Nayar, and C.B. T\imbdeRashtriya Chemicals and Fertilizers Ltd., Thai, India

INTRODUCTION :

Rashtriya Chemicals & FertilizersLtd.,is the largest fertilizer producerin India having a total capacity of 1.0Million MT of N., 0.12 Million MT ofP2Oe and 0.045 Million MT of K2O. Thaifertilizer complex of RCF, locatedabout 100 km. south of Bombay is oneof the biggest in Asia having 2 x1350 MTPD Ammonia plants designedby Hal dor Topsoe, Denmark and 3 x1500 MTPD Urea plants designed bySnamprogetti, Italy. These plantswere commissioned in 1984-85.

Urea technology of Snamprogetti isbased on the ammonia strippingprocess. In this process, unconvertedcarbamate is decomposed into ammoniaand CO« in the stripper operating atalmost the same pressure as that ofthe Urea reactor by application ofheat through steam and action byexcess ammonia fed into the reactor.

About 80% of total carbamate isdecomposed in stripper. The stripperis one of the most critical equipmentsin the Snamprogetti Urea process.Simultaneous failure of two strippersgave most anxious moments to theUrea staff.Anal y ses of these failuresand timely corrective actions helpedto overcome this crisis.

STRIPPER-FUNCTIONCONSTRUCTION :

AND

In the Snamprogetti Ureaprocess,the stripper is basically ahigh pressure decomposer, operatingat about 147 bar. It receives thesolution containing about 33% Urea,33% NH, and 13% CO2 as carbamate(NH4COONH2). In the stripper, thesolution is heated up with 22.6 barsaturated steam as it flows down thetitanium tubes by forming a fallingfilm. The CO2 content of solution is

345

reduced to about 5.5% by thestripping action of excess ammonia asit boils out of the solution.

Due to the corrosive nature ofcarbamate solution, the tubes of thestripper are of titanium materialwhereas the tube sheets and thedished ends are cladded with atitanium lining. The stripper shell isfabricated with ASTM A-516 Gr.70steel with a multilayered construction.There are 9 outer shells each of 14.9mm in thickness and an inner shell of12 mm thickness. The inner shell islined with a 3 mm thick titaniumliner. All the shell plates are weldedto dished ends. These strippers weredesigned by Snamprogetti andfabricated by Kobe Steel, Japan.These strippers were designed asreversible type to obtain a maximumlife from Ti tubes.

THE BROAD SPECIFICATIONS OFSTRIPPER ARE :

Shell side Tube sidePressure(bar)Operating 22.6 143.0Design 25.5 162.0

Temperature(°C)Operating 220.0 Inlet 190.0

Outlet 210.0

Design

Tube

230.0 225.0

OD x Thk x L27 x 3.5 x 4500 mm

No of tubes - 2966Surface Area - 838.6 M2

MATERIALS OF CONSTRUCTION :

Shell - C.S.CASTM A-516 Gr.70)Tubes - Titanium (SB-338 Gr.3)Ferrules - 25/22/2 Cr/Ni/MoChannels& tubesheets - C.S./Ti clad.(SB-265

Gr.1).Design Code - ASM E Section VIII

Div.2 and TEMA-R.

HISTORY OF FAILURES :

The stripper, being one of the mostcritical high pressure items in theUrea plant, is checked everyalternate year for thinning of theinternal titanium tubes. During theannual shutdown in April 1991, whilechecking the condition of the Unit-11(Stream-I) stripper, some insulationdamage was noticed near the 1CNAnozzle (Fig.1). After removing theinsulation, it was observed thatseveral cracks had developed in andnear the 1CNA nozzle and near aportion of the lower channel aroundthe nozzle. Cracks were also foundnear the circumferential welding ofthe channel shell to the bottom head.The protective Zinc coating on theouter surface of the stripper up tobottom manhole portion was alsomissing. No such paint damage wasobserved on the topside of thestripper.

The appearance of these cracksonly around the 1CNA nozzle on theouter C.S.shell created a scare sincefailure normally starts from inside inany equipment where processconditions are severe andnegligence/maloperationscan result infailures. The extent of thepenetration of these cracks was alsou n k n o w n . Hence fu r therinvestigations and analysis wereplanned.

Since Kobe Steel, Japan hadfabricated this stripper as perSnamprogetti's approved drawings, itwas decided to call Kobe's technicalexperts as well as Snamprogetti'sengineers for investigations.

To assess the extent of the damage,all the insulation was removed tocarry out magnetic particle testing(MPT) of the total outer surface anddye penetrant test(DP test) of theinside surface.

346

Following were the observations ofthese tests:

1. There was no defect detected inthe inner titanium lining.

2. Extensive external multi-directionalcracks were observed around the1CNA nozzle (dummy inlet nozzle) andon the circumferential weld of thehead to channel shell (Fig.2).

3. One crack was also observed inthe circumferential seam weld of thetube sheet to the channel shell.

To check the extent of damage tothe inner layers C.S. shell, crackaffected plate portions of therespective layers were removed byarc air gouging using 9 mm graphiteelectrodes and 500 amps directcurrent with the following obser-vations :

9TH LAYER (OUTER MOST LAYER) :

Crack affected plate of size 1380mm x 1340 mm was removed from thearea around the 1CNA nozzle bygouging.

8TH LAYER :

After removing the 9th layer plateportion, cracks were observed justbelow the 9th layer cracks (Fig.3).Also a lot of corrosion deposits werefound near the cracks. Consequently,a plate from the 8th layer of size1000 mm x 840 mm was gouged andremoved from around the 1CNAnozzle.

7TH LAYER :

On the 7th layer, cracks wereobserved in a different location fromthe cracks observed in the 8th layer(Fig.4). Corrosion deposits wereobserved on the surface.

A plate of size 770 mm x 900 mmwas gouged and removed from the

1CNA nozzle area.

6TH LAYER :

Cracks were observed in the bottomportion initiating from the headcircumferential weld (Fig.5). A plateof size 580 mm x 205 mm was gougedand removed from just above thehead circumferential weld.

CRACKS ON 1CNA NOZZLE :

In the 9th layer, minor cracks wereobserved in the radial direction onthe 1CNA nozzle weld. The extent ofthese cracks on the 1CNA nozzle weldkept increasing as the 7th layer wasapproched. A large circumferentialcrack approx. 300 mm long wasobserved in the 1CNA weld. The crackwas gouged to the depth of 55 mmfrom the 7th layer under theguidance of Kobe experts. Aftergrinding of the affected weld,a network of hairline cracks werenoticed in the 1CNA weld and parentmetal.

CHECKING THE STRIPPERS OF OTHERUNITS :

After noticing the condition of theabove mentioned stripper, it wasdecided to check the condition of thestripper in two other streams. Theshutdown was taken in Unit-21(Stream II). The insulation covershowed some decolouration. However,after removing all the insulation, itwas noticed that the paint was stillintact around 1CNA nozzle and otherareas. BY MPT, it was proved thatthere was no crack on the outsidesurface of the vessel.

In Unit-31, (Stream III) a colourchange in the stripper insulationcover was observed around thenorth side area of the 1CNA nozzle.After removing the insulation, crackswere observed where the nozzle waswelded to the shell. Onecircumferential crack was observed

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from the 8 O'clock position to the 3O'clock position. One of the cracks inthe nozzle was found to be 112 mmdeep by Ultrasonic test (UT)measurement.

FAILURE ANALYSIS :

In order to investigate the causesof the failure, a thoroughinvestigation was carried out. NOTexaminations like Ultrasonic testing(UT), Magnetic Particle Testing (MPT),Radiography testing (RT) and DyePenetrant (DP) checks were carriedout by a third agency. Two samplesof the plate were cut from the toplayer of the cracked zone of thestrippers and sent for metallurgicalanalysis. Chemical analysis indicatedthat the plate material was asspecified. The material used forchannel shell layers was normalisedshowing ferrite and pearlite struc-ture (Fig.6). Equiaxed structure wasseen on circumferential surfaces whilebanded structure was seen in crosssection of shell layers. Cracksfollowed the grain boundaries.However, structural analysis indicatednon-homogenity of the structure(Fig.7).

From the metallurgical analysis, aconclusion could be drawn that thefailure is intergranular, originatingfrom the welds due to corrosion byseepage of ammoniacal solution inassociation with residual stresses inthe stripper channel layers. Also byanalysing the development of cracksin each layer, it was observed thatall cracks were initiated from theexposed surface or from underneaththe cracks on the top adjacent layer.

This conclusion was also confirmedby another independent agencyinvestigating the failure. Itsconclusion was that the failurehad taken place due to stresscorrosion cracking (SCC) caused byammonium carbamate solution andresidual stresses.

SOURCE OF AMMONIACAL SOLUTION(AMMONIUM CARBAMATE) :

In the stripper, the solutioncontaining urea, unconvertedcarbamate and excess ammonia comesfrom the urea reactor. Beforeentering the stripper, a butterflycontrol valve (HV-03) is installed justadjacent to the stripper solution inletnozzle (1N) in order to controlparameters in the reactor andprovide desired differential pressurebetween the reactor and the stripper(fig.SA & 8B). This line does not haveany support between the reactorand the stripper since the line is ofcomparatively short length. Becauseof variations of the conditions of thefluid flowing in the line, there usedto be minor leakages in the upstreamor down stream flange of the controlvalve (HV-03). It is a normal practiceto put a source of low pressuresteam, near the leakage point toavoid any urea/ammonium carbamatedeposition which may corrode thecarbon steel material of the flanges.However in Unit-11 and Unit-31, theleakage sometimes was excessive. Inorder to control gaseous emmissionsof ammonia/carbamate, coldcondensate was put on the source ofleakage. The 1CNA nozzle is locatedjust below the HV-03 control valve.The condensate containing dissolvedammonia/carbamate, appeared to haveseeped through the damagedinsulation portion which causedsoaking of the insulaton withammon i cal/carbamate solution. Whilechecking the condition of theinsulation, it was noticed that therewas no aluminium foil on the outersurface of the stripper and theinsulaton wool was in direct contactwith the metal surface. It can be saidthat the decolouration of the paint orsurface corrosion was mainly becauseof attack by ammoniacal/carbamatesolution trapped in the insulationmaterial.

348

To analyse residual stresses whichcould have caused the stresscorrosion cracking (SCC), fabricationdetails of the equipment werechecked. The stripper was fabricatedby Kobe Steel, Japan as per ASMESection-VIII Div.2. There was noobserved deviation from code. As theshell was fabricated in multilayers,stress relieving of the circumferentialweld between the channel and thehead was not required. Also, channeland 1CNA nozzle welding stressreliveing was not necessary as percode. So, the residual stresses causedduring fabrication could not beremoved. It is felt that the attack ofammoniacal carbamate solution on theplate material along with residualstresses resulted in stress corrosioncracking in this area. There are manyother observations as given belowwhich support SCC failure in thestripper:

1) No cracks near the weld of theshell plates were observed. This isbecause these plates are weldedindividually at three locations versuswelding of all plates to the nozzleetc. This procedure does not createas much stresses as that of completewelding (Fig.9).

2) There were no cracks propagatingin the dished end regions whichwere heat treated.

3) No crack development has beenreported in monolayer stripperswhich are post heat treated.

REPAIR OF STRIPPERS :

After discussing with Snamprogettiand Kobe Steel, it was decided thatstrippers in Unit-11 and Unit-31should be taken out of service andthe bottom channel be replaced. Aftera lot of technical discussions withLarsen & Toubro (L&T), Bombay,India, the job was given to them.

The Unit-11 stripper was sent forrepair first. The details of repairwork carried out by L&T are :

RECTIFICATION/INVESTIGATION :

The investigation of the extent ofdamage and the rectification work ofthe subject stripper was entrusted toLarsen & Toubro, Bombay, a renownedIndian fabricator having expertise inmanufacturing vessels with exoticmaterials.

A thorough investigation of theequipment by non destructiveexaminations and other tests couldonly identify and segregate thecomponents which requiredreplacement and those which could bereused.

For the bottom channel, theinternal titanium strips covering thecircumferential weld seams werecarefully removed by L & T.Thereafter the weld between themultilayered channel shell and thetubesheet was carefully cut ensuringno damage to titanium parts. Gasgouging of weidment was done up toabout 100 mm depth and arc airgouging for the balance thickness.Purging of the titanium surfaces frominside of the channel with argon wasdone during gouging of the outerlayers of the channel shell»

A special arrangement for Argonpurging was made inside the channelshell, near the circumferential seamsto keep the titanium surfaces coolwhich protected them from oxidation.As an alternative, water cooling wasalso tried to cool the titaniumsurfaces. The weld between thechannel shell and the hemisphericalhead was also parted taking similarprecautions.

Carbon steel layers of the bottomchannel shell were removed bygouging/grinding without damagingthe inside Ti clad CS shell,

349

so that it could be reclaimed inusable condition. CS layers of thechannel were initially gougedlongitudinally, till reaching thesecond layer and thereafter weremachined.

L & T fabricated the substitutecarbon steel channel shell bymul ti w all technique. Monowallconstruction was not considered toavoid a longer procurement time. Themultiwall channel shell could befabricated using readily availablematerials.

Nozzle 1CNÂ was removed bymachining/grinding with utmostcaution to avoid damaging titaniumportion.

As the CS portion of nozzle 1CNAwas found unusable, a new forged CSneck was procured on urgent basisand the same was welded to the newmultiwall channel.

*

After machining the ID of the newmultiwall channel shell (manufacturedby shrink fitting 6 shell courses of28+18+22+30+30+30 mm), the same wasshrink fitted on the reclaimed Ti cladCS lined shell. This was done aftertaking a skim cut on the outerdiameter of the CS portion of thereclaimed Ti clad CS lined shell.

The leak detecting tubes (weepholes) were also reclaimed bycarefully removing the outer couplingand grinding off the weld betweenthe leak tube and the Ti clad liner.

Extreme care had to be taken forrepair/rectification work to avoiddamage to the exchanger componentshaving titanium materials. Titaniumcomponents were handled avoidingiron contamination by covering themwith plastic sheets/brown papers andby avoiding exposure to dustyatmospheric conditions.

The titanium surface of the inner

clad shell was continuously monitoredduring various stages of fabricationto avoid decolouration and/orexcessive temperatures to ensure thatno oxidation of the titanium surfacestook place.

Ail weld seams including those onthe titanium liners were examined byliquid penetrant testing (PT) to lookfor defects, prior to starting weldingoperations for rectification.

The tubesheet and hemisphericalhead were examined for the presenceof cracks, using NOT methods such asLiquid penetrant testing (PT),Magnetic particle testing (MPT) andUltrosonic testing(UT).

Various NOT checks were also doneduring fabrication on the completeequipment including strict qualitycontrol on the repair work. This wasdone to enable certifyingacceptability of the components. Someof them are given below.

- PT examination of all exisiting weldsfrom outside and on the titaniumliners from inside.

- RT of longitudinal seams of eachshell course of multiwall channel.

Grooves were made on theseparating lines of the shrink fittedshell courses of the multiwail channeland filled by welding and PT/MTexamined.

- PWHT (post weld heat treatment) ofchannel shell after shrink fitting allshell courses concentrically and afterwelding CS forging for nozzle 1CNAon to the same was carried out. PWHTis not a mandatory requirement fromcode point of view. Yet L & T carriedout the same to remove any residualstresses caused during fabrication.

- PT/MT examinations of weld edgeson tubesheet, hemispherical headand channel shell. (The weld edges

350

had to be weld built up at a fewspots and re-prepared for buttwelding).

- UT examination of a 100 mm wideband at the edge of hemi- sphericalhead to detect defects if any.

- UT examination of CS nozzle neckto multiwall channel shell welding.

After completion of thecircumferential seams between themultiwall channel shell and thetubesheet/hemisperical head,titanium insert plates were fitted.Thefitting required high skill and a lotof precautions to ensure that no gapsremained underneath. This wasrequired to ensure that weldsconnecting titanium cover strips didnot fail under channel side pressure.All titanium welds were visuallyinspected to detect decolouration andPT examined to ensure freedom fromdefects.

- All tube to tubesheet welds weresubjected to a helium leak detectiontest by filling and pressurisinghelium on the shell side.

Titanium cover strips werepneumatically and helium leaktested before hydrotesting. A Heliumleak test was also repeated afterhydrotesting.

- Equipment was finally subjected tohydrotesting on both the shell sideand tube side at pressures of 34.7bar and 221.9 bar respectively.

- Rectification also required a numberof special operations to be carriedout, some of which are listed below :

i) Cutting of special threads on CSforgings for nozzle neck 1CNA wasrequired. This was done to match thethreads on the reclaimed titaniumliner.

ii) In order to dismantle the

threaded titanium liner, dry ice wasused to shrink the iiner.This wasrequired prior to unscrewing theliner since it was badly jammed. Thethreading on the reclaimed titaniumliner of nozzle 1CNA was founddamaged. The repair was done byweld buildup and subsequentreth read ing.

iii) In order to ensure matching withthe existing pipe lines, it wasnecessary to maintain the originaldimensions, specifically for nozzlelocations and projections. Thisnecessitated limiting the reduction inheight of the tubesheet hub to 6 to8 mm while parting the same from theexisting channel by removing theexisting weld metal including heataffected zone.

Rectification of the hemisphericalhead and the subsequent machiningfor preparing the weld edge resultedin a marginal reduction in the length.To match this, the same additionallength was provided on the multiwailchannel.

Also, weld buildup had to becarried out on the CS portion of theinner Ti clad CS shell on both ends(Fig.10).

Extreme precautions had to betaken during the weld build up toavoid damage to the titaniumcladding. Titanium cladding wasstripped off upto about 35 mm awayfrom the edges to enable the CS weldbui I tup on the CS portion to increasethe length. Arrangement/monitoringwas done to ensure that thetemperature of the titanium surfacedid not increase above 200°C. Thestripped off portion of titanium wasmade up using extra wide titaniuminsert strips and cover strips.

- Fabrication of the multiwall channelinvolved shrink fitting of the shellcourses with strict control on gapsbetween the different layers.

351

This necessitated controlling thecircumferences of the fabricatedindividual shell courses within about0.5 mm and a highly skilled operationof shrink fitting.

- In order to salvage the titaniumleak tubes (weepholes), cutting of theCS layers of existing channel wasdone in four/five sectors. Also, inorder to compensate for the reductionin length of the weephole tubes, thedepth of the counter bore on thechannel shell had to be increased.

- In order to carry out investigationson site, a number of deep scoop ingswere done by gouging circumferentialseams of the channel to headincluding the adjoining base metal.The sizes of the scooped out areasranged between 125 mm to 150 mmlong, 35 mm to 50 mm wide and 40 mmto 90 mm deep. One spot wasscooped out near the tubesheet tochannel shell joint while 8 spots werescooped out near the hemisphericalhead to channel joint (Fig. 11).

To repair the above scooped outareas which possesed linear cracks asrevea led on s u b s e q u e n tinvestigations^ specially developedtechnique was adopted. Existing weldmetal and heat affected zones wereremoved completely and verified byan etching test. The linear defectswere removed by grinding andcleared by PT examination. Thescooped out areas/grooves were builtup using special electrodes ofspecific sizes. Controlled preheat andinterpass temperatures were applied.An Initial layer was deposited using3.2 mm dia. electrodes andsubsequent layers with 4 mm dia.electrodes.

In order to make up thedimensions, weld deposition was alsodone on the weld edge of thehemisherical head. Weld repaired/builtup areas were post heated at 200 to250 °C for four hours immediately

after the welding. They were thenallowed to cool to room temperature.The weld repaired portion and aband of about 100 mm width fromdished end edge were PT and UTexamined after 24 hours to detectdelayed cracking. Subsequently, therepaired areas were also RT examined.All precautions were taken to ensurethat no damage occured to thetitanium surfaces during weld repair/build up by employing argonpurging, monitoring of colour,temperatures, etc.

- After separating the inner Ti cladshell a few cracks were noticed onthe CS* portion. These were groundout and the areas were weld built upwith due precautions ensuring nodamage occured to titanium surfaces.

- Titanium surfaces were thoroughlycleaned and subjected to Ferroxyltest to ensure freedom from ironcontamination.

UREA STRIPPER UNIT-31 :

With the experiences of the Unit-11stripper failure, the Unit-31 stripperinspection job was started. Onremoving insulation from the bottomchannel shell, visual cracks wereobserved near the 1CNA dummy inletnozzle. The crack location was identi-cal to that in Unit-11, but the crackswere of less severe nature.

CRACKS LOCATION :

i) 1 CNA dummy inlet nozzle tochannel shell weld.

ii) Outermost (9th) layer of themuitilayered channel shell.

iii) 1CNA dummy inlet nozzle body.

INVESTIGATIONS :

Ultrasonic testing of the 1CNAnozzle body was done to determinethe crack depth. The depth of five

352

radial cracks (going transversethrough the nozzle to the channelshell weld) was determined as shownin Fig.12. The UT indicated a depthof 112 mm in the crack No.2 (2ndfrom left) and 40 to 45 mm depth asdetailed in Fig. 13.

REPAIR JOB AT SITE :

Bharat Heavy Plates and Vessels,the only company manufacturing themultilayered vessels in India,werecalled for carrying out thespecialised repair job on site. Thejob was carried out as per the repairprocedures given by Kobe Steel.

The outermost 9th layer plate ofsize 800 mm x 650 mm covering thecracked area in the plate was takenout by gouging (Refer to Fig.12). Thecracks in the nozzle body were alsogouged. Cracks at location Nos.1,3,4,&5 (Ref.Fig.13) disappeared aftergouging up to 40 mm depth in thenozzle body. The cracks at locationNo.2 remained even after gougingupto a depth of 80 mm. The cracksin the 8th layer and subsequentlayers of the channel shell weregouged till the cracks disappeared.The radial crack No.2 (Ref.Fig.13) inthe nozzle to channel shell weld jointwas left beyond 80 mm depth, sincecrack removal by gouging couldaffect the integrity of the titaniumliner inside.

All the cracks were welded withE-7018-1 electrodes.dried between200-250° C. Proper pre-heating(100-150 °C) and post heating(100-150'C) with 1 hr. soaking timefor the nozzle weld were strictlyfollowed. For p re-heating and postheating, special LPG burners wereused. All the cracks in the nozzleweld, nozzle body and multilayeredchannel shell wrapper plates werewelded in proper sequence. Duringwelding, Magnetic Particle Inspectionwas carried out at every stage tocheck for any propagation of cracks

in the weld. As stress relieving wasnot possible after welding, all theweld beads were peened using apneumatic blunt chisel. MagneticParticle Inspection was carried out inorder to check for any delayedcracking after completing all thewelding jobs. No indications of crackswere observed.

TESTING :

After completing the job, thestripper was hydraulically tested ata pressure of 161.9 bar (Operatingpressure 142.2 bar) for one hour. Noleakage was observed. The MPI wascarried out again after hydraulictesting. No crack indications wereobserved.

ACOUSTIC EMISSION MONITORING OFUNIT-31 STRIPPER :

An acoustic Emission Technique(AET) was used to monitor on-line thestatus of repaired cracks.

EXPERIMENTALS :

I- channel Ounegan/Endevco make3000 series AE equipment was utilisedfor recording the AE signals. TwoPZT sensors of 175 KHZ frequency(resonant type) along withpreamplifiers of 40 dB fixed gain andwith a band pass filter were used torecord the signals. Three rods werewelded onto the outer most layeraround the nozzle to act aswaveguides. The AE system has afixed voltage threshold of 1 volt.Threfore,the total gain values forboth the channels were selected at 78dB to cut off the background noise(including flow induced noise).

In absence of any baseline dataand in order to classify the AEsources, it was decided to comparethe event/count rate for signalsrecorded after 1 month with the firstmeasurements as the locations ofpossible AE sources were known.

353

Otherwise, with a 2 channel system, itwould have been very difficult tolocate the sources in a cylindricalvessel.

The AE signals were recorded onOctober 18,1991 and November 12,1991.The parameters like pressure,temperature etc. were the same. Alsothe gain and threshold settings of AEunit were the same as during thefirst measurement.

RESULTS AND DISCUSSION :

The process parameters during theAE recording are given in Table-1.Parameters which can affect AEbehavior of material are pressure andto some extent temperature. Theyremained fairly constant during therecording period.

TABLE -1

SPECIFICATION OF UREA-31 STRIPPER

1. Vessel Material

2. Inner Lining

3. Op. top Temp.

4. Op. bottom Temp.

Carbon Steel

Titanium

196'C

206 'C

5. Operating Pressure 141.3 bar

Cumulative events vs.time plots forrecordings on Oct. 18,1991 andNovember 12,1991 are shown in Figs.14 & 15 respectively. As per thestandard recommended practice, asource is considered active if itsevent/count continues to increasewith increasing or constantstimulation. A source is considered tobe critically active if the derivativeof event/count with respect tostimulus continuously increases withincreasing stimulation or with timeunder constant stimulus (Fig.16).

As can be seen in Figs. 14 & 15, theevent rate has increased substantiallyin 25 days under constant pressure.For example on Oct.18, 1991, a total of354 events were recorded during 16hours compared to a total of 312events on November 12, 1991 recordedduring 8 hours (i.e.half of theprevious recording period).Therefore, the AE source (i.e. crackpropagation in all probability) iscritically active. If the source iscritically active, it is indicative ofquestionable structural integrity andshould be evaluated by other NOTMethods.

The experimental evidence clearlyindicates that the source of AE iscritically active which means that thecracks have propagated. This wasalso confirmed by UT. Additionally,the substantial increase in eventrate may be concluded as rapiddeterioration in structure with time.However, it is not possible tocalculate the crack propagationvelocity on the basis of availabledata.

CONCLUSIONS :

It can be concluded that thefailures of two strippers were dueto stress corrosion cracking as aresult of seepage of ammoniacalsolution through the damagedinsulation in combination withresidual stresses in layers of theshell.

The failure could have been avoidedif :

1) The butterfly control valve HV03,was located away from the stripper.

2) No cold condensate was used onthe leaky flanges.

3) Insulation was covered afterwrapping aluminium foil on the shellposition all around.

354

4) Insulation was covered withaluminium sheets with no leakage onjoints.

5) Stress relieving of thecircumferencial joint between theshell channel and head and alsobetween the shell and nozzles wascarried out.

After the repaired strippers inUnit-11 and Unit-31 were installedwith the above precautions, theywere inspected during a recentannual shutdown. The repairedsurface was in good condition.

ACKNOWLEDGEMENT :

The authors wish to acknowledgewith gratitude the support andencouragement given by theManagement of Rashtriya Chemicalsand Fertilizers Ltd. for thepresentation of this paper.

DISCUSSION

G.R. Prescott, Consultant: You showed a detail ofthe leak detecting tube, and if I heard you correctlyyou said it was welded to the titanium liner. Is thatcorrect?Nayar: Yes.Prescott: How does it detect leaks in that case?Nayar: I will try to explain the leak detection withthe help of a slide (see sketch below). There arenine layers of carbon steel clad with 12 mm carbonsteel and 3 mm titanium inner layer (see Diagram 1below). The clad plate joints are welded afterremoving the titanium layer in the vicinity of thewelds (Diagram la). The weld is covered by afiller plate welded to the titanium liner (DiagramIb). The titanium leak detecting tube is welded to

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355

H.P.Pota S.K. Nayer

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Figure 1. Urea stripper.Figure 2. Developed view of cracked area

near nozzle 1CNA.

356

Figure 3. Crack In 8th layer plate.

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t A *TIME (HOURS) -

10

Figure 15. Cumulative events vs. time fortest carried out on Nov. 12,1991.

ACTIVE)

(INACTIVE)

STIMULUS —»•

Figure 16. Classification of AE sources.

359