Repair of a Thick-Walled AmmoniaSynthesis Converter Containing a Leak

After eight years of service, a leakage was observed within a 175 mm thick girth

seam of an ammonia converter running at 400°C. The cracked weid seam was cutout completely and welded anew. To avoid any undesired formation ofresidual

stresses by the subsequent post-weld heat treatment, a new procedure oflocal heattreatment was developed.

Albert HeuserBASF AG, Ludwigshafen, Germany

DETECTION OF THE DAMAGE

The synloop of the NHg plantNo. 4 at Ludwigshafen, using theC.F. Braun process, has two con-verters in series. Started up in1982r the plant was in serviceabout 8 years. On January 18th 1990the alert shift personnel noticedwhite spots on the ground below thesecond converter and run-off trackson the sheet metal covering of theheat insulation after light rain. Ahot spot on the sheet metal co-vering and explosimeter indicationspointed to a leak in the lowerconverter region. This paper des-cribes the damage, repair and firstexaminations performed. A secondpaper entitled "Hydrogen Attack in2 1/4 Cr-1 Mo Steel Below Nelson'sCurve, Caused by Artraionia SynthesisGas" by G. Wagner, A. Heuser and G.Heinke [1.] gives the results of thedifferent examinations and descri-bes the reason for the damage.

BASF Aktiengesellschaft,

Ludwigshafen/Rhein, Germany

The plant was immediatelytaken out of operation; the synloopwas partially still in the hotstate, filled with hot nitrogen andthen run down slowly - over severaldays. This plant procedure was la-ter to prove to have been very ad-vantageous.

After the heat insulation hadbeen taken off in the area of thebottom girth weid, a small spot wasfound on the converter wall, wherethe Zn dust paint was noticeablywhitened. By means of a magneticparticle (MP) test, a crack about8 cm long in the circumferentialdirection was found on the outerconverter surface within the areaof the light spot in the paint.From its central region, which wasonly about 15 mm long, gas (N2 atthis point in time) escaped becauseof the small residual pressurestill present in the converterduring this test.

By means of a first ultrasonic(ÜS) test ing at a few areas onlyfrom the outside with oblique acou-stic incidence from above, it wasfound that

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- a crack from the inside, whichhad penetrated more than halfthe wall thickness, was presentover about l m of the cir-cumference on both sides of theleakage point,

- on both sides of the leakage,with a 90° offset to it, therewere indications of cracking,start ing from the inside, to adepth of at least 30% of thewall thickness, and

- an incipient crack of onlyslight depth had to be presentopposite the leakage point.

Thus, there was a virtuallyall-round crack starting from theinside in this weid seam, which ex-cluded an ad hoc repair of the con-verter on site. The converter manu-facturer was commissioned to carryout the repair.

A first ultrasonic test, car-ried out from the outside, on theupstream converter C 701 gave noindications of similarly seriousdefects.

PROCESS AND OPERATING CONDITIONS OFTHE NH-3 SYNTHESIS

The operating conditions ofthe NHg conversion are shown in theprocess diagram (Figure 1). The gasfrom the synthesis loop (with about4 % of NH3 at the inlet, pH2 = 115bar) is first heated to 400°C inthe heat exchanger W 721. It entersthe first converter C 701 frombelow, flows upwards between theshell and the catalyst basket andthen downwards through the catalystbed. As a result of the exothermicformation of NH3, the temperaturerises. The gas leaves the converterat 500°C through a pipe, joined di-rectly to the basket, centrally inthe nozzle and flows through thetubes of heat exchanger W 721 lo-cated directly underneath the con-verter. It is cooled down again to400°C in the heat exchanger andthen passes into the converterC 702 in a similiar manner. The gascoming into contact with the wall

of this converter contains about11% of NH3 at pH2 = 103 bar. A syn-thesis gas enriched to 16% NHg lea-ves the second converter C 702 at480°C and enters the steam boilerW 722, which is located directlyunderneath the converter.

DESCRIPTION OF THE CONVERTERS

The operating conditions -wall temperature 400°C, hydrogenpartial pressures of 103 and 115bar - make it necessary to use asteel which is resistant to hy-drogen attack. With the aid of theNelson diagram (Figure 2) , the ma-terial selection resulted in thechoice of 2 1/4 Cr-1 Mo steel,12 CrMo 9 10 according to theGerman material code, at a cleardistance (> 80°C) from the criticalNelson curve.

The converters (Figures 3 and4) consist of one (C 701) or two(C 702) forged cylindrical shellrings - with a girth weid in themiddle in case of C 702 - to whichthe hemispherical, forged heads arejoined by two further girth weids.All the weids had been made withfiller materials of the same compo-sition by the narrow-gap submerged-arc technique. The agglomeratedpowder UV 420 TTR and the sub-merged-arc filler wire of the samecomposition S l CrMo 2 made byThyssen were used. Bxcept for theclosing seams, all the weids had aback-weld pass from the inside. Forthe closing seams there was aninward-projecting backing aid whichwas turned out of the shell on alathe and ground off after weidingto be flush with the forging sur-face. Thus, there was a notch-freeweid root at the inside surface.

During manufacture, the ves-sels were first finished except forone of the bottom girth weids, andthe two parts were annealed sepa-rately in a furnace at 700°C; theaustenitic basket, transversely di-vided in the cone, was then movedin and the last vessel girth weidwas made - as the "closing seam".The post-weld heat treatment (PWHT)

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of the closing seam was done like-wise at 70Q°C (Figure 5) in a parthousing temporarily constructedaround the converter. On converterC 701, the closing seam was theupper girth weid, and in converterC 702 it was the lower girth weid.

INVESTIGATIONS ON CONVERTER C 702

During the night before theconverter C 702 was transported tothe pressure vessel manufacturer,two boat samples were taken fromthe crack area ascertained from theoutside, and two drill cores (dia-meter 35 mm) through the entirewall were drilled from the basemetal above the closing seam.

Based on the results of metal-lographic, microfractographic andanalytical examinations of thesesamples, it was possible to esta-blish a first damage analysis evenbefore the converter arrived at therepair workshop. In particular,however, a determination of the hy-drogen content along the drill corelength, i.e. across the vessel wallthickness (Figure 6) , demonstratedthat the slow run-down procedurehad acted as an extremely effectivehydrogen effusion annealing, sincethe total hydrogen values - mole-cular and atomic - was at the levelof new material, at values of < lppm. Significantly higher valueswere found only in a layer at most2 mm thick on the inner surface,where molecular hydrogen was oc-cluded in microporosities, as wasfound later Tl] .

As a result of this examina-tion, it was decided to dispensewith a time-consuming hydrogen ef-fusion annealing which would other-wise have had to be carried out be-fore the start of the repair work.This meant a time saving of 10days.

For the repair, the defectiveweid was cut out; flame cut above,mechanical cutting off below theweiding seam. Thus a ring of about100 mm width was available for fur-ther examinations.

The ring was sent to BASFMaterials Technology and dividedthere into segments. On these seg-ments, the depth of penetration ofthe crack or cracks was determined(Figure 7) by means of ultrasonicthrough transmission in the axialdirection from the turned surface.This showed the real extent of thedamage. In addition to the deepcrack which led to the leak, whichitself was only 10 mm long, a fur-ther crack region offset by 120°was found, where the residual wallthickness was no more than 15 to20 mm. The total cracked area was36%. The segments were further sub-divided to enable metallographicsections and to get samples for thedetermination of mechanical-techno-logical data.

The remaining weids were te-sted from the inside by means ofMP-testing with fluorescent powder.It was found that all the weidsshowed separations of a depth of upto 2 mm. Therefore, all the weidswere ground to a depth of about2 mm.

During the first internal in-spection of the equipment, thespotty appearance of the wholeinternal surface was striking(Figure 8).

EXAMINATIONS OF CONVERTER C 701,OTHER EOUIPMENT AND PIPING IN THESYN-LOOP

Since no crack indications hadbeen found in the US-tests from theoutside on C 701, this converterwas not removed from the plant, butopened and emptied for a moredetailed examination of all weids.Crack sequences up to a maximumdepth of 7 mm were likewise foundhere in the closing seam. All otherweids showed small cracks of adepth of l mm. All the crackswere ground out.

The butt weids of the pipingand the heat exchangers W 721 andW 722 exposed to conditions similarto those of the converter wallswere MP-tested - where accessible,

245

from the inside - but nothing wasfound. In the case of weids in-accessible from the inside, it wasestablished by ultrasonic testingfrom the outside that it was atleast impossible for major cracksto be present.

REPAIR OF CONVERTER C 702

Since the important damage inboth converters was restricted tothe closing seams, a stress ana-lysis was carried out. By means ofa finite element (FE) calculation,it was demonstrated that all thejoint weids between the hemisphe-rical head and the cylindricalshell section show near to the in-ner surface of the vessel higherstresses due to the internal pres-sure than does the cylinder wall(Figures 9 and 10) . The axialstresses - acting perpendicularlyto the girth weids and the cracks -show values higher than the tan-gential stresses in the cylindricalregion.

Optimization of PWHT

For a reconstruction of thelocal PWHTs of the closing seams,carried out during the fabricationof the two converters, the annea-ling records and various notes bythe manufacturer and the annealingcompany were checked. The most im-portant point found here was, thatin case of converter C 702, thelower girth seam became the closingseam for fabrication reasons (Fi-gure 4) . As a consequence of thisPWHT - radiant elements in thelocal furnace (Figure 5) - the heatwas only partially introduced intothe vessel wall, because thecylindrical support was alreadywelded on. Therefore the formationof an unsymmetrical and narrow tem-perature field could be presumed.The recorded temperatures corres-ponded to the instructions for theheat treatment of the weid.

For a simulation of the annea-ling process by calculation, thefollowing important conditionsamong others were taken intoaccount:

- annealing temperature: 700°C(actual ± 10 °C)

- holding time: 5 hours 30 minutes(actual: 5 hours 20 minutes)

- heating rate: 12°C/h (actualbetween 350°C and 600°C: 14°C/h)

- cooling: 20°C/h to 20°C (actualbetween 600°C and 350°C: 17QC/h)

- 500 mm wide insulation (25 mmthick*), 950 mm above the clo-sing seam on the outside, i.e.beyond the enclosure (Figure 5)

- assembly weid of the austeniticinternals, not yet made, upperpart of the internals displacedupwards by 280 mm

- insulation of the vessel insideto fill the space between thetwo internal parts and theinside of the closing seam

- air between the internal partsand the vessel wall as insula-ting mediumimpressing of temperatures inthe region of up to 700 mm aboveand below the closing seam, i.e.partial heating-up of the cylin-drical support

- heat transfer coëfficiënt cor-responding to virtually completeinsulation from the surroundingair in the region between theannealing elements and innerwall of the enclosure

- temperature in the vessel inte-rior (air) : linear rise from20°C to 200°C during the hea-ting-up phase, then constant200°C.

The overall FE model developedin this way (detail for the lowergirth weid of C 702 in Figure 11)contained 1434 eight-node elementswith a total of 4797 degrees offreedom for calculating the non-steady temperature field and 924elements with 6406 degrees of free-dom for the stress calculationscarried out for the treatment atvarious points in time. For the FEanalysis, the real elastic-plasticmaterial behavior of weid metal andbase metal was taken into account(bilinear) and the stress reliefcapacity of weid metal and base

Assumed

246

metal was included. The materialdata required for this purpose weredetermined in tests up to 700°C onresidual material from an originalweiding classification sample (Fi~gure 12).

During the PWHT process, thestresses generated up to the end ofthe 5' hour holding phase subsidealmost completely owing to thestress relief, which is relatively"fast" at 700°C. The component isplastically deformed. In the coo-ling phase, the redeformation leadsto a build-up of very considerabletensile stresses owing to the de-creasing stress relief capacity ofthe material and to the un-symmetrical temperature field inthe weid region. Figure 13 showsthe change in the stress distri-bution across the vessel wall inthe closing seam during the annea-ling process, taking the axialcomponent as an example. This app-lies analogously to the tangential,radial and von Mises-stresses.

The picture obtained for thedistribution of the various stres-ses across the wall thickness afterthe end of annealing was as follows(Figure 14) : particularly in thecircumferential direction and evenmore in the axial direction, valuesof tensile stress were reachedwhich represent a considerable lo-cal load on the material. Thus, theaxial stress on the inner vesselwall reached values which are abovethe yield point of the weid metalat room temperature.

Beyond the primary loads ofpressure and temperature, residualstresses represent additional localloads on the material. Since theconverters had been subjected to apressure test with 273 bar at roomtemperature before start-up, thiswas capable of causing an addi-tional change in the residualstress profile. A corresponding FEcalculation was therefore carriedout.

The change in the axial re-sidual stresses across the wall(Figure 15) shows that the pressure

test again leads to plastic flowphenomena, especially on the insideof the vessel wall. As a conse-quence of the resulting stress re-arrangement s, a reduction in theresidual stresses generated duringannealing takes place on the insideof the vessel wall. Similar FE cal-culations for the original PWHT ofC 701 showed a much lower level ofresidual stresses compared withC 702 due to the proper temperaturefield (absence of cyclindricalsupport).

These results show that PWHTin a manner comparable to the ori-ginal fabrication was unsuitablefor repairing. Furnace annealing ofthe complete equipment would cer-tainly have given the least resi-dual stresses, but there were va-rious reasons against carrying outfurnace annealing.

In order to find an optimizedPWHT for the repair weid to achievethe lowest possible residual stres-ses, a number of variants of theheat treatment were numerically si-mulated.

The most important result ofthe simulation calculations wasthat the generation of unfavorableresidual stresses during local an-nealing treatments can be avoidedwith heat input on the inside andoutside of the closing seam and al-so heating of the entire converterto a base temperature. To preventexposure of the converter materialto unnecessarily high temperaturesin the range of temper embrittle-ment, it was decided to carry out alocal PWHT, the entire vessel beingheated to a base temperature of 400to 420°C, which is the operatingtemperature (Figure 16).

In the relevant FE calcula-tion, the total external surfacewas assumed to be insulated. Afterthe phase of heating-up to 400°C,additional local annealing with AT= Q°C- across the wall in the regionof the repair weid was provided. Anannealing zone was simulated with awidth of 480 mm above and 400 mmbelow the closing seam on the out-

247

side and 85 mm above and below theseam on the inside. The time curveof the annealing here correspondedto the original fabrication. Theresult of the FE calculation showsthat, in this PWHT variant, the re-sidual stresses after the heattreatment are drastically reducedas compared with the original fa-brication (Figure 17).

Repair procedure

The repair weiding of con-verter C 702 was carried out usingthe same wire/powder combination asin the original fabrication, butwith the difference that weid metalwas first buffered on in the rootzone, machined and used as backing.After the narrow-gap submerged-arcweid had been made, the backing andthe root weid material were ma-chined out to a depth of about15 mm, and a root-seal pass fromthe inside was made with pre-heating.

For the PWHT, the entire con-verter including the cylindricalsupport was insulated with 80 mmthick mineral fiber mats. Hot airat ~420°C was introduced by meansof a high-speed gas burner throughan upper manhole nozzle of 600 mmdiameter, in order to heat the en-tire vessel to the base temperature(Figure 18) .

The weid seam zone of the con-verter was provided around the en-tire circumference with a frameworkof ceramic resistance-annealingelements, as envisaged in the FEsimulation. The weid seam zone washeated from the inside over a widthof 200 mm to the left and 200 mm tothe right of the weid, again bymeans of ceramic resistance-annea-ling elements.

For the annealing, 54 controlzones on the outside and 20 controlzones on the inside of the wallwere set up in the weid seam zone.One NiCr-Ni thermocouple per con-trol zone for measuring and con-trolling the temperature was fitteddirectly to the material surface bymeans of miniature spot-welding

equipment. The weid seam zone onthe inside was insulated over awidth of 1,220 mm with 25 mm thickceramic fiber.

The heat treatment was moni-tored by means of 120 thermocouplesin total, which were fitted in 11measurement planes (I to XI inFigure 18) on the inside andoutside óf the wall. Overall, aclose approach to the desired tem-perature/time curve was achieved.

SUMMARY

After 8 years of service aleakage was observed within a175 mm thick girth seam of anammonia converter running at 400°C.An ultrasonic inspection showedthat there was a more or less all-round large crack. It was decidedto move the whole converter to theshop of a pressure vessel manu-facturer for repairing. The crackedweid seam between bottom and coursewas cut out completely and weldedanew. To avoid any undesired forma-tion of residual stresses by thesubsequent post-weld heat treat-ment, a new procedure of local heattreatment was developed which hadbeen derived by a detailed finiteelement stress calculation. The re-paired converter was running againafter 14 weeks. Some minor cracksfound in a second converter hadbeen removed in the field.

REFERENCES

|1| Heuser, A.; Wagner, G.; Heinke,G. :Hydrogen Attack in 2 1/4 Cr - lMo Steel below Nelson's curvecaused by Ammonia Synthesis GasAmmonia Safety Symposium, LosAngeles, November 1991

Ik J B Albert Heuser

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converter C 701

closing seam

PH!: 115 bar 103 barNH3: 4 % 11 %

converter C 702

• closingseam

103 bar 95 bar11 % 16 %

!! I,400°C 480 °C

W 722

feed water synthesis gas

Figure 1. Process scheme, NHU-IV-plant,part of conversion.

O 20 40 60 80 100 120 110 160 180 200

hydrogen partial pressure [ bar ]

Figure 2. Operating limits for steels in hydrogenservice to avoid decarburization and fissuring.

200 mm

vessel material: 21/4Cr-1Mobasket: 18Cr-10Ni

pressure:temp.:

design210 bar

432 °C

service-170 bar

S400 °C

mechanicalcut

vessel material: 2'/4Cr-1Mobasket: 18CMONI

pressure:temp.:

design210 bar432 °C

service-170 barS400 "C

Figure 4. Converter C 702.

Figure 5. Local post weid heat treatment,converter C 702.

0,2 0,3 0,50,30,30,7 i j0,9

*0,4 0,3 0,50,40,40,4 0,90,4 0,4 0,5 0,3 0,3 0,4 f,6*

specimen for hydrogen estimation samples

Figure 3. Converter C 701.

Figure 6. Hydrogen content distribution(molecular and diffusible) over the wall ofconverter C 702 (parent material), allindications in ppm.

249

lm 180°

leak

Figure 7. Converter 2: crack path and partitionof cracked welded seam.

Figure 8. Part of inside surface of converter C702.

inside -^ wall thlckness •»- outsldi

Figure 9. Stress distribution (tangentialcomponent) due to service (210 bar, 400°C),converter C 702.

weldfid seam

wall thlckness •»- outside

Figure 10. Stress distribution (axial component)due to service (210 bar, 400°C), converter C702.

local furnace

Figure 11. Structure of FE-net for simulation offirst fabrication.

400

300

"200

100

4 6holding time [ h ]

Figure 12. Time dependent stress decrease atconstant plastic elongation (1%) for differenttemperatures.

250

tensile

+600-,

t-400

4-200

O -

3 -200 -

-400 -

f -600 -J

compression outside

Figure 13. Change of residual stressdistribution (axial component), during postweid heat treatment, first fabrication, converterC 702.

tensile

+600-,

+400

E +200 -

T O -

'S-200 -

-400

-600

155 mm

compression inslde - wall thickness - outside

Figure 14. Residual stress distribution afterpost weid heat treatment, first fabrication,converter C 702.

tensile

l +600 -,

' +400 -

"E +200 -

§ °8 -200

-400 -

-600

compression inside

155 mm

Iduring pressure test (273 bar, 20°C)|

~| during service (210 bar, 400°C)|

outside

Figure 15. Change of residual stressdistribution (axial component), after post weidheat treatment, first fabrication, converter C702.

700-

600-

500-

cu

S

g. 300-

o

200-]

100-

5 h holding period *°'Sh

690°C

heallng<20°C/h

/

/

//

1 10 t

>v/

. 1— 1

ƒ/ t/-f

i

l

1 \>

emp. basisor converter00°C **j°C

coolingmax. 20°C/h

ƒ*

y\\

4t

\p£250°C coollng In air

0 10 20 30 40 50 60 70 80

duration of heat treatment [ h ]

Figure 16. Post weid heat treatment afterrepairing of converter C 702.

tensile

+ 600-,

+ 400

+ 200 -

O -

-200

-400

f -600 J

compression

155 mm

first fabrication

inside - wall thickness - outside

Figure 17. Residual stress distribution (axialcomponent) after post weid heat treatment,converter C 702.

690 "C +10 °C

i n ra iv v vi vii viii ix400 "C

X XI

Figure 18. Post weid heat treatment ofconverter C 702.

251