Post-Wel d Heat Treatment of a RepairedSynloop Waste Heat Boiler

Repair of leaking tube-tubesheet connections often requires post-weld heat treatment. This cancreate a problem, since the tubesheet is hindered in expansion by the exchanger head and shell.

A local conductive heat treatment procedure can overcome this problem.

T. L. HuurdemanDSM Fertilizers, Geleen, The Netherlands

H. C. SchrijenDSM Research, Geleen, The Netherlands

located at Geleen in the Netherlands, DSMFertilizers, a division of DSM, operates twoammonia plants, each with a nameplate capa-city of 1360 mt/d of ammonia.One of these plants was designed and con-structed by M.W. Kellogg, based on Kellogg'sreduced energy amronia technology for steamreforming of natural gas. The plant was com-missioned in July 1984.During the first five years of operationleaks developed in the secondary reformerwaste heat boiler and in the synloop wasteheat boiler.

Repairwork on both boilers required post-weldheattreatment due to the high hydrogen par-tial pressure and the high metal temperatureduring normal operation. According to theNelson curves hydrogen attack could be expec-ted.

However, heattreatment of a tubesheet invol-ves risks due to the high thermal stressesinvolved when only the tubesheet and not thecomplete boiler is heated.These risks showed up very clearly during theheattreatment of the repaired tubesheets ofthe secondary reformer waste heat boiler.

The heattreatment was done by heating thecomplete tubesheet with electrically heatedmats.This method had several disadvantages:- heating of the complete tubesheet resultedin high thermal stresses causing crack for-mation in areas that were originally soundand free of cracks;

- a complete heat cycle took about 100 hoursand lasted considerably longer than thetime needed for the actual repair work.

Therefore a new method for heattreatment hasbeen developed which only-supplied heat tothe repaired sections of the tubesheet (par-tial heattreatment).The method has been developed by DSM in co-operation with an outside contractor specia-lized in heattreatmnet of equipment.After thoroughly testing by DSM Research, themethod was twice successfully used during therepair of the synloop boiler.

The method implies less risks and has theadditional advantage that the heattreatmentis less time consuming.

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This paper describes the development of thenew irethod and the experiences gained withheattreatment of repaired tubesheets on bothboilers. Also design improvements for a newsynloop boiler are discussed.

Description of the steam system

HP-steam (125 bar) is generated in three boi-lers, downstream of:- the secondary reformer- the high temperature shift converter- the ammonia converter.

tubesideinlet temp,outlet temp,pressure

shell-sideinlet temp,outlet temp,pressure

synloopboiler

processgas489 °C316 °C208 bar

water water/steam190 °C 328 °C320 °C 328 °C125 bar 125 bar

Case history of the secondary reformerwaste heat boiler

Although the plant has been designed byKellogg, the boilers are not the conventionalKellogg design used in other units.DSM selected boilers with thin tubesheets anda reinforcing structure behind the tubesheets.Tubes have been welded to the tubesheet byinternal bore welding.

All three boilers have natural circulation ofwater and are connected to one common steam-drum.

The secondary reformer waste heat boiler has300 straight tubes and a central core tubewith an internal gas by-pass valve for outlettemperature control. Tubes and tube-sheet ma-terial is 13 CrMo44 (1 Cr, 0.5 Mo).Tube dimensions: 51 mm OD, 5 mm wallthickness.

The synloopboiler has a U-tube bundle with260 tubes.Tubes and tube-sheet material is 10 CrMo 910(2 1/4 Cr - 1 Mo).Tube dimensions: 31.8 mm OD, 3.6 mm wallthick-ness.

Normal operating conditions

secondary reformerwaste heatboiler

tubesideinlet temp,outlet temp,pressure

shell-sideinlet temp,outlet temp,pressure

processgas1013 °C686 °C35 bar

water/steam328 °C328 °C125 bar

Condensate from the surface condenser to-gether with process-condensate and deminera-lized water is used as boiler feedwater.Condensate from the surface condenser passesthrough a mixed bed polisher. For the regene-ration of the mixed bed polisher hydrochloricacid and caustic soda are used.

In December 1984, during a regeneration ofthe mixed bed, hydrochloric acid enteredthe boiler feed water system by mistake.

This resulted in a catastrophic damage ofthe secondary reformer waste heat boilertubes:- 148 tubes had a deep inward bulge, thesize of an egg, directly after the fer-rules at the hot gas inlet of the boiler;

- 49 of these bulges were bursted open,creating a leak from the 125 bar steam/water side of the boiler to the processgasside.

The boiler was repaired by:- cutting all tubes loose from both tube-sheets;

- welding a 1 meter tubepiece to every tubeat the cold end;

- pushing the tubes in place again and re-moving the extending damaged section;

- rewelding of all tubes to the tubesheet;- postweld heattreatment with electricallyheated mats of both tubesheets.

Heattreatment of the inlet tubesheet, withthe tubes not welded to the outlet tubesheet,created no problems. However, the heattreat-ment of the outlet tubesheet, with the tubewelded to both tubesheets, created tremen-dous problems. After the heattreatment manycracks were found in the tube - tubesheetswelds, even in welds of the inlet tube sheet,due to high thermal stresses. This requiredextensive repairwork.

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Succeeding heattreatment was carried out inthe same way but new with adapted heating andcooling-down rates. Altogether the heating ofa complete tube sheet with electrically heatedmats proved to be a very risky method withrisks of uneven temperature distribution andhigh stresses on the tubes resulting in weldcrack formation.Therefore research was started to develop amodified heating method.However, before a suitable method was deve-loped another repair in the boiler was neces-sary in June 1986.

During a scheduled turnaround a ruptured tubewas discovered.Probably this rupture occurred during shut-down, since the boiler had not caused problemsduring operation.The tube failure had been caused by over-heating due to excessive build-up of loosescale at the waterside of the tube.Water from the leaking tube quenched the inlettubesheet resulting in many cracks in the ad-jacent tube-tubesheet welds and in the tube-sheet itself.

After repair of the damage, heattreatment wasrequired.Heattreatment of the complete tubesheet wasrejected because of the risks involved.Instead of using electrically heated mats, aninductive heating method was used, supplyingheat only to one single tube-tubesheet weldat a time.This method was not successfull since it wasnot possible to reach the required tempera-tures in every zone of the weld and the methodwas very difficult to control.Despite the inadequate heattreatment the plantwas restarted and the boiler has been in ope-ration since April 1986 without further pro-blems.

However, the development of a method for par-tial heattreatment of repaired tube-tubesheetwelds was urgently required.

Theoretical background

One of the reasons why Cr- and Mo-alloy steelsare used in hydrogen-containing media at hightemperatures and pressures is that thesemetals are resistant to the so-called Nelsonhydrogen attack. The resistance to this formof intergranular cracking is provided by thepresence of stable mixed carbides.However, welding leads to the formation ofinstable carbides in the filler metal and the

heat-affected zones. The interaction ofhydrogen at a certain pressure and tempera-ture causes intergranular cracking and inter-nal decarburization.

During the welding procedure, the stablemixed carbides are dissolved and Fe_C andFe.,C carbides are formed in the wela and inthe heat-affected zone. The low energy offormation of these carbides implies insta-bility under the current process conditions.

The following reaction takes place:Fe2C + Fe3C + 8 (H) > 2 CH4 + 5 Fe +

(cracks)

This means that the metal will have to besubjected to heattreatment after welding toimprove the structure of the metal by for-ming stable mixed carbides of the (MbCrFe)_Cand the (MoCrFeKC type.

•J

The IIW research about carbide formation re-ported in Doc.1159-80 has shown that e.g.13CrMo44 has to be heated at 675 °C for 20minutes.Therefore it is not possible to avoid theheattreatment by means of temper bead wel-ding.

It should be noted here that the temperatu-res of this heattreatment may differ fromthose specified in the various codes forstress relieving cycles. For example in theGerman code, a stress relieving temperatureof 600-700 °C is specified for ICr,0.5Mb.A temperature of over 650 °C is required toobtain resistance to hydrogen attack.A combination of these two heattreatmentsleads to an heattreatment temperature of650-700 °C.Post weld heattreatment of an entire tube-sheet with all its welds under nonexpansion-prohibiting conditions reduces weld stressesand improves the structure of the metal.It has been found that post weld heattreat-ment of an entire tubesheet, as is usuallyrequired, may cause unforeseen seriousdamage due to the complex tubesheet-tube-stiffener-etc. geometry.The heat distribution of such a complex con-figuration can hardly be approached or pre-dicted, as has been confirmed in straingauge and temperature measurements in prac-tical situations.

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In principle, partial post weld heattreatmentof a repaired weld leads only to the desiredimprovement of the micros tructure and reduc-tion of hardness, since annealing underexpansion/shrinkage-prohibiting conditionsresults in deformation and stresses duringcooling. In general post weld heattreatmentof a complete exchanger should therefore bepreferred.The degree of deformation and the stressmeasured depend on the geometry and the free-dom to expand. The residual stress resultingfrom cooling from about 710 °C is at most theyield stress and the resultant plastic defor-mation amounts to 0,6 - 0,7 % (Figure 1).

Partial post weld heattreatment is thereforeperformed to improve the microstructure andnot to relieve stresses. This annealingmethod imposes stringent requirements on theintegrity of the repaired zone.

The mentioned stresses do not constitute aproblem because brittle fracture and planestrain cannot occur under the current processconditions with a tube sheet thickness of20 mm. Moreover, a 'sound' weld is capable ofabsorbing the plastic deformation resultingfrom this annealing cycle.

It should be noted that the mentioned stresslevel applies to the starting condition only.During operation relaxation of this "locked-in" stress will occur. The degree of relaxa-tion depends on the operating temperature.

Local heattreatment procedure

Local heattreatment can be effected by meansof induction, conduction or flame softening.However, in the situation under review, flamesoftening is not allowed due to the directcontact between the metal and the flame.

Local inductive heating was carried out bymeans of mediumfrequency inductive heating.In this method a water-cooled coil is incor-porated in the tube and the tubesheet. A re-presentative mock-up model was used for thetests. From the many tests carried out underprocess conditions it became clear that thismethod is extremely difficult to control.The majority of the tests failed.

It was decided to develop a local conductiveheattreatment in cooperation with an externalheattreatment contractor. A special test rigwas built to perform the tests.

A copper rod was inserted in the tubesheetand was externally heated by means of aCooper element (Figure 2).Heated copper bars were also inserted in thesurrounding tube-tubesheet weld connectionsto obtain even temperature and stress distri-butions and gradients. The copper bars wereprovided with a thermocouple to enable tem-perature control to prevent the bars frommelting. Molten copper may cause liquidmetal embrittlement. The geometry of the barswas adapted to the tube-tubesheet weld.Allowance was made for differences in expan-sion and the resulting gap was filled withthermal putty. Each copper bar was indivi-dually temperature controlled. The tubesheetgeometry, tube dimensions, materials, fillermetals, temperatures and welding parameterswere the same as in the practical situation(photo 1).

The following procedure was agreed upon:- to use a maximum heating/cooling rate of250 °C/hour;

- to maintain a temperature of 650-700 °Cfor 30 minutes for the entire weld, i.e.the weld itself, the HAZ of the tube andthe HAZ of the tubesheet;

- to record the temperatures measured atseveral points around the circumference,also at the backside of the test sheet;

- to control the temperature of the copperbars to prevent them from melting.

Three heating tests were carried out(photo 2):- two in tube A;- one in tube B to obtain an impression ofa tube at the edge of the tubesheet.

Photos 3 up to and including 8 give an im-pression of the heattreatment test with thetest rig.

The locally heattreated welds were subjectedto several tests.

- No cracks were observed in non-destructivetests.

- The Vickers hardness values measured inthe weld varied from 180 to 205 Hv.

- Investigation of the structure of the me-tal with the aid of replicas revealed anormal tempered ferrite-bainite structure.

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- Residual stress measurements were carriedout at the narrow and the broad ligamentsof the tubesheet with the aid of stress re-laxation method (photos 9 and 10).The method used is the so-called Pfender-method.In this test the decrease in distance be-tween two fixed points on the test specimenis measured after grinding a groove betweenthe measuring points. The decrease in dis-tance is a measure for the internal stress.The results are given in tables 1 and 2.

Summarizing the test results can be concluded:

- The post weld heattreatment tests were suc-cessfull. Desired temperatures were reached.

- Partial heattreatment does not lead tocracking.

- The desired structure and hardness valuesare obtained.

- The residual stress level in the tubesheetis highest at the narrow ligament, where itamounts to 185 N/mm2. This is about 50 % ofthe yield strength of the starting material.

The question arose as to the possibility ofmultiple heattreatrtent cycles.

Its was decided to perform the total post weldheattreatment cycle ten times under the fol-lowing conditions:

- a maximum cooling rate of 250 °C/hour;- a maximum heating rate of 200 °C/hour;- an annealing time of 1 hour;- an annealing temperature of 650 - 700 °C;- record and control the temperature with theaid of thermocouples at several pointsaround the circumference, at the back aswell as the front of the tubesheet;

- control the temperature of the sheet source;- perform tests after the 5th and the 10thannealing cycle.

No unusual phenomena were observed in magne-tic particle tests carried out after the 5thand the 10th heattreatment cycle.

The micro structure of the weld metal was thatof a tempered ferrite-bainite with an averagehardness of 160 Hv.

A sample was taken from the weld after it hadbeen subjected to several heattreatment cyc-les. This sample, which was taken from thebroad ligament and may have incorporated

filler metal and part of the heat-affectedzone, was subjected to a tensile test.Another test bar obtained form a non-affectedpart of the test sheet served as a reference.

The results given in table 4 show a slightdecrease in mechanical properties. However,even these decreased properties still meetthe requirements of lCr,0.5Mo specified inDIN 17155.

The results of the measurement of residualstress after several heattreatment cyclesare given in table 3. The position of themeasuring points is given in figure 3.

The residual stress measured in tangentialdirection at the narrow ligament was so highthat we may assume exceedance of the yieldpoint and plastic deformation. A low residualstress was measured at the broad ligament.No plastic deformation had occurred here,therefore it is assumed that the thermal ex-pansion of the material in this zone causespartial elastic deformation of the surroun-ding material.

Based on these results can be concluded:

- In the case of sound starting materialsmultiple partial heattreatment of repairedtube-tubesheet welds has the desired effectfor the material structure.

- No cracking occurs and the hardness andmechanical properties of the annealedmetal meet the requirements.

Case history of the synloop boiler

In 1986 it became evident that there was aleak in the synloop boiler. Ammonia was foundin the boiler water and the steamcondensate.Also hydrogen accumulated in the surfacecondensor, creating a safety hazard of avery specific nature. Therefore the plant wasshut down in April 1987 to repair the syn-loop boiler.During a pressure test 5 leaks were detected.All leaks were found in the tube materialdirectly after the internal bore weld.Magnetic particle inspection revealedanother 15 tubes with cracks.Most of these cracks were found in the tubematerial directly adjacent to the weld.Also cracks in the tube-tubesheet welds werefound.

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Investigations of the cracks in the tube ma-terial revealed that in manufacturing theboiler sagging of the weld material into thetube had taken place. This weld material hadbeen removed later on and exactly at thoseplaces crack formation was found.Since the minimum required tube wall thicknessis 2.8 mm, with an actual wall thickness of3.6 mm, cracks no deeper than 0.8 mm could beremoved by grinding. Spots, where grinding inexcess of 0.8 mm was necessary to remove thecracks, were repaired by welding.

The post-weld heattreatmsnt was carried outwith the new developed conductive heattreat-ment method (see photo 11).Heat was only supplied to the repaired areas.The method proved to be very successful andthe whole procedure took considerably lesstime than a heattreatment of a complete tube-sheet with electrically heated mats.

Directly after the plant was restarted thesynloop had to be shut down and depressurizedthree times. This was caused by:- a gasleakage and fire on a flange connec-tion in the cold-shot line to the ammoniaconverter;

- a leaking manhole cover on the ammoniachiller;

- a gasleakage and fire on a flange connec-tion of a by-pass temperature control valveof the synloop boiler.

After these three temperature/pressure cyclesof the synloop the boiler was leaking again.The leakage stayed within reasonable limitsuntil a plant trip in January 1988 when theleakage increased. Therefore it was decidedto shut down the plant in April 1988.

A leak test with 40 bar nitrogen and magneticparticle inspection revealed:- 4 leakages;- 35 cracks in tube-tubesheet welds;- 5 cracks in ligaments between tubes.After grinding out the cracks, 11 spots hadto be repaired by welding.

Subsequent magnetic particle inspection ofthe repaired spots and their vicinity showedno defects, however. X-ray examination re-vealed two defects.After repair the heattreatment was started.Heattreatment was done in three sections ofthe tubesheet.Heating rate and cooling rate were the sameas used during the repair in 1987.

Heat rate: max. 200 °C/hourCooling rate: max. 250 °C/hour to 350 °C,

thereafter cooling with therepaired spots covered withinsulation.

The repaired zones had to reach a tempera-ture of 680 - 720 °C during one hour.This temperature was reached without pro-blems in all three areas.

Magnetic particle inspection of the tube-sheet after heattreatment revealed 96 cracks.All cracks were mainly found in tube-tube-sheet welds and in the ligaments in theneighbourhood of the heat treated areas.During grinding cracks were found justbeneath the surface of the material, thecracks running parallel with the surface.Detection of these cracks with magnetic par-ticle inspection is not possible. The natureof these cracks is not clear and is subjectfor further investigation. These smallcracks propagate during thermal- and/orpressure cycles of the equipment.Therefore it was necessary to decreaseheating- and cooling rates for further heat-treatments .

After grinding 35 spots needed repair be-cause the grinding depth was too large.The repaired spots were checked by magneticparticle tessting. No defects were found.However, X-ray inspection revealed againtwo defects that had to be repaired.

Heattreatnent after the second repair wasdone under the following conditions:- heating rate 200 °C/hour up to 350 °C

150 "C/hour between 350 °Cand 700 °C;

- holding a temperature of 680 - 720 °Cfor one hour;

- cooling rate 50 °C/hour down to 350 °C100 °C/hour from 350 °C to100 °C;

- cooling to ambient temperature in stag-nant air.

The total heattreatnent was performed infour separate cycles.

With thermocouples on the tubesheet thetemperature gradient was measured. Fromthese measurements, it appeared that theboiler-head and -shell absorbed much heat,resulting in a temperature gradient of4 °C/mm.

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Magnetic particle inspection after the secondheattreatment series revealed only minorcracks that disappeared after slight grinding.

Finally hardness was measured in the repairedareas. To prevent hydrogen embrittlement theVickers hardness has to be < 250.However six welds and 2 ligaments showed aVickers hardness between 250 and 300.Since the hardness measurement method usedshowed to be not very accurate (plus oreminus 20 VPN) and given the fact that thetubesheet had already sub-surface cracks, itwas decided to refrain from further heattreat-ment to reduce the hardness.

The plant was restarted and the boiler was inoperation for three months without problems.However, in August 1988 a small leakage wasdetected again. The leakage stayed constantuntil July 1989 when it suddenly increasedafter a plant trip. The plant was kept on-stream until November 1989, then the boilerwas replaced.Inspection of the orginal boiler showed thatthe leakage was in a non-repaired area.

The partial heattreatment method proved tobe very successful for heattreatment of re-paired sections of the synloop boiler tube-sheet .The fact that new leaks developed was relatedto the bad condition of the tube sheet mate-rial.

New synloop boiler

During the repairs and the inspections itbecame evident that the tubesheet materialand the welds were in bad conditon. Moreleaks had to be expected in the future.Out of 60 months of operation the boilershowed leakages during 35 months. Thereforeit was decided to order a new boiler.Various designs were studied. Finally, itwas decided to order the boiler with thesane manufacturer as the existing boiler.Main reason for this decision was that thisrequired minimal changes in the piping lay-out of steam-, water- and gassystems.Furthermore, other designs could create pro-bably other problems and therefore an alreadyknown design was selected.

However, some changes were made in the de-sign of the new boiler:

- internal bore welding was no longer usedfor the tube-tubesheet connections.Tubes were inserted in the tubesheet andwelded to the tubesheet (see figure 4);

- tubes were hydraulicaly expanded in thetubesheet after welding and after postweldheattreatment;

- the tubesheet is covered with an 8 mmInconel 600 weld overlay.In case of a leakage this material allowstube plugging without heattreatment;

- tubesheet thickness increased from 18 mm to50 mm.The tubesheet thickness was increased asfar as the tubesheettemperature allowed.The tubesheet temperature had to staybelow 380 °C to prevent severe nitriding.The increased tubesheet thickness improvedthe effect of hydraulic expansion of thetubes;

- the tube-tubesheet weld was designed forfull pressure, therefore a groove had tobe machined around the tube holes;

- the width of the ligaments was increasedby 4 mm. This resulted in an increase ofthe boiler diameter from 1800 mm to2236 mm.

The new boiler was installed in November1989 and has been in operation without pro-blems up to now.

Summary

When leaking tube-tubesheet connectionsare repaired in an aimonia plant oftenpost-weld heattreatment is needed to improvethe microstructure of the heat affected zoneand weld material to prevent hydrogenattack.Post weld heattreatment of complete tube-sheets creates a problem, since the tube-sheet is hindered in expansion by the sur-rounding exchanger head and shell.This can cause severe damage.

A local conductive heattreatment has beendeveloped. The method was extensively tes-ted in the laboratory and has been usedtwice successfully during heattreatment ofa repaired synloop boiler.

This local heattreatment leads only to thedesired microstructural improvement andsufficient reduction of hardness of thematerial and does not relieve stresses.However stress relaxation will take placeduring operation.

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The level depends on the operating tempera-ture.However, the plastic deformation caused bythe heattreatment is well within the limitsthat the material can cope with.

Advantages of the local heattreatment are:- damages as often occur when an entiretubesheet is heated are prevented;

- the complete heattreatment cycle takes lesstime.

Acknowledgement

The authors gratefully acknowledge the as-sistance of Piek Gloeitechniek, Urmond,The Netherlands in developing this methodfor local post weld heattreatment.

H.C. Schrijen

Photo 1. Laboratory test rig.

Photo 2. Tube-tubesheet geometry.T.L. Huurdeman

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Photo 3. Thermocouples on tubesheet fortemperature control.

Photo 6. Tubesheet and copper bars withheating elements Insulated with ceramicfibre.

Photo 4. Copper bars Inserted In the tubesheet. Photo 7. Heat treatment In progress.

Photo 5. Copper bars with heating elementsInstalled.

Photo 8. Reverse side of tubesheet during heattreatment.

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1

2

3

4

165 N/mm2

40 N/nun2

185 N/mm2

40 N/mm2

Table 1. Residual stress measured at weld A

Photo 9. Tubesheet with stress relaxationmeasurement points.

Photo 10. Detail of tubesheet with position ofstress relaxation measurement points.

la

Ib

2a

2b

3

4a

4b

-60 N/mm2

0 N/mm2

- 165 N/mm*

80 N/mm*

185 N/mm*

20 N/mm2

70 N/mm2

Table 2. Residual stress measured at weld B.

Photo 11. Repaired tubesheet with heatingelements and thermocouples Installed.

1

2

3

252 N/mm2

42 N/mm2

315 N/mra2

Table 3. Residual stress measured after 10 heattreatment cycles.

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Starting material

DIN 17155 (13Cr«o44)

yieldstrength

386.6

2 295

RTO N/mm2

tensilestrenght

516.1

440-580

A*

25.6

20

z%

73

Table. 4 Results of tensile test.

*-Temp.

Figure 1. Residual stresses and deformationduring cooling

heating element

heating elementtempérature controk

thermal putty

Figure 2. Copper bar used for partial heattreatment Inserted In tubesheet.

j tensile testspecimen

Figure 3. Residual stress measurement ontubesheet ligaments.

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HVDRAUUC EXPANSION->• •*—

Figure 4. Tube-tubesheet connection: newboller.

Max Appl, BASF: You mentioned the change of thetubesheet design for the syngas loop boiler. Would yourecommend this sort of change or modification to the boilerafter the reformer too?Huurdeman: We happen to be in the position that we don'thave to think about that, since we already have a spareboiler. There's always a risk about using this method forthat boiler also. We have a boiler in operation in the secondammonia plant with that type of construction but that is a60-bar boiler, not a 125-bar boiler.S. E. Alban, ICI: Do you have any more details ortheories concerning the deterioration mechanism of the 21/4

CrMo tubesheet?Huurdeman: During the inspection made after we found theleak in the boiler, we concluded that the tubesheet materialwas very bad. We found a lot of subsurface cracks. Whenwe started grinding to remove the cracks that were visible,we even found more cracks below the tubesheet surface. As Ialready mentioned, it is part of future investigation for us todetermine if replacing a tubesheet of such a boiler isimpossible. You have to order a new boiler, and that's whatwe did. But the old boiler is still on site and we will dosome test work on it to verify the nature of these subsurfacecracks.

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