This is a standby paper for the AlChE Ammonia Symposium. Standbypapers are not presented to or discussed by the Committee attendees.

Failure and Repair of the Shell of aPrimary Waste Heat Boiler on a

1,100 ton/d Ammonia PlantA primary waste heat boilershell installed on a 1,100 ton/d Kelloggammonia plant failed to operate. Inspection and repair procedures,results of failure analyses, and methods for avoiding future failuresare outlined.

A. F. Pariag and I. E. WelchFertilizers of Trinidad & Tobago Ltd., Point Lisas, Trinidad

andG. E. Kerns

Amoco Research Center, Naperville, IL 60566

INTRODUCTION

Fertilizers of Trinidad and Tobago (FERTRIN)operates two 1044 ton/day Kellogg Ammoniaplants (01 and 02) in Point Lisas, Trinidad.The plants have been operating since 1982.Each plant has two vertical bayonet andscabbard tube primary waste heat boilers(PWHB).

The boilers are of typical Kellogg designwhere the process gas is contained by acarbon steel shell. The shell is protectedby a refractory lining with a stainlesssteel liner on the inside. A water jacketcools the outer surface of the shell.

Both as-fabricated 01 plant PWHB's had beenstored in a laydown yard for 4 years priorto plant start-up. On arrival at FERTRIN,both 01 plant PWHB bundles had been removedfor an internal shell inspection. Nodefects were found. However, neither thewater jacket nor the refractory lining hadbeen removed for a detailed shellinspection.

From start-up, FERTRIN has deviated fromKellogg procedure, and has operated theboiler with water jacket overflow ratherthan jacket level control. This procedureensures that the jacket is always full ofwater. Vents were installed on theoverflow lines during start-up of theplant to stop gurgling and agitation in

the water jacket. Both boilers haveoperated trouble tree except for flangeleaks. Attempts to pull the bundles inMay 1984 to correct flange leaks failedbecause bundles were catching against theliner.

On January 9, 1985 at approximately 12:30p.m., the 01 plant was shutdown because ofa leak in the shell of one of the wasteheat boilers.

FIELD INSPECTION AND REPAIR

The upper three feet of the water jacketwas removed in the general area ofsuspected failure. The outer surface ofthe shell, the gas outlet nozzle, and thetop flange welds were cleaned using powerbrushes. A major crack was immediatelyevident at the gas outlet nozzle weld.Dye penetrant and magnetic particleinspections were done in these areas andfurther cracks were identified. Straightbeam and shear wave ultrasonics were used tcdetect delamination and additional crackingin the shell plate, nozzle and associatedwelds. The same areas were inspected usingthe Exxon attenuation procedure to detecthigh temperature hydrogen attack. Hardnessreadings were taken from the bottom of theshell flange to 965 mm below the flange.The readings were consistent on the shellplate and slightly higher at the flangeweld.

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A large section with extensive crackingaround the outlet nozzle was removed, andnew rolled plate sections rewelded in itsplace, as shown in Figure la. The weldpreparation consisted of 37° bevels forthe vertical welds, and for the horizontalwelds 45° on the upper edge and 10° on thelower. Other minor cracks were groundout, and the areas rewelded.

Prior to welding, the shell was heatsoaked at 204°C for five hours to removehydrogen from the metal. The temperaturewas then reduced to 52°C and Gas TungstenArc Welding (GTAW) was used for the firsttwo passes with filler rod ER70S-3. A 1.6mm thick carbon steel backing strip wasused where possible. This precaution wastaken primarily to prevent weldcontamination from the refractory.

The temperature was-then raised to 204°C,and maintained during subsequent fillerpasses by Shielded Metal Arc Welding(SMAW). E7018 electrodes were used with amaximum oxcillation of three times thewire diameter. Immediately after welding,without allowing the metal to cool, postweld heat treatment was done at 593°C plusor minus 14° for two hours. The channelhead and bundle were supported by overheadcrane during the heat treatment. Thisavoided excessive loading on the weldedarea.

METALLURGICAL ANALYSIS

A complete metallurgical analysis was doneon the sections that were removed. Thedetails of the analyses are as follows:

SPECIMEN LOCATIONS

Dye penetrant and magnetic "partieleinspection of the shell OD on the wasteheat boiler had revealed cracking in aregion extending approximately 1370 mm inthe circumferential direction, and 480 mmin the vertical direction. A majorsection of the 44.5 mm thick ASTM A516 Gr.70 shell was removed for metallurgicalexamination. The section was locatedadjacent to (i.e. upper left of) therefractory-lined 711 mm OD process gasoutlet nozzle. The section size was 380mm long (circumferential direction), 44.5mm thick, and varied in height from 255 to890 mm. Additional sections were cut toinclude:

(a) the butt weld between the

shell and top flange, and(b) a water jacket support

bracket, vertically fillet-welded to theshell OD.

Figure Ib shows the orientation ofvisually-observed cracks, and the locationof sections cut from the shell prior tofinal repairs. As shown in Figure 2,visual examination of the sectionsrevealed obvious cracking. Severalmetallography specimens were prepared fromeach shell section. In addition, oxidedeposits from the shell OD (within thewater jacket area) were analyzed.

The orientation of cross-sectionalmetallography specimens (transverse orlongitudinal) was that of the polish planerelative to the longitudinal axis of theshell.

CORROSION DEPOSITS

X-ray diffraction analysis of themagnetic, dark brown scale on thewater-cooled portion of the shell ODrevealed primarily magnetite (Fe-OJ, witha small amount of wuestite (FeO); Energydispersive x-ray analysis (EDXA) revealedapproximately 5 wt% Na, 2 wt% Si, 2 wt%Cl, 1 wt% Al, 1 wt% Mn, with small amountsof Ca, K, S, and Cu. The balance was Fe.The Ca, Si, and Al contamination wasrelieved to be caused by refractory liningmaterials. Sodium chloride found in thedeposit was believed to be as"a result ofcontamination of the surfaces by thecoastal atmosphere. The plant is locatedapproximately 0.8 kilometers from the sea.

METALLOGRAPH1C RESULTS

The microstructure of the steel shellexhibited banding, with ferrite andlamellar pearlite phases. Surfacedecarburization was evident to a depth ofapproximately 1 mm on the shell OD.

Typical of an annealed carbon steelcontaining approximately 0.25 wt% C, themicrostructure is shown in Figure 3. Asshown in Figure 4, cracking on the shellOD exhibited localized corrosion, withsubsequent oxide-filled cracks propagatingnormal to the surface. Figure 4 showssimilar cracking in weld metal between theshell and top flange. The crackpropagation was clearly discontinuous,with regions of the crack broadened by

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corrosion, as seen in Figure 5. Figure 6shows that secondary cracks werefrequently initiated in these regions ofapparent arrest.

The corrosion deposit in the cracks wasanalyzed using both energy and wavelengthdispersive x-ray analysis. The resultsindicated an iron oxide, with an oxygenlevel between Fe30» and Fe203. An x-raydot map for oxygen is shown fn Figure 7.Nucleation of cracks appeared to beassociated with fracture of a protectiveoxide, with subsequent cycles ofmechanical fracture and healing, as shownin Figure 8.

Examination of metallography specimensfrom the six sections shown in Figure Ibrevealed cracks in weld metal, weldheat-affected zone, and base metal. Inseveral cases, crack initiation occurredin weld metal, with propagation throughthe heat-affected zone into base metal.Figure 2b suggests that high profile welddeposits may influence crack initiation byconcentration of macro-stresses on theshell OD surface.

SURFACE ANALYSIS

The shell OD surface was cathodicallystripped of oxide deposits using ENDOX 214solution and inspected. Figure 9 showsthat pitting attack and micro-crackingexisted on the surface of weld metal atthe joint between the shell and topflange.

To determine the cracking mechanism abovethe water jacket, the fracture surface ofthe circumferential weld crack (shell toflange joint) was exposed by mechanicalcutting from the shell ID surface. Theweld fracture surfaces were cleaned usingthe ENDOX technique, and examined using ascanning electron microscope (SEM). Astep-like progression was very evident, asseen in Figure 10. A metallographiccross-section was then prepared by cuttingnormal to the weld fracture surface,through a region of step-likeprogressions. Examination revealedfrequent changes in fracture plane, withassociated secondary cracking (Figure 11).

Since the weld crack did propagate intothe base metal of the shell, the effect ofthe banded microstructure in producing thestep-like fracture pattern was alsoinvestigated. A 50 mm long, 25 mm wide, 3

mm thick specimen was cut from the shellsection just below the shell to flangeweld (section 2, Figure 1). The plane ofthe specimen was parallel to the shell ODsurface with its longitudinal axisparallel to that of the shell. As seen inFigure 12, fracture by slow bendingproduced a ductile, dimpled fracture modeunlike the behavior seen in Figure 10.

CONCLUSIONS

Analysis of the failure revealed anextensive surface cracking pattern on theOD surface of the waste heat boiler shell,extending downward to the center-line ofthe process gas outlet nozzle. Themechanism clearly involves localizedcorrosion, aggravated by periodicmechanical stress. The cracking involvesa slow, step-like progression, and is notrestricted to the water jacket portion ofthe shell. Thermally-Educed surfacestresses due to liquid level fluctuation,local boiling, or splashing within thewater jacket was initially accepted as thefailure mechanism. However, suchconditions of corrosion and surface stresswere not obvious at the flange weld abovethe water jacket.

PREVENTION OF FURTHER OCCURRENCES

The design shell temperature of the wasteheat boiler is 343°C. The designconditions on the jacket water are 49°C onthe inlet and 100°C on the outlet. Thetotal make-up rate was designed at 5443kg/hr (91 litres per minute) which is acombination of boil off and blowdown. Thesurface temperature of the shell duringoperation is significantly lower than343°C because of constant heat removal.It is closer to 100°C as supported byremote Infra Red measurements.

From the beginning of operation, thiswater make-up procedure was not followedand the philosophy adopted was to addwater at a rate sufficient to maintain aconstant overflow. The resulting wateroutlet temperature was in the region of93°C. This condition could only havehelped to reduce thermally inducedstresses due to liquid level fluctuation,local boiling, or splashing. Thus, theoperating philosophy is being maintained.

After the failure, the level of the jacketwater was increased by 51 mm by extendingthe weir around the overflow point. It

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was thought that an increase in levelwould reduce the possibility of thermallyinduced stresses due to liquidfluctuation. Two vent pipes wereinstalled at the top of the jacket toallow steam to be vented.

To alleviate the corrosion problem,FERTRIN is considering: (l) coating theupper exposed area of the shell of theboilers during the next turnaround, and(2) the use of oxygen scavengers to reducethe corrosivity of the cooling water. Inaddition, the water jacket will be raisedsuch that the water will cover the shellup to, and including, the flange weld.

The secondary waste heat boiler (102C) issupported off the structure by springsupports. However, the outlet nozzles ofthe primary waste heat boilers (101CA/CB)

A.F. Pariag 1. E. Welch

Flange

•s, j. 610 mm-270 mm610 mm130 mmJ

Shell outletnozzle S2

Figure 1 a. Sections of shell which were replacedon primary waste heat boiler 101CB.

are rigidly connected to 102C (see Figurele). If the 102C's spring supports arenot adjusted properly, then thepossibility exists that some stresses maybe transmitted from 102C into the shell of101C through the connecting nozzles.Thermal cycling during start-up andshutdown could aggravate this problem ifit exists. This is currently underinvestigation.

Although not discussed in this paper,cracks were also found in the shell of theother boiler on the 01 plant. The crackswere in the same general area as in theboiler that failed. This may be relevantto the possibility of crack propagation bymechanical stress. In addition, no crackswere found when the boilers on the 02plant were inspected. These boilers areidentical to those on the 01 plant.

Shell to flange weld Range\ /\

1370 mm circumferential dim.

Double fillet-welded attachmentrim for waterjacket

BracketCrack (typ)

.Process gasoutlet nozzle S2

*-Shell OD 1380 mm

Figure 1b. Cracking pattern and location of anal-ysis sections on primary waste heat boiler 101CB.

Spring support

Waterjacket

; Process gasoutletnozzle S2

Primary waste Secondary waste Primary wasteheat boiler heat boiler heat boiler101-CA 102-C 101-CB

Figure 1c. Relationship between 101CA/CB and102C.

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Flgyre 2a. Cracking on OD surface, visible atsaw-cut edge of section 2.

Flgyre 3. OD surface cracks in base metal ofshell, longitudinal cross-section through section1 (dark phase is pearlite; light phase is ferrite).Etchant: Nital.

c.

Figure 4a. O D surface cracks in base metal,Figure 2b. Crack at fillet weld below attachment transverse cross-section through section 1. Etch-rim, left edge of section 3. ant: Nital.

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'" k '• A'

- > ^ _

Figure 4b. OD surface crack in weld metal, longi-tudinal cross-section through shell-to-flangeweld, section 2. Etchant: Nital.

Figure 6. Arrest points and oxide-filled second-ary cracks, transverse cross-section through sec-tion 2. Etchant: Nital.

MnS Inclusion

Figure 5. OD surface crack into base metaJ, trans-verse cross-section through section 2 showingarrest points and secondary cracks.

Figure 7a. Localized corrosion and cracking ini-tiated on shell OD surface, transverse cross-section through section 1.

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Figure 7b. X-ray dot map showing oxygen distri- Figure 9. Pitting and cracking on surface of weldbution (white) in Figure 7a region. metal, shell-to-flange joint, section 2.

Figures. Oxide fracture and crack initiation,longitudinal cross-section through section 4.Etchant: Nital.

Flgyre 10. Step-like pattern, fracture surface olshell-to-flange weld, section 2.

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Figure 11. Longitudinal cross-section at fracturesurface of shell-to-flange weld, section 2. Etchant:Nital.

Figure 12. Fracture surface, longitudinal slowbend specimen near shell-to-flange weld crack,section 2.

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