Inspection of an Anhydrous AmmoniaAtmospheric Pressure Tank

A 25,000-ton anhydrous ammonia atmospheric pressure storage tank was inspected after 11years of service. Inspection revealed one construction defect and 25 groups of predominantly

transgranular stress corrosion cracks in the tank floor at lap weld.

Syed B. AH and Robert E. SmallwoodCynamid, Westwego, LA

INTRODUCTION/BACKGROUND

A 22,700 metric ton (25,000 short ton)ammonia storage tank was internallyinspected after eleven years of service atAmerican Cyanamid's Fortier Plant locatednear the Greater New Orleans area,Louisiana. This paper discussesdecommissioning, inspection, testing,recommissioning details and various processand safety improvements incorporated.

There are two, 25,000 short ton each,ammonia storage tanks at the subjectfacility. In 1986, external inspections ofthe storage tanks and fracture mechanicsanalysis (rupture before leak) studies wereconducted. The inspections revealed thatthere are no major problems. However, thefracture mechanics analysis studies couldnot be rigorously completed due to lack ofsome actual Charpy V-Notch toughness data.The studies were conducted based on mostconservative theoretical estimates. Thestudies revealed that the tanks aremarginal in respect to fracture mechanicscriteria. Ammonia stress corrosioncracking had been considered improbable inan atmospheric pressure tank, but recentEuropean reports indicated otherwise.(l>2)Risk assessment studies were then conductedusing guide frequency, Figure 1. Thesestudies revealed that uninspected tankspose an unacceptable risk. The studiesrecommended that a full inspection andcorrection of any detected defects is

necessary to reduce risk to an acceptablelevel. We also had an interest ineliminating use of the second 25,000 shortton ammonia storage tank.

We decided to inspect the subject tank inconjunction with hydrostatic and acousticemission tests. Engineering drawings,critical paths, plans, contracts,procedures, cost, etc. were thenfinalized.

TANK DESIGN

The subject tank was commissioned in 1978.The tank is 45.1 M (148 feet) in diameterand 21.6 M (70' 8-1/4") high. The tank wasdesigned to API 620, 5th Edition,Supplement No. 3. The tank was originallyhydrostatically tested to a liquid level of14.7 M (48' 4"), but construction notesreveal a second hydrostatic test at near16.2 M (53 feet). As shown in Figure 2,the first seven shell rings are constructedof ASTM 537 Class 2 quenched and temperedsteel while the last two shell rings wereconstructed of A-573 Grade 70 normalized.The shell stiffner, top compression bar and0.61 M (24 inch) bottom manway are alsomade of A-537 Class 2 material.

The tank is insulated with Alumisealconsisting of multilayer aluminum sheathingwith dead air spaces between the layers.

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Pumps are provided at the tank to transferammonia within the complex and forshipments via the Mississippi River and anintrastate pipeline.

DECOMMISSIONING STRATEGY

Various decommissioning methods wereevaluated. Due to the availability of asecond 25,000 short ton tank, problemsassociated with handling of aqua ammoniaand no major time constraint, the followingstrategy was developed and used fordecommis s ioning.

A. Pumping

Use existing pumps until they loseprime. Install a diaphragm pump topump out the ammonia heel.

B. Evaporation

Evaporate remaining ammonia heel byrecycling warm ammonia vapors from theexisting refrigeration compressors andrecover liquid ammonia.

C. Nitrogen Purge

Use nitrogen for final purge, runexisting refrigeration compressors aslong as possible and recover liquidammonia.

D. Flare

Minimize use of an existing flare forfinal purge out.

E. Air Purge

TANK DECOMMISSIONING

The tank decommissioning started in lateMay 1989 using the following sequence.

1. Existing ammonia transfer pumps wereused until they lost their suction ataround 817 metric tons (900 short tons)of ammonia left in the tank.

2. MAY 26, FRIDAY, 3 DAY WEEKEND.Existing barge (shipment) loading pumpswere used until they lost theirsuction. There may have been 180-272metric tons (200-300 short tons) ofammonia left in the tank based on frost

lines.

3. Tried to use newly installed diaphragmpump to pump out remaining heel. Thepump does not prime.

4. Started recycling hot vapors from theexisting tank refrigeration compressorsinto bottom of the tank. Removingliquid ammonia from the ammonia surgedrum on discharge of an ammoniacondenser and transferring it to thesecond ammonia storage tank.

5. MAY 30 AND 31.Still removing liquid. Ammonia levelin the tank was about six inches, i.e.163 metric tons (180 short tons). Notinterested in trying the diaphragm pumpagain. A test shows removing about.014 cubic meters (3.6 GPM) of liquidammonia.

6. JUNE 1.Hooking up steam hoses to theinsulation drain connections, shown inFigure 3, to warm up the tank.

7. Removed one of the insulation panels onnorth end of the tank. Found a lot ofice build up. Figure 3 again.

8. JUNE 2, FRIDAY.Set up a steam header and hoses to heatup tank wall. Removed three morecomplete insulation panels; ice presentin all panels. Set up several steaminjections, Figure 4.

Setting up additional contingency plansfor liquid boil off. Adding warmammonia vapors into the existingammonia transfer and barge pumpssuction and discharge lines to boil offammonia.

9. JUNE 4, SUNDAY.Frost line 25.4 - 38.1 mm (~1 to 1-1/2inches) lower than Friday, June 2.

10. JUNE 5, MONDAY.Frost line 76.2 - 101.6 mm (3-4 inches)lower than Friday; still using bothrefrigeration compressors. Setting upadditional steam connections. Settingup nitrogen injection piping as well.Taking tank's skin temperature readingsas well, Figure 5.

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11. JUNE 6.Frost line dropped about 25.4 mm (1inch) in 24 hours.

12. JUNE 7.Removed Pentax trip valve from theliquid outlet line for overhauls.Frost line has dropped another inch.Liquid level in the tank may be aroundthree inches. Maintaining 304.8 mm (12inches) of water column pressure in thetank. Removing about .015 cubic meters(4 GPM) of ammonia liquid.

13. JUNE 8.Decided not to purchase truck nitrogen;will use nitrogen from an in-houseplant, 1,000 SCFM of nitrogen isavailable from the plant.

14. JUNE 9, FRIDAY.Had a project meeting with an oilremoval contractor to finalizedetails. Frost line is at about 76.2 mm(3 inches).

Stopped warm vapors recycle andshutdown one refrigeration compressor.Tank pressure dropped towards 254 mm(10 inches).

15. JUNE 12, MONDAY.All frost lines have disappeared.

Shutdown compressor to observe increasein tank pressure. Started running onecompressor off and on to hold tankpressure.

NITROGEN PURGE

JUNE 12, 5 P.M.Started nitrogen purge from bottom of thetank to top to establish a plug flow.

JUNE 15.Have consumed about .05 M cubic meters (1.8MSCF) of nitrogen so far. Increasednitrogen flow towards 35.4 cubic meters(1250 SCFM) from 25.5 cubic meters (900SCFM). Checking vapor samples forammonia. Purging all lines with nitrogen.Used about .07 M cubic meters (2.5 MSCF) oftotal nitrogen, worth about $10,000, ifpurchased.

Compressors shutdown.

JUNE 16, A.M.Started sending vapors to an existingflare. Removed two more insulation panelsto finalize scope and cost of insulationrepairs.

Samples taken to check ammoniaconcentration - 0.5% ammonia in top of thetank and 2% in bottom. Ready to start airpurge.

AIR PURGE

Started air purge on June 16. Tankisolation work started. Schedule updated.

OIL REMOVAL

JUNE 19. MONDAY.Air purge complete. Tank isolationcomplete. Opened 0.61 M (24 inch) manway,Figure 6. Oil removal contractor arrives.Took an oil sample thru the manway. Oillooks very good; very little ammonia smellinside the tank. Oil removal using avacuum truck started around 7 P.M.

JUNE 20 AND 21.Oil removal in progress. Used one drum ofcitric acid and a detergent for finalremoval of oil and cleaning. Insulationrepair contractor arrives on June 21.

JUNE 22.Entered tank. Still have some puddles ofoil/water at various spots. Using mops forremoval. Oil removal work completed inabout 3 days. Removed 17 cubic meters(4,500 gallons) of oil.4 P.M. - Closing up the 0.61 M (24 inch)manway for a

hydrostatic test.7 P.M. - Started filling up the tank withtreated (coagulated)

river water, about 1000 GPM (3.785Cu. meters) rate.

HYDRO/ACOUSTIC EMISSION (AE) TESTS

JUNE 23.Acoustic emission contractor arrives.5 P.M. - Water level is up to 5.94 meters(19' 6").

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Insulation contractor setting up holes foracoustic emission test sensors; willinstall 92 sensors, Figure 7 shows aninstallation of a typical sensor.

Finalized hydrotest/AE details, Figure 8.

Decided to internally hydroblast the tankwall and floor for internal inspectionsinstead of a sandblast to avoid masking ofsmall cracks, if present.

JUNE 24.Sensors calibration started. Increasingwater level in the tank and monitoring AEsensors.

JUNE 25.Conducting AE tests. Water level increasedto 11.89 meters(39 feet), 100% of AE test load. Minor AEequipment problems.

JUNE 26.Water level up to ultimate hydro level of14.7 meters (48' 4").

AE test concluded in about 3 days. Nomajor noises detected. Will startdraining/pumping water via a nearby standbyidle cooling tower sump and a drain systemto an existing effluent treatment facility.

JUNE 28.Water level down to about 7.92 meters (26feet). Level dropped about 6 feet in 16hours; may take another 72 hours to empty.

Details and plans with an internalinspection contractor finalized and set up.

AE TEST DATA

PERCENT

OF TOTAL NO. OF COLOR

(0-100%) SENSORS CATEGORY CODE RECOMMENDATION

17.4 16 ZERO GREEN NO INSPECTIONS NEEDED.

26.1 24 A BLUE FOR FUTURE INSPECTIONS.

29.3 27 B YELLOW FOR FUTURE INSPECTIONS.

27.2 25 C RED INSPECT NOW.

TOTAL = 92

JULY 1 AND 2.Draining of water completed.(24 inch) manway reopened.

0.61 meters

Setting up roller scaffolding, trolleys,etc. inside the tank for cleaning andinspections. Started sidewalls hydroblast.

JULY 3.Pumping hydroblast water out to the coolingtower sump. Using a rust inhibitor afterthe cleaning.

JULY 4.Holiday.

JULY 5.Hydroblasting/cleaning floor.

JULY 6.Finished hydroblast/cleaning .Nite - Internal inspections work started.

JULY 9.Internal inspections completed in 60 hours(430 manhours).

JULY 10.Internal repair work started.

JULY 14. FRIDAY.Internal repairs, etc. work completed.

JULY 17 THRU 20.Putting everything back together forrecommissioning.

JULY 21. FRIDAY.Started nitrogen purge.

JULY 21 THRU 28.Making up piping, etc. Insulation repairwork completed on July 28.

JULY 29, WEEKEND.Started ammonia purge from the top using-7.2°C (19°F) vapors from the ammonia plantrefrigeration loop. Had troublemaintaining good tank pressure. Startedventing vapors from the bottom of the tankthru the flare.

JULY 31.Switched to warmer, 13.3°C (56°F), ammoniavapors from the ammonia plant refrigerationsystem.

AUGUST 1.Found 56% ammonia going to the flare.

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AUGUST 2 THRU 5.Conducted an Operational Readiness Review(ORR) on August 2 to detectsafety/operation problems and to correctthem before introducing liquid ammonia. Nomajor problems found. Ammonia purgecontinues.

AUGUST 6.Started tank cooling using liquid ammonia.Tank back on line.

PROCESS/SAFETY IMPROVEMENTS

Various improvements listed below wereincorporated during the tank outage.

All major process controls and alarmswere brought into the Ammonia PlantControl Room via a process controlcomputer to improve surveillance. SeeFigures 9 A/B.

The backup ammonia flare system wasautomated to improve use and functionof the flare.

All ammonia thermal relief valvedischarges were connected to the flare.

Dual independent pressure sensingsources were provided to improvereliability of pressure monitoring.

All connecting liquid ammonia lineswere checked, painted and reinsulatedto prevent atmospheric corrosion. Alllines looked good and no problems werefound.

A remote operator was added to thefirst main ammonia liquid dischargevalve. Now we have dual control valveson the discharge line.

All first valves on the tank weretested, painted and reinsulated. Onlyone vapor valve indicated atmosphericcorrosion.

New bolts were used and qualityassurance inspections conducted beforeusing them - had 100% pass rate.

Provided a temperature indication/alarmon top of the tank to backup threelevel indications and to alarm highlevel, etc.

Installed vibration trips on ammoniacondenser fans to prevent damage.

Provided autostart for the ammoniatransfer pumps to prevent low pressurein the yard ammonia header.

Non-return check valves were added toall ammonia lines around the storagetank.

Storage tank relief capacity waschecked. Increased relief capacity bykeeping dual relief valves in service.

Expansion joint on the outlet linereplaced; old looked good.

The cooling system for thereciprocating refrigeration compressorswere revamped to improve cooling.

The inerts removal system was revampedto increase capacity.

Alternate backup power supply providedfor the pumps.

The project was implemented in 11 weeks vs.18 weeks planned and well under budget.There were no injuries to any construction,engineering or inspection personnel. Theproject could have been implemented in lessthan 11 weeks if the second ammonia storagewas not available.

TANK INSPECTION

Holes were cut through the insulation and92 sensors were attached to the tank's sidewalls with magnets. The tank was slowlyfilled with water over a two day perioduntil the water level reached 14.73M(4.8'4"). While the water entered the tankeach sensor was continuously monitored forsound emissions. The data was,evaluated onsite by the Monpac procedure. Exceptfor one "C" Monpac source emission nosignificant acoustic emissions were found.The tank was then emptied of water.

The tank was reentered and all seam weldsin the floor and side-walls and thebottom-to-side-wall seams were hydroblastedto remove scale and deposits for about 3"on each side of the welds. Each weld seamwas then wet fluorescent magnetic particletested for defects with an AC yoke. Theyoke poles were placed in at least twopositions 45° to each other and the

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procedure repeated at half pole spaceintervals. Only three types of defectindications were found: (1) a 250mm (10")long section of the floor to side wall weldseam which had not been welded, (2) threesmall crack-like (7mm long) indications onthe upper two shell rings where clips hadbeen welded and removed, and (3) 25 sets ofcracks in and adjacent to the bottom plateweld seams.

The 250mm (10") missing weld seam was atthe same location as the "C" Monpac source(Figure 2). A vacuum box test showed soapbubbling when a vacuum was applied.However, no evidence of an external leakthrough the missing weld was found and weconcluded that the air source

causing the soap bubbles was due to airtrapped in the space created by theinternal and external fillet welds andbottom and side wall plates.

The three areas on the upper plate courseswere lightly ground and the crack-likeindications were no longer detectable.Acoustic emission testing did not indicateany problem at these locations. Thesethree flaws were due to the welding andremoval of installation clips and were tooshallow to be of concern.

BOTTOM PLATE CRACKS

Figure 11 shows the location of 25 sets ofcracks found in the tank bottom by wetfluorescent magnetic particle inspection.The cracks appeared in a random fashion inabout two-thirds of the bottom. It wasnoted that water remained on the tankbottom after the acoustic emission tests inthose locations where no cracking wasfound.

At each crack location the crack extendedinto the weld. In most cases the othercrack end extended about 25 to 38mm intoeither the top or bottom lap welded plate;in a few cases the crack extended into bothplates. The cracks were always transverseto the lap weld. Cracking appeared both onthe sides and ends of plate in anapparently random manner. The lap weldssurrounding Plates 43 and 26 had seven andfive cracks, respectively but the cracksdid not always extend onto these twoplates. Near the crack ends in the plateseveral small parallel smaller cracks weresometimes noted. Figures 12-15 show some

of the cracking found. All of the cracksfound were sprayed with dye penetrant anddeveloper. In some cases the dye penetrantmethod did not fully define the crack andin several cases it is doubtful if thecrack would have been originally detectedby dye penetrant inspection. Often dyepenetrant and wet fluorescent magneticparticle inspection did not reveal thesecondary cracking present. Several of thecracked areas were examined by in-situmetallographic techniques which revealedsecondary cracking in all cases.

Several areas that were not cracked werealso examined by in situ metallographictechniques. No cracks were found in theseareas. Since the area examined by insitumetallography was only a very small portionof the entire tank's bottom, it is quitepossible that additional cracks exist whichare too fine to be detected by wetfluorescent magnetic particle inspection.

A section 250mm x 380mm containing twocrack sets from Plates 93 and 88 wasremoved. In Figure 10 these two cracksonly about 38mm apart were counted as onecrack set. Figure 16 shows that the cracksare barely visible to the unaided eye afterpolishing and etching for in-situmetallography. At 15X magnification(Figures 9 and 10) the cracks are morereadily visible and are shown to bediscontinuous. What is not apparent fromthe low magnification observation is thelarge number of short shallow parallelcracks also present. However, thesesecondary cracks were readily observed byin-situ (Figure 10) and laboratorymetallography (Figures 20 - 28). The twocrack groups shown in Figure 16 were cutout and one group (labeled A) wascross-sectioned in

the weld, the HAZ and base metal. Only asingle crack with considerable branchingpenetrated through the weld depositterminating at the junction of the bottomplate and weld deposit (Figure 20). Thecrack was wide, filled with deposits, andwas primarily transgranular. Only a singlecrack penetrated about 80% of the lapbottom plate thickness 7mm from the weld.This crack was full of deposits and waspredominantly transgranular but unlike thecrack in the weld, little branchingoccurred (Figures 22 and 23).

The cross section taken 25mm from the weld

147

showed 12 different cracks of which fourare shown in Figure 16. The largest crack(Figures 17 and 18) only penetrated aboutone-third of the plate thickness and hadnumerous short secondary cracks The maincrack was wide and contained deposits. Theother cracks found in this cross sectionwere of two types; some were wide andcontained deposits (Figure 28), whileothers were very tight and did not appearto contain deposits (Figure 29). Bothpredominantly integranularr andtransgranular cracks were found; the crackpath through the steel did not appear toinfluence the crack width.

The second group of cracks shown in Figure16 were submitted to Spectro Chem, Inc.(Baton Rouge, La.) for scanning electronmicroscope (SEM) and energy dispersivex-ray fluorescence spectroscopy (EDXS).Unlike the first group of cracks describedabove, there were two large cracks within7mm of the weld. The crack was bent openedand the fracture surface was examined withthe SEM. The cracks in the plate materialand weld were transgranular (Figures 29 and30). The EDXS spectrum of deposits in thecrack found only iron, manganese andchlorine.

PLATE PITTING

The surface of the plates outside thehydroblasted area was covered with a thinblue black mill scale. Pitting was evidenton the inside tank surface throughout thetank but was not found on the back side ofthe section removed for analysis. Figures4 and 6 give a fairly accurate picture ofthe discoloration and condition of thefloor plates. These pits were rust colorand at the most less than 0.13mm (0.005")deep (Figure 31). The pits were found tocontain iron, manganese, chromium,aluminum, silicon, sulfur, and chlorine.It is believed that the pitting occurredduring hydrotesting or while the tank wasbeing constructed. Aluminum, silicon,sulfur, and chlorine are elements presentin plant water, although aluminum andsilicon could have also been depositedduring metallurgical sample preparation.At the time of the original hydrotest,chromâtes were still being used as acorrosion inhibitor in the plant; theexisting records do not indicate ifinhibited water was used in the originalhydrotest. There does not seem to be anyrelationship between the pitting corrosion

and stress corrosion cracking.

CHEMICAL ANALYSIS

The chemical analysis of both platesamples, the weld filler metal, and theoriginal mill test reports are shown inTable 2. The chemistry of the two platesamples most closely match that of Heat No.92321 whichis not surprising since over three-fourthsof the plate came from this heat. Residualelements were low in both plates and areconsidered average for these steels. Thechemical composition reported here wouldmeet the chemical requirements of all ASTMlisted carbon steels of similar strengthlevel that are killed and made to finegrain practices.

The ASTM A20 carbon equivalent for weldingwould be 0.39 for both plates. This isconsiderably less than the maximum carbonequivalent of 0.47 allowed by A20. Thusone would not expect welding problems andnone appeared to have occurred.

MECHANICAL TEST

Tensile tests were run on the plate samplesremoved from the tank and are compared tothe original mill test reports as shown inTable 3. The yield strengths in all testswere considerably higher than the specifiedminimum of 290 MPa (42 KSI). Thepercentage elongation in 51mm (2") for theplate samples was much greater than thepercentage elongation in 203mm (8")reported by the mill. The percentagereduction in area found was above 60% forboth sample plates which indicates goodductility. A tensile specimen was baked at204°C for two hours and then tested; littlechange in yield or tensile strengthoccurred but the percent elongation andreduction in area decreased about 10%.This baking was used to drive any hydrogenout of the steel andone would expect anincrease in elongation and reduction inarea had hydrogen been present. The smalldecreae in ductility apparently occurredcould have been due to an aging process.

HARDNESS TESTING

Figure 32 shows average hardness valuesobtained at different location on a crossof the lap welded joint. The results aretypical for materials and joint design.There are no indications that localized

148

conditions exist that would promote thestress corrosion cracking noted.

OTHER NONDESTRUCTIVE TESTS

The thickness of each plate in the floorand side walls was measured with anultrasonic thickness gauge. Allthicknesses measured were greater than theminimum specified plate thicknesstolerances allowed in ASTM A20. Noevidence of external corrosion either underthe insulation on the side walls or tankbottom were found.

Ko significant corrosion was observed atthe holes cut in the insulation or whereinsulation panels were removed. Theexterior side wall surface was mildlyrusty. The insulation was removed from allnozzles and no significant corrosion wasnoted.

RESIDUAL STRESS MEASUREMENTS

Residual stress measurements were made intwo vertical welds on the tank's north andeast sides about 1.2M off the floor. Thestrain gauge hole-drilling technique perASTM E837 was used. Measurements weretaken parallel and transverse to the welddirection in three locations--weld center,13mm from the weld center in the HeatAffected Zone (HAZ), and 38mm from the weldcenter in the parent metal.

Table 4 shows the test results. Twoconclusions can be drawn from this data andsimilar tests on other tanks.

1) The only direction for substantialtensile residual stress is parallel tothe weld. Transverse stresses wereeither compressive or essentially nil.

2) The tensile and compressive stresses inboth directions dissipate to 20% orless of the yield strength in theparent metal 38mm from weld center.

The original fracture mechanic calculationswere reviewed based on this residual stressdata. Since the residual stressestransverse to the weld are low orcompressive and the parallel residualstresses are much higher, the worst crackwill always begin and propagate transverseto the circumferential weld loaded in hoopstress. The original calculations assumedthat a worst case crack was parallel to a

vertical weld.

The original calculations assumed that theresidual stresses are equal to thematerials yield strength. While thisapproximation is reasonably correct in theweld, the residual stresses areconsiderably less in the HAZ and parentmetal. The worst case crack will betransverse to a horizontal weld where thehydrostatic stresses are double thoseparallel to the weld.

Using this new information about residualstresses, the critical crack lengths wererecalculated using the Charpy V-notchvalues used in the original platecalculations. In both cases the criticalcrack length was such that the tank wouldleak before rupture.

The original calculations for weldsdemonstrated very clearly that weld defectsare much more of a concern regardingbrittle fracture of the vertical weld thansimilar defects in the horizontal weld.After tank construction this point wasaddressed since the vertical welds were100% radiographed, while the horizontalwelds were only spot radiographed.However, stress corrosion cracking is morelikely to occur in the horizontal weld thanvertical weld if exposed to the samecorrosive environment since both theapplied and residual stresses are greatesttransverse to a horizontal weld thanelsewhere in an atmospheric pressure tank.

In a pressurized tank the applied stressestend to become equal on both vertical andhorizontal welds in all directions but theresidual stresses are still much greater inthe parallel direction to the weld thantransverse to the weld. Thus, one wouldexpect any stress corrosion cracks to betransverse to the weld direction and thisis what was observed in this tank.

REPAIRS

As mentioned earlier, the three smallcracks found in the upper plate courseswere lightly ground and the crackdisappeared. The .25M (10") section ofmissing weld was welded with low hydrogenelectrodes and dye penetrant examined inthe root pass and after the final pass.

Except for the two sets of cracks removed

149

with the bottom plate sample, all of theother crack sets in the floor were grounduntil cracks were no longer apparent by dyepenetrant testing. The ground areas werethen weld overlaid with low hydrogenelectrodes until the original floor platethickness was obtained. In two casesgrinding penetrated the floor before thecrack disappeared. The repaired area wasdye penetrant examined both on the rootpass and after the final pass. In aboutone-third of the floor repairs additionalcracks were found after welding and thesewere grounded out and repaired. After allwelding was completed, the repaired areawas ground flush with the surrounding plateand re-dye penetrant tested.

No suitable carbon steel plate that hadbeen impact tested could be found in timeto replace the section removed from thefloor. A 4.7mm (3/16") Type 304 stainlesssteel plate was bent to shape and welded inplace using Type 309 filler metal. Dyepenetrant examination was done afterwelding and no additional cracking wasnoted in the surrounding carbon steelplate.

CONCLUSIONS

1. Stress corrosion cracking, mostprobably due to ammonia, occurred inthe tank bottom. The cracks appear tohave originated in highly stressedareas of the plate material runningtransverse to the weld direction.Multiple cracking occurred in theparent metal with most of the cracksbeing very shallow. Only one or twocracks occurred at each location in theweld and HAZ but the cracks were quitedeep and in at least two casespenetrated the plate.

2. Corrosion occurred after cracking inmost cases; some very shallow crackshad no corrosion product within thecrack.

3. No feature of the steel could be foundthat would have caused stress corrosioncracking except for the high residualstresses running parallel to the weld.Even though a non-standard grade ofsteel was used for the tank bottom, itis expected that other carbon steelsused in ammonia tank floors would havealso experienced such failures.

4. It can only be concluded that at sometime, probably while the tank was firstfilled with ammonia, that the rightcombination of oxygen, water, ammonia,and possibly carbon dioxide was trappedlocally at a few locations near weldsto cause cracking. ' Suchconditions will only occur on tankstart-up and are unlikely to occuragain until the tank is reentered.While the presence of oil in the tankcannot be completely ruled out as acontributor to cracking, the oil isvery low in sulfides and does notappear to have contributed to thefailures.

5. There was no evidence of leakage ateither the missing weld or in thecracked floor plates. It appears thatcorrosion products or debris within thecracks prevented leakage.

6. The tank met the "leak-before-rupture"criteria. The most critical cracklocations would be cracks transverse tothe horizontal plate course welds.

7. Dye penetrant examination is not assensitive for crack detection as wetfluorescent magnetic particleinspection. Hydroblasting Is thepreferred method of deposit, scale andslag removal for wet fluorescentmagnetic particle inspection.

8. Acoustic emission testing detected onlythe one significant defect found in thetank wall. It was ineffective inlocating floor cracks.

9. Based on similar inspections with otherammonia storage tanks, the low numberof weld defects found is somewhatsurprising.

10. No external corrosion of anysignificance was found.

ACKNOWLEDGEMENT

Mr. Roy P. Galliano set and followed allphases of the tank inspection. Mr. RobertG. Webber gathered the residual stress dataand made the necessary fracture mechanicscalculations and analysis which made thispaper possible. All metallurgraphicspecimens and photographs were made by Mr.Louis Dufrené.

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FOOTNOTES

( 1) Byrne, J. R.; Hoir, F. E.;Williams, R. D."Stress Corrosion in a 12 K TonnesFully Refrigerated Ammonia StorageTank".AIChE Ammonia Symposium - December1988.

( 2) Salve, R. A.; Henser, A. H."The Structural Integrity of a 12,000Tonnes Refrigerated Ammonia StorageTank in the Presence of StressCorrosion Cracks - An AssessmentStrategy"AIChE Ammonia Symposium -San Francisco, 1989.

( 3) Davies, R."Acoustic Emission on a Basis forPlant Integrity Monitoring"Presented at the Seiken Symposium,October 27-28, 1986, University ofTokyo.

( 4) BSI Published Document PD 6493,1987 Draft Revision"Guidance in Some Methods for theDerivation of Acceptance Levels forDefects in Fusion Welded Joints".

( 5) Lunde, L. , Nyborg, R."The Effect of Oxygen and Water onStress Corrosion Cracking of MildSteel in Liquid and Vaporous Ammonia",AIChE Ammonia Symposium, 1986.

( 6) Lunde, L., Nyborg, R."SCC of Carbon Steels inAmmonia--Crack Growth Studies andMeans to Prevent Cracking"Paper No. 98, Corrosion 89,New Orleans.

.102. .10« . . 10«

Robert E. Smallwood Syed B. All

EFFECTS

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151

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152

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Figure 98. Storage tank operating data display.

*•*- Indicate C»ckB

Figure 11. Bottom of 25K ammonia storage tank

Figure 10. Missing 10" fillet weld on south side of tanThis was the only significant construction defectfound. (About 1/10X)

Figure 12. Two cracks (counted as one crack set foundadjacent to the Junction of Plates 37,38 and 43.Plates 37 and 38 are on top of Plate 43.About a/4X)

153

A

Figure 13. Cracks found adjacent to the junction ofPlates 38,43 and 44. Plate 38 is on top ofPlates 43 and 44. (About 1/7X)

Figure 14. Crack that extends into both plates. Notethe pitting found in the plate's mill scale.(About 1/5X)

Figure 15. Crack that extends into both plates(different crack from Figure 6) aftergrinding. Note that grinding haspenetrated Plate 32 (arrow); this crackappears to be through wall. (1/5X).

Figure 16. Two groups of cracks in Plate 93 after in-situmetallurgraphy. These two crack groups areshown as one crack in Figure 3. The crack groupmarked "A" was sectioned at the three locationsshown by black arrows and metallurgicallyexamined In this laboratory. The crack groupmarked "B" was sent to Spectra Chem for SEManalysis. (About 3/4X)

'

Figure 17. Close up of Crack Group A in Figure 16.Note numerous cracks at edge of weld(arrows). (15X)

Figure 18. Cracking Plate 93 (left) and weld (right) inCrack Group B in Figure 16. There are two largecracks; one Is readily apparent, the other is notand is marked by arrows. (15X)

154

i iFigure 19. In- situ metallurgraphic replica of area near the

crack end In Crack Group B in Figure 16.(31X, Nital Etch)

•/y-;*v: "^,-c^'.^'' • •"-V ; '• • . - #>« - ; ' ' ";>;.4> -,;".', . v;--

"••• ' 'V v'^S» '!',A

Figure 22. End of crack in heat affected zone CrossSection H in Figure 16. This crack was about80% through Plate 93. (100X Nital)

f

Figure 20. End of crack in weld from Cross Section W inFigure 16. Plate 93 is at left. (100X, Nital)

Figure 23. Mixed mode cracking near end of crackshown in Figure 22. The crack was full ofcorrosion product prior to etching.(500X, Nital)

Figure 21. Mixed mode secondary cracking adjacent tomain weld crack. The main crack and secondarycracks were full of corrosion product prior toetching. (500X. Nital)

Figure 24. Four of the 12 cracks found in the crosssection marked P in Figure 16. The large crackcould be detected by dye penetrant inspectionbut the smallest cracks were only apparent bymetallurgical examination. (31X, Nital)

155

Figure 25. Corrosion product filled crack at the end ofCrack 6 in Figure 24. (500X, Unetched)

^ ? ""„ , ••* .

* - , . , ,„. *

Figure 26. Same area as shown in Figure 25 after etching.The crack is full of corrosion products. Note themix mode of cracking. Many of the fine cracksapparent in- Figure 25 cannot be distinguishedfrom the grain boundaries. (500X, Nital Etch)

Figure 27. Crack 7 In Figure 24. This intergranular crackand about half the other cracks found in CrossSection P were wide and contained corrosionproducts. (200X Nital Etch)

Figure 28. Crack No. 8 in Figure 24. This mixed modecrack and about half of the other cracks foundIn Cross Section F were very "tight" andappeared free of corrosion product

Figure 29. Transgranular stress corrosion crack surfacein bottom plate. (SEM 750X)

156

Figure 30. SEM micrograph of the crack tip in theweldment after carefully opening themajor crack. The arrow points out depos-its and/or corrosion products at the tipof the primarily transgranular crack.(SEM, 290X)

Tabla 1. Linear elastic fracture mechanics calculations

Figure 31. Unetched cross section of pits In plateshowing scale and deposits. (SEM,200X).

I.BOB B- 5

36 HT TIP OF CRflCK

VFS > 236 10.E40

Figure 32. Energy dispersive X-ray fluorescencespectrum generated from depositsand/or corrosion products at the tip oftransverse crack across the weldmentfrom the ammonia storage tank floor.

VALUES REPORTED ARE AVERAGE OF THREE

MEASUREMENTS MADE 0X110 INCHES APART

ROCKWELL HARDNESS "B" INDICATED BY ARROWS

HRB 91 HRB 86 HRB 80

Shell

1

3

1

1

1

1

3

3

3

3

Charpy

Plate 54J(40 fflbs)

flute 41JC30 fflbs)

Sean 201(15 fflbs)

Sean 27 J (20 fflbs)

Sean 20J(15 fflbs)

Sean 27J(20 fflbs)

Sean 20J(15 fflbs)

Sean 27J(20 f fib»)

Sean 20J(1S fflbs)

Sean 27.1(20 fflbs)

AssunedAspect

2:1

2:1

2:1

2:1

6:1

6:1

2:1

2:1

6:1

6:1

DepthOf Crack

213BS(8.4')

137m(5.4")

11.9«»(0.47")

18.5m(0,73")

4.8nn(0.19-)

6.6nn(0.26")

11.9»(0.47")

11. Son(0.73")

4.8mn(0.19")

6. Ira(0.24")

CousaentsLeak before rupture

Leak before rupture

Surface flaw

Surface flaw

Surface flaw

Surface flaw

Surface flaw

Leak before rupture

Surface flaw

Surface flaw

Shell Ring Ho. 1 was 19.9mai (0.784") thick.

Shell Ring Ko. 3 was 15.5mm (0.612") thick.

Table 2. Chemical analysis of A573 grade 70 3/16" plate

Top BottomPla

Heat Ho.!Z380

Carbon

Manganese

Phosphoroui

Sulfur

Silicon

Aluminum

Copper

Molybdenum

Chromium

Nickel

Vanadium

Titanium

Niobium

0.157 0.156 0.110 0.22 0.21 0.17 0.20

1.06 1.04 0.69 0.99 1.05 1.05 1.01

0.018 0.016 0.020 0.010 0.010 0.010 0.010

0.018 0.019 0.020 0.021 0.015 0.020 0.028

0.28 0.26 0.42 0.22 0.21 0.27 0.23

0.01

0.10 0,11

0.04 0.04

0.09 0.08

0.10 0.10

0.04 0.04

0.13 0.13

0.06 0.05

0.13 0.13

0.14 0.13

<0.01 <0.01 0.03

<0.01 <0.01 0.03

<0.005 <0.005 0.006

Seventy-five (75) 4.8mm x 2.29M x 9.14K (3/16- x 90' x 360') A573Grade 70 plates were ordered for the tank and mill test reportswere found for 74 plates.

One plate came from Heat 77733.

Four plates came from Heat 49270.

Between two to nine plates caaa from Heat 77380.

Between 59 to 66 plates came from Heat 92321.

Table 3. Mechanical propertiest fHRB 79 HRR 86 /HKB iy HKBjöO 1/133 144 164 141 150 150 138 133 153 142

4 t / /v

Figure 33. Knoop microhardness measurements{100 GM. load}

\ Bottom Plate 1

7 Heat 77733 1

Beat 77380 2

Pet A573

Yield Screneth

352 MPa(51.0 KSI)

407 K?a<59,0 KSI)

51.8 to 57.0 KSI

377 to 403 MPa56.7 to 58.4 KSI

290 KP*{42 Hin.)

486 HPa(70.5 KSI)

534 MPa(77.4 KSI)

70.2 to 73.9 KSI

540 to 516 MPa78.3 to 74.9 KSI

483 to 620 HPa70.0 to 90.0 KSI

331 in 51oa<2-)

203^(8 )

3mm(8 *

191 in 203nn(S">

16Z In 203aa(8*)

157

Tabla 4. Residual stressas In tank welds

Stresses ParallelTo »«Id

Weld Center

Heat Affected Zone

Parent Hetal

345 HPa 310 HFa(50 KSI) (45 KSI)

414 HPa 207 HPa(60 KSI) (30 KSI)

138 HPa 69 HPa(20 KSI) (10 KSI)

Stresses TransverseTo Ueld

-241 HFa(-35 KSI)

34 HFa(5 KSI)

-245 HPa(-50 KSI)

310 HFa(-45 KSI)

-34 HPa(-5 KSI)

103 HPa(-15 KSI)

Positive stresses are tensile.Hegative stresses are conpressive.

Table 5. Fracture mechanics analysis for stress corrosioncracking In 25K ammonia tank

(6:1 aspect ratio surface crack)

Location

1st Course PlateTransverse toHorizontal Veld

3rd Course PlateTransverse toHorizontal Weld

19.9 mm(0.784 In.)

15.5 mm(0.612 in.)

178 nra(7.0 in.)

114 mm(4.5 in.)

119 mm(4.7 In.)

94 mm(3.7 In.)

Max Appl, BASF: My questions concern your fracturemechanics calculations. Did you use real material valuestaken from actual plate material used for the construction orfrom samples cut out of the tank during inspection? Or, didyou rely totally on literature or spec values?Smallwood: No samples were cut out of the tank walls.The original mill test reports had Charpy V-notch valves ofplate materials over 1/2-in.-thick (13-mm-thick). Whereactual data did not exist, we used literature values.Bill Salot, Allied Signal: Since the tank is 70 2/3-ft-high (21.2-m-high), why was the water fill height limited to48.3 ft (14.5 m) during the acoustic emission test?

Smallwood: The foundation design limited the full heightduring the acoustic emission test.Salot : Was differential thermal expansion considered whenthe decision was made to install a stainless steel replacementsection in the carbon steel floor? Does it affect the leak-before-break conclusion?Smallwood: Calculations were made prior to installing thestainless steel replacement sections in the tank floor todetermine if differential thermal expansion would present aproblem or would affect the look-before-break conclusion.There was no significant increase in stress or risk in usingstainless steel replacement sections.

158