Leak in an Atmospheric AmmoniaStorage Tan k

A Freon test detected a 75-mm crack, not the predicted pinhole-size defect. Failure to locate andrepair this leak would have destroyed the insulation barrier at a replacement cost of Canadian

$1.5 million.

Robert C. A. WiltzenSherritt Gordon Ltd., Fort Saskatchewan, Alberta T8L 2P2, Canada

The text of this paper deals specifically withthe successful location and repair of a leakin an atmospheric ammonia storage tank locatednear Watson, Saskatchewan. Watson is a townof approximately 1200 people in a farmingcommunity 700 km east of Fort Saskatchewan.Sherritt has successfully operated two similartanks at the Fort Saskatchewan site.

HISTORY

In 1980, Sherritt Fort Saskatchewan com-menced construction of a world scale 1,000tonne/day Ammonia and 909 tonne/day Urea plant.In 1983, the complex was commissioned and beganproduction.

In order to facilitate storage of theanhydrous ammonia, two new atmospheric storagetanks were also constructed; one of 36,400tonnes at Fort Saskatchewan, Alberta and asecond one of 27,000 tonnes at Watson,Saskatchewan (50 km east of Humboldt). Thesewere in addition to an existing 18,000 tonnetank at Fort Saskatchewan.

The original tank at Fort Saskatchewan hadbeen decommissioned and internally inspectedin 1987. No leakage was noted and a numberof minor construction related defects wererepaired.

Both tanks at the Fort Saskatchewan complexhad operated successfully with no evidence ofleakage.

LEAK DETECTION

Shortly after start-up in 1984, the normallybright Alumaseal insulation of the Watson tanktook on a dark, dull discoloration. Theinsulation barrier consists of a thin multi-layer aluminum skin on both the walls and roof.Over a period of time this became progressivelyworse. A leak was suspected and a program wasinitiated to; first, verify if in reality therewas a leak, and secondly, the source. Therewas also a great deal of speculation as towhether the discoloration was related tostart-up, piping, nozzles, floor, wall or roofleakage.

The first inspection attempt was to isolateall nozzles, piping and flanges. This was doneby removing the insulation around the nozzles,chocking and using ammonia detection tubes tocheck flange gaskets. No leakage was located,however strong ammonia was detected in thenortheast quadrant of the tank. Additionaltesting was carried out by "poking" small holesthrough each of the 1.5 m x 6 m panels of thealumiseal insulation shell and samplingammonia concentrations. Slightly higher

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concentrations were found on the north half ofthe tank, however, nothing conclusive wasachieved from this testing.

SCHEDULE

On May 25th, 1988, it was decided to proceedwith the decommissioning of the tank to carryout an internal inspection to locate the sourceof leakage and make permanent repairs.

The project took on a rather uniquechallenge; no local facilities existed, theskilled labour, materials, purging equipmentand testing had to be brought in from the FortSaskatchewan complex. It was also determinedthat the insulation which had deterioratedwould require patching and resealing.

According to the best information available,the most likely source of leakage would be apinhole or small crack in the overlapping floorplates. No record of any wall leaks wasdocumented and only two sources of roof leakagecould be located in consultation with otherusers.

Draining of the tank began in the middle ofMay. Nitrogen purging, blinding, air purging,scaffold installation and an internal highpressure wash of the floor was completed. Thetank was ready for inspection by the first weekof July. At the same time, vacuum relief,isolation valves and related equipment wereserviced.

DECOMMISSIONING. DRAINING AND PURGING

Since no sump existed in the tank floor, itwas necessary to dig an external pit and installtwo portable pumps. When suction was lost tothese pumps, the tank was allowed to heat-upby drawing vapors off the top and re-injectingwarm ammonia vapors into the bottom. The basethermocouples were used to record a continuoustemperature read-out and as the interiorreached 0°C, warm nitrogen was introduced.This procedure continued until inerts reacheda level which would no longer allow thecompressors to operate. Purging then continuedto flare with a total nitrogen volumeequivalent to one and one-half times the tank.Air purging to flare followed until theappropriate nozzles and manholes could beopened to allow safe entry.

In order to address environmental concerns,provisions had to be made to collect andseparate the oil residue from the water usedfor the internal high pressure wash. The wallswhich were clean and lightly rusted did notrequire washing. Although consideration wasgiven to a detergent or solvent wash, it wasrejected. This proved beneficial since thelight film of oil remaining on the floorprevented rusting and assisted in the vacuumbox testing of the welds.

HELD PREPARATION

In addition to the high pressure washing ofthe floor, it was necessary to power brush boththe floor and shell welds to remove slag, rustand other soil materials tracked in during thecourse of the inspection.

INSPECTION OF FLOOR AREAS

Vacuum Box Testing

A vacuum box test of al l f l oor wel ds reveal edno leakage.

Visual Inspection

A visual inspection of all floor areas withhigh intensity lights revealed a number ofconstruction related flaws to a maximumdepth of 4 mm; all were repaired by welding.

Magnetic Particle Testing

A magnetic particle test of all floor "T"joints revealed no defects.

Gas Testing

Ten 3 mm diameter holes were drilled throughthe floor; gas samples revealed 0% to 3%ammonia.

The results of these tests were conclusive;no leakage existed in the floor.

INSPECTION OF WALL AREAS

A magnetic particle test of all wall "T"joints and a visual examination of all areaswith high-intensity lights revealed no defectsand no repairs.

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INSPECTION OF DOME AREAS SUMMARY

Since no leakage could be located in thefloor or tank walls, a procedure was developedto carry out a Freon test on the dome panels.Twelve 9.5 mm tubes were inserted through theinsulation of the roof panels as test pointsand five automatic Halogen leak detectors werepurchased. The tank was sealed off by closingall nozzles and pressurizing with compressedair to 5 KPa. Freon was then injected and thepressure increased to 6.2 KPa. Within 25minutes, Freon was detected under the insu-lation and within 50 minutes total saturationof all test points occurred.

The results of this test verified that theleakage was occurring in the dome within anarea of 20 panels near the top of the dome onthe north side. The removal of 8 panels locateda construction related flaw which was inreality an inadequately repaired area that haddeveloped into a 75 mm long crack.

We can only speculate as to how or why theflaw may have occurred. A patina scale on theinside surface of the repair might indicatethat a frayed or damaged welding cablecontacted the roof causing a burn through. Itwas, however, very evident that the correctrepair procedures were not followed and thesubsequent inspection/testing of the vesseldid not reveal the problem. The crack couldhave been very small initially, and propagatedover a period of time to that found with theFreon.

The area was cut out and repaired by patching.A second Freon test revealed no furtherleakage.

OTHER INSPECTION METHODS

Prior to and during the course of theinspection, consideration was given to othermethods which might have assisted in theidentification of the leak.

A quote was received to carry out an AcousticEmission inspection, however, this was ruledout as impractical because the leak wasexpected to be a pinhole. Facilities were alsoavailable to perform a hydro test, but inhindsight, this also would not have locatedthe leak.

Sherritt's experience in the inspection oftwo atmospheric ammonia storage tanks nowparallels that of the industry. No ammoniarelated flaws have been evident.

Failure to locate and repair this leak wouldultimately have led to the destruction of theinsulation barrier at a replacement cost of1.5 million dollars Canadian.

The project was completed in 22 weeks at acost of $300,000 Canadian.

Although the use of Freon in this applicationwas considered rather unorthodox, it wassuccessful. The leak was initially perceivedto be the size of a pinhole.

The 75 mm crack, which was considered acomparatively large defect, certainly showedup dramatically during this test. Whether ornot it would work in the case of a pinhole iscertainly speculative, however, the additionof more than the 12 test points used here wouldprobably be successful in locating consider-ably smaller leaks.

Robert C.A. Wiltzen

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COVER PATCH. ,/B" DIA. HOLE DRILLED TO 3/8"

SECTION VIEW OF FLOOR PLATE

Figure 1. Section view of floor plate.

PATCH (EXTERNAL!

GOODALL VIIBOO AMMONIA TUBING

20 GA ALUI

HOOD SUPPORT »^*v;*\V^wwj32 GA AL'Wïgff^tXY/ j l /1 il y

HOOD SUPPORT-

WOOD SPACER'

/ / ' VB ROOF /ANEk'FLUSH PATCH ^m/ ' /

SILICONE SEAL

\BACI$-Uf\PLÀ<i;E

SECTION OF HOOF SHOWING TYPICAL TEST POINT FOR FREON TESTING

Figure 3. Section of roof showing typical test point for freontesting.

SECTION VIEW OF HOOF PLATE

Figure 2. Section view of roof plate.

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DRAIN TANK

INSTALL PIPING

PUMP OUT HEEL

WARM AMMONIA IN

PURGE KITH NITROGEN

INSTALL BLINOS

OPEN TANK S AIR PURGEINSTALL SCAFFOLDING

MODIFY SCAFFOLDING

H.P. WASH INTERIOR

H.P. INSPECT INTERIOR FLOOR WELDS

M.P. INSPECT INTERIOR SHELL WELDS

SERVICE V.R. & VAC. VALVES

VAC. 60X INTERIOR FLOOR WELDS

INTERIOR REPAIRS TO FLOOR

MAINT. CONTRACT ÂLUMISEAL INSULAT.

DRILL FLOOR HOLES, TEST FOR NH3

FREON TEST

REMOVE ALUMISEAL ROOF PANELS

REMOVE SCAFFOLDING

REPAIR LEAK IN ROOF

CLOSE MANHOLES

REPAIR ALUMISEAL HOOF PANELS

NITROGEN PURGE

REMOVE BLINDS

REINSTALL NOZZLES

HAY JUNE JULY AUGUST SEPTEMBER OCT .5 23 30 6 13 20 27 4 11 IB 25 1 8 15 22 29 5 12 19 26 3

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— h=)Figure 4. Schedule —Watson 27,000 tonne ammonia storage tank.

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Figure 5. Sherritt Gordon 27,000 tonne storage facility,Watson Figure 7. Dôme of tank alumiseal panel removed to locate

peak.

Figure 8. Area of leakage, dye penetrant test showingcracked area.

Figure 6. Portable scaffold erected to Inspect wall areas. F l e x i b l e S t a i n l e s s S t

Figure 9. Area of leakage as seen from inside the tank.

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Jay Shah, Chicago Bridge & Iron: In your inspection didyou find any stress corrosion cracks? Was the inspectionwith wet MT or regular MT?Wilt/en: There was no evidence of stress corrosioncracking as determined with wet magnetic particle testing. Aconsiderable amount of dye penetrant testing was carried outin the selective areas of mechanical damage remaining in thefloor plates from construction. Although no stress crackingwas noted in these areas either, it was recommended to repairthe deeper ones (the maximum depth recorded was 4 mm).Herbert Inhaber, Ecology and Environment: Did you tryto test ammonia concentrations in the air outside the tank atvarious locations outside the tank, prior to actually goinginside and checking?Wiltzen: Ammonia concentrations were sampled on severaloccasions through holes "poked" through the insulationbarrier on both the walls and roof. Although these tests wereinconclusive, they indicated slightly higher concentrations onthe north quadrant of the tank. The darkest areas ofdiscoloration on the outer skin were also in the same area.Richard Osiowy, Simplot Canada: You mentioned thatit was beneficial to use only high-pressure wash because itleft an oil residue on the plates.

Wiltzen: Yes.Osiowy: And in what way did it aid you?Wiltzen: The oil film remaining after the high-pressurewash had a double benefit; it helped seal the vacuum box tothe floor during the testing and it prevented rusting of thewelds.Osiowy: How did you dispose of the water and the oil thatyou did wash out?Wiltzen: The oil-contaminated water was of concern, bothfrom the disposal and the environmental points of view. Theoil was first separated in an oil field separation tank obtainedon rental from a supply company. Oil wicks were then usedto remove any oil residue remaining in the water. Since thiswater ended up in a nearby dugout in which a family ofCanada geese were nesting, it was imperative to remove oilcompletely.Osiowy: Do you have any idea of how much water washandled?Wiltzen: Since there was more than an adequate amount ofwater, both from the dugout and several artesian wells on theproperty, the volumes required were not recorded. No solventor detergent was used in the washing procedure.

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