2001/2002: primary reformer failure/fileser… · per day stamicarbon urea plant. current operating...

Primary Reformer FailureThe Agrium Fort Saskatchewan Nitrogen Operations experienced a massive reformer failure

following a short maintenance outage and routine startup. The events which led up to the failure,the methodology, scheduling and alterations made to bring the reformer back online, and the

safeguards incorporated to lessen future occurrences of this type of failure are discussed.

D. H. Timbres and Mark McConnellAgrium Fort Saskatchewan, Alberta, Canada

Introduction

At the Agrium Fort Saskatchewan NitrogenOperations in Fort Saskatchewan, Alberta,Canada on Nov. 16-17, 1998, a massive

reformer failure was experienced. This failure followeda short maintenance outage and what was considered aroutine plant restart. Almost all the catalyst tubes failedin the radiant section, and the air-steam preheat coilNo. 1 (shield coil) was damaged. This resulted in 39days of lost production in the ammonia plant and 40days lost production in the urea plant.

The events which lead up to the failure, the method-ology, scheduling and alterations made to bring thereformer furnace back on line, and the safeguardsincorporated to lessen future occurrences of this type offailure are discussed in this article.

Background

Agrium Inc. is a leading global producer and mar-keter of fertilizer and a major retail supplier of agricul-tural products and services in both North America andArgentina. The Corporation produces and markets fourprimary groups of fertilizers: nitrogen, phosphate,potash, and sulfur.

Agrium, Fort Saskatchewan Nitrogen Operationswas commissioned in 1983 and produces ammonia andurea fertilizer for the Western Canadian and exportmarkets. The site comprises a 1,000 metric ton per day

nameplate Kellogg ammonia plant and a 907 metric tonper day Stamicarbon urea plant. Current operatingcapacities are 1,350 metric tons per day ammonia and1,280 metric tons per day urea.

The primary reformer furnace in the ammonia plantis a Kellogg design with 260 radiant tubes, 5 radianttube rows, with 52 tubes per row. The radiant tubeswere HP-Nb modified material and installed over aperiod of 1991 to 1993, with the majority (207 out of260) of the tube installed new in 1993. The reformerfurnace layout is shown in Figures 1 and 2.

There are six arch burner rows with 22 burners perrow for a total of 132 arch burners. In addition there aresix tunnel burners, which are not operated, and 13steam superheater burners.

The convection section consists of three coils in the"hot leg," the air-steam shield coil, the main air-steamcoil, and the mixed feed preheat coil. In the "cold leg,"there are four coils, the steam superheat coil, thefeedgas coil, the boiler feed water preheat coil, and thefuel gas preheat coil. All aforementioned coils are inthe exit heat direction.

Primary Reformer Startup Procedure

The plant procedure for startup consists of introduc-ing a nitrogen purge to the front-end by means of tem-porary hose connections to the following locations:

• Purging the process gas line,• Purging the steam to process line, and

AMMONIA TECHNICAL MANUAL 268 2002

Figure 1. Plant overview (primary reformer at left).

Figure 2. Closeup of primary reformer furnace.


ARCH BURNER FIRING PATTERN

NORTH

ROW #6 ROW #5 ROW #4 ROW #3

61

41

51

43

53

WEST 45

55

47

57

49

59

*• 33

*• 5

•• 35

•• 9

•• 21

•• 17

•• 29

•• 25

•• 1

•• 13

•• 66

• 63

•• 37

•• 11

•• 39

•• 15

•• 7

•• 27

•• 3

•• 23

•• 31

•• 19

•

•• 20

•• 32

•• 24

•• 4

•• 28

•• 8

•• 16

•• 36

•• 12

•• 38

•• 64

ROW #2

• 65

•• 41

•• 2

•• 26

•• 30

•• 40

•• 18

•• 10

•• 34

•• 6

•• 22

•

ROW#1

•• 60

50

58

48

• 56. EAST• 46

•• 54

•• 44

•• 52

•• 42

•• 62

SOUTH

AFTER BURNER NUMBER 66IS REQUIRED WHEN ADDING

IS LIT, NO SETADDITIONALB

PATTERNURNERS.

Figure 3. Arch burner firing pattern.


• Purging the steam to air coil line.The operators follow an arch burner-firing pattern

(see Figure 3) with a warmup rate of 100°C/h(212°F/h).

The radiant box is preheated by lighting specific archburners between each of the radiant tube rows until theflue-gas temperature reaches 400°C (752°F), followingwhich steam is introduced into the radiant tubes. Bypreheating, you prevent condensation of the steam onthe cold tubes/catalyst. Also, you minimize tempera-ture shock of hot steam on cold catalyst.

At a 400"C (752°F) flue-gas temperature, steam isintroduced to the process at 40% rate. The temperatureis then steadily increased by lighting more arch burnersuntil the exit tube temperature reaches 700°C(1,292°F), at which point natural gas is introduced tothe process.

Additional arch burners are then lit.At 750-800°C (1,382-1,472°F) flue-gas temperature,

air is introduced to the secondary reformer. When lightoff has been confirmed by a temperature rise across thesecondary reformer, the system is gradually brought toup operating conditions.

From Nov. 16 through Nov. 17, 1998, the primaryreformer furnace was already in a warm condition witha flue-gas temperature of just under 400°C (752°F).Night shift was continuing to warm up and was gettingready to introduce steam at 400°C (752°F).

The panel operator directed the field operator to lightadditional arch burners to warm up the reformer. Thefield operator did this exact operation a number oftimes over the next few hours.

What was later discovered was that the field operatordid not perform checks inside the radiant box afterlighting the additional burners. That is, the field opera-tor did not visually look at the burner(s) that had beenlit to confirm there was no flame impingement on thetubes and to visually confirm the tubes were not exces-sively hot.

In addition, the panel operator was incorrectlyobserving the transfer header outlet temperatureinstead of the flue-gas temperature.

So as additional arch burners were being lit, the tem-perature in the radiant box continued to rise, but, as yet,no steam had been introduced to the radiant tubes.

At about 10:35 PM on Nov. 16, steam should have

been introduced into the radiant tubes.At about 2:47 AM the next morning, the field opera-

tors looked inside the radiant box, and observed thecatalyst tubes were a glowing bright yellow and dis-continued firing. It was later noted that the flue-gastemperature had reached a maximum value of 1,071°C(1,960°F). [The melting point is typically quoted atabout 1,343°C (2,426°F) and complete melting will befinished at 1,400°C (2,552°F), respectively known asthe solidus and liquidus melting temperatures ("therange of melting"). It is probable that localized flue-gastemperatures were much higher than temperaturesreported by thermocouples at the outlet of the radiantbox. Uneven firing or the relatively high air leakagerate at low firing rates could cause this.]

The time line for the temperature and number ofburners lit is shown in Figure 4.

Description of Damage

Following a cool down of the primary reformer fur-nace, further observations within the radiant boxshowed that approximately 50% of the tubes had com-pletely failed. Tube failure was observed mainly atwelds. Molten metal had solidified within the radianttube catalyst. See Figures 5, 6 and 7.

No risers had failed, but overheating was evident.See Figure 8.

In the "hot leg" of the convection section, the air-steam preheat coil No. 1 (shield coil) showed evidenceof overheating and distortion. See Figure 9 and 10.None of the tubes had ruptured, however. The air-steamNo. 2 coil was intact with no evidence of damage, aswas the mixed feed coil.

In the "cold leg" of the convection section, none ofthe four coils (steam superheat, feedgas preheat, boilerfeedwater preheat, and the fuel gas preheat coils) weredamaged.

Some minor damage was noted to the radiant furnacewall insulation and refractory.

Repairs to Primary Reform Furnace

On Nov. 17, 1998, a management team was assem-bled to conduct the demolition and repairs to the pri-mary reform furnace and air-steam preheat coil No. 1(shield coil), and to bring the plant online as soon as


Timeline of the November 16-17,1998 Incident

1200

1000 -

g

S

Nw16 Nov-16 Nov-16 Nov-16 Nov-16 Nov-16 Nov-17 Nov-17 Nov-17 Nov-17 Nov-17 Nov-17 Nov-1721:00 21:30 22:00 22:30 23:00 23:30 00:00 00:30 01:00 01:30 02:00 02:30 03:00

Transfer Header Temperature (TI-2179) -Flue Gas Temperature (Tl-2119) • Number of Burners

Figure 4. Time line for temperature and number of burners lit.

Figure 5. Overview of failed radiant tubes.


Figure 6. Closeup view of failed radiant tubes.

Figure 7. Molten metal found within the radiant tube catalyst.


Figure 8. Riser tubes (riser tube on right has been mechanically cut forremoval).

Figure 9. Overview of air-steam preheat coil No. 1 (shield coil) damage.


Figure 10. Another overview of damage to air-steam preheat coil No. 1 (shieldcoil).

possible. In addition, since a scheduled maintenanceturnaround had been planned for the following June(1999), those additional maintenance items werebrought forward for completion at the same time as themajor repair.

Because of the magnitude of the problem, KelloggBrown and Root (KBR) were contacted for assistance.To oversee progress in procurement and expediting, anAgrium senior purchasing agent was placed in theKBR office in Houston. The senior purchasing agenttraveled to KBR on November 18, 1998. The decisionwas made early hi the repair process to replace all 260radiant tubes, all 5 riser tubes, all 5 outlet manifolds,and all radiant tube spring hangers. This decision wasmade despite the observation that some radiant tubesstill remained intact. Time was of the essence and themanagement team felt there was not time to test radianttubes, riser and collection manifolds for usefulness.

On Nov. 18, 1998 after senior Agrium managementand Agrium's insurance agents had viewed the damage,blinding and scaffolding started as the first stage ofdemolition. A local contractor was hired to commence

removal of damaged items from the radiant box.For the major work, a second local company was

hired to set the stage for repairs, plus do additionalwork on the convection section of the furnace.

By Nov. 21, 1998, KBR, with input from Agrium,had narrowed vendors for the cast radiant tubes, castrisers, and new cast collection headers to two vendors(one in the U.K. and the other in the U.S.). Two ven-dors were considered optimum due to the fast supplyand delivery each offered. Placing the order with onevendor would have increased delivery time.Demolition was well underway at this point.

On Nov. 22, 1998 a letter of intent was sent to thefirst vendor, and 114 radiant tubes, 6 risers, and 5 col-lection headers with the second vendor. This supplierprovided 10 extra radiant tubes and one extra riser tube.The arrangement with the two suppliers guaranteed aninitial receipt of radiant tubes by Dec. 10, 1998 (3weeks and two days after the incident) and final deliv-ery of materials by Dec. 20, (4.5 weeks after the inci-dent) allowing a staged-in repair.

To help expedite the replacement radiant parts, the


Agrium management team became aware very early onin the procurement process that one part of the radianttube replacement was going to be a bottleneck. Thatpart was the top of the radiant tube known as the tubetop. The tube top is essentially the inlet section of theradiant tube, made entirely of wrought components.

To meet early delivery schedules, the tube tops wereremoved from all damaged radiant tubes. As well,Agrium had in inventory 20 complete spare radianttubes, and all the tube tops were cut off from the inven-tory tubes and sent to the vendors as well. The addi-tional tube tops were insurance against any unusabletube tops that might surface in inspection and/orassembly of the new replacement radiant tubes.

Again, Agrium personnel were sent to the tworespective vendors facilities to expedite manufacturingand delivery of radiant tubes, risers and collectionheaders.

The transition assemblies (between riser and outlettransfer header) were contracted to a third vendor, andthe air-steam preheat coil No. 1 (shield coil), includingintermediate tube supports, was contracted to a fourthcompany. Both the latter were located in the U.S. anddelivery was scheduled as soon as individual itemswere fabricated. Also, delivery was timed to coincidewith the delivery of the cast radiant components.

Due to some long-term process requirements for theradiant section of the furnace, replacement tubes weremade with a slightly larger internal diameter (from 4.0in. ID to 4.3 in. ID) and the tube metallurgy of the radi-ant tubes was changed from HP-Nb modified to HP-Nbmicroalloy material. (This larger internal diameterchange in the radiant tube decreased the pressure drop.)

The riser tubes remained as HP-Nb modified materi-al with no alteration in physical dimensions.

The outlet collection headers were altered fromwrought Incoloy 800HT to the cast equivalent.

In addition to the physical components, none of theradiant tube catalyst was recovered, and new replace-ment catalyst was purchased. The catalyst replacementwas unidense loaded. To facilitate the quick delivery,the 156 radiant tubes from the U.K. vendor had to beair freighted to Canada hi three airfreight loads. Afreight forwarder was hired to contract the Russianbuilt planes to haul the materials to Canada. Figures 11,12 and 13 show the type of aircraft, the stacking of

tubes hi the plane hold and the unloading of the radianttubes at Edmonton, Alberta, Canada for transport toFort Saskatchewan (a distance of 50 miles).

Due to sequential delivery of items, all radiant tubeswere individually stabbed into the furnace. See Figure14. There was no time for building harps outside thefurnace and then installing completed harps into thefurnace.

The replacement schedule was organized for 2-10 hshifts 7 days a week, with 4 h for any X-ray work. Allwelded joints were X-rayed.

Following installation of each complete row of radi-ant tubes (and riser and transition can assemblies), thecatalyst was loaded into the radiant tubes and the pres-sure drop readings completed. This was done whileconstruction continued on other rows, until all fiverows of radiant tubes (and riser and transitions canassemblies) were completed.

Following along with the repairs to the radiant sec-tion, the air-steam preheat coil No. 1 (shield coil) wasrepaired.

All mechanical work was completed by Dec. 22,1998.

Scaffolding and blinds were removed by Dec. 23,1998 and the major contractor demobilized. Preheatingcommenced Dec. 24, 1998, and, by early Dec. 26,1998, ammonia production began - just 39 days afterthe initial damage to the furnace.

The job was completed with no lost tune injuries orserious medical injuries.

Corrective Actions

Following the repairs and startup of the plant opera-tions, a full enquiry was undertaken as to the cause ofthe failure. This enquiry resulted hi the following cor-rective actions:

• It is mandatory to have an overall operations coor-dinator during startup.

• It is mandatory to have two operators on the panelduring startup.

• Log books would be kept of each area of the plantto improve communication between crews.

• Written procedures for lighting burners.• An automatic shutdown system was installed to

protect against overheating during, startup. (The ammo-nia plant did have a high temperature shutdown system


Figure 11. Russian cargo plane used to transport radiant tubesfrom U.K. to Canada.

Figure 12. Packaging of radiant tubes in hold of Russian plane.

Figure 13. Offloading of radiant tubes at EdmontonInternational Airport for transfer to Fort Saskatchewan.


Figure 14. Stabbing of tubes into pri-mary reformer furnace.

on the process side. As there was little flow from thenitrogen purge, this system did not "see" the high tem-peratures. The new system is designed specifically tolook at flue-gas temperatures (voting system) andsteam flow (voting system) and trip if temperature istoo high with low steam flow).

Closing Comments

• The automatic protective systems in an ammoniaplant may not adequately protect equipment duringstartup conditions.

• Common practice at the Fort Saskatchewan plantwas to rely on procedures to protect the plant duringstartup.

• When relying on procedures, all necessary stepsshould be taken to ensure they will be correctly fol-lowed.

• The importance of field checks must be reinforcedto the operators.

• If the negative consequences of a procedure notbeing followed are too great to accept, the need foradditional layers of protection into the system shouldbe considered. This could mean designing an automat-ic trip system.

QUESTIONS AND ANSWERS

Rob Gommans, Gommons Metallurgical Services:Were tubes protected from de-icing salts during roadtransport?D. EL Timbres, Agrium: Yes. For road transportation,all the catalyst tube openings were protected withflange protectors, and plastic caps over the inlet andoutlet pigtails.The catalyst tube assembly was thenplaced into individual plastic bags, and finally loaded

onto wooden bearing blocks, blocked and strappeddown to the trailer. The load was further protected witha tarp over the total assemblies. For air transport,flange protectors and plastic caps protected the cata-lyst tube openings over the inlet and outlet pigtails. Theassemblies were loaded onto wooden bearing blocks,blocked and strapped down. Following unloading ontoroad trailers at the airport; the load was tarped fortransport to the plant site.


2001/2002: primary reformer failure/fileser… · per day stamicarbon urea plant. current operating...

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