Remaining life assessment of refinery furnace tubes using finite element method
Aiche 42-024-primary reformer failure- agrium
1. Primary Reformer Failure
The 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 McConnell
Agrium Fort Saskatchewan, Alberta, Canada
Introduction
At the Agrium Fort Saskatchewan Nitrogen
Operations in Fort Saskatchewan, Alberta,
Canada on Nov. 16-17, 1998, a massive
reformer failure was experienced. This failure followed
a short maintenance outage and what was considered a
routine plant restart. Almost all the catalyst tubes failed
in the radiant section, and the air-steam preheat coil
No. 1 (shield coil) was damaged. This resulted in 39
days of lost production in the ammonia plant and 40
days lost production in the urea plant.
The events which lead up to the failure, the method-ology,
scheduling and alterations made to bring the
reformer furnace back on line, and the safeguards
incorporated to lessen future occurrences of this type of
failure 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 and
Argentina. The Corporation produces and markets four
primary groups of fertilizers: nitrogen, phosphate,
potash, and sulfur.
Agrium, Fort Saskatchewan Nitrogen Operations
was commissioned in 1983 and produces ammonia and
urea fertilizer for the Western Canadian and export
markets. The site comprises a 1,000 metric ton per day
nameplate Kellogg ammonia plant and a 907 metric ton
per day Stamicarbon urea plant. Current operating
capacities are 1,350 metric tons per day ammonia and
1,280 metric tons per day urea.
The primary reformer furnace in the ammonia plant
is a Kellogg design with 260 radiant tubes, 5 radiant
tube rows, with 52 tubes per row. The radiant tubes
were HP-Nb modified material and installed over a
period of 1991 to 1993, with the majority (207 out of
260) of the tube installed new in 1993. The reformer
furnace layout is shown in Figures 1 and 2.
There are six arch burner rows with 22 burners per
row for a total of 132 arch burners. In addition there are
six tunnel burners, which are not operated, and 13
steam superheater burners.
The convection section consists of three coils in the
"hot leg," the air-steam shield coil, the main air-steam
coil, and the mixed feed preheat coil. In the "cold leg,"
there are four coils, the steam superheat coil, the
feedgas coil, the boiler feed water preheat coil, and the
fuel gas preheat coil. All aforementioned coils are in
the 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
4. • 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 arch
burners between each of the radiant tube rows until the
flue-gas temperature reaches 400°C (752°F), following
which steam is introduced into the radiant tubes. By
preheating, you prevent condensation of the steam on
the 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 is
introduced to the process at 40% rate. The temperature
is then steadily increased by lighting more arch burners
until the exit tube temperature reaches 700°C
(1,292°F), at which point natural gas is introduced to
the 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 light
off has been confirmed by a temperature rise across the
secondary reformer, the system is gradually brought to
up operating conditions.
From Nov. 16 through Nov. 17, 1998, the primary
reformer furnace was already in a warm condition with
a flue-gas temperature of just under 400°C (752°F).
Night shift was continuing to warm up and was getting
ready to introduce steam at 400°C (752°F).
The panel operator directed the field operator to light
additional arch burners to warm up the reformer. The
field operator did this exact operation a number of
times over the next few hours.
What was later discovered was that the field operator
did not perform checks inside the radiant box after
lighting the additional burners. That is, the field opera-tor
did not visually look at the burner(s) that had been
lit to confirm there was no flame impingement on the
tubes and to visually confirm the tubes were not exces-sively
hot.
In addition, the panel operator was incorrectly
observing the transfer header outlet temperature
instead 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 the
catalyst tubes were a glowing bright yellow and dis-continued
firing. It was later noted that the flue-gas
temperature had reached a maximum value of 1,071°C
(1,960°F). [The melting point is typically quoted at
about 1,343°C (2,426°F) and complete melting will be
finished at 1,400°C (2,552°F), respectively known as
the solidus and liquidus melting temperatures ("the
range of melting"). It is probable that localized flue-gas
temperatures were much higher than temperatures
reported by thermocouples at the outlet of the radiant
box. Uneven firing or the relatively high air leakage
rate at low firing rates could cause this.]
The time line for the temperature and number of
burners lit is shown in Figure 4.
Description of Damage
Following a cool down of the primary reformer fur-nace,
further observations within the radiant box
showed that approximately 50% of the tubes had com-pletely
failed. Tube failure was observed mainly at
welds. Molten metal had solidified within the radiant
tube 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 evidence
of overheating and distortion. See Figure 9 and 10.
None of the tubes had ruptured, however. The air-steam
No. 2 coil was intact with no evidence of damage, as
was the mixed feed coil.
In the "cold leg" of the convection section, none of
the four coils (steam superheat, feedgas preheat, boiler
feedwater preheat, and the fuel gas preheat coils) were
damaged.
Some minor damage was noted to the radiant furnace
wall 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
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5. 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-17
21:00 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.
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6. Figure 6. Closeup view of failed radiant tubes.
Figure 7. Molten metal found within the radiant tube catalyst.
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7. Figure 8. Riser tubes (riser tube on right has been mechanically cut for
removal).
Figure 9. Overview of air-steam preheat coil No. 1 (shield coil) damage.
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8. Figure 10. Another overview of damage to air-steam preheat coil No. 1 (shield
coil).
possible. In addition, since a scheduled maintenance
turnaround had been planned for the following June
(1999), those additional maintenance items were
brought forward for completion at the same time as the
major repair.
Because of the magnitude of the problem, Kellogg
Brown and Root (KBR) were contacted for assistance.
To oversee progress in procurement and expediting, an
Agrium senior purchasing agent was placed in the
KBR office in Houston. The senior purchasing agent
traveled to KBR on November 18, 1998. The decision
was made early hi the repair process to replace all 260
radiant tubes, all 5 riser tubes, all 5 outlet manifolds,
and all radiant tube spring hangers. This decision was
made despite the observation that some radiant tubes
still remained intact. Time was of the essence and the
management team felt there was not time to test radiant
tubes, riser and collection manifolds for usefulness.
On Nov. 18, 1998 after senior Agrium management
and Agrium's insurance agents had viewed the damage,
blinding and scaffolding started as the first stage of
demolition. 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 additional
work on the convection section of the furnace.
By Nov. 21, 1998, KBR, with input from Agrium,
had narrowed vendors for the cast radiant tubes, cast
risers, 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 supply
and delivery each offered. Placing the order with one
vendor would have increased delivery time.
Demolition was well underway at this point.
On Nov. 22, 1998 a letter of intent was sent to the
first vendor, and 114 radiant tubes, 6 risers, and 5 col-lection
headers with the second vendor. This supplier
provided 10 extra radiant tubes and one extra riser tube.
The arrangement with the two suppliers guaranteed an
initial receipt of radiant tubes by Dec. 10, 1998 (3
weeks 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
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9. Agrium management team became aware very early on
in the procurement process that one part of the radiant
tube replacement was going to be a bottleneck. That
part was the top of the radiant tube known as the tube
top. The tube top is essentially the inlet section of the
radiant tube, made entirely of wrought components.
To meet early delivery schedules, the tube tops were
removed from all damaged radiant tubes. As well,
Agrium had in inventory 20 complete spare radiant
tubes, 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 unusable
tube tops that might surface in inspection and/or
assembly of the new replacement radiant tubes.
Again, Agrium personnel were sent to the two
respective vendors facilities to expedite manufacturing
and delivery of radiant tubes, risers and collection
headers.
The transition assemblies (between riser and outlet
transfer header) were contracted to a third vendor, and
the air-steam preheat coil No. 1 (shield coil), including
intermediate tube supports, was contracted to a fourth
company. Both the latter were located in the U.S. and
delivery was scheduled as soon as individual items
were fabricated. Also, delivery was timed to coincide
with the delivery of the cast radiant components.
Due to some long-term process requirements for the
radiant section of the furnace, replacement tubes were
made with a slightly larger internal diameter (from 4.0
in. ID to 4.3 in. ID) and the tube metallurgy of the radi-ant
tubes was changed from HP-Nb modified to HP-Nb
microalloy material. (This larger internal diameter
change 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 from
wrought Incoloy 800HT to the cast equivalent.
In addition to the physical components, none of the
radiant tube catalyst was recovered, and new replace-ment
catalyst was purchased. The catalyst replacement
was unidense loaded. To facilitate the quick delivery,
the 156 radiant tubes from the U.K. vendor had to be
air freighted to Canada hi three airfreight loads. A
freight forwarder was hired to contract the Russian
built 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 radiant
tubes at Edmonton, Alberta, Canada for transport to
Fort Saskatchewan (a distance of 50 miles).
Due to sequential delivery of items, all radiant tubes
were individually stabbed into the furnace. See Figure
14. There was no time for building harps outside the
furnace and then installing completed harps into the
furnace.
The replacement schedule was organized for 2-10 h
shifts 7 days a week, with 4 h for any X-ray work. All
welded joints were X-rayed.
Following installation of each complete row of radi-ant
tubes (and riser and transition can assemblies), the
catalyst was loaded into the radiant tubes and the pres-sure
drop readings completed. This was done while
construction continued on other rows, until all five
rows of radiant tubes (and riser and transitions can
assemblies) were completed.
Following along with the repairs to the radiant sec-tion,
the air-steam preheat coil No. 1 (shield coil) was
repaired.
All mechanical work was completed by Dec. 22,
1998.
Scaffolding and blinds were removed by Dec. 23,
1998 and the major contractor demobilized. Preheating
commenced Dec. 24, 1998, and, by early Dec. 26,
1998, ammonia production began - just 39 days after
the initial damage to the furnace.
The job was completed with no lost tune injuries or
serious medical injuries.
Corrective Actions
Following the repairs and startup of the plant opera-tions,
a full enquiry was undertaken as to the cause of
the 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 panel
during startup.
• Log books would be kept of each area of the plant
to 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
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10. Figure 11. Russian cargo plane used to transport radiant tubes
from U.K. to Canada.
Figure 12. Packaging of radiant tubes in hold of Russian plane.
Figure 13. Offloading of radiant tubes at Edmonton
International Airport for transfer to Fort Saskatchewan.
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11. Figure 14. Stabbing of tubes into pri-mary
reformer furnace.
on the process side. As there was little flow from the
nitrogen purge, this system did not "see" the high tem-peratures.
The new system is designed specifically to
look at flue-gas temperatures (voting system) and
steam flow (voting system) and trip if temperature is
too high with low steam flow).
Closing Comments
• The automatic protective systems in an ammonia
plant may not adequately protect equipment during
startup conditions.
• Common practice at the Fort Saskatchewan plant
was to rely on procedures to protect the plant during
startup.
• When relying on procedures, all necessary steps
should be taken to ensure they will be correctly fol-lowed.
• The importance of field checks must be reinforced
to the operators.
• If the negative consequences of a procedure not
being followed are too great to accept, the need for
additional layers of protection into the system should
be 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 road
transport?
D. EL Timbres, Agrium: Yes. For road transportation,
all the catalyst tube openings were protected with
flange protectors, and plastic caps over the inlet and
outlet pigtails.The catalyst tube assembly was then
placed into individual plastic bags, and finally loaded
onto wooden bearing blocks, blocked and strapped
down to the trailer. The load was further protected with
a 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. The
assemblies were loaded onto wooden bearing blocks,
blocked and strapped down. Following unloading onto
road trailers at the airport; the load was tarped for
transport to the plant site.
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