The document summarizes the failure of the second bed outlet pipe in an ammonia synthesis converter at a fertilizer plant in India. Catalyst was escaping from the damaged pipe and causing issues downstream. The pipe was replaced with a Johnson screen made of more corrosion-resistant Inconel 600. The repair took 25 days and successfully addressed the catalyst carryover problem. The original pipe, made of stainless steel, failed after 6 years due to hardening from high temperature hydrogen and nitrogen exposure in the converter.

1. FAILURE IN SYNTHESIS CONVERTER
Prem Baboo
Sr. Manager (Prod)
National Fertilizers Ltd. India
PREFACE
National Fertilizer Limited operates two number Ammonia plants at Vijaipur
each having installed capacity of 1520 MTPD. Ammonia line-1 plant was
commissioned in 1987 & employs Haldor Topsoe design featuring steam reforming
of Natural gas in Primary reformer, hot potassium carbonate (Benfield) system for
carbon dioxide removal and synthesis loop with operating pressure of 220Kg/cm2.
The plant is running successfully without major breakdowns.
This article describes the failure of second bed outlet pipe in Synthesis
converter, problem associated with catalyst carry over and subsequent repair by
installing Johnson screen in the converter.
CONSTRUCTIONAL DETAILS OF SYNTHESIS CONVERTER
Synthesis Converter, R-1501 supplied by M/s Kobe Steel, Japan contains
Haldor Topsoe patented axial and radial flow catalytic basket of S-200 series has
heat exchanger in first bed. The basket has two beds filled with iron promoted
catalyst. Synthesis gas enters via annular space between inner and intermediate
shell radially into the catalyst bed and leaves the bed radially after reaction through
axially placed perforated center pipe. To prevent high temperature hydrogen attack
& nitriding complete basket and its internals are made up of Austenitic Stainless
steel grade 304 material.
Center pipe of second bed is made out of 22mm thick rolled plate having
approximately 24,000 holes of 9.5 mm diameter on its periphery. Test certificate of
the manufacturer revealed that after fabrication & drilling of holes complete
perforated pipe was solution annealed. Upper & lower ends of center pipe have no
perforations such that the bottom end is welded to catalyst support plate and the
upper end passes through glands connected to tween deck plate top. The perforated
portion of the center plate was covered with two layers of Incoloy wire mesh of size
16 and 18 gauge to prevent escape of catalyst along with outgoing gas. The wire
meshes were clamped to the center pipe with bands & buckles.

2. EVENTS OF FAILURE
Problem first surfaced in 1993 when level in Ammonia Separator, B1501
started hunting and subsequently its outlet flow decreased. Level in separator is
maintained through outlet control valves 15LV-22 A & B. Both control valves are
meant for parallel operation and have ball type isolation valves at upstream and
downstream.
Fig. – Schematic Diagram of Synthesis loop
On dismantling & inspection of control valves catalyst dust was found in its
cage, which probably resulted in lesser flow through control valves. The cage of the
control valve was of spiral design having 3 mm openings. These were replaced by
concentric type cage with 4mm opening fabricated in local workshop.
It was suspected that catalyst was escaping from converter second bed and
the same was possible only after damage to second bed outlet pipe and its wire
mesh. M/s Haldor Topsoe, designer & OEM of basket, suggested replacing the center
pipe with Johnson Screen.

3. Regular problem of catalyst carryover was faced every time the plant
stopped and the separator & its level gauge stand pipe required thorough flushing.
Catalyst & its dust used to get accumulated in Let down vessel, B1502 and was
removed regularly during shutdowns. In year 2001 the catalyst was found in
Ammonia Receiver, V1 in Urea plant and two stainless steel pieces belonging to
converter second bed outlet pipe were found inside Synthesis Loop Boiler during
inspection.
PREPARATION & PLANNING
During plant turnaround in May 2002 the job of replacement of second bed
center pipe with Johnson screen in Ammonia Converter was undertaken. Elaborate
arrangements with two number 4500 Nm3/ hr capacity blower, filter and separators
were made for unloading of catalyst from Converter Basket.
REPAIR OF CONVERTER
Dismantling and Catalyst removal:-
Synthesis Converter was depressurized, cooled and purged with nitrogen
and top cover was removed after opening holding bolts with chain blocks. First bed
cover was removed & False-lid was installed
in its place. Outlet header to catalyst
unloading blower was connected to the false-
lid with two number 6” pipe.
Catalyst unloading was carried out in
Nitrogen atmosphere by outside agency M/s
Contract Resources. Blower suction pressure
was maintained at 0.6 ata absolute
throughout catalyst unloading activity to
achieve satisfactory performance of the
system.
After unloading catalyst from first bed
top cover was reassembled and the first bed
was taken out. It was placed vertically on
stools and firmly held with converter
structure. Catalyst of second bed was
unloaded from basket similarly through
manholes of Tween-deck.

4. Fig. : Second Bed of the Basket
Inspection of First Bed :-
First bed of converter basket was inspected from inside and following
observations were made.
• Wire-mesh of the center pipe was found loose and its clamping bands
were found fallen down.
• One of the thermocouples was found damaged.
• Condition of wire-mesh at outer screen was good.
• No defects were found in weld joints inside the first bed and the
hardness on bottom support plate was in the range of 600 to 650 HV.
Inspection of Second Bed :-
Complete scaffolding was erected inside second bed for thorough inspection
and following observations were made.
• Center pipe was found broken at 150mm from the bottom almost in
complete circumference and its wire mesh was in damaged condition.
• Wire mesh of the second bed inner shell was found slightly damage in
two number bottom segments as plug weld joints with the screen
were found broken at a number of places.

5. Fig. – Broken Center pipe of second bed
Fig. – Another view of Center pipe
Removal of the old center pipe of second bed :-
Removal of center pipe and assembly of Johnson screen was carried out
as per procedure given below.
• The expansion bellow was removed by grinding off the fillet welds
at cold-shot pipe and Tween-deck.
• Original center hole in the tween deck was increased from 540mm
to 640 mm with the help of plasma machine and the uneven
surface of hole was smoothened by grinding.
• Stuffing box below the tween deck was removed with plasma
cutting.
• Finally when the center pipe was removed with crane cold-shot
pipe also came out with it. Weld joints between brackets of cold-
shot pipe & the bottom portion of center pipe got broken resulting
in removal of cold-shot pipe along with center pipe.

6. Installation of new center screen in second bed :-
• HTA’s procedure for installation of Johnson screen required
welding of base ring with 30 mm long portion of existing center
pipe. Accordingly, 30mm long pipe piece was left on catalyst
supporting plate and the remaining portion was grinded off.
Fig. : Welding details of Johnson Screen with Catalyst support plate
• Weld joint between left over center pipe piece and second bed
supporting plate was checked by D.P test and found all right.
• Hardness of catalyst supporting plate and tween deck were
checked and found in the range of 700~800 HV and 600~750 HV
respectively. Hard nitride layer on top portion of catalyst
supporting plate and tween deck plate were removed by grinding
to achieve required hardness of 180~200 HV suitable for welding.
• Height of the guide brackets on the cold shot pipe were
maintained as 12 mm and 95 mm on 10” and 12” pipe portions of
cold-shot pipe respectively.

7. • Cold shot pipe was reinstalled in position and its guide brackets
were welded with remaining portion of center pipe.
• New Johnson screen was received in two parts. Both parts were
welded together maintaining the straightness of the assembly.
• Base ring, supplied along with
Johnson screen, was placed on
the remaining 50mm portion
of center pipe, aligned, leveled
and welded. The base ring
was then welded with catalyst
supporting plate.
• Johnson screen was placed,
leveled and welded with the
base ring.
• To repair broken weld joints of inner shell wire-mesh 5mm thick,
2”x2” size Inconel-600 plates having 10mm center hole were used.
These plates were plug
welded to fix the wire mesh
with in position. Bottom weld
joint of wire-mesh with
catalyst support plate was
found broken at two places
and was repaired by welding
Inconel-600 plates to the shell
such that the wire mesh
remains in pressed condition.
• New stuffing box was installed

8. and welded on the tween deck.
• The expansion bellow was installed around the cold shot pipe and
inside the new center screen as per modified drawing received
along with Johnson screen. After installation of expansion bellow it
was observed that i) the radial clearance between cold shot pipe
and expansion bellow
shell was 28.4mm
whereas originally it
was only 3.2mm, ii)
the height of
expansion bellow
above tween-deck was
found 338mm against
original dimension of
190mm. It was
apprehended that
expanded bellow
might touch the inlet
scrolls of exchanger in
the first bed bottom during thermal expansion.
• Matter was discussed with M/S Haldor Topsoe and accordingly
radial clearance between cold shot pipe and sleeve of expansion
bellow was reduced from 28.4 mm to 3.2 mm by welding an
additional ring with the sleeve of expansion bellow. In addition
position of expansion bellow was shifted down by 148.0mm to
bring it back to original position and welded with Johnson screen.
• Root run of all weld joint were checked by D.P. test. Filler wire of
AWS Class ERNICR3 was used for welding all Inconel 600 joints
and AWS Class ER308L was used for stainless steel AISI304 weld
joints.
Repair of Bottom cover:-
Bottom cover of converter was opened for inspection and replacement of
glands. Cracks were observed in tapped holes in Inconel overlay portion of the
stuffing box. Radial cracks were starting at the threads and propagating towards the
center of outlet pipe. Holes of 4mm were made at the end of cracks and welding was
carried out.

9. New studs were installed in place of 8 no. missing studs. Glands were
repacked with new gland packing. All studs and nuts were tack welded to prevent
their loosening during operation.
Catalyst loading & final box-up :-
Catalyst loading was carried out with hoppers and crane. All gaskets and
glands in the basket were replaced and Converter top & bottom covers were
boxed up.
CONCLUSION
Failure of second bed center pipe in Ammonia Converter resulted in carry
over of catalyst in successive downstream equipments & vessels in Synthesis section
even upto Urea plant. Though no problem was faced with respect to reaction
chemistry in Converter, catalyst carryover resulted in frequent problems in
downstream equipments.
Failure of center pipe was attributed to nitriding and subsequent hardening
of stainless steel in high temperature nitrogen atmosphere that exists in Ammonia
Converter. The damage was further aggravated by high temperature hydrogen
attack after initial nitriding of protective layer. Though center pipe was solution
annealed after drilling of holes during manufacturing but it failed after 6 years of

10. operation. New Johnson screen is made up of Inconel-600 material that has superior
resistance against nitriding. There are incidences of failure of Johnson screen also in
Ammonia Converters but after sufficient strengthening in design by Haldor Topsoe
these are expected to perform well.
Converter basket repair was completed in 25 days and catalysts carry over
successfully arrested thereafter.