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.

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.

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.

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.

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.

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.

• 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

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.

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

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.