Failure of Weld of F-11 Bottom Flanges with Micro Alloy ...

Failure of Weld of F-11 Bottom Flanges with

Micro Alloy Catalyst Tubes

of Primary Reformer

As a part of Ammonia plant revamp, FFBL replaced all its catalyst tubes with superior Micro Alloy

material and Low Alloy Flanges. This combination proved to be a failure due to corrosion of the weld

joint between the tube and bottom flange.

Anwar Mahmood Shahid

Fauji Fertilizer Bin Qasim Limited

Naveed Hisam

Fauji Fertilizer Bin Qasim Limited Introduction

his paper provides an insight about the dissimilar metals weld failure phenomenon

observed in the welds of F-11 bottom flanges with 25-35 Micro Alloy (MA) catalyst tubes. Further, it describes non destructive inspection performed on defects, installation of box up sleeves to operate furnace with cracking in welds and detailed failure analysis. It also elaborates steps taken for successful replacement of bottom flanges to rectify the problem of Primary Reformer at Ammonia Plant of Fauji Fertilizer Bin Qasim Limited, Karachi, Pakistan (FFBL).

History:

FFBL was established in 1995 to operate a Urea-DAP Fertilizer Complex in Karachi, Pakistan with the following design capacities:

Ammonia: 1270 MTPD

Designed by Bechtel, USA. Urea: 1650 MTPD

Wet part Designed by Stamicarbon, Netherlands

Dry part by Hydro Agri, Belgium.

DAP: 1350 MTPD Designed by Grande

Parroise, France. The 1965 vintage Ammonia plant was operated by Olin Chemicals in Lake Charles Louisiana USA until 1992. Thereafter it was relocated to Karachi where it was commissioned in 1999 by FFBL.

Figure 1. Overview of Twin cell Primary Reformer

T

652011 AMMONIA TECHNICAL MANUAL

62 AMMONIA TECHNICAL MANUAL2012

Ammonia Process: Reforming of natural gas feed stock is carried out in Primary Reformer to produce hydrogen for Ammonia synthesis. The Primary Reformer is a Foster Wheeler designed twin cell, two tier tubular furnace with staggered catalyst tubes arrangement. The two radiant chambers are called North and South chamber. It has 432 catalyst tubes, 216 in North & South chamber each, which were originally made of HK 40 grade material with Carbon Steel bottom flanges. Process gas conditions are as follows: Inlet Pressure (kg/cm2) 35.6 Delta P (kg/cm2) 1-2 Inlet Temp (oC) 480 Outlet Temp (oC) 765 TMT (oC) 870 (1060 for glowing tubes) Inlet chemical composition (Mole % Dry basis):

N2:8, CO: nil, CO2: 1.46, CH4: 89.93

Outlet chemical composition (Mole % Dry basis):

N2:1.03, CO: 8.13, CO2: 11.87, CH4: 8.68, H2: 70.29

In Foster Wheeler design Furnaces, catalyst is placed above a perforated cone to avoid catalyst draining in the gas stream. The cone is partially filled with refractory. The bottom flange of catalyst tube is outside the radiant chamber and exposed to atmospheric temperature (see Figure 2). These aspects create a dead end for gas flow and lower bottom flange temperature leading to formation of steam condensate during normal plant operation. This condensate is converted to carbonic acid by reaction with carbon dioxide present in the process gas.

Figure 2. Catalyst Tube Arrangement Problems Faced at FFBL: In construction phase, FFBL used 75 % in-service old catalyst tubes and 25 % new HK-40 catalyst tubes. After commissioning at FFBL, severe reliability problems were observed with Primary Reformer. Two fire incidents / shutdowns occurred due to tubes failure and few catalyst tubes were nipped during operation due to leakage in intermediate weld joints. Year 2001, 2002 and 2003 was very hectic for FFBL with respect to catalyst tubes handling. FFBL ordered 40 catalyst tubes (material HK-40) in year 2002 with improved bottom flange material i.e. F-11.

66 2011AMMONIA TECHNICAL MANUAL


Figure 3. Cracked Catalyst Tubes

Ammonia Plant BMR: FFBL launched Ammonia plant BMR study in 2003. The process engineering study of the Ammonia Plant was awarded to Haldor Topsoe AS, Denmark (HTAS) to identify bottlenecks for plant reliability, increased production and energy efficiency. Besides replacement of several equipments, HTAS also proposed to replace existing HK-40 catalyst tubes with upgraded Micro Alloy thin walled tubes having larger inner diameter. FFBL developed detailed ordering specifications and drawings of Micro Alloy catalyst tubes for procurement and replacement. The specs / drawings were forwarded to HTAS for their review. Flange material compatibility aspect was not highlighted by FFBL to HTAS nor by HTAS to FFBL, during review. The top and bottom flanges of these Micro Alloy catalyst tubes were maintained as F-11 as with previous catalyst tubes of HK-40 material. Accordingly 216 new tubes of North chamber were replaced in Jan-2004 during Annual Turnaround. These were procured from MetalTek USA. This resulted in new 216 Micro Alloy tubes in the North chamber and 216 HK-40 tubes in the South chamber. The reformer condition was then considered very well to increase the plant load safely.

FFBL Management decided to replace the South chamber HK-40 tubes with Micro Alloy tubes to obtain a safe and uninterrupted increased production. Order was placed with Schmidt-Clemens Spain and the tubes were replaced in Jan 2006. HTAS had already recommended conducting a detailed engineering study thru the OEM i.e. Foster Wheeler for complete debottlenecking of the primary reformer for smooth operation at 125% load. A detailed combustion and flexibility engineering study was carried out by Foster Wheeler in 2005 – 2006, who recommended a few other modifications related to cross over piping and convection coils. This detailed engineering was carried out by Foster Wheeler considering the Micro Alloy catalyst tubes with F-11 flange in both chambers. FFBL provided updated Micro Alloy catalyst tube drawings to Foster Wheeler for reference but no concern was raised by FW regarding the material of catalyst tubes’ top and bottom flanges i.e. F-11. All the proposed modifications by FW in the convection section, mixing headers and outlet header supports were completely executed by FFBL in May 2007 annual turnaround. Thereafter the whole reformer started performing well for 125% front end load. Post BMR Problems: On 19-Aug-2008 a process gas fire was observed at the lower end of the Primary Reformer catalyst tubes in the middle of the North cell. It resulted in damage to adjacent casing plates and structural material. However, timely observation of the flame enabled us to take immediate remedial action and the Ammonia plant was immediately shutdown.



Figure 4. Crack in Bottom Flange Weld

After inspection of the catalyst tubes, we observed a circumferential crack on the weld joint of tube number 324 (covering 40% of circumference) between Micro Alloy catalyst tube and F-11 flange. The crack was located at the weld toe of F-11 flange side. The plant was started after removal of defective catalyst tube and plugging the relevant sockets in headers. In view of above, it was planned to carry out Radiographic Test (RT) of some of the bottom flange’s welds in upcoming Annual Turnaround of Jan 2009 to assess the general health of bottom weld joints. During Annual Turnaround in Jan 2009, visual inspection of a few bottom flange joints was carried out from the inner side after removing catalyst. It showed severe corrosion grooving (up to 2 mm) along the toe of weld joints in subject welds in F-11 flange metal. Cracks were also observed extending along the weld toe in F-11 flange material behind the grooving. Radiography of a few random joints also confirmed existence of grooving problem in other tubes. However, due to cracking within the groove, detection of cracks with RT inspection was difficult due to masking. Accordingly, the scope of inspection was extended to ultrasonic scan of bottom weld of all catalyst tubes to gauge crack depths. Stork Cooper Heat, already engaged at our plant on some other job, was asked to assist in Ultrasonic Test (UT) scanning. Bottom flange joint of 50% tubes was radio graphed and UT was done on all bottom flange welds. Results were unexpectedly horrifying

and revealed defects. The severity of cracking was high in 54 joints towards F-11 flange. There was linear indication (similar to LOF / crack) in the root of the weld along F-11 flange weld fusion line. A narrow dark band was also found along the linear defect in radiograph indicating some metal loss toward flange side weld root.

Figure 5. Corrosion and Grooving

In order to check the depth of cracks in welds of certain tubes, bottom flange welds were grinded up to sound metal and checked with DPT. Minor linear indications propagated as 2-5 mm upon deep grinding / DPT in North Chamber tubes. However, in south chamber the cracks were found eliminated after 2 mm depth. The results were less aggressive in South chamber tubes as compared to North chamber tubes due to age factor i.e. lesser on-stream time as these tubes were installed two years after installation of tubes in North chamber. Summary of RT and UT results is as follows:

29%

4%

67%

South Chamber Catalyst Tubes Status in UT

Crack Depth b/w 0.5 ~ 1.0mm

Crack Depth > 1.0mm

Crack Depth < 0.5mm

40%

7%

53%

North Chamber Catalyst Tubes Status in UT

Crack Depth b/w 0.5 ~ 1.0mm Crack Depth > 1.0mm Crack Depth < 0.5mm



Out of Box Approach

Interim Arrangement – Installation of Box up sleeves: We did not have the time to perform in situ weld repairs and heat treatment of 54 flanges. Nor we had any spare flange for replacement. An idea of external sleeves installation triggered to have some interim arrangement to let the plant run for one year and gain time for detailed investigations and permanent solution of the problem. The sleeves were wrapped around weld joint and welded with Inconel filler wire (ERNiCr-3). We had only SS-304 pipe available to fabricate sleeves. We shared this idea immediately with HTAS, Foster Wheeler, MetalTek and Schmidt-Clemens. Principally all agreed to it as a short term measure only. FFBL decided to opt this solution. External backup sleeves fabricated from Dia 5″ Sch 40 (6 mm thick) SS-304 pipe cut in two halves were installed on 54 nos. (50 in North and 4 in South chamber) most defected flange welds.

Figure 6. Sleeve Installation In-Progress

SLEEVES INSTALLED AROUND WELD JOINT

Back

Figure 7. Catalyst Tube, Sleeve and Flanges Upper edge of sleeve circumference was fillet welded to the Micro Alloy tube and the lower circumference edge was fillet welded with F-11 flange. Preheating with oxy acetylene flame, intermediate heating and slow cooling under insulation were done on these welds, which were checked by DPT after cooling. The decision to install sleeves was extremely difficult as no experience of such sleeves installation in the industry was available. The calculations were done in FFBL for checking the strength of sleeve. The job was done adequately and Reformer was put back in service and kept operated safely for one more year without any problem.

Table 1: Catalyst Tubes Main Features:

Specified

Requirement

North Chamber

Tubes (M/s Metal

Tek - USA)

South Chamber Tubes

(M/s Schmidt-Clemens)

Catalyst tube Material

25Cr-35Ni Nb Micro Alloy Centralloy 4852 Micro

Flange Material ASTM A 182 F-11 ASTM A 182 F-11

Filler Metal ERNi Cr3 Inconel 82

Type of Weld J preparation 20o,with no root gap

V-Groove 75o with 3mm root gap

Root Pass Autogenous With Filler metal

Weld Process GTAW GTAW

Pre Heat Min 150 oC Min 150 oC

PWHT No 725 oC for 2 hours

F-11 Bottom Flange



Root Cause Investigation: Extensive correspondences with well renowned designers, licensors and manufacturers were carried out for detailed metallurgical analysis, root cause determination and remedial actions to avoid recurrence. The study covered suitable material selection for bottom flanges replacement and weld-ability of new flanges with in-service MA catalyst tubes. Following companies were contacted in this context:

1. MetalTek USA – Manufacturer of North

chamber catalyst tubes. 2. Schmidt-Clemens – Manufacturer of South

chamber catalyst tubes. 3. Foster Wheeler – Designer of Reformer 4. HTAS – Process Licensor 5. Stork Cooper Heat The outcome of these investigations and detailed failure analysis reports are summarized below: Visual and NDT Findings: Visual inspection, dye penetrant test, magnetic particle test and radiography test were performed on various samples of tubes of both tube manufacturers i.e. S-C and MetalTek. Following were the main findings:

a. Deep grooving found in the weld root at F-11

flange base metal. b. Cracks observed initiating from groove and

propagating along the fusion line between weld of Micro Alloy tube and F-11 flange.

c. Heavy surface deposits of carbonaceous material were detected.

d. No abnormality was reported on external surface of weld.

(Cross Sectional View) (Magnified View)

Figure 8. Cross Section View

Figure 9. Microscopic View

Figure 10. Hardness Evaluation Weld – MA Tube

MA Tube

Groove

F-11 Flange

MA

Tube

Weld

Metal

F-11

Flange

Groove on tube

inner wall.

Metal loss

within flange,

adjacent to

fusion line.



Figure 11. Hardness Evaluation F-11 Flange - Weld Damage Cause Analysis: Two types of defects were observed i.e. corrosion grooving and the cracking. The primary degradation observed for both manufacturers’ tubes manifested as grooved metal loss. This metal loss was along the inner circumference of the flange parent metal at the root face of flange to tube weld. From the groove region, cracking was evident along the fusion line between the weld metal and flange metal. A. Grooving: Grooving, the metal loss, is attributed to following two mechanisms.

1. Carbonic Acid Corrosion:

It is usually result of dissolved gases i.e. oxygen and carbon dioxide in combination with moisture. Carbonic acid is formed when CO2 dissolve in condensate. This carbonic acid lowered PH and promoted corrosion of alloy steel. Water condensate / carbonic acid corrosion mechanism needs sufficient moisture or condensate to be present. Although the normal

tube operating temperature is high inside the furnace, the referred bottom flange section is outside the furnace radiant section and external skin temperature measured at this region is 45°C to 55oC i.e. closer to ambient temperature. The Carbonic acid has more tendency to react with iron in high energy zone. Accordingly, it reacted with iron in the HAZ of F-11 flange as per following reaction:

H2CO3 + Fe FeCO3 + H2 This action produces corrosion and liberates atomic hydrogen on the metal surface. This atomic hydrogen diffuses inside the metal to region of high tensile stress.

2. Galvanic Corrosion:

Galvanic corrosion occurs at the junction of dissimilar metals when they are joined together and exposed to suitable electrolyte (i.e. Water Condensate). For Galvanic corrosion three conditions are required i.e.

Presence of electrolyte Different materials or alloys An electrical connection with anode and

cathode. This difference in metallic structure of tube (Cathode - Noble) and the flange (Anode – Less noble), presence of electrolyte (H2CO3) and the connection of both metals lead to initiation of galvanic corrosion. In our case all these conditions were applicable. General corrosion was also observed throughout the flange inner surface.

B. Cracking due to Hydrogen Embrittlement in Dissimilar Welds:

The phenomenon generally occurs from ambient temperature to 150oC with the decreasing trend at temperature above 82oC. The variables



affecting this phenomenon are presence of H2, embrittlement susceptibility of steel micro structure and presence of residual stresses. Atomic hydrogen, liberated during carbonic acid corrosion, can diffuse into the steel and adsorb at above mentioned stress riser / high energy zones. This atomic hydrogen combines to form molecular hydrogen that diffuses very slowly. This molecular and atomic hydrogen, in adequate concentration, build up very high pressures in the locations where they are present. With time, these pressures can increase to a point where it results in brittle failures by reducing tensile strength thereby initiating the crack further deep into the material. Key Findings: Although, the process gas temperature is high in catalyst tube (465 oC to 765 oC), the bottom flange portion has a dead end for process gas flow. Bottom flange is exposed to atmospheric temperature and normally has a temperature between 45 oC – 70 oC. Accordingly, steam condensation is unavoidable during normal plant operation. This condensate reacts with Carbon dioxide, present in the process gas, and produces carbonic acid. Carbonic acid is highly reactive with iron in high energy zone i.e. the HAZ of F-11 flange. The Primary Damage mechanism on the bottom flange weld was grooving on F-11 flange side fusion line caused due to Carbonic Acid attack on highly stressed / dissimilar zone. This in turn liberated atomic hydrogen which was responsible for initiating fine cracks due to development of high pressure points in the matrix. The top flange welds of tubes were also inspected using visual and radiographic testing however NO indications were found in the subject joints. The reason for this could be attributed to the fact that although having the same metallurgy and welding pattern, the

condensate corrosion was not present. Thus atomic hydrogen was not liberated due to corrosion. Therefore Hydrogen induced cracking was not initiated in any of the top flange tube welds as observed in Bottom welds.

Recommendations: SS-304, 304L and Incoloy 800H material were proposed by engineering companies for replacing the flange for better corrosion resistance. Although SS-304 / 304 L is suitable replacement flange material being compatible with Micro Alloy tubes, we opted for Incoloy 800 H having better match with Micro Alloy tubes for reliability and durability of the system. The weld integrity between the Incoloy and Micro Alloy is superb due to same microstructure and thermal coefficient. Also, the Incoloy material is much resistant to chloride SCC, in case chlorides happen to enter in process stream. Subsequently in 2011 and 2012 turnarounds, RT and DPT was performed on various flanges after unloading catalyst. The weld quality found excellent with no indication of corrosion. Replacement of Bottom Flange of 432 Catalyst Tubes: It was a big challenge to replace bottom flange of all of the 432 catalyst tubes in one Turnaround. There were two main options for execution.

1. In-Situ Replacement, in vertical position of

tubes, by lowering catalyst tubes by 18″ without removing them from the chamber. It had a potential of extensive welding defects due to location and congested space.

2. In-Yard Replacement, in horizontal position of tubes, after removing all catalyst tubes from the chambers and shifting them in an open yard. It involved extensive rigging and shifting of catalyst tubes with a potential of safety risk.



Welding Mock Up Tests: The whole job was centered around the quality of bottom flange weld joint. Accordingly we planned to test welders and select the best of all the lots. We prepared a large number of structural mock ups to simulate in-situ catalyst tube lowering, flange cutting, beveling, alignment and welding in vertical position of tubes. The execution contractor engaged their welders 20 days before commencement of Annual Turnaround. Approximately 150 joints were prepared for practicing of welders. 70 good quality welders were engaged by the contractor for welding mock up test. Each weld joint underwent proper alignment check, DPT, heat / current / speed monitoring during welding and RT of root and final weld. 34 welders were selected exclusively for flange welding job both in vertical and horizontal position. The weld quality was very good in horizontal position of tubes than the in-situ vertical position of tubes.

Figure 12. Mock Up Arrangement

Figure 13. Mock Up Arrangement

We also engaged cold cutting / beveling machines during mock up and found them very quick, accurate and safe. Three machines were engaged for cutting / beveling of the catalyst tube bottom weld joint. These machines contributed a lot for saving total duration of turnaround. Methodology / Planning: Based on very good welding quality in horizontal position of tubes, it was decided to replace all flanges in an open yard by removing all catalyst tubes. All mechanical works including NDEs (DPT+RT) were planned for 23 days with day and night shift. This strategy worked very well and with the untiring efforts and extensive monitoring of FFBL Engineers, we had a smooth & quick job execution in 19 days with four days saving.

Figure 14. Work in Yard



Figure 15. Flange Replacement with Incoloy 800H

Figure 16. Inlet Pigtails

Figure 17. Catalyst Tubes

Figure 18. Outlet Pigtails

Each catalyst tube was explicitly marked with tags and reinstalled on its original location after flange replacement in yard. Besides flange replacement, every catalyst tube had to pass through another activity of removal of residual cut piece of inlet & outlet pigtail from sockets and its build up / grinding / DPT for accurate depth. The sockets of inlet & outlet headers also

went through build up / grinding / DPT for getting accurate depth before insertion of new pigtails. All of the inlet pigtails were replaced with new ones. Whereas 50% outlet pigtails were reused to save time due to extensive welding of bottom stress collar joint.

Resources Mobilization: A large number of resources were mobilized on this job including the following:

6 Cranes, 3 Trailers (40 Ft each), 2 Fork

Lifters, 35 Welding Machines. 3 Flange Cutting / Beveling Machine (Cold

Cutting Machine), Pneumatic Guns for tubes’ bolts opening / tightening, Gadgets for quick, accurate and uniform flange cutting, alignment and replacement.

Extensive lighting / illumination in yard and in situ.

Development of eight work stations in yard to place catalyst tubes horizontally. The tubes were placed on large size beams to facilitate fabrication / welding and NDE works.

3 Power generators to ensure uninterrupted power supply.

Fire proof Canopies on both ends of catalyst tubes to minimize environmental effects on welding quality.

Spread beams for safely shifting catalyst tubes in horizontal position from one work station to another for subsequent activities.

Extensive housekeeping by dedicated teams. Allocation of barricaded areas for tools,

accessories, casing plates, pigtails and gas cylinders.

Safety Aspects: Following actions were well taken up for this job:

Extensive safety talks, hazard monitoring,

immediate corrective actions, strict work permit compliance and superb housekeeping throughout the job.



Usage of relevant PPEs by all team members. Adequate illumination for comfortable

working in night shift. Application of Equipment worthiness

procedure on cranes, folk lifters, trailers, welding machines and chain blocks.

Extensive use of fire proof cloth canopies to avoid spreading of spatters.

Lesson Learnt: We suffered not only financial losses, but

also the costly man hours of our employees in investigations, material arrangement, meetings, mock ups and execution of this activity.

A thorough review of flange material during the Engineering Phase of BMR in year 2003 could have avoided this failure.

All details, especially the material up gradation must be thoroughly studied in design phase to avoid any subsequent upset.



Date post:	23-Oct-2021
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

Failure of Weld of F-11 Bottom Flanges with Micro Alloy ...

Documents