Case Study – Ammonia Plant Reformer Tube Replacement
Langfields was contacted in 2015 by a fertiliser company about site welding of a small quantity of new reformer tubes to existing equipment in an ammonia plant on a short lead time. While the reformer tube to pigtail butt welds were performed successfully, defects developed in the hot collector sockets after the completion of the pigtail to hot collector socket welds as illustrated in Figure1.
Figure 1. Defects in the hot collector sockets developed as a result of possibly irregular service conditions
The outcome of the Root Cause Analysis of this failure was a complete retubing of the reformer, as well as replacing the sockets on the hot collector at a later date, due to metallurgical issues of the hot collector. The metallurgical concern with the hot collector was possibly caused by temperature fluctuation due to irregular service conditions (rapid shutdown resulting in rapid cooling of the reformer and hot collector).
This complete retubing of the hot collector meant significant down time for the ammonia plant due to the large (160) number of reformer tubes and associated welding (2 welds in each tube) in challenging site conditions, requiring precision welding due to the high temperature operation as well as high thermal stresses associated with the reformer tubes and connecting pigtails and hot collectors in service.
Langfields Technical Solution
While Langfields already had experience in reformer fabrication since 1980s, the productivity, precision and repeatability demand in conjunction with the large number of welds lead to substantial development work. The result of this is a cutting edge mechanised orbital welding solution that is entirely unique to Langfields within the industry and requires significant welding engineering knowhow as well as welders’ training to execute. In addition, site conditions prohibited the use of interpass cooling and mandated the use of compact equipment due to the vicinity of welds as well as restricted access to them on site. Previous trials during the small batch replacement of the reformer tubes consolidated the procedural requirement in terms of consumable grade and welding process to be considered for the improved solution.
One of the many challenges when welding reformer tubes is the significantly restricted access due to highly congested area. This restricted access can easily lead to human error, even with highly skilled welders, thus leading to inconsistent weld quality when using a traditional, manual welding approach. This means a small mistake can lead to a welding failure with significant consequences.
Figure 2. Welding system used for the pigtail welding to the reformer tubes and hot collector
Therefore, the unique welding solution developed by Langfields in 2015 was orbital (mechanised) GTAW welding for both the reformer tubes to pigtail butt welds and the pigtail to hot collector socket welds – welding system used is shown in Figure 2. Orbital welding allows consistent welding around the circumference without the need for stopping within a pass, reducing the number of weld starts and facilitates a stringent control of welding parameters, critical for the metallurgical integrity of the weld, as well as the Heat Affected Zone (HAZ) of the Alloy 800H base material.
Langfields’ method of using a low profile welding head allows the welder to maintain the welding parameters consistently around the circumference of the weld, even if sections of the weld has severely restricted access and visibility caused by existing plant equipment, supports, etc. Constant welding parameters around the full circumference of the weld is critical for the successful operation as it is well established through historic incidents and failures in ammonia and hydrogen plants, weld quality plays a critical role in extending the life of reformer tubes and pigtails.
Benefits of Langfields Reformer Tube Welding Solution
The unique process developed by Langfields with orbital welding systems provides weld profiles with minimal stress raisers: good blending of weld passes and smooth transition of the weld toes to base material as the macro sections in Figure 3 illustrates.
Figure 3. Weld profiles developed with orbital GTAW welding system by Langfields for a) and b) the pigtail to reformer tube butt welds and c) and d) the pigtail to hot collector socket welds
Langfields was able to maintain this advantageous weld profile, particularly critical for the socket welds in site production as illustrated in Figure 4. This profile reduces the risk of developing cracks at the weld toe due to thermal fatigue and / or stresses in service.
Figure 4. Socket weld profile in a) developed in procedure qualification and b) maintained in production on-site
The Heat Input is kept within 0.7-1.5 kJ/mm range from root to cap in both the butt and socket weld. This low Heat Input range, in combination with the strict control of parameters and weld shape reduces the risk of solidification cracking and liquation cracking, a common feature of welding Alloy 800 material.
Further developing the orbital GTAW solution for another shut down in 2018 (160 butt welds and socket welds each), the number of passes for the pigtail to reformer tube butt welds was reduced from 13 to 8 passes while maintaining the weld quality required, albeit with welding equipment from a different manufacturer. This has lead to 40 % saving in welding times on the reformer tube butt welds, giving the Langfields welding solution for reformer tubes a an overall, the Langfields welding solution gives a 60-80 % timesaving on welding.
Throughout the various projects in supporting our clients in orbital GTAW welding of reformer tubes and pigtails, distinct approaches were developed in the consumable selection for new installations and partial replacements, primarily due to the hot collector defects encountered in 2015.
Table 1 Room temperature strength comparison of Alloy 82 and Alloy 617 consumable grades
While the preferred consumable grade for the Alloy 800 reformer tubes is Alloy 617 (ERNiCrCoMo-1), due to its high temperature strength and creep strength, using this consumable grade to weld existing equipment with compromised metallurgical structure increases the risk of defects in the HAZ as shown in Figure 5. In the development work performed by Langfields in supporting client requirements it was found that welding existing equipment using Alloy 82 (ERNiCr-3) gives better weld and base metal integrity. This is due to the Tensile Strength difference between the two consumable grades shown in Table 1.
Figure 5. Microcracking observed in the Heat Affected Zone (HAZ) when welding onto “aged” base material
The Alloy 82 welding solution was adopted therefore for repairs and replacements where welds are performed on equipment already in service and with a high probability of metallurgical degradation of the base material, even though the creep strength of the Alloy 82 is lower than that of the Alloy 617. While this solution may not match the lifespan of new equipment it is a reliable approach for unexpected repairs or repairs near the end of the lifecycle of these tubes and hot collectors prior to complete replacement.
Langfields
Langfields has a long and varied history providing specialist automated welding solutions to some of the most complex problems at the heart of the plants that we service. Langfields’ dedication to the latest fabrication and welding techniques belies a history that dates back to the 1800’s. We have extensive experience working with non-ferrous metals and our ability to work at short notice has earned us an enviable reputation in the area of mechanical breakdowns and shutdowns.
We have 4 weld procedures qualified specifically for our automated reformer tube welding solution with a team of welders also trained and certified, ready to be deployed in short notice anywhere in the world to respond to the critical timing of such operations.
Recent projects include major shutdowns in the Middle East and Europe and our ability to mobilise large teams of specialist welders at short notice to work globally has cemented our reputation as the welding contractor of choice for this type of specialist work.
For more information please contact Ben Cohen (bcohen@langfields.com) or Wayne Griffiths (wgriffiths@langfields.com).
