Permanently eliminate naphthenic acid corrosion in your vacuum tower with a repair method that’s better, faster and cheaper than the traditional approach.
Installation of automated weld overlay
It’s common knowledge that materials with a surface content of 3% molybdenum (moly) are resistant to this type of corrosion. For years, the remedy to prevent pressure boundary wall loss in these units was to use an automated machine welding process to install a two-layer weld overlay.
First, a "butter layer" of 309 stainless steel, used in high temperature corrosion resistance applications, was installed, followed by a second layer of 316 or 317 stainless steel. The idea of this approach was that the butter layer would help minimize dilution, ensuring that the surface content of the second layer would achieve the desired moly content.
However, there are some drawbacks to this slower, less-effective method. Primarily, the schedule is impacted since the weld and inspection times are doubled; there is also an increased risk of hoop shrinkage, which can affect the installation of tray support rings and internal attachments.
While other repair methods, such as thermal spray applications, have been considered, their long-term viability is questionable at best since the application is a mechanical bond. Unlike a weld metal overlay, which is fused to the vessel wall, thermal spray is a metallic coating that is prone to spalling. If disbonding occurs, there is a potential threat to pumps and other downstream equipment; pressure boundary restoration of the vessel wall is not an option.
Case study for a better option
Recently, BHI Energy, a project management and welding services company, worked with a Canadian refinery to tie-in to a single-layer, 8 feet high band of overlay that had been installed in 2013. Our scope was to tie-in to this band and extend it up an additional 12 inches. Before welding, a visual inspection of the existing overlay was performed. After more than 10 years of continuous service, no pitting or relative indications were found. The ripples of the weld beads and grinder marks of the original installation were still visible. Based on these observations, it is safe to conclude that this one-layer repair is permanent and will extend the service life of this equipment.
Furthermore, if the base metal thickness is below the required minimum thickness, BHI has established weld build-up procedures that allow the alloy material to be considered part of the pressure boundary. This restores the thickness of the vessel wall without compromising the desired surface chemistry.
Continuous advancements and quality assurance
For more than a decade, BHI Energy has continued to enhance this one-layer ASME code-compliant repair method for vacuum towers by improving materials and welding procedures. Its single-layer repairs have consistently delivered surface molybdenum chemistries of more than 3%, and as high as 10% depending on the material employed. Quality control and strict adherence to welding procedures are paramount. In addition to BHI’s superior automated equipment, it also offers a unique, transparent method of monitoring weld parameters in real-time — the results of which are shared daily with a client’s technical team. This technology, coupled with its diligent quality control program, ensures that chemistry goals are achieved.
BHI has several case studies and references available upon request. To learn more about this exclusive repair option, BHI welcomes the opportunity to explain its approach to reliability teams.
For more information, visit bhienergy.com or call (281) 753-9080.
