Are you are talking about an external weld build-up while the column is in operation? Repairs of this type are not addressed by the NBIC because you need to verify defect removal and integrity of the repair. On line repairs are case-by-case and should be discussed with the Jurisdiction or Insurer of the equipment, or look at the API code on repairs and alterations.
Also use ASME PCC-2 as a resource to help guide you. Article 2.2 speaks to external weld build up. I also urge caution if you plan on doing this with the vessel online, especially if it's the vacuum tower. You should do AUT across the entire area you plan on welding to check for any low spots where burn through is a real possibility. Alternatively to weld build you could try and weld a fillet patch over the corroded area, welding the seam where you know you have good metal. See PCC-2 article 2.12. Also look to API 2201 and PCC-2 article 2.10 to get your mind jogging about what to consider for welding on in service equipment. You can do this kind of repair, you just need to make sure you are very careful. And do not commit to something outside of your technical experience. If you need to, seek help from someone who can come to your site, not just some people on an internet forum. Profit is never more important than the safety and well-being of people.
The NBIC does not currently prohibit restoration by encapsulation of a degraded area which is a means of implementing an ASME code restoration without breaching pressure boundary. let me know if you need additional information on this method of restoration. The method can be implemented in a timely fashion and is quite cost effective.
Welded Encapsulations are one of many topics addressed in ASME PCC-2 Repair of Pressure Equipment and Piping Part 2. They're used commonly enough, and easy to fabricate out of materials readily available at most refineries and upstream facilities. Nothing secret or special about them, but it does help to have a pretty thorough understanding of vessel design, and occasionally, design by analysis.
