The application of emerging fitness-for-service standards in conjunction with advanced modeling and ultrasonic thickness (UT) inspection is demonstrated with the recent assessment and repair of a CO2 absorber vessel. UT inspection discovered four regions of localized metal loss on the internal surface of a CO2 absorber vessel shell. Of the four regions, two were directly adjacent to major structural discontinuities, including two nozzles, one of which contained a reinforcing plate or repad.
In order to define the critical locations of metal loss and estimate a corrosion rate, thickness data for the regions of metal loss was provided in the form of 1 inch by 1 inch equally-spaced, rectangular thickness grid from two separate inspection dates. Based on the estimated corrosion rate, and the specified operation interval, the rate of metal loss was determined to be significant enough to require repair. This conclusion was based on the fact that in certain locations, the metal loss was estimated to grow through-wall before the end of the specified operation interval.
Computational analysis using guidelines per API 579-1/ASME FFS-1  was used to evaluate an appropriate repair procedure. This included evaluation of repair plate placement and sizing using advanced modeling techniques including elastic-plastic material behavior and contact interaction. The effect of future metal loss was included based the estimated corrosion rate. The result of this assessment was a repair design that provided sufficient protection against excessive plastic deformation and allowed for continued operation through the specified operating interval. Thus, repairing the vessel based on fitness-for-service (FFS) criteria allowed for continued use of the vessel and avoided costly replacement. The lessons learned provide insight into the improved design of vessel repairs.