During a turnaround, a visual internal inspection of an aging deethanizer showed various forms of damage. The vessel was scheduled for replacement at the next turnaround, and the refinery decided to take extra measures to assure safe operation during the 2 year interim. A combination of partial inspection and engineering evaluation was used to assess the deethanizer. The overall assessment of Fitness-For-Service was based on both existing Codes and the guidelines by the Materials Properties Council (MPC). The approach included the following steps: (1) calculation of critical crack sizes; (2) automated ultrasonic testing of selected welds and plates; (3) engineering evaluation of results; and (4) reinspection and evaluation after 6 months. The initial inspection showed severe corrosion at down-comers, blistering in various locations, and crack-like surface indications. The large areas of corrosion were evaluated using the API 510 procedure, and were found to be acceptable. The blisters were evaluated both as Locally Thinned Areas and as cracks, and were also found to be acceptable. Because only a partial inspection was performed, a statistical method was used to estimate the longest and deepest cracks in the entire vessel. The estimated largest sizes were considerably smaller than the limiting sizes for fracture, but the possibility of slow crack growth could not be ruled out. No crack growth was detected by the re-inspection, however, growth in blister size was detected, and the corrosion rate was found to be high. The evaluation showed that the vessel met all requirements by the MPC guidelines and would remain in compliance with Codes until the next turnaround. However, because of the high corrosion rate, the refinery decided to move forward with the plans to replace the deethanizer.
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