Repair and FFS of Vacuum Column for Corrosion Under Insulation

One of the vacuum column made of Carbon steel (SA 285 Gr. C) was found with severe corrosion under insulation during turnaround inspection. The equipment was in service for the past 46 years. The corrosion was around the full circumference of the column at a location immediately above the stiffener ring. The wall loss was extensive, which mandated a welded full circumferential patch to encapsulate the corroded region. A full circumferential lap patch with reinforcement plug welds was designed as per ASME PCC-2 [2], fabricated and installed at site. During the dye penetrant examination of the patch plate welds, fine cracks were noted at the toe of the fillet welds. A subsequent PAUT (Phased Array Ultrasonic Testing) of patch plate welds, showed that the cracks were in column shell (base material) surface initiating from toe of the fillet weld of the patch and beneath the fillet weld. The cracks found in first few sections of welds were ground and re-welded with appropriate pre-heating and post-heating. However, some cracks were still observed in base material, although with reduced magnitude compared to previous welding. Despite proper controls in place during the welding, the challenge was, some minor cracks were re-occurring at the base material surface below fillet welds. A multi discipline review was carried out to understand the potential root cause for the cracks, detailed assessment of risks and impact due to the cracks. A risk based approach was followed, in evaluating cracks for acceptance based on API 579-1/ASME FFS-1 [1], for continued service of the column and to minimise the extension of turn around duration. Later, a long-term plan was made to replace the equipment.