A set of Delayed Coker Drums was placed in service in 1968, and now have 27 years of service in what may be the most violent and severe cyclic loading conditions of any petrochemical vessel. As part of the continuous evaluations of their integrity, surface contour measurements were performed to document the growth of the bulges.
Sequential drum profiles required enhancements in the survey technology to upgrade it from relative to absolute measurements. A second survey after six months initially suggested a dramatic growth in the vessel. This was subsequently corrected for indexing to the bottom seam and for scaling but served as an impetus for obtaining greater inspection detail. The profile shown in Figure 1 is perhaps the worst case growth observed. In terms of the original radius of 126 inches, the worst case bulge in November represented an overall diametric and hence circumferential growth of 8.4%. In six months the growth increased to 9.5% at this contour. Other contours did not increase this much.
In response to this profile information, insulation was removed and a thickness survey was performed vertically through the bulges and around the circumferential welds. As shown in Figure 2, significant thinning was found on either side of the circumferential welds on both drums and at most elevations. Typically, the wall had thinned by 15% over 6 inches from the weld. The bulges showed negligible thinning, contrary to what had been expected.
Inspection further determined that small horizontal cracks near the welds were perhaps joining together to form longer cracks on the outside. The significance of this will be discussed in the analysis results of Part #3.
References:
- "State of the Art Coke Drum Inspection and Analysis", J. Richards; R. Boswell; R. Clark; API 60th Fall Refining Meeting; Task Force on Coke Drum Inspection, Fall 1995.
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