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Inspectioneering Journal

Structural Characterization of a Coke Drum

Part 2

By Richard S. Boswell, P.E. at Stress Engineering Services, Inc., and Tom Farraro at CITGO Petroleum Corp.. This article appears in the May/June 1996 issue of Inspectioneering Journal

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.

Figure 1- Growth of the drum between surveysFigure 1- Growth of the drum between surveys

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.

Figure 2 - Thickness surveys of both drums showed significant thinning near the circumferential weld seams in both drums at all elevations.Figure 2 - Thickness surveys of both drums showed significant thinning near the circumferential weld seams in both drums at all elevations.

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:

  1. "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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