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Weld Overlay of DCU Coker Drums

The practice of overlaying DCU coker drums is intended to restore the fatigue life of a damaged coke drum. The premise is that by restoring the cylindrical geometry of dented and bulged drums, strain concentrations are mitigated or eliminated. Some researchers have suggested the fatigue life of the damaged drum may be increased by a factor of 10. The data in support of this practice has not been detailed in the industry literature and the expected improvement is not demonstrated using standard practices. A number of engineering concerns may be listed.

One apparent concern regards the remaining fatigue life of material already exposed to thermo-mechanical strain cycling. Although weld overlay is applied to the damaged location and restores the geometrical integrity of the area, it does not restore the existing material to its original fatigue life strength. This material remains susceptible to continued damage progression from on-going thermo-mechanical strain cycling (at a reduced level since geometric strain concentration is reduced). However, other strain concentrations are introduced by the weld overlay.

Failure Mode in Weld Overlay Areas

DCU coker drum failures have been relatively benign according to industry surveys (API Coke Drum Survey, 1996; API TR 934-G). The failures have been cited as being of "no major consequence other than business interruption". Anecdotal data indicates shell pressure envelope failures occur in weld seams of either short length through-wall cracks or sometimes of considerable length along the weld line.

By applying weld overlay restoration over an area, through-wall cracks may develop over the area resulting in a potential "blow out". The energy release will be potentially much greater through the large area opening in comparison to a narrow line opening.

 

through-wall crack


Evaluation of Weld Overlay Approach

The weld overlay repair is appealing but its application is a trial type repair concept with only limited factual feedback reported in the literature. The use of overlay notionally resets the fatigue capability of the drum material. However, the overlay is applied on existing fatigue damaged material. This damage is not abated in subsequent operation; ongoing thermo-mechanical strain cycling exposure continues as previously, but only without the geometric strain / stress concentration incurred by a grossly distorted shell profile.

Weld overlay application, however, creates strain / stress concentrations on a macro and micro level because of the as-welded condition of the overlay surface, metallurgical notches and internal defects contained in the weld deposit. There are other considerations not discussed in the lightly detailed assessments provided to date. Equipment operators experiencing crack indications in a weld-overlay patch should report this for immediate FFS assessment and alert industry associations such as the API and, also, the regulator.

When a weld overlay repair is proposed, appropriate engineering data needs to be developed and presented for review and evaluation. A rigorous risk assessment should then be completed with the assistance of more than one subject matter expert in attendance.

John Aumuller, P. Eng., Ph. D.