Many of the Cr{1-1/4 to 2-1/4}-Mo{1/2 to 1} pressure vessels in the refining and petrochemical industries such as process reactors, distillation columns, separators, pressurized storage vessels, and heat exchangers are typically vertical columns, most often supported by a circular skirt. Typically, design considerations for these vessels and support skirts are for operating under continuous “steady-state” conditions, where temporary stresses due to short-term “transient” events such as start-up and shutdown are often ignored. Consequences of dynamic and cyclic loading play a very significant role in their life and performance. For Coke drums, survey data from API shows that the skirt-to-drum attachment weld and adjoining area appears to be the most problematic, frequently experiencing low-cycle fatigue cracking due to concentrated stresses.

A methodology for repairing the skirt attachment weld of Cr-Mo pressure vessels is provided. When designing a repair approach, consideration should include material and aged condition, extent and location of defects, welding process and consumables, and codes, standards, and regulatory guidelines. When repair by weld metal buildup to rebuild a skirt-attachment weld configuration is considered, weld procedure qualification and adequate mock-ups should be performed in order to ensure a sound repair. Further, when invoking a code compliant repair without post-weld heat treatment by controlled deposition welding or temper bead techniques, proper training of welder operators should be conducted to ensure the techniques are implemented properly.

A case study is provided for a Coke drum, where the original design and fabrication of the skirt attachment included an initial SAW weld metal buildup on the 2.25Cr (P5A) cone followed by an SMAW/GTAW attachment weld to the 1.25Cr skirt (P4). During a plant shutdown, a surface breaking crack was detected in the skirt to shell attachment weld by Dye Liquid Penetrant Testing (D-LPT) and confirmed with Magnetic Particle Testing (MPT). Subsequent examination by Phased Array Ultrasonic Testing (PAUT) discovered a large number of volumetric indications, oriented towards the knuckle section internally. The repair approach consisted of 1) Completely remove the existing skirt and the attachment weld (knuckle) in segments, 2) Inspect the cone for remaining flaws, 3) Excavate and repair flaws in cone using temper bead technique, 4) Rebuild knuckle area for skirt to cone attachment with an increased radius using temper bead welding techniques, 5) Install new skirt sections using controlled deposition welding technique. Temper Bead and Controlled Deposition repair welding techniques were utilized to avoid conventional post-weld heat treatment requirements, significantly improving the turn-around time in the field.

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