Anyone familiar with coke drums knows that cracks are inevitable.
Robust solutions for coke drum cracks using skirt section replacement
Due to the cyclical service of this type of equipment and the often-mismatched plate thicknesses between shell courses, cracking of the circumferential weld seams is not uncommon. Improved inspection techniques over the past few years have led to more and more of our clients’ requesting repairs for their drum-to-skirt attachment welds.
There are several different designs for skirt attachment welds: scallops, lap-joints and inside radius fillets; but one factor is shared by all — when a crack occurs on the drum behind the skirt, the only way to perform a repair of this magnitude is to remove a portion of the skirt. If the cracking is extensive, the decision to replace the entire top portion of the skirt may be preferred.
Depending on the diameter of the drums, the skirts are divided into six or eight equal sections. After performing a strength calculation for the drum, typically half of the sections can be removed. For example, if the skirt is divided into eight sections, the odd sections are removed first, leaving the even sections to support the drum. The base metal crack repairs are then completed, and the new skirt sections are installed. Once the new sections are welded both at the top and bottom horizontally, the even sections are removed before repeating the crack repair process. Following the completion of the crack repairs, the four remaining skirt sections are installed, and the vertical welds between the new sections are performed.
BHI Energy has successfully executed the restoration of more than two dozen skirt section replacements worldwide. However, it’s crucial to remember that various factors must be considered when undertaking this type of repair. The attachment weld is responsible for supporting the weight of the drum and is in an area where higher temperatures make it susceptible to thermal fatigue. Often, this joint will need to be redesigned to allow welder access. Since the design of the weld joint is vital to the life of the drum, it is extremely important to select a contractor with engineering expertise to identify this challenge and provide recommendations.
In addition, skilled welders, automated welding equipment and carefully engineered welding procedures are paramount. If a temper-bead weld repair is to be performed, BHI has incorporated data logging technology into its welding procedures. BHI’s unique real-time data-capture process provides transparent quality control by ensuring that the as-welded temper-bead groove repair is consistent with the parameters defined in its welding procedures. By keeping a close eye on heat inputs, BHI can make real-time adjustments that lead to optimal temper, lowering hardness readings and extending the life of the drum.
Technology and craftsmanship are essential, but pre-job planning is equally important. BHI engineering offers an illustrated step-by-step execution plan that keeps the project on track and eliminates all confusion. Unlike the control traveler, this plan provides a more comprehensive view of the refurb so the client can easily track and verify progress.
Although this restoration process is invasive, it has become a routine service provided by BHI. An entire skirt section replacement can be completed in three weeks or less. Additional ancillary benefits should be considered, such as the enhancement of skirt material, alteration of joint design from scallops to lap-joint and optimization of keyhole/slot design for improved performance.
For more information, visit https://www.bhienergy.com/markets/oilgasindustrial/ or call (218) 753-9080.
