When one of the six coke drums at a large refinery suffered a 5-footlong through-wall crack, operations were brought to a sudden halt. Why the failure? Approximately three years prior to this incident, the refinery owner had contracted an unqualified provider to install a temper- bead structural weld overlay in this drum. Unfortunately, the contractor did not possess the equipment, skills or processes required to perform this very technical project.
Coke drums are cyclical vessels that experience drastic swings in temperature and loading during operation. These variations often lead to bulges and cracks in the drum's base metal. An alloy 625 temper-bead structural weld overlay is an excellent repair option for this type of damage since the thermal expansion rate of alloy 625 is comparable to that of the drum's base material. In addition, the use of a temper-bead weld procedure allows the required post-weld heat treatment (PWHT) task to be avoided.
A temper-bead weld process is a complicated repair that requires diligence to execute successfully. This type of weld procedure mandates that the welding parameters be maintained, and that the monitoring of heat inputs and bead placement be strictly adhered to. In this instance, the poor installation of 1,970 square feet of structural weld metal overlay (Figure 1) resulted in 134 linear feet of deep cracking, including the through-wall crack that stopped production (Figure 2).
BHI Energy was called upon to remove the faulty overlay, repair any cracks in the base metal and then install a proper alloy 625 temper-bead overlay.
BHI leads the overlay industry in equipment technology, and we have used and perfected the following features over the past 10 years:
- Automated welding: Unique to BHI, our "Lock Out" software locks in a parameter window that limits operator adjustment, reducing human error.
- Closed-loop micro-stepper motor technology: This technology ensures precise indexing for a smoot-has-welded surface profile that avoids stress risers and produces an even heat-affected zone (HAZ) for proper tempering (Figure 3).
- Digitally programmed starts and stops: Bead placement is critical to the tempering process, and BHI's digitally controlled equipment ensures proper bead placement.
- Automated fail safes: Fail safes stop operation if the machine fails to travel or feed wire. These collision avoidance characteristics help protect BHI clients' pressure vessels during installation and facilitate multiple machines on a single track.
These unique equipment advantages combine with BHI's execution strategies and detailed inspection test plans to produce the most cost-efficient and comprehensive repair program in the coking industry.
Results
Evident quality and superior production resulted in this owner increasing BHI's scope by 56 percent. Following the successful execution of this project, BHI followed up with an R&D project to explore the effects of surface profile on crack propagation and drum life. They identified that the out-of-control indexing witnessed before its repairs had a negative effect on the tempering process, creating hardness issues in the base metal since the jagged HAZ could not be properly tempered.
Since 2005, BHI's industrial team has built its expertise by executing multiple schedule-critical and code-compliant coke drum and vessel restoration projects globally, especially those focused on the life-extending repair of delayed coker unit coke drums.
For more information, visit www.bhienergy.com or call (281) 753-9080.
