Coke drums used in the delayed coking process have historically been subjected to severe operating conditions which result in numerous mechanical problems. The Coke Drum is far from your typical pressure vessel. However, the industry standard foundation, shimming, grouting and anchor bolt design continue to be identical to a typical pressure vessel. There is a better solution!
The standard Coke drum cycles every 12-24 hours depending on many economic factors. It is often easy to predict and design for cyclic service. Delayed Coking, however, is an inherently difficult and complicated cycle to predict. Each cycle can produce very different temperature and stress profiles in the critical regions of the shell, bottom head and skirt.
Given the unpredictability of the channeling that occurs inside a coke drum throughout the cycle, hot spots can occur at random on different parts of the drum each and every cycle. The more cycles a coke drum has seen in its life, the more you are likely to find problems such as cracking, degradation of the foundation or grout, shim plates moving or being ejected, drill alignment problems, broken anchor bolts, etc. The list of potential problems is numerous and daunting.
As a standard coke drum foundation design gets older, these problems become more and more likely to occur. Costly downtime for repairs become more frequent and a constant discussion about repair or replace is often had between the managers, maintenance, operations, and the bean counters (and they all have a different opinion!).
These repairs, however, only provide a band-aid solution to the problem. Our unique solution provides a way to reduce the stresses on the skirt, shell, baseplate, foundation, shims and anchor bolts by allowing the baseplate to slide freely on customized shim plates. When the skirt and vessel is allowed to grow and shrink due to thermal expansion during the cycle, the stresses are greatly reduced.
In 2005, a major refinery on the US Gulf Coast approached Becht Engineering with a challenge to develop a method for replacing the deteriorating coke drum shims and to re-level the coke drums to allow for the installation of Delta Valves during an upcoming T/A. The challenge was to be able to conduct this shim replacement activity prior to a major turnaround (T/A) while the drums were still in operation. The incentive for this proposal was estimated to be a reduction in T/A time by 12 days which would result in a savings of several million dollars in lost production.
Close Up of Severely Corroded Shims with Anchor Bolt Missing.
View of Failed/Ineffective Shim Stack at Coker. Shims Have Been Ejected from Under Drum. Result is No Support For Drum.
Becht Engineering accepted this challenge and ultimately developed a plan and system to successfully accomplish this goal. This was accomplished by releveling the drums and installing temporary shims while the units were operating which allowed for the removal and replacement of the deteriorated drum shims. This was considered to be an “industry first” to be able to accomplish this entire activity prior to the T/A with the drums in operation. The specially designed final shims looked as good as new 3 years later (see the image below) and continue to perform well to this date.
Shims and Anchor Bolts in Excellent Condition After 3 Years of Operation.
In the past 10 years, we have rehabilitated over two dozen coke drums currently in operation. On each project, a new shim design that permits thermal expansion and contraction of the skirt was used. These were often put in place on units with a prior history of anchor bolt and shim failures. All the modified installations have performed reliably. With our skilled engineering team, Becht Engineering can create a customized solution to replace and repair a failed foundation with our custom design shims during a turnaround, or even while the unit is still online!