Delayed Coking Unit (DCU) Process, Thermal Cycling and Bulge Mitigation Techniques
Delayed Coking Unit (DCU)
In recent years, fracking technologies and its supply of lighter crude has shifted the slate available to refiners. Despite this shift, heavy crude continues to constitute the largest portion of available feedstock and most likely will continue into the future. According to AFPM, the United States has the largest concentration of delayed coking units of any market in the world with more than 60 refineries using the technology to destroy vacuum residuals and increase distillate yields. Prior to the delayed coking process, the eventual feedstock known as vacuum reduced crude (VRC) was processed through several units. First the originating crude has salts and dirt removed in the desalting process. Next, the crude distillation process heats the feedstock into initial products such as gasoline, jet fuel, light ends, and most importantly to the coker, atmospheric reduced crude. Finally, the atmospheric distillation process provides further separation leaving VRC to be moved to the delayed coker for processing.
Constructed first by Standard Oil of Whiting, Indiana in 1929, delayed coking is the only main process in a modern refinery that is a batch-continuous process. The DCU batch process produces solid carbon material known as petroleum coke primarily in a few forms: shot, sponge, and needle. Depending on the physical and chemical properties that the various forms of petroleum coke display, the coke can be used in a wide variety of ways including feedstock for the petrochemical industry, fuel grade material for the aluminum and steel industries or even used for gasification. In today’s market, approximately 80 percent of worldwide production is fuel grade petroleum coke.
Coke Drum Materials and Process
The delayed coking process utilizes coke drums usually arranged in pairs that can range in size from small (15’ diameter) to large (>30’ diameter). The pairing of these drums allows for refiners to operate the process continuously dependent on unit feed rate, drum size, and throughput capacity. Designed and built to the ASME Section VIII, Division I, older drums were constructed of carbon steel but modern drums are fabricated from low carbon chrome-moly alloy steels internally cladded with 410ss or 405ss for internal protection from corrosion due to high sulphur. Specifically, most drums are fabricated using 1.25Cr-0.5Mo and 1Cr-0.5Mo alloy plate although there are numerous examples of construction using 2.25Cr-1Mo, 3Cr-1Mo, and C-0.5Mo materials.
The coking process is cyclical and typically begins with one drum of a pair being pre-heated from ambient temperature to ~250F-350F via steam injection and usually constitute to most significant amount of the cycle time. The next phase begins by feeding the drums ~750F-850F residual oil. This is followed by additional steam injections to promote the thermal cracking process. Lastly, water is injected to cool and aid in solidification of the coke product. The coke is then hydro-cut and the drum de-headed to unload the coke product into storage areas. The cycle times for the coking process can vary greatly from 9 hours to 36 hours and begins again in the subsequent drum of the pair. Most coker operators currently run cycle times of approximately 16 hours but cycle times have been observed as low as 10 to 12 hours. The shorter cycle times result in increased thermal cycles due to the shortening of the heating and quenching cycles.
Low Cycle Thermo-Mechanical Fatigue
Thermo-mechanical fatigue (TMF) is fatigue damage that occurs in components, such as the coke drums, due to coinciding exposure to cyclic mechanical strains and thermal cycles. TMF can occur in an environment that experiences cyclical temperature gradients in areas with complex geometries such as the coke drum skirt attachment welds or areas of dis-continuous material properties such as seam welds. The nature and magnitude of strains under TMF conditions are often very complex due to the number of variables involved in the delayed coking process. Coke drums of all materials are subject to low cycle thermo-mechanical fatigue, which causes accumulative damage, and most often thought to be the principal reason for bulging and subsequent cracking. Originally, Weil and Rapasky studied the phenomenon for 20 years beginning in the 1930’s asserting that radial bulging was found to be directly attributable to the quenching portion of the operating cycle. They also observed severe bulging in the lower portion of the drums that experienced the highest quench rates producing high thermal gradients. Even today, the initiation mechanisms of bulging in coke drums is still not fully understood although we can mainly attribute them to thermal stresses. The area that is clear is that delayed coking’s thermal cracking process has a wide range of variables that affects the severity, growth rate, and frequency of bulging in the drums.
Bulging in the Coke Drum
According to the 1996 API Coke Drum Survey, 57% of operators reported bulging in shell sections. Furthermore, the drums that experienced bulging 87% had also experienced cracking. The severe low cycle thermo-mechanical load makes coke drums susceptible to shell bulging during the operation of the drum. The high stresses over time cause localized bulging and contribute significantly to cracking. The bulging is categorized as a deformation that forms partially or completely circumferential in the drum. The bulging can occur both inwardly (towards the ID) and outwardly (towards the OD) as well as be observed for rippling that may occur in the shell material. In most cases, the deformation of the bulge is measured for depth and sharpness indicating the severity and potential for recurring issues to the shell courses impacted. Localized bulging and the associated fatigue stresses caused by relatively minor distortion can have catastrophic impacts on fatigue life especially in cases where bulging occurs near circumferential weld seams.
Structural Weld Overlay for Life Extension Utilizing AZZ Machine Technology
Beginning in the early 1990’s, AZZ Specialty Welding initiated the first Structural Weld Overlay’s (SWOL) in coke drums. With aid from partners in the industry, SWOL process was developed as an engineered repair method utilizing advanced techniques in laser mapping, fatigue analysis, and field machine welded execution. AZZ SWOL extends the cyclical life of the drums by significantly slowing bulge growth rates and retarding crack growth with strategically designed weld deposits guided by analysis. Once properly assessed and designed, temperbead welding procedures are evaluated for use with AZZ designed machine welding Gas Metal Arc Welding systems. These systems utilize state-of-the-art waveform controlled power supplies and are built by AZZ engineers with sensor enabled parameter monitoring and digital control systems, the machine welding process has the ability to be effortlessly field modified to handle any geometry and difficult welding position unique to the drums. Utilizing the same equipment, AZZ SWOL can be installed both to the ID or the OD of the drum depending on the compressive stresses that are needed to slow bulge rates and reduce bending stresses.
The AZZ SWOL repair method has been implemented by owners experiencing low cycle thermo-mechanical fatigue in order to reduce inspection cycles, extend the life of the coke drums and gain greater predictability on operations. According to experts familiar with these issues, “long range studies have demonstrated how structural weld overlay repairs have lasted 10+ years, 3-5x longer than other repair strategies.” The results observed in these cases are exceedingly reliant on a high quality overlay design, stringent control of the machine welding technique affecting the metallurgical properties, and precise control of the surface profile quality. In recent times, because more providers have entered the field the failure to adhere to these rigors throughout the process has had negative effects to the repair performance. In some cases causing damage to the vessel. Although the outcome is dependent on the rigor of design and quality of implementation, the AZZ SWOL repair method has been found by owners to be both logistically and financially effective. Now with over a hundred field installations worldwide, AZZ SWOL has proven that we are the world leader in coking unit life extensions.
AZZ Specialty Welding – The World Leader in Coking Unit Life Extension
Whether the coker needs to be repaired emergently or during a planned turnaround, AZZ has unparalleled innovative leadership in the coking unit backed by proven results. AZZ Specialty Welding has the world’s largest portfolio of coker life extension projects and qualified procedures to handle coking unit specific challenges. Find out more about our coker life extension solutions and engineered technologies at azz.com/WSI or follow us on LinkedIn.
American Fuels and Petroleum Manufacturers, AFPM. Technical Papers. Retrieved from https://www.afpm.org/data-reports/technical-papers.
Weil, N. A. and Rapasky, F. S., 1958, "Experience with Vessels of Delayed-Coking Units", Proceedings American Petroleum Institute, pp. 214-232, vol. 38
1996 API Coke Drum Survey, 2003, American Petroleum Institute, Washington, DC.
API 934J,2018, American Petroleum Institute, Washington, DC.
Samman, M and DuPlessis, P. Refcomm 2017, Galveston. 10x Life Improvement in Coke Drum Life Using Weld Overlay