March 13, 2018 at 2:28 pm #28295
Feed Throughput to the Delayed Coker Unit can be increased by reducing the “off time” interval of the drum cycle operation. One way being accredited by a specific DCU Licensor is done by providing external heat (for example by installing an electrical resistance) to the junction weld of the coke drum and its supporting skirt. This heat input allowing to approach (the external wall temperature) to the temperature of the drum interior and by this way an early introduction of hot feed to drum. As well, less product vapors from the “On Service” drum are required to be deviated to the Drum being preheated, this representing additional production not going to Blowdown but direct to Fractionation and Tank Farm.
Is there anyone having experience with this improvement?
How long time off has been reduced by doing this change?
Is it a reliable change and what is the Cost/Benefit ratio value for this improvement?
Thanks in advance for sharing your experiences.
March 14, 2018 at 2:56 pm #28298
Coke drums develop cracks over time due to low cycle fatigue. The stresses on the drum during water quench and during feed in provide the most damage. Providing electrical resistance heating to the skirt to shell attachment is a way to heat that section of the drum up hot enough that the stresses the drum experiences in this area during feed in are smaller. Lower stresses allow for more cycles before failure.
On an 18 hour fill cycle, a coke drum will experience 5000 cycles in approximately 20 years. On a 12 hour fill cycle, those same 5000 cycles will be complete in just over 13 years. On the shorter fill cycle, the likelihood of the stresses in the coke drum being higher than when operating on an 18 hour fill cycle are much higher. With higher stresses, the number of cycles until a through wall crack will reduce. There are cases where a through wall crack occurred in less than 2000 cycles. Using electric resistance heating coils on the skirt to shell attachment area should reduce the stresses caused by the hot feed entering the drum even if the drum was adequately preheated. Lower stresses will result in more cycles before a through wall crack. Stresses during the water quench phase are typically higher than for the feed in phase and those stresses may determine the reliability of the coke drum and this heating technology does not address those stresses.
I am aware of some large coke drums on short fill cycles that do not have this heating technology. The stresses are high and the drums are experiencing fatigue damage that will have to be repaired at some point. All coke drums wear out and have to be replaced at some point. There is a balance between maximizing throughput and maximizing reliability.
My opinion is that this heating technology is part of a coke drum reliability program when operating on short fill cycles. The other part is managing the stresses during the water quench phase.
You must be logged in to reply to this topic.