Coke Drum External Cracking 2

[jnleroux]

Does anyone have experience with external cracking in the lower third of their coke drums? Also, how did you address or plan to address the situation?

Thanks,
John Leroux

 

[jte]

jnleroux-

No, coke drums never crack! Ok... welcome to the forum. It's not uncommon for coke drums to crack. It is uncommom for them to get past their 5th birthday without crack.

It would help if you filled your "Personal Profile" so we would know a bit more about your background. It helps to focus our respones. Click on the "jte" at the top of this post to see my background. You'll no doubt get several responses in this forum, but also check out the "Boiler and Pressure Vessel" forum.

I'll guess that the cracks you are describing would be associated with weld seams, particularly circumferential seams. Chances are that you are looking at a situation which will progressively worsen. Consider replacing the lower 1/2 of the shell and, since you probably also have cracks in the head-skirt junction, the heads. Or just get new drums.

For a more detailed evaluation, I'd throw some money at one or more of the following outfits:

http://www.stress.com/tier3.php?pid=206&sid=5&iid=1&nname=Fitness

http://www.bechtengineering.com/processPower/experience/experienceProcessPowerPressureVessels.htm

http://www.equityeng.com/client/pet-refining.html

http://www.cbi.com/services/vertical-plate-coke-drums.aspx

jt

 

[JJPellin]

I am not a vessel expert. But we regulary find cracks in our coke drums (we have 6). One set of drums was made with a design weakness that seems to have resulted in more cracks. On those drums, the superstructure is supported on the drum rather than from a separate steel frame. These drums also have more attachement points for pipe supports, platforms, etc. Many of the cracks in these drums were associated with those attachements. We normally cut out and repair cracks in place. We are currently running 12.5 hour drum cycles. The current plan would be to replace most of our drums after about 20 to 25 years. We do mapping of our drums at every Turnaround to check for bulges or distortions. We scaffold the inside and do a full check for cracks at each TA, also.

 

[jnleroux]

Thanks for the information.

My original post was lacking some information.

We have two Cr-½Mo Coke drums that were installed in 1979. External cracking in the bottom section of the drums were removed by grinding/weld repair, however PWHT resulted in new cracks. These new cracks were removed without weld repair. At the next outage, the cracking returned, with the same grind/weld/PWHT/crack removal sequence.

I am considering two options for the future of our Coke drums.

1. Replace the bottom 1/3 of the drums.
2. Replace the drums completely.

Obviously, cost is the main driver for considering option 1. I know that many refineries have replaced Coke drums due to cracking/bulging in the past 10 years.

I would like to know if anyone has replaced the bottom section of the drum and what sort of issues did they encounter (additional cracking above the new section, need to replace top section after a period of time, etc.). Alternatively, it would be helpful if anyone has a good reason to stay away from a partial replacement.

I appreciate any help that the group can provide.
Thanks,
John Leroux

 

[JJPellin]

I have never heard of a partial replacement. We replaced two drums complete in 2001 and plan to replace two more in 2009. We have done partial replacement of fractionating collumns to repair damage or extend capacity. But not coke drums. Sorry, I can't help.

 

[jnleroux]

JJPellin,
That type of information helps since I am looking for industry experience.
Thanks,
John

 

[jte]

jnleroux-

Replacing the lower sections of drums has been done sucessfully in at least two refineries that I'm aware of. See the CB&I link in my previous post for more info and I'm sure they'd be happy to provide a reference list. Call the references. They may be competitors, but I've found that even competitors are willing to provide general details about jobs they've done with specific contractors. (And, no, I won't be on CB&I's reference list!)

jt

 

[JJPellin]

I don't know if drum size plays a role in the decision to replace complete or do a partial replacement. The drums we replaced in 2001 were about 90 feet tangent to tangent by 24 feet in diameter. The drums we are replacing in 2009 are about 110 feet T-to-T and 29 feet in diameter. I don't know if it is practical to deliver the larger drums in one piece. When we did the smaller drums, I believe they were made in Japan, shipped whole. This presents some challenges from a shipping standpoint. We are close to the Mississippi river and were able to barge them up from the Gulf. If we were much further inland it would have been tricky to work out the shipping details.