I worked on a 4 drum coker and it was built in 1967. As of right now all four drums are still in service. We have had several cracks in the drums and have been successful with welding the cracks. Our engineers informed us about 5 years ago that the drums were ten years beyond their lifespan and needed to be replaced. Once bids were received, they decided to continue welding the cracks. That having been said, drums are not manufactured like the drums that were in place at the coker I worked at. They seem to be a one of a kind drum. They were thicker at the bottom than at the top and the panels were not welded vertically, but rather horizontally. They were lined too. Drum’s lengths between charging and idle differed by more than 18 inches too. I never measured the circumferences, so I can’t help you there. Good luck in replacing your drums, if that is truly necessary.