I concur with Fallah, the number itself doesn't seem out to lunch. Depends what your process conditions and requirements are.
Depending on the specific application though, you might be better off with a smaller diameter and a longer can; I say this because your thickness will increase with respect to inner diameter: At 1440 psig design you would have an estimated shell thickness of 8.375 inches @ a 1/16" Corrosion Allowance (assuming carbon steel)! Welding at these sizes is either
1) Ridiculous with standard ASME Section IX 30 degree weld tapers (you're talking like 32 square inches of Weld! That is like 21 sub-arc passes!) or
2) fancy / proprietary and therefor very very expensive due to Technology monopolies in that market.
At the same specs w/ a 120" ID shell in lieu of proposed 220" ID gives you an estimated thickness of only 4 inches, which is only about a third of the welding required for the 8 inch. Your amount of actual steel is likely not that different either in this scenario (ie your Heuristics are not accurate at these sizes), maybe double but your circ seam welding is likely much much easier and more manageable, even if you are quadrupling your amount of circ seams. God forbid your fabricator has to do a weld repair on a circ seam like that, what a huge nightmare... you know your price will include a few weld repairs if the shop is reputable because welders and even sub arcs make mistakes. It happens. At 4 inches, you have half the passes required to weld, at a third of the mean weld area to be filled. You are welding 6 times less per Circ seam! That means you are welding 4/6=75% as much as with the other option. And at this larger size you would likely be looking at introducing 2 long seams, as many plates wouldn't be able to get that circumference from standard sizes... Now you have 12 times the long seam weld per unit length of shell, with maybe 1/4 of the amount of total shell length, again 3x more welding on the large size. That labour will kill you.
Very few people can build vessels 6 meters in Diameter; you require billions of dollars worth of equipment to properly roll, maneuver, weld, heat treat, paint... etc etc etc. You bet you'll be paying for the maintenance on this specialty equipment. On the other hand, most medium to large well known fabricators could easily fabricate a 120" ID unit.
So, I suppose there are a few things to consider. If it was me, I would give the vessel to all vendors you are interested in having fabricate it, and have them size it. They will provide you will the cheapest option based on their equipment and your design specifications. It could be the 4:1 ratio, but I'd guess at this size it isn't, to be honest with you. I suppose your site constraints might govern as well.
All of these sizes are guesstimated.
EDIT: I see you are a student, my apologies. Anyway, a few logistical things to consider, from a Mechie that Builds these things for a living. But you are right, that L/D Ratio is pretty common and a good place to start at, anyway. There are a hundred things that could swing the economics a bunch of different directions.
Take a gander at my quick sketch for what I mean re Weld sizes on these seams.
Edited by MTumack, 10 November 2015 - 05:53 PM.