Thicker shell or add stiffener?

[rww88]

Let me begin by stating that my query is a hypothetical one and very general in nature; consequently I cannot supply specifics like material specifications and exact dimensions. Who knows, maybe their exists no available general answer or rule-of-thumb. The situation is one in which an economic decision must be made between increasing the thickness of a cylindrical shell or alternatively adding a single exterior stiffening ring to a thinner shell, in both instances to provide additional support and prevent buckling due to a given external pressure. Let's assume that it is allowable for the stiffening ring to be attached by intermittent double fillet welds. Let's further assume all the materials are low-alloy carbon steel and no special PWHT or MDMT conditions are relevant. I'm sure that such an economic analysis would depend on the production volume, i.e. the number of identical shells to be fabricated, or would it? Considerations I suppose would be the cost to purchase, bend, and weld the stiffener verses the additional cost associated with the thicker cylindrical shell? I do not have a clue as to the cost of labor for bending (shell or stiffener) and the material and labor cost to produce welds. I am not lazy; if anyone can direct me to a good resource I am am willing to do the research. Thus far I have no success in that regard. Alas, I am rumbling.

 

[racookpe1978]

Too little information to know, or even to guess. To begin to guess.

What are the penalties for a thicker shell? Weight, cost of material, cost of shipping the heavier weight item, difficulty of rolling the thicker shell - perhaps a few others. If your application was weight-limited (aerospace, airplanes, extremely expensive material) then that might matter.

But, most fabricators rolling pressure vessels can roll just about any reasonably thick material, so the cost of the material is likely less than the cost of welding and certifying the outside rib or ribs. Ribs add weight and material as well. Ain't no free lunch anywhere but politics. And even there, it is the lawyers and lobbyists who get the free lunch, and the taxpayers who pay.

If your welding cost per man-hour is very low (China fab) then the material cost could be acceptable.

 

[SnTMan]

rww88, like racookpe1978, I doubt you are going to find a neat simple answer. Some of my experiences in the shell & tube game:

If the shell is not "too long" or the thickness increase is not "too great" then thicken the shell. If nozzle reinforcing, nozzle loading, or perhaps other structural considerations such as Zick can benefit, thicken the shell.

If not, add a ring. If the design is well advanced such that thickening the shell causes one to practically start the design over, add a ring. *

Rings can usually be burned in segments from plate, such that the cost of the ring itself is not too great.

If serial production is involved, that may favor a thicker shell, as the long seam and round seam welding are (presumably) semi-automatic processes whereas the attachment of the ring is not.

I suspect a given shops' "culture" will have an influence as well.

Look forward to others' input also.

Regards,

Mike

* On Edit: Hate to say, but I've done this more than a few times

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[drawoh]

rww88,

Have you tried doing a couple of designs, doing the analysis and getting quotes on it?

Are you constrained by engineering costs, fabrication costs, or weight?

--
JHG

 

[rww88]

Thanks so much to all. Very good (and intelligent) responses. I will use all of these thoughts going forward.

 

[chicopee]

Reading this post makes me remember an analogy with bridge girders. Bridges built before 1990's would have their girders stiffened at their webs and you'll see that as you drive on the interstate system. During and after the 1990's, stiffeners were no longer used but the webs became thicker. Thicker webs made a lot of sense as you could get the steel supply thicker to fabricate the webs and dispense with fabricating and welding stiffeners. Time is money.

 

[JStephen]

It entirely depends on the details. And two different fabricators may figure the economics differently, with different results. If in doubt, estimate the cost both ways and pick the cheapest (what fabricators might do). If you're buying the fabricated item, let the people selling it make the call as to which is used.

 

If it's anywhere close, decide in favour of design simplicity. Labour is costlier than material.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."