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U-2(g) - Stiffining for Internal Pressure

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UKCats

Chemical
Dec 26, 2001
85
Can I use structural stiffing members in Pressure Vessel design to restain internal pressure??? I think not.

UG-27 and UG-32 provide formulas to detmine shell and head thickness for internal pressure. I am working with a Fabricator that claims you can use stiffing members with an FEA for internal pressure under the U-2(g) clause in the Introduction of ASME Section VIII, Div. 1 in lieu of the formulas in UG-27 and UG-32.

Help ! ! !
 
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An interesting case... My initial thought was the old "you can't use FEA to overrule a thickness determined when a formula is provided in VIII-1." But... UG-27 and UG-32 do not provide rules for the geometry which you are dealing with (to include the stiffening members). Thus, the Code doees not provide a specific design rule/formula and this then reverts to U-2(g).

Having said that, I doubt that a stiffened shell would be economical: A cylinder is the optimum shape for a shell and uses the available material most efficiently. By the time you are done with the additional engineering and fabrication effort I'd have a hard time believing that you will come out ahead.

You didn't specify what type of formed head you are dealing with, but the only optimal head is a hemi-spherical one. I could see some potential advantage to adding stiffeners to a F&D or 2:1 SE head. I still don't think the economics would work out, though.

Ultimately, the customer as well as the AI has to be convinced - and that may be difficult to do, both from the mechanical and process engineering perspectives.

So, it sounds to me as though a vessel may have been fabricated of plate which was too thin and now the fabricator is grasping at straws. For a thin shell, I'd consider a weld build-up which can be done in a semi-automated manner. We do this in the field often enough to rebuild corroded shells. Same for a head, though less automated.

Another option if the thinning is localized is to evaluate it per VIII-1 Appendix 32.

jt
 
jte, perhaps the vessel has stiffeners for some other reason, and the designer wants to use them to reduce thickness.

In any case, it is definitely uneconomical to design this way if there is no other motivation.

You don't need FEA, necessarily- should be solvable using conventional shell formulas, at least for cylinders.
 
The vessel is 10-ft diameter, 20 psig MAWP, with a flat bottom head. They are welding I-beams to the bottom to stiffen the bottom head. The shell and top head are per UG-27 and UG-32. The material is Everdur, which is a copper alloy. I ran a Code Calculation and without the Stiffeners, it would be 4-inches thick, w/o the 1/4-inch CA. With the I-beams, the thick thickness is around 7/8-inch thick w/CA. Our other MOC choice is Zirconium. So there is an economical approach but I don't think it's the intent of U-2(g).

There AI has signed off on 3 previous vessels but I've seen AI's have their licenses suspended before for mis-applying the code. They are not always right nor do they know where to turn. State Boards are usually not technically that deep.

Does anybody know of a Interpretation out there address this??? It must have been asked before.
 
It makes a lot more sense with the extra details. I don't know of any applicable code sections or interpretations other than the one quoted above. However, I would say they are taking a reasonable approach. The code sections for unstayed heads are intended for flat plates without stiffeners, and once you add the stiffeners, you have a different stress situation entirely.

It is not unusual to have flat-bottom tanks with atmospheric or low-pressure applications. But in these cases, the bottoms are supported on grade, or on grillage beams (similar to your situation) or in some other way. It would be very very unusual to have a 4" thick bottom slab on a 10' tank with 20 PSI pressure, especially with some costly alloy. Of course, it simplifies things all around if the bottom can be a standard head shape or coned.

If you believe your procurement documents don't allow a stiffened plate, that's a different issue- but it's a contractual issue, rather than a code issue.
 
UKCats-

So the calc's you ran were presumably per UG-34? I still feel that the fabricator is within the intent of U-2(g) since AFAIK the code does not address a stiffened flat head.

I'm not at all surprised that the head thickness changes drastically: When you figure it's governed by bending stresses and the resistance to bending is a function of thickness squared (stress) or cubed (deflection). Adding beams as ribs is a qood way to add bending strength - and good engineering judgement is not prohibited. This situation is very much like a roof under a snow load: Would you rather have 2x4's every 16 or 24" and 3/8" plywood or fewer 2x4's and 1" plywood?

Where I'd be very careful in the hand calc's (very similar to a reactor bed support) or FEA is in the way the moment is transferred into the shell. You may need to thicken the shell substantially at the flat head-shell joint to handle the moment which the beams will try to transfer. Perhaps one alternative would be to cut the beams just short of the shell and let the loads transfer through shear... Just thinking out loud.

With just a flat circular plate, what you have is a nonlinear problem due to geometry since either with FEA or hand calc's you are likely to have large deflection for the flat head. When it deflects it quickly starts to carry substantial load through membrane stress rather than bending. See Roark's Formulas for Stress and Strain by Young, 6th ed Article 10.11 "Effect of Large Deflection; Diaphragm Stresses" pg 457 and formulas (1) and (2) on page 477. It's an iterative solution. With beams added, you'll be back into linear since the deflection should be less than around half the beam height.

jt
 
mY GESS IS THAT YOU CAN APPL7Y THE INTERNAL STIFFERNERS AND GET AWAY WITH THEM,
IN THE OLD DAYS THEY WERE CALLED GIRDERS,
NOW YOU ARE TO FIND OUT WHAT THAT MEANT!!!
iF YOU CAN NOT FIND THEM, PLEASE LET US NOW AND WILL ADVISE.
I HAD A GREAT DINNER JUST NOW AND I AM SO SATISFIRED THAT I AM WILLING TO HELP AND MAKE.. MY OWN POINTS... AGAIN
GENBLR
 
Note that App.13 of Div.1 may be a useful basis for designing a reinforced flat head. The stress formulae may not be used as such, but at least the stress limits and some general rules are applicable. It also confirms that a reinforced (stiffened) flat wall resisting to pressure is indeed allowed by code, as noted by others above.
Personally wouldn't go to FEA to design such a head: UG-47 may be used to calculate the thickness between stiffeners and the stiffeners themselves may be calculated as beams. As far as the participating width of wall to the inertia of the stiffeners is concerned, UG-29 has something on this subject.

prex

Online tools for structural design
 
Thanks prex... It looks like the Scope of Appendix 13 says it all. It refereces U-2(g), mentions that stiffing is not limited to non-circular vessels, and says stiffing can be used for pressure stresses. Diagrams in Appex 13 shows stiffing for internal pressure.

Great website...
 
The stiffened flat head that you have described is quite commonly used for vessels supplied (and ASME stamped) for water treatment services. The bottom is typically a flanged-only head, stiffened with I beams. Hope this helps.


Steve Braune
Tank Industry Consultants
 
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