U-2(g) - Stiffining for Internal Pressure
U-2(g) - Stiffining for Internal Pressure
(OP)
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 ! ! !
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 ! ! !





RE: U-2(g) - Stiffining for Internal Pressure
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
RE: U-2(g) - Stiffining for Internal Pressure
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.
RE: U-2(g) - Stiffining for Internal Pressure
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.
RE: U-2(g) - Stiffining for Internal Pressure
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.
RE: U-2(g) - Stiffining for Internal Pressure
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
RE: U-2(g) - Stiffining for Internal Pressure
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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
RE: U-2(g) - Stiffining for Internal Pressure
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
http://www.xcalcs.com
Online tools for structural design
RE: U-2(g) - Stiffining for Internal Pressure
Great website...
RE: U-2(g) - Stiffining for Internal Pressure
Steve Braune
Tank Industry Consultants
www.tankindustry.com