Stresses Due to Supports 3

[JStephen]

I have a situation where supports generate compressive stresses in a head, with the stresses being counteracted by pressure when the vessel is pressurized. What is the allowable stress in this case?

There are two issues here. One is the allowable stress itself. I see where Section VIII indicates maximum tension in general and maximum axial compression in cylinders, but doesn't give an allowable compressive stress in spheres or heads. Second is the question of whether compressive stresses that cannot occur similtaneously with pressure are limited to the ASME values. Any insight?

 

[desertfox]

Hi JStephen

As far as I am aware the allowable stress value doesn't change,if you have compressive stresses reacted by tensile stresses, then the stress in the vessel at that point is just the algerbrac difference, assuming I am not misunderstanding your post.
The maximum compression is cylinders might be to do with buckling but thats only a guess.

desertfox

 

[TGS4]

JStephen,

You ask a very astute question. I would reply by asking you a question - what failure mode(s) are you protecting against? The limits on tensile stresses are to provide margins against yield and ultimate - a tensile failure. However, in your situation, what failure modes do you expect? Is it buckling, in which case you have a completely different set of criteria, or is it something else?

In this type of (complex) loading, you need to set aside the notion of blindly following prescriptive rules (and their associated "allowable" stresses), and thinking about how the material will actually behave, and how it can fail.

 

[BPVFEA]

JStephen,
Following clauses from ASME Sec VIII Div.2 would help you to find allowable compressive stress & stress occuring.

4.3.10.2 Stress Calcuation for Head with Pressure & External Loads (Refer the point-d also.)

4.4.12.4 Allowable compressive stresses

BPVFEA

 

[prex]

JStephen,
Question 1: ASME VIII has rules for buckling of heads, but of course only with an external pressure acting on the whole head. Your condition is likely one with the possibility of local buckling, and there are no rules for that in ASME: you'll need to set up your own analysis methods.
Question 2: the answer is in the concept of operating states of the vessel. If I understand you correctly, you have two fully separated states: one with internal pressure only (covered by ASME rules), the other one with support loads (only?). You need to satisfy code's general rules for all of the operating states and this means that you need to find your way to solving the buckling issue as stated above.

prex

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[JStephen]

Thanks for the input, y'all.

If you'll refer to the item posted on saddle design, I think it is the S12 and S13 stresses which are compressive circumferential stresses. The reference uses 1/2 the yield as an allowable. That is similar to the situation I was looking at, and sounds like it will work the way I had it planned.