Vessel with non-circular (hexagonal) OD 2

[ME231]

Is there any logical way (besides FEA) to take credit for additional material available when considering a shell with a non-circular OD in comparison to a regular cylindrical shell? In order to clarify, if I were to compare a cylinder with ID=3" and OD=4.5" (i.e. wall thickness of .25" all around) and a second shell made from a hexagonal bar with a bored ID=3" and OD such that wall thickness is at least .25" at the min thickness section, is there a way to qualify the latter for higher pressures at similar temperatures?
I did consider Sec VIII Div 1 Appendix 13, but the it does not seem to apply to hexagonal cross-sections.
Thanks much to all for your thoughts!

 

[ME231]

Sorry for the typo on my first post, I meant an OD of 3.5" with wall thickness of .25 all round!

 

[jte]

ME231-

I would suspect that you will see no credit for the excess thickness: The classic stress = PR/t formula can be easily derived from a free body diagram of a 180° section, determining the force the pressure exerts and the cross sectional area available to resist that force. I can take your section and cut it such that the stress remains the same. Thus, I see no additional strength provided by the excess thickness.

If I were to run a FEA of this, I would expect to see higher stress in the “thin” sections due to the bending moment imparted by the existence of the non-uniform shell. Then I’d have to classify the stress to determine how much is primary membrane, how much is primary bending... But primary membrane is primary membrane and will not be allowed more than the basic allowable stress.

jt

 

[boilerone]

jte is right. For a cylinder of even wall thickness the Hoop stress (which calculates tensile stress on the wall thickness) is the same all over, for a shell of uneven thickness then the thicker cross-sections have lower stress but the thinner cross-sections still have higher stress and, you know, it is always the highest stress that governs so you cannot take the credit of the thicker wall sections of a hexagonal outside shape.

 

[ME231]

thank you both for your insights