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ASME External factor of Safety 2

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tjrush2000

Mechanical
May 2, 2001
9
Where can one find information do confirm the factor of safety is 4 for external pressure disign in the ASME Code.
 
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tjrush2000-

The simple answer is that you won't find any confirmation of a "safety factor" of 4 for external loads. I don't have a reference handy, but you'll find that the safety factor is between 10 and 20. Take a simple drum, design it to Code rules (Div. 1 or Div. 2 - they are the same), and then build a simple FEA model and calculate the Eigenvalues. The lowest Eigenvalue should be between 10 and 20. The difficult part is in evaluating the effect of imperfections on buckling load. This explains the high Eigenvalue. Buckling behavior is highly geometry dependent which relates to fabrication tolerances and things like nozzles. Local load effects (say a platform clip) are also not considered in the Code calculations. In addition, compressive stresses due to radial external pressure are evaluated completely independently from those due to longitudinal compressive stresses (including wind and seismic). Thus the "factor of safety" when the vessel actually arrives in the field is quite a bit less than 10 to 20. Experience shows that this amount of conservatism is adequate.

More detailed external load analysis can result in reduced material requirements for the cost of more engineering and better fabrication. See Code Case 2286 for this option. Look for a paper to be published at the ASME Pressure Vessels and Piping Conference this year which will explore this issue in more detail.

jt
 
"Pressure Vessel Design Handbook", by Bednar, pg47,48 states that the external pressure charts are based on a factor of safety of 4, however, the calculation of Pa has a 4/3 term in it that reduces the final design pressure to a factor of safety of 3.

jte is correct that the code can give an answer many times lower than the eigenvalue but the fea model does not account for fabrication imperfections. The code charts are based on information from actual test models.
 
I'm really surprised of what jte states, as the german code has a theoretical formula (no imperfections accounted for in it of course) to calculate the (elastic) buckling pressure of cylindrical shells, and the specified factor of safety is 3: so really the factor of safety is 3, with the effect of imperfections included in it!
For the plastic buckling range a different formula is used, whereas the ASME code curves account for both phenomena in a single approach.
Anyway App.3 to Part D of ASME II gives a complete description (with bibliographic references) of the basis for external pressure charts, and the specified factor of safety is indeed 3 for cylinders in the elastic range (with respect to theoretical critical pressure). The imperfections are limited in such a way that they can't reduce the critical pressure to less than 80% the theoretical value. prex

Online tools for structural design
 
prex is "right on the dot"
 
In 1980, I've studied the ASME codes to become an authorized PV's(fired and unfired) inspector. I stopped doing this type of work in 1989 however,i still have all of my assignments tucked somewhere so i believe that i can help you out in a few day about researching for this factor of safety of four(4) which is by the way correct.
 
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