ASME VIII-2 Design by Rule vs Design by Analysis with nozzles

[Klikoos]

Hello everyone,

"4.5.2.1 Nozzles shall be circular, elliptical, or of any other shape which results from the intersection of a
circular or elliptical cylinder with vessels of the shapes for which design equations are provided in
paragraphs 4.3 and 4.4. The design rules in this paragraph shall only be used if the ratio of the inside
diameter of the shell and the shell thickness is less than or equal to 400. In addition, the ratio of the
diameter along the major axis to the diameter along the minor axis of the finished nozzle opening shall be
less than or equal to 1.5.
4.5.2.2 Nozzle openings that do not satisfy the criteria of paragraph 4.5.2.1 and other geometries shall
be designed in accordance with Part 5."

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-ASME VIII division 2, 2007​

I find this odd because design by rules sets absolute minimums with stress calculations whereas design by analysis just asks for stress analysis.

I do not see why it makes sense to abondon design rules solely based on shell thickness vs diameter ratio. Would anyone care to explain?

Cheers,

Alex

 

[TGS4]

The validity of the rules was only demonstrated in the range you listed. At higher d/t ratios, other failure modes (buckling) will come into play. ASME PTB-1 explains this in more detail with the link to the Bildy analysis backstopping the method.

 

[jte]

In addition to TGS4's comment, I'll point out that I disagree with your comment (unless I misinterpreted your meaning):

I find this odd because design by rules sets absolute minimums with stress calculations whereas design by analysis just asks for stress analysis.

DBR establishes one acceptable design, not "absolute minimums". Keep in mind that a person could ignore all or the rules in Part 4 and design the entire vessel strictly using Part 5 and be perfectly Code legal. Ref: VIII-2 4.1.1.5 and 4.1.5.1.