ASME VIII, Div 2 Stress Classification Lines

[engpes]

I have a nozzle type application where the complex geometry does not meet the ASME VIII, Div 1 requirements, so I am having to analyze to ASME VIII, Div 2 appendix 5.

I have run an FEA and I am trying to determine if I am required to draw the stress classification line (SCL) perpendicular to the highest stress (ie 45 degrees from the base of the nozzle neck). The attached picture shows how I am drawing my SCL.

As I interpret ASME VIII, div 1 app 5, the SCL should be taken at this angle and not simple perpendicular to the shell thickness and perpendicular to the nozzle neck thickness.

If I linearize the stress at the 45 degree angle my stresses are excessive, but if I go only horizontal and vertical at the nozzle base, they are not excessive.

I interpret ASME VIII, div 2 annex 5-A.3(c)(1) to indicate that is must go at the 45 degree angle which is perpendicular to the highest stress component.

Is my interpretation correct? Am I being overly conservative? Thanks in advance. This is a major rush.

 

[TGS4]

No no no. An SCL that goes through the junction ring is completely inappropriate for Protection Against Plastic Collapse. Please get someone who knows what they're doing to do this for you.

Please note the admonition in 5.2.1.2:

"For components with a complex geometry and/or complex loading, the categorization of stresses requires significant knowledge and judgment. This is especially true for three-dimensional stress fields. Application of the limit load or elastic-plastic analysis methods in 5.2.3 and 5.2.4, respectively, is recommended for cases where the categorization process may produce ambiguous results."

Plus, your references to the Code are all over the map, and generally incorrect. Not encouraging for a profession and a technique that require precision.

 

[engpes]

Please be specific in your criticisms.

 

[BenStewart]

I suggest you look at PTB-3, it is a must have document! Like the rest of the PTB series for that matter.

It's difficult to know what you mean by "If I linearize the stress at the 45 degree angle my stresses are excessive, but if I go only horizontal and vertical at the nozzle base, they are not excessive" - what failure mechanism are you assessing, how did you linearise stresses, what loads are being considered (internal design pressure, design piping loads, operating pressure load ranges...), how did you categorise your stresses, etc... these are all important questions when trying to give a proper response.

I often see that it is the failure mechanism and stress categorisation that gets people - protection against plastic collapse is an assessment of primary stresses - the crotch corner is a location which would typically "fail" due to cyclic internal pressure loads, which is where secondary stress come into play, again returning to which failure mechanism are you assessing here?

BTW...All failure mechanisms should be assessed.

Cheers,
Ben

 

[TGS4]

ASME VIII, Div 2 appendix 5

It is Part 5, not appendix 5.

ASME VIII, div 1 app 5

ASME Section VIII, Division 2, Part 5. ASME Section VIII, Division 1, Appendix 5 has a topic of Flanged-and-Flued or Flanged-Only Expansion Joints.

If you are doing an evaluation of an ASME Section VIII, Division 1 vessel, you need to be aware of article U-2(g) from that document. Please see my article on the topic -

http://becht.com/blog/performing-an...ision-1-to-qualify-an-article-u-2-g-component

The application and location of SCLs requires substantial experience and expertise - hence my reference to 5.2.1.2. Engineers implementing the elastic stress analysis method to demonstrate Protection Against Plastic Collapse need to be aware and understand the limitations and appropriate use of the technique. You need to be familiar with concepts of structural components vs transition components (rings), gross structural discontinuities, how to perform the linearization, and finally how to appropriately classify the linearized stresses. Further, please note the admonition in 5-A.3(c):

Application of the limit load or elastic-plastic analysis methods in Part 5 is recommended for cases where elastic stress analysis and stress linearization may produce ambiguous results.

An SCL that goes to the corner of a nozzle is inappropriate, and will likely be invalid - based on the sum total of the validation requirements listed in 5-A.3(c)(1) through (5). The appropriate locations is shown in Figure 5-A.1.

Note that you will also have to demonstrate that you have satisfied all of the other Failure Modes in Part 5: Local Failure, Buckling, Ratcheting, and Fatigue.

Specific enough? Please get someone who knows what they are doing to either do this for you or mentor you in the proper application of the techniques. Please also consider taking a training course on the topic.