Stress Classification Line Through Nozzle Crotch Region 1

[m_ridzon]

Below is a section view of a 3D vessel/nozzle junction. When doing FEA for ASME pressure vessels, I have always taken a stress classification line (SCL) through the nozzle crotch region (i.e., red line in the image). I would also take blue lines for completeness. I would linearize the stresses and qualify the locations to ASME requirements. This is what I was taught many years ago while working at a well-known nuclear power OEM that shall remain unnamed.;-) And the reasoning dated back to an old Welding Research Council publication. A new colleague at a new company recently debated the topic with me, asserting that the red classification line shall never be taken. Rather the blue lines are acceptable linearization lines. So I have two questions:

[ul]
[li]Is the red classification line an inappropriate cut to take? If so, why?[/li]
[li]If YES to above, how are the stresses in the crotch region qualified to code?[/li]
[/ul]

 

[TGS4]

The red line is not a valid stress classification line, because it is inside the "transition ring" - at least for the purposes of the failure modes of: Protection Against Plastic Collapse, Protection Against Local Failure, and Protection Against Failure From Cyclic Loading: Ratcheting.

The criteria for the orientation and location of stress classification lines according to ASME Section VIII, Division 2, Part 5 are provided in Annex 5-A. Paragraph 5-A.4 discussed the Selection of Stress Classification Lines. 5-A.3(c) provides some guidelines.

I also apply the guidance in WRC 429, Area III (starting on on Page 11).

... classification (linearization) of stresses in the transition region between two basic structural elements is inappropriate. The, the acceptable global location is at the interface of the basic transition elements, nominally the tangent of the blend radius or within the blend radius.

Also note that your geometry is akin to Example 4 (in WRC 429), with some blend radii.

Nevertheless, it is worthwhile to read (and potentially re-read) paragraph 5.2.1.2. of ASME Section VIII, Division 2, which is also relevant to this discussion.

 

[m_ridzon]

Thank you for this feedback. However, unless it's tucked inside one of your citations, nobody thus far has been able to tell me how the crotch region is to be qualified to code. That stresses in that chunk of metal cannot just be ignored. So if SCLs are forbidden there, then what method is used?

 

[GD2]

OP,
Have you gone through Annex 5-A of Sec VIII Div 2?

GDD
Canada

 

[TGS4]

That stresses in that chunk of metal cannot just be ignored.

As far as the failure modes of: Protection Against Plastic Collapse, Protection Against Local Failure, and Protection Against Failure From Cyclic Loading: Ratcheting - actually, they can be. Read the sections of WRC 429 that I referenced.

unless it's tucked inside one of your citations

That's pretty much the point of references. You need to do the reading yourself.

 

[IdanPV]

TSG4,
What about Protection Against Failure From Cyclic Loading (Fatigue Assessment)?
During my career I was exposed to several articles/sources of information where the stress value was taken directly from the model and was classified as ΔSp,k.
For example:

https://www.pveng.com/wp-content/uploads/2016/06/FatigueLife_FEA.pdf

page number 26.

Do you aware of this practice? Is it permissible?

 

[TGS4]

Fatigue is a "stress-at-a-point" failure mechanism, and therefore it is both appropriate and necessary to use the localized stresses (for example, inside a transition region). However, when it comes back to calculating K[sub]e[/sub], you need to revert to the nearest applicable, and validate and validated stress classification line to determine when ΔS[sub]n,k[/sub] is greater than S[sub]PS[/sub].

 

[m_ridzon]

That's pretty much the point of references. You need to do the reading yourself.

I don't think I have access to all of these things you cited. So that's why I asked. Is WRC 429 publicly available? If so, can you point me to it? I already looked around the web, but couldn't find it.

 

[TGS4]

You can access it through Accuris (formerly IHS). Or, you could always use Google. If by "publicly available", you mean for free - that I can't help you with. You could always try your luck with Linda Hall...

 

[NRP99]

Check this thread-thread794-491477 and my references at the last post.

 

[DriveMeNuts]

The British PD5500 code uses the crotch corner stress as the basis for nozzle reinforcement. That may answer some of your questions.
There is also an explanation document which goes into further detail. It is something like PD5600.