WRC 537 vs FEA 2

[E-Boogie]

Good afternoon,

I have used Compress to model a vessel and the applied external loads to nozzles. All of my nozzles meet limits of reinforcement.

When I run calculations, I get deficiencies on 3 nozzles. Each of these nozzles are in a head of the vessel. The deficiency says, "Load case 1: WRC 537: g = rm / t < 5 (ratio not covered by WRC 537; rm / t = 5 used.)" Upon reading WRC 537, I believe that it is due to the gamma ratio being less than 5 which falls outside of the data range. Compress must be using gamma = 5 to calculate the WRC 537 stress because in the vessel report there is a WRC 537 section complete with values stating that the nozzle is not over stressed.

I can eliminate this deficiency by changing the nozzle from a HB nozzle to a pipe and flange with a repad. Since the repad is on the head, a new head will need to be purchased and cut up for the repad. This is a substantial cost that I would like to avoid.

The question: Will an FEA be sufficient to prove the integrity of the nozzle since the HB nozzle will not fall in the range of data available in WRC 537?

Nozzle in question: 10" 600# HB, SA105
OD= 15"
Wall Thickness = 2.5325"
rm = mean radius
t = thickness

WRC 537 gamma ratio
g = rm / t = ((15-2.5325)/2) / 2.5325 = 2.4615 < 5

Thanks for the help.

 

[SnTMan]

E-Boogie, the answer to your question is, yes, if all the involved parties agree.

Edit: If not, a thicker head may make more sense than pads.

Regards.

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[E-Boogie]

Thank you very much SnTMan. I am contacting my customer right now.

 

[E-Boogie]

I would imagine almost all, if not all of the HB and up nozzles will be outside of the range for gamma. Is there another method other than WRC 537 to check these nozzles?

 

[SnTMan]

E-Boogie, for closed-form calculations for nozzle in heads, I am not aware of anything aside from WRC 107 which is actually implemented by WRC 537. I expect you'd run into the same limitations or worse with it. There is at least one "canned" FEA analysis, Nozzle Pro but I have no experience with it. There are probably others. Nothing available thru Compress?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[JStephen]

For gamma outside of that range, would it be reasonable to consider it as a "rigid attachment" rather than hollow?
For the proposed repads, would it be feasible to use thicker plate and machine/grind one side or the other to fit the curvature rather than forming it?
Just some ideas.

 

[E-Boogie]

Compress does use NozzlePro and I plan to use it to prove these nozzles.

I would guess that you did not create these calculations, but do you have any idea why the heavier nozzles are not covered in WRC 537 or 107? Is it assumed that they will see lower stress, so it is safe to assume the lowest gamma ratio (5) when running these calculations?

 

[SnTMan]

E-Boogie, you'd guess correct I honestly do not know. I'd guess that originating in the past with WRC 107 that the focus was on common pipe necks only. Exactly what happens with the calculations when the parameter limits are not met is, I suppose, anybodys' guess, aside from perhaps the authors. I suppose it is also up to the client, if any, to accept or reject such results.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Try with:
1) hand calculation
2) consider "rigid" nozzle (thick in relation to its diameter).
3) reduce the load on the nozzle

Regards
r6155

 

[DriveMeNuts]

In set mode option, set nozzle analysis to WRC 107. This uses the solid attachment graphs only, enabling you to avoid the "deficiency".

The deficiency is in relation to the limits of the graphs of WRC 537. As adding extra thickness only makes the nozzle stronger, I would have thought Compress would have flagged it up as a "warning". The deficiency does say that rm / t = 5 will be used instead, so Compress are just leaving it with you to make sure you to make the judgement.

If you were to reduce the thickness of the nozzle neck to get WRC 537 to say yes, then using the thicker HB nozzle would only make the nozzle stronger.

 

[TGS4]

The problem with ignoring this limit is that there is a provision in ASME Section VIII, Division 2, Part 5 (which is the criteria upon which the results calculated by WRC 537 are judged) that states that elastic analysis can only be used for rm/t>4. Simply put - thick-walled components should not be designed using elastic analysis.

Trying to artificially do an end-run around this hard limit is not permitted. The same goes for when you try to perform an FEA on this. If rm/t<4, then the rules do not permit elastic analysis - even if the software still lets you.

See 5.2.1.3.

 

[E-Boogie]

TGS4,

We are a Div 1 shop and do not have copies of Div 2. Can you share a portion of it with me or a link to where I could view this portion of the code?

You are saying that the FEA will not be adequate as it violates the requirement set in Div 2, correct? AND analyzing the nozzle as a solid attachment is also not permitted, correct? Is there a set of equations to work this problem?

All,
Thanks a bunch for the help

 

[TGS4]

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.

5.2.1.3 The use of elastic stress analysis combined with stress classification procedures to demonstrate structural integrity for heavy-wall (R/t<4) pressure-containing components, especially around structural discontinuities, may produce non-conservative results and is not recommended. The reason for the non-conservatism is that the nonlinear stress distributions associated with heavy wall sections are not accurately represented by the implicit linear stress distribution utilized in the stress categorization and classification procedure. The misrepresentation of the stress distribution is enhanced if yielding occurs. For example, in cases where calculated peak stresses are above yield over a through thickness dimension which is more than five percent of the wall thickness, linear elastic analysis may give a nonconservative result. In these cases, the elastic–plastic stress analysis procedures in 5.2.3 or 5.2.4 shall be used.

An elastic FEA may not be adequate for nozzles with very small R/t ratios.

Whether or not the nozzle could be evaluated as a solid attachment is more of an engineering judgement thing for me. The WRC 107/537 calculations are supposed to be based on solid attachments - but check the actual Bulletin itself, yourself, to confirm.

 

[DriveMeNuts]

My interpretation of 5.2.1.3 is that it is applicable to the component being assessed?
If you had a very thick nozzle neck and a thin parent shell and you can easily 'judge' that the thin shell will always fail before the thick nozzle, then do you still need to revert to an elastic-plastic analysis?

If you do have to, then all of the WRC 107 graphs with a solid attachment do not comply with ASME VIII Div 2 Part 5 elastic rules.

 

[TGS4]

Your interpretation aligns with my thoughts. It's for the component under consideration. Just don't throw an SCL (or let your software throw an SCL) into such a nozzle.