Asme B16.9 fittings proof test vs FEA results 1

[LupusLupis]

Hi all,
I am requested to perform FEA analysis on TEE 20" equal ASME B16.9 (40mm tick body/30mm thick wall) company want to redouce body-tickness/wall-tickness ratio as per proof test Asme B16.9.
The point is that every FEA analysis I've performed show that the ratio is worse than tee tested as per Asme B16.9 par.9, does anyone confirm this result?

sumarized I can say that ratio body-tickness/wall-tickness follow this result scale:
Asme B31.8 > Asme B31.3 > FEA - Asme VIII D.2 rules > proof test Asme B16.9

 

[LupusLupis]

Hope someone is interested in this matter, I add result of my calculations body/wall thick increment % for straight tee 20" 30mm wall thick:
A31.8, 157% - A31.3, 153% - EN10253-2typeB 120% - FEA 100% - A16.9, 33%

 

[LupusLupis]

To clerify: FEA analysis was performed with ANSYS 14 (meshing with sweep option/quad) and postprocessing results with Von-Mises linearized SCL near croch section (as shown in Asme VIII D.2 part 5), Membrane and Membrane+Bending are imposed to be < 1.5 * admissible stress (in this case admissible is Yield strengt of material having to find ratio of pipe w/t and tickness of tee).

 

[vpl]

I have to admit I am totally confused. Lupus, from what I understand you have a Tee where your company wants to reduce the wall thickness using FEA to support the results. Unfortunately, trying to "mix and match" codes just doesn't work. You need to decide what code you're going to use and then design only to that one. Otherwise you're right, it just won't pass.


Patricia Lougheed

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[LupusLupis]

vpl:
Point is not to mix codes (of course every one have different philosophy) but to compare results of different calculation and test as I described, I have to prepare a relation and describe results, just I need a general confirmation (maybe someone with a specific experience in this matter) for the following results for a typical case:
body/wall thick increment % for straight tee 20" 30mm wall thick: A31.8, 157% - A31.3, 153% - EN10253-2typeB 120% - FEA 100% - A16.9, 33%

 

[MJCronin]

Bravo,.....vpl !

 

[TGS4]

LupusLupis - If I understand you correctly, you have performed a linear-elastic stress analysis of a component. You choose to NOT use the limits that are listed in the Code. And now you are confused as to why the results are different from a proof test. Is this correct?

For the proof test case, are you dealing with the factored or unfactored pressures?

 

[LupusLupis]

Ok I know tha matter look not so clear I try to be explain better step to step, I am sorry If it will be a bit long but I see I have no other way, this is the way of company not my choice I just have to comply their rules:

1-When traders got offer for a tee, they get from client only geometry + wall thickness + material + reference code ASME B16.9/MSSP 75 no other information about design matters, they use to offer a fitting tested as per proof test ASME B16.9 chap.9 (in the past they did many proof test to comply with this code) and normally the thickness of plates of starting material is 1.5 times of the specified wall thickness, clients accept for 80% of times this way to proceed other times they ask for ASME B31.8/31.3 or EN calcuations.
2-The company think to redouce this starting plate thickness to be more competitive on the fitting market and asked to use FEA analysis to reach this goal and asked me (new there) to perform a FEA analysis.
3-Calculating the MAWP of the equivalent pipe using an allowable stress = 50% of yield strenght of material I proceeded with FEA anaysis with the typical geometry of tee ( I know how change geometry of tee with tickness of the starting plates).
4-Results of many analysis (in this way) was that the starting material thickness have to be not less than 1.8 times of wall thickness and cannot reach thinner value like the proof test ASME B16.9 (1.5 times) I guess because the proof test allow local plastic deformation and this cannot be using FEA with ASME VIIIDiv.2 part5 rules that company imposed.

My question: Anyone agree with these results? Any suggestion how to solve this matter?

 

[TGS4]

I need you to be very specific with your answer, please. Exactly which articles in Part 5 are you using to evaluate your FEA?

 

[LupusLupis]

The following guidelines are used from many engineers here around.

Applied the MAWP of equivalent pipe on equal TEE model (using a starting plate thickness 1.5times of pipe tickness), used ASME VIII.Div.2:

1-Mesh as per fig. 5.A.7
2-SCL around the branch on TEE considering a nozzle on a cylindrical shell as per fig.5.A.11 (total 8 SCL on 2 plane sections of branch axis)
3-Applied table 5.6 on nozzle: Pm<S and Pl+Pb<1.5xS (considering S=50%of yield because not an ASME material).

Conclusion of these steps Pm<S is verified everywere but bending Pb is very high on couple of SCL on branch therefore Pl+Pb > 1.5xS, note that using a different S situation does not change because I got the pressure from MAWP of equivalent pipe with the same S.

 

[TGS4]

Please advise why you are classifying the through-thickness bending stress distribution as Pb and not Q. Furthermore, Pm is not something that you will typically verify with FEA, it is a hand calculation. You will, however, verify the Pl limit with FEA.

Please also confirm that you are complying with all of the requirements of 5.2.2, 5.3.2, 5.4, and 5.5, especially 5.5.6. What you have described above is merely a cherry-picking of parts of rules, not the entire procedure.

(Just as a heads-up, you are applying Division 2, Part 5 wrong. I am trying to gently nudge you in the direction of doing it correctly. Please work with me - it will help you)

 

[catasebi]

Hi there,
I was reading all the comments and I have something to add:
- What is the reason of choosing allowable stress 50%SMYS? Even if your material is not listed in ASME, to define the allowable, you can use the method from ASME II Part D Table 10-100 : allowable stress = min(1.5xSMYS;SMTS/2.4) - you can use this formula when the design code is ASME VIII Div.2.
- You say you simulate a proof test acc. ASME B16.9 para.9. Pay attention when you do the FEA, because the test pressure is calculated to reach SMTS, the analysis must be elastic-plastic. This means you need to define material stress-strain curve acc. ASME VIII Div.2 Appendix.3.D. You can use elastic method when the stresses don't go over SMYS.
- For hand calculations, for crotch area, you should use ASME B31.8 para.831.6 & Appendix F.
- What are your design conditions (pressure, temp. ...)? Cause I believe proof test condition is just a final verification.
- the way you interpret the assessment for elastic method seem to be correct, so
Pm - is correct, this is the Hoop stress and can be calculated by hand. Anyway, when you do FEA Pm is smaller close to reinforcing areas.
Bending - consider it as Pb, is no point in consider it a secondary stress (Q). you can consider Pb as Q when you have thermal stresses but in your case this is not applicable

 

[TGS4]

Anyway, when you do FEA Pm is smaller close to reinforcing areas.
Bending - consider it as Pb, is no point in consider it a secondary stress (Q). you can consider Pb as Q when you have thermal stresses but in your case this is not applicable

I completely disagree with this. Near to a discontinuity, the linearized membrane equivalent stress will be classified as PL. Furthermore, Pb ONLY exists in a flat plate. In this geometry, there is no flat plate, so no Pb. Through-thickness linearized bending is secondary - Q. Q does NOT require thermal stresses.

I do, however, agree that elastic stress analysis is NOT the way to go with this type of evaluation, although I disagree with catasebi that elastic stress analysis is only acceptable if the stresses are less than yield.