Assessment of existing large bore steam duct acc ASME VIII div 2

[JVT_Stress]

Hello,

I'm new to eng-tips, but have been reading the posts on this B&PV forum a lot in the past when i got stuck on a problem.

Our client wants to make modifications on an existing large bore steam duct (OD 2820mm WT=10mm), the duct has been designed and constructed according to ASME VIII div 1 ed2010.
Design conditions FV at 20°C / +0,05Mpa at 110°C (saturated steam). I'm considering external nozzle loads also.
My company has requested from me to make an assessment of this steam duct, and evaluate the feasability of the proposed modifications.

Since the original design code is ASME VIII div 1 and the duct contains 2 inclined 45° DN800 (OD813mm) branches on the circumference, with a small hillside offset, I'm considering to use the criteria of Appendix 46-4 and use DBA according to ASME VIII div 2 part 5 to assess both plastic collapse and collapse from buckling.

I have requested to original calculation notes, but up until now I have not received them.
So my approach is to first evaluate the existing steam duct without modifications, and then evaluate the steam duct with the modofications.

For this evaluation I'm using elastic plastic analysis.
According to Appendix 46-4 I need to use a beta factor of 3.5 for elastic-plastic analysis for plastic collapse, and the "design margin of the division 2 assessment procedure" for demonstrating protection against collapse from buckling.

My conclusion after the calculatiuons:
-Check for plastic collapse with beta =2.4 -> PASS (+0.05Mpa/110°C/external nozzle loads at 110°C)
-Check for plastic collapse with beta =3.5 -> FAIL (+0.05Mpa/110°C/external nozzle loads at 110°C)
-Check for instability due to vacuum with beta = 2.4 -> PASS (FV/20°C/external nozzle loads at 20°C)(imperfections not yet included, only applied F&M on DN800 branches are used to initiate buckling )

My question is the following:
If I understand correctly appendex 46 is new in the 2019 edition, it is a usefull guideline for using FEA in ASME VIII div 1. However, based on my assessment of the existing steam duct from 2010, my presumption is that in the original design of the steam duct a beta factor of 2.4 was used instead of 3.5 for demonstrating protection against plastic collapse?
How do I need to deal with this? If I'm going to analyse the modified steam duct with the requiered beta of 3.5 the calculaltion will fail, and in order to meet the appendix 46 requirement I need to add additional supporting and/or reinforcemnt? And challenge the original design, which is not my intention.
Is it correct to state that appendix 46 needs to be used only for new equipment and not for modifications of existing pressure equipment according ASME VIII div 1?

Does anyone have good advice or tips?

Sorry for my lengthy post, but I want to state the problem as clearly as possible.

Best regards,

 

[IdanPV]

Is it correct to state that appendix 46 needs to be used only for new equipment and not for modifications of existing pressure equipment according ASME VIII div 1?

My opinion is - no, I think you will have to use BETA of 3.5.
I will share my thought about it:
You are right that Appendix 46 is new, and appears only in the 2019 Edition.
But, even before Appendix 46, if someone wanted to use FEA in VIII-1 he had to do it by using VIII-2 Part 5 with VIII-1 Allowable Stresses (ASME II-D Table 1A)
Beta of 3.5 came from the design margin of ASME VIII-1, the design margin is used to determined the allowable stress - basically UTS/3.5.

So, if you want to stay coincident with the Code, you should use VIII-1 allowable stresses which means Beta of 3.5 (as it is the design margin).

This is only my opinion and my interpretation of the situation

 

[TGS4]

I agree with IdanPV - you would be using the U-2(g) clause and its "as safe as" provision. You can't get away from the beta=3.5. Even the FFS standard would send you in that direction.

 

[JVT_Stress]

Thank you very much IdanPV & TGS4 for the quick response and clarification, the use of Beta = 3.5 for protection against plastic collapse is clear for me now.

What about protection from plastic collapse due to buckling, if my interpretation of Appendix 46-4(c)(3) "design margin of the division 2 assessment procedure" is correct I can use a lower Beta for elastic-plastic buckling assessment. Following ASME VIII div 2 part 5.4 this design margin can be beta=2.4 for class 2 components for a type 3 buckling analysis.

 

[r6155]

Wait the original design calculation.

Regards

 

[TGS4]

JVT_Stress - I would be comfortable using that factor for demonstrating Protection Against Collapse From Buckling, for a Type 3 buckling analysis.

 

[JVT_Stress]

Hello all,

Thank you all very much for the advise.

After reading the code once more again, turns out i was to harsh in my judgement of the existing design.
I had forgotten about the factor of 0.88xBeta in table 5.5 of ASME VIII div2 part5.

B*(P+D)
0.88*B*(P+D+T)
(no other design load cases need to be considered in my case)

Problem was thermal expansion, but with 0.88*3.5*(P+D+T) for design P=+0.05Mpa at T=110°, turns out not to converge but with a load factor of 0.98! (fully corroded + 8% under tol)
which is much better for a first trail calculation of the existing steam duct, and in my opinion confirms the existing design.

PS: at r6155, indeed i'm looking foreward to the original design calculation.