ASME Section VIII Div 2: Load Case Combinations for thermal gradient 1

[3rav]

Hi all,

In Table 5.5 for Elastic-Plastic Analysis:

2.1(P+Ps+D+T)+2.7L+0.8Ss

where (Table 5.2):
T - Is the self-restraining load case (i.e. thermal loads, applied displacements).
This load case does not typically affect the collapse load, but should be
considered in cases where elastic follow-up causes stresses that do not relax
sufficiently to redistribute the load without excessive deformation.


In my vessel there is temperature gradient which cause plastic strain (temperature gradient is from 50 C deg to 500 C deg).

My questions are:
1. Thermal loads - what is that mean? Is it temperature?
2. If T is not a temperature, how can I include it in my FEA analysis?

Regards
Rafal

 

[BJI]

1. Yes, loads from restrained thermal expansion, whether it be through thickness thermal gradients or relative displacement of various components, like piping or attachments.

2. I think you have already established that T is thermal loads (caused by temperature of course), or applied displacements. If you have already calculated the plastic strain from thermal loads then it sounds like you have already included it in your analysis.

 

[TGS4]

Think of the thermal loads as the alpha * delta T and/or delta alpha * delta T. In all cases they are imposed displacements.

Out of curiosity, how are you implementing the 0.88*Beta factor on the thermal loads?

 

[3rav]

Yes, I included thermal loads in my analysis.

1. To be sure, I need to multiply the gradient temperature by 2.1 like internal pressure or own weight (eg 500C*2.1)?

2. If so, should I apply a material curve (epsilon-sigma from ASME II-D) for the base tempature or multiply by factor 2.1?

Regards
Rafal

 

[TGS4]

In my opinion, the only way to implement a factor on the thermal loads is to factor the coefficient of thermal expansion (alpha). Any modification to the temperatures is going to mess up other material properties that are temperature-dependent.

 

[3rav]

Hi,
Thank you for the good advice.

I have another question about welds:

1. In the case of welded joint analysis, better to do Elastic Stress Analysis (SLC as ASME VIII DIV2 5.2.1) or as shown in the article:

http://journal-dl.com/item/591088713fbb6e13744158c9

?
2. It is acceptable to perform Elastic-Plastic analysis for the whole model (5.2.4; 5.3.3) and SCL for welds?

Paper Details

DOI: 10.1115/PVP2016-63759
Authors: Seipp, Trevor G.
Publish Date: 2016
ISBN: 978-0-7918-5039-8

"ELASTIC-PLASTIC ANALYSIS METHOD
The next incremental expansion of this concept is for
the elastic-plastic analysis method. The sole question is:
how to de-rate the strength of an elastic-plastic material?
We propose that, identical to the Limit-Load Analysis
Method, the weld volume plus 1*t adjacent to the weld be
modeled as a weaker material. That weaker material is
calculated in the same manner as the non-weld material:
using Appendix 3-D, however, both the yield strength and
the ultimate tensile strength shall be multiplied by the weld
joint efficiency factor for that weld."

Regards
Rafal

 

[TGS4]

In ASME Section VIII, Division 2 - per Table 7.1 (and 7.2), Part 5 is only permitted when the examination is sufficient to permit a weld joint efficiency of 1.0. Partial weld joint efficiencies are not permitted in DBA. Therefore the method described in that paper is not permitted in Part 5.

(However, that method is permitted in API 579-1/ASME FFS-1, but only for fitness-for-service situations.)

 

[3rav]

Hi,
Thanks for the clarification.

2. It is acceptable to perform Elastic-Plastic analysis for the whole model (5.2.4; 5.3.3) and SCL for welds?

Regards
Rafal

 

[TGS4]

Why would you want to mix and match methods like that?

 

[3rav]

Hi,

I think that it is possible to show what are the stresses in the cross section of the weld.

Regards
Rafal

 

[TGS4]

You can either choose the elastic method, our the elastic-plastic method, but you cannot mix the two.

 

[3rav]

Hi,

Which section (point) of the Elastic-Plastic analysis can be considered as leading in the evaluation of the weld joint?

Regards
Rafal

 

[TGS4]

It's well described in that paper. It's handled in the definition of the reduced strength assigned to the weld material. The rest is managed by the LRFD method.

 

[3rav]

Hi,

Could you tell me where exactly this is "It's handled in the definition of the reduced strength assigned to the weld material" (which section (point) in ASME VIII Div2).

Regards
Rafal

 

[TGS4]

It's not in VIII-2, but it's explained in that paper. As I mentioned before, partial weld joint efficiency is not permitted in VIII-2.

 

[3rav]

Hi,

Thank you very much for your answer.

I have another question: Does asme provide/describe creep FEA analysis?

Regards
Rafal

 

[TGS4]

Yes. See API 579-1/ASME FFS-1.

 

[3rav]

Hi,

In Table 5.5 of ASME VIII Div2 the Local Criteria is:
1.7(P+Ps+D), no T (thermal loads) but in "Design Method Combining API and ASME Codes for Subsea Equipment for HP/HT Conditions up to 25,000-psi Pressure and 400°F Temperature", they recommend taking into account the thermal load:

"When temperatures greater than 250°F are considered, thermal loads also need to be considered when evaluating for protection against local collapse."

https://www.spe.org/media/filer_pub...48-9ab5-d2b8b09f655b/16_paper169813_apr14.pdf

My question:
Should I take into account thermal load in Local Failure?

Thank you in advance

Regards
Rafal

 

[TGS4]

The load case combinations are minimum requirements. The User is always permitted to require additional load cases, if they deem them to be of interest.