ASME VIII Div.2 - Part 5 - table 5.3 - how to treat temperature and thermal laod

[pvros]

Dear All,
I am seeking your guidance on table table 5.3 of ASME VIII, Div.2.
On table 5.3 are reported several loads combination in design and operating condition with related allowable stressed.

1 - Self-restraining load T
Recently, I have seen several reports where temperature is neglected (while I was use to see in reports combination (12) (Po+To), but never (3) (P+T) of table 5.3).
Explanation is that when equipment is in temperature, temperatures are uniformly distributed and we can assume load T as negligible (as also described in of table 5.2).

Even if I may agree, in heat exchangers I believe that differential temperature on tube-sheet or across pass partition is a load that cannot be neglected.
What is your opinion on that?

2 - How to correlated allowable stress of table 5.3 with stress classification of table 5.6?
Max allowable stress in design condition as per table 5.3 is Spl.
However, for example, at the junction between head and shell I can classify the stress as Q as per table 5.6.
It means that in that location stress in design condition can be neglected and that can be analyzed them in operating condition only?
It also mean that differential thermal load (for example) can be analyzed in operating condition and can be neglected in design one?

Hope I have not made too much confusion.
Thanks

 

[TGS4]

In the context of Table 5.3, for demonstrating Protection Against Plastic Collapse (for the limits on S and S[sub]PL[/sub]), Design Load Combination (3) rarely, if ever is applicable. And that is my opinion, based on the reasoning that you provided (from Table 5.6), that stresses caused by differential thermal stresses rarely result in primary stresses.

In my opinion, it doesn't have anything to do with the supposition that the temperature is uniform and therefore there are no "thermal stresses", although it is possible that such an argument could be successfully made.

In your example of a heat exchanger, with differential temperature across a tubesheet or across a pass partition plate, those loads are very real. The stresses are likewise very real, and cannot be neglected on the basis of them being "small". However, I would have a difficult time categorizing the stresses as primary. Nevertheless, refer to paragraph 5.2.1.2. If you question that categorization, the only recourse is to perform a Limit Load analysis or Elastic-Plastic Analysis.

 

[pvros]

Thanks for your valuable answer, as usual.
I am fully align with you.