Design temperature consideration

[chevproe]

Hello everybody,

I have a question regarding the consideration of Design temperature for Reactor whose details are as per mentioned below :
Design temperature (Int.) : 454°C
Vacuum condition : (-)0.1MPag @ 454°C
Base Material : 2.25Cr-1Mo-V (SA -336 F22V)
Design code : ASME Sec VIII Div. 2, 2013 Ed.

When I refer ASME Div. 2 , Para 4.1.5.2 (d)(1), it is mentioned therein that
“A design temperature greater than the maximum temperature listed for a material specification in Annex 3-A is not permitted. In addition, if the design includes external pressure (see paragraph 4.4), then the design temperature shall not exceed the temperature limits specified in Table 4.4.1.”
As per Table 4.4.1, Maximum permitted temperature for Carbon and Low alloy steels is 425°C only, which is higher than 454°C applicable to the subject Reactors.
My understanding for the limitation on maximum temperature in Table 4.4.1 is that it is intended to restrict the material allowable compressive stress within the time-independent temperature range. Also, Code para 4.4.3.2 states that

“ The equations for the allowable compressive stress may be used in the time-independent region for the material of construction as provided in Table 4.4.1. If the component as designed is in the time-dependent region (i.e. creep is significant), the effects of time-dependent behavior shall be considered”.

This para may be interpreted to be allowing the design temperature exceeding the limits in Table-4.4.1, provided that creep effects and time dependent behavior are taken into consideration.
I checked Section- II Part –D tables and found that the threshold for time dependent behavior for SA-336 F22V is 450°C.

May the experts on this esteemed forum please advise whether I can proceed with the stated 454°C design temperature.
My opinion is that it may be possible to consider 454°C for vaccum design temperature since the vaccum condition is a short term loading, not occuring frequently, and with lower stress levels, hence the creep damage might not be significant.
However, I am a novice for Div. 2 design and so not sure how to proceed.
Requesting the experts on this forum to please advise!!!!!

 

[TGS4]

For that material and that temperature, you are definitely in the creep regime. Compressive stresses will be the least of your worries - creep-fatigue interaction is the primary concern. You will need to be shooting Code Case 2605.

Fyi - that Code Case is wickedly difficult to properly implement. I would highly recommend retaining the services of an expert in the field f who had experience with Code Case 2605.

 

[chevproe]

Thanks for your response...TGS4...
Whilst I understood that creep fatigue analysis shall be required to be done by an expert, i was trying to understand why Code has placed a limitation on maximum permissible temperature for Vacuum conditions (i.e. 425-C for Carbon and Low ally steels).
Coudl you please shed some light on this aspect?

 

[TGS4]

Creep bucking. Which behaves differently from regular buckling, which the Code external pressure charts are based on.