<< in 2017 edition of ASME section VIII div 2 I could not find 3 times allowable stress for Pm+Pl+Q.can you pls make a screen shot of this? >>

See ASME VIII-2 Para. 5.5.6 / Clause 5.5.6.1 (Ratcheting Assessment - Elastic Stress Analysis):
S_PS limit for "Pm+Pb+Q/Pm+Pl+Q" is defined at point 5.5.6.1(d).

In addition, see Figure 5.1, which is very intuitive in relation with ASME VIII-2 stress categories and qualification.



<< when we talk about the Yield limit shall we consider it in the Operating Temperature or in the room temperature for FEA analysis? >>

Very good question. As I wrote above, Blast is an Accidental Load, with very low occurrence probability. Therefore, it would be unrealistic to assume the Design Conditions (typically being "upset" working conditions, and NOT quasi-permanent long-term operating ones) when we analyze Blast Load.
I would recommend to use the OPERATING PARAMETERS (for both Pressure and Temperature), because Blast would occur unexpectedly during operating service circumstances.
However, you should liaise with HSE Engineer on the Project, to investigate if there might be some fire scenarios previously to blast/explosion escalation. It may happen Vessel temperature to increase prior to explosion above the Operating level due to environment conditions (e.g. fire case).

Connected somehow with this idea, try also to use a Realistic Dynamic Drag Pressure and avoid conservative over-estimations. It is really HSE Discipline role to provide accurate input data in this regard...push them if they try to pass the responsibility...



<< in ASME section VIII-2 part 5(2017) has been introduced 3 ways for elastic Plastic evaluation.in the first place in such a case which of these 3 methods are the most suitable for FEA analysis? >>

It depends on the analysis requirements.
I would take as a minimum:
1) Protection against plastic collapse - see the Assessment Procedure described at Para. 5.2.4.4.
2) Ratcheting Assessment - Elastic Stress Analysis - see Para. 5.5.6.2 for the SIMPLIFIED Elastic-Plastic Analysis, which is based on Elastic Stress Analysis and does not necessarily require nonlinear FEA.
3) Ratcheting Assessment - Elastic-Plastic Stress Analysis - see the Assessment Procedure at Para. 5.5.7.2.

Please be aware that Abaqous is a GENERAL FEA package, although with high performances, but I am not aware if it has included the specific Pressure Vessel analysis capabilities as per ASME VIII-2.
On the contrary, PRG FEA Package (FE Pipe, Nozzle Pro modules) has been specifically developed for Pressure Vessel and Piping analysis as per ASME VIII-2 Part 5 requirements. The FEA modelling templates are included for the most typical "nozzle-shell/head" junctions, the loading cases are automatically selected to comply with ASME VIII-2 Part 5 Elastic approach, and, probably, the latest version includes also elastic-plastic analysis tools (I am not sure about this because I use with my Company an older version).

Well, these are my thoughts and personal opinions. Maybe will help.
Good luck!

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