Should Bolt Preload be classified as a Primary and Secondary Stress? 2

[chaboche]

I seek advice whether bolt preload should be included as a primary stress when performing an assessment using the stress limits of ASME VIII Div 2 Appendix 4. Should bolt preload be included against the general primary membrane, and membrane plus bending stress limits of ASME 4-133 and 4-134, or is preload a seconadry stress?

I note that ASME VIII Appendix 4 Subsection 4-141 gives the allowable stresses limits to assess the bolt itself, which includes preload. However, should the preload be included as a primary stress when assessing the remainder of the vessel?

Thanks for your help, any advice or references would be appreciated.

 

[vesselguy]

I believe you got the ideas mixed up. Primary and secondary stress classifications in the Code is applicable to the shell theories and not for the bolt, i.e., for the shell and head. If you're implying the transfer of load from the bolt load throught lugs or other mounting brackets then calculate it as normal using WRC-107 or other local stress methods. When you use those methods, the stresses are defined.

 

[chaboche]

Thanks for your reply vesselguy,

I have performed a stress analysis of the component using FE software. Would I be correct to include the calculated stresses due to preload, in the assessment of the shell and head, according to the primary stress limits?

I wasn't sure whether preload should be classed as producing secondary stresses?

 

[prex]

Bolt preload is what ASME VIII calls the gasket seating bolt load, I assume: this is to be considered as a primary stress for the flange, but is applied only in a separate loading condition at room temperature. Just look at how App.2 treats it to be convinced of that (only primary stresses are considered in Div.1).
When you go to the design condition with pressure applied, of course bolt preload is not any more considered as, at least for a flange per App.2 (gasket inside the bolt circle), the preload is cancelled by flange deflection (once again you can check this in App.2).

prex

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[chaboche]

Thanks prex,

I have looked up Appendix 2 and indeed I see that ASME states that preload (gasket seating load) should be considered separately to operating conditions. The worst case should then be considered. I also note that it there are even more cases to consider when a tube sheet is included, to identify the worst case loading.

Thanks very much for your help to resolve my query.