Hi,

If the field "Bolt initial tightening stress" is omitted Caesar returns always same value of 272MPa no matter what bolt material is used.
Why this happens, and how is possible to have 272MPa initial stress when allowable bolt material stress in this particular case is 129MPa - see attachment.

Thanks.

One can create any desired bolting (over)stress with a large enough wrench. Personally, I think they should use an even bigger value for values that the analyst neglects to fill in. If, for example, all of the bolting prestresses defaulted to 1 MPa, the routines would not generate an error message and bad things could happen in the field. This way, the routine generates an error message to remind the analyst his work is not yet complete.

This is because the bolt material has nothing to do with the default value for this particular item. If you leave it blank (which you shouldn't do), then CAESAR II defaults it to 45000.0 / SQRT( Bolt_Diameter ).

Notes:
1) CAESAR II works internally in Imperial units, so the units on "bolt_diameter" are "inches".

2) This is a rule of thumb from a Taylor Forge Bulletin.

Frankly speaking, the central point in a flange calculation should be the "Assembly Bolt Load".
It is intuitive to feel that higher loads will be required depending on the internal pressure at operation. Re-tightening the bolts may be required after operations starts, but there is a warning...not re-tighten too often as this might affect the gasket and produce more leakage!

Surprisingly, this number (bolts load) does not affect the ASME VIII Div1 App 2 calculation as App2 is not mentioning such "user" bolt load. Why?

You may think that App2 (Taylor- Forge works) is a procedure focused on flange dimensioning case(and, in my opinion, we are forcing this reality by trying to adapt this procedure for flange checking case).

Anyway, ASME VIII Div1 appendix S is clear saying "in any event, it is evident that an initial bolt stress higher than the design value may and, in some cases, must be developed in the tightening operation, and is the intent of this Division that such a practice is permissible [...]"

I try to summarize my understanding:
- DIV1 App 2 is giving the minimum required bolts load Wm1, Wm2 and W, not the bolt actual loads. The allowable bolt stress is a low number just to overdimension the bolts.
- DIV1 App S gives the possibility (they say "may" and "in some cases must") to work with other bolts load, which is greater than max (Wm1, Wm2, W).