Tighten Bolts Beyond Yield to Almost Break Point/A Better Factor of Safety than ASME VIII 1 App 2? 5

[Spoonful]

Dear All,

Here is article I found quite interesting, stating that it is advantages to tighten bolts beyond yield point, to almost its breaking point. There point is one bolt it tightened to that level, it will work against its clamped members to prevent the lose of clamp load. Please see attached. I thought it might be interesting to see what others opinions about this.

Base on this article, that bolt load need to be stressed over not only over allowed design stress, but also its yield stress, and even over its tensile stress. When designing flanged joint using ASME VIII 1 app2. calculated bolting stress for both gasket seating and operating condition are compared with allowable bolting design stress. Therefore, based on this article, we can use bolts tensile stress to compare calculated stress, and we are still on the safe side? If so, most of the realistic design cases will not fall because of bolting stress.

Please share your thoughts

Regards

Spoonful

 

[Spoonful]

Another article

 

[XL83NL]

Interesting artciles, but

Base on this article, that bolt load need to be stressed over not only ..., and even over its tensile stress.

whats the point of breaking a bolt when you need it?

 

[SnTMan]

Spoonful, design of flanges is one thing, making up and in service are another.

It is well known that the higher bolt pre-load that can be applied, the less influence the external loads will have on the bolted joint, particularly retention of bolt load. However, speaking of gasketed joints, the joint members, flanges and gaskets, have be able to withstand the applied bolt loads.

ASME Code recognizes the diffrence between bolt stresses used in design and those used in service, see Appendix S of Sec VIII, Div 1. Also please note that bolting design allowable stresses are used in Appendix 2 only for establishing required bolt area. Yes, you or your software can then calculate an applied bolt stress from the selected area and the design conditions, but this is a work of fiction. Again, see Appendix S.

Also in practice the applied bolt stress during tightening is often not known with a high degree of precision. The external loads are also often not known with a high degree of precision. The temperqature effects must be considered as well, as the bolt yield decreases.

Tighten the bolts as tight as the joint will withstand, and no tighter

Regards,

Mike

 

[stanweld]

Spoonful,
The papers address structural steel bolt tightening. It has long been recognized that in tightening to slip critical requirements, bolt tensioning is often somewhat greater than the yield strength of the bolt. This is one of the rasons for prohibiting the reuse of previously tightened (pretensioned & slip critical installations) A-325 and A-490 bolts when they are removed.

The same cannot be said for bolted flanges in ASME VIII applications. Listen to SnTMan.

 

[TGS4]

Well said SnTMan! LPS for you!

The gasket in a pressurized bolted flange connection and the effects of changing and differential temperature are variable not considered in the article.

There have been multiple ASME PVP and JPVT papers written on this topic and the general sense is that higher bolt loads for certain types of gaskets are beneficial. Note that certain gaskets (PTFE, for example) are severely harmed by excessive bolt loads.

To ensure a leak-free (not really, but more of a 10^-6 vs 10^-3 leakage rate) joint, the elastic (and sometimes inelastic) interaction between the gasket, the flange, and the bolt must be considered. In my opinion, exceeding the yield point in the bolts could be troublesome, because post-yield, the tangent modulus is less than the elastic modulus, which results in a weaker response to changes (pressure and temperature).

 

[mcclain]

Keep in mind that the "apparent" tensile strength of a bolt is considerably less due to the added torsional stress during torqueing, i.e., torqued tension vs. direct tension.
See KulaK, Guide to Design Criteria for Bolted and Rivets, AISC 2001.

 

[SnTMan]

TGS4, thank you

 

[Spoonful]

Thank all for you kind share of your knowledge.

 

[racookpe1978]

Key point and tricky phrase alert!

Bolted FLANGES (between piping flanges and gaskets and the next flange) are NOT structural joints.

If you have a piping flange AND a specific gasket (Flexitallic or the like, or a fiber or a rubber gasket for lower pressures) then you MUST follow that gasket manufacturer's EXACT torque limits on bolt size, torque build up (at 1/3, 2/3, and then final torque) requirements, and the lubrication requirements that torque spec assumes will be used. YOU have NO choice about these values if you are going to use a "piping flange" rather than a "structural bolting" connection, or if you are trying to achieve a purely mechanical seal (like on a steam turbine casing or hot gas gas path connection on a combustion turbine burner) that does NOT use common piping elements and parts.

 

[chicopee]

Another point to consider is that when bolts are preloaded beyond their yield point, they should not be reused.