Bolt Torque Target Stress 1

[uglyspec]

Hello,

Many gasket manufactures advertise bolt stress value from 30,000 psi to 60,000 psi for bolt torqueing purposes. Is there any concern that if I torque a stud to 50-60 ksi stress you may actually yield the flange that is being bolted? In other words the high torque value of the studs would actually bend the flange over around the flange perimeter. For reference; an SA-105 flange have minimum yield strength of 36,000 psi


Thanks,

 

[TGS4]

That is definitely a consideration in the bolt stress calculations. I don't have any concern about that particular failure mode.

 

[ricklts]

Plastic deformation of the flange can and does occur, hence the reason there are 'low stress' gaskets from most of the manufacturers,and especially in CL150 and CL300 flanges.

It's not the individual bolt torque you should consider but the total resultant bolt load (clamping force) generated by the bolting at whatever the required torque value specified.

To properly address your concern, please specify flange size and rating, the type of gasket, flange materials, and applicable code. Operating/Design conditions would also be nice.

Since you mention A105, my first reaction is to consider an ANSI/ASME design, rather than API 6A. 6A specifies 55,000 psi bolt stress for it's flanges, but 6DSS requires the same flanges used subsea to be loaded with 75,000 psi. Go figure.

 

[pdiculous963]

Yes, plastic deformation does occur, but it will shakedown following a few reassembly cycles. I suggest reviewing PCC-1 Appendix O for published flange bolting limits. These are actually based upon a deflection criterion, rather than a stress criterion. The background to this Appendix is outlined in WRC 538.

There is also a PVP paper (PVP2006-ICPVT-11-93075) which essentially outlines that excessive bolt loading really can not lead to mechanical failure of the flange, assuming fatigue is not a concern.

 

[uglyspec]

The flanges I am most concerned about yielding are ASME Appendix 2 flanges where you are utilizing customer spiral wound gasket. ASME PCC-1 Appendix O seems to only relate to ASME/ANSI gasket. Do you agree?

 

[pdiculous963]

Yes, I agree regarding the scope of Appendix O, however my initial point is the same. Local plasticity does occur, but it shouldn't be a concern. If you see significant rotation, then you should be concerned.

 

[TGS4]

If you are concerned with flange rotation of Appendix 2 flanges, then make sure that they meet the flange rigidity requirements. That should be sufficient.

 

[r6155]

The best choice is measurement of bolt elongation, never the torque.

Regards
r6155

 

[j13m]

+1 for a reasonable amount of plastic deformation being expected in flanges.

 

[Mech2325]

If the gasket manufacturer suggests this, u may not be taking the risk of going for a higher bolting stress which may crush your gasket thickness being used. The gasket thickness is generally specified as 3.2mm and it's calculation is not being covered by ASME App. 2. It is left upon gasket manufacturer to look for it. As per my thinking,the gasket maybe at risk with it.