Bolt Preload and Sect. VIII - App. 2 Flange Stress

[ValveEngineer123]

I am designing a flanged connection to the rules of Sect. VIII - App. 2 (actually Sect.III, App. XI, but its the same procedure). I am using a corrugated stainless steel gasket at the connection, for which app. 2 gives a minimum seating stress of 7600 psi. Using this gasket stress, I calculate the flange stresses and find them to be within acceptable limits. I also have a bolt preload specification to meet (%70 of bolt yield stress). When I load the bolt to this value and back calculate the induced gasket stress, I get an actual value of over 30000psi. My customer has questioned why the value of 30000psi is not used in the app. 2 flange stress calculations. When I use the 30000psi value in place of 7600 psi, the flange stresses are well above allowable and even yield in some cases. I know the flange is not actually yielding in reality as I can physically load/unload the flange bolting and not measure any permanent deformation. It appears that the flange stress equations stated in app. 2 are intended to be used as a conservative method for overdesigning the flange and bolting area rather than an accurate method of predicting actual flange stresses including the effect of preload. In addition, the code specifically avoids the issue of bolt preloading and basically tells you to 'ensure that preload plus in service loads do not yield the components of the connection'.

Does anyone know of any other method (aside from FEA or experimental methods) of accurately calculating flange stresses, accounting for the combined effects of preload and in-service loads?

 

[SnTMan]

ValveEngineer123, it is true, Appendix 2 contains design RULES, not analysis. I don't know of any "standard" analysis.

The only things I might suggest is to 1) Calculate the gasket stress using the actual width, not "b", and contact the gasket manufacturer to make sure the gasket is not overloaded (I doubt it), and 2) perform rigidity calculations per Appendix 2 if you have not already.

You might also review Appendix S for the Code's take on bolt-up, etc.

These are S VIII, D 1 references, not S III.

Regards,

Mike

 

[unclesyd]

Just for curosity what is your bolt size and material?

 

[ValveEngineer123]

SnTMan: Do you mean use the actual gasket width to calculate my actual gasket stress based on the preload? If so, this is what I have done to obtain the 30ksi value. Or are you referring to using the actual gasket width as opposed to 'b' in the bolt load/flange stress calculation?

unclesyd: The bolting is actually threaded studs (SA564 Gr.630 Cond.1100), on one end threaded into an internally threaded part, and a hex nut (SA 194 Gr.8M) on the other end.

 

[SnTMan]

ValveEngineer123, yes, I meant to calculate gasket stress based on actual dimensions and preloads, as it sounds like you have done.

Regards,

Mike

 

[unclesyd]

If I'm reading you right at 70% yield that will give you a bolt stress of around 94,000 psi.

Two things.
Why such a high bolt stress?

We never used 2H nuts, let alone 8M nuts above 70,000 psi bolt stress.
Are you using fine threads?

Is you gasket very narrow in respect to diameter?

I agree with SnTMan that the gasket is OK as 7600 psi is a minimum. Under certain conditions 15,000 gasket seating stress is not out of the question, though 30,000 still seems high.

 

[ValveEngineer123]

I have found the yield stress of the studs to be 115ksi, as per sect. II, Pt.D, table 3. 70% of this value gives me a preload stress of 80.5ksi. The studs have UNC threads. The bolt preload requirement was specified by my customer (they have also specified a minimum gasket preload stress of 20ksi). I have questioned them on it and they are now re-examining their specification in light of the issue raised about flange stress. I believe the intent of the requirement was to ensure the flanged connection had enough preload to prevent any in-service gasket leaks. I myself thought the bolt stress seems high, however when I calculate the required torque to develop that preload using the typically unreliable 'nut factor', I get the exact value that my experienced assembler says he has been using for years, with good results.

I was a bit concerned with the 8M nuts myself, however my calculations show that there is sufficient engagement that the bolt will fail prior to stripping the nut threads.

The gasket OD=2.719", ID=2.406", width=.156". So yes, it seems as if the gasket is narrow with respect to diameter.