Design of Large Diameter Tees - Equal or Reducing Outlet

[dapatel]

thread378-456875
I have been involved in the R&D of tees design for pipeline and plant application in the 80s and 90s. We developed parametric equations to work out range of header and branch thickness for high yield steels, but the manufacturers could never produce tees for say X65 strength. The tees were much closer to X56 to X60 due to high thickness for the tees for 100 barg application. Off-course, these can be designed using the FEA, but for production and time constraints, this was not be the best option at the time. I have also used ASME B 31.3 method of area compensation to design tees and found to be a good way of designing tees quickly at the start of the project, with manufacturers input, and help the stress analysis team from the outset.

I found that tees manufactured with thickness closer to matching pipes, although might pass the burst test requirements of ASME B 16.9 or MSS-SP-75, did not pass ASME fitness for service FEA analysis, since the crotch area on most tees and the flanks on an equal tees have very high SCFs. The tees have to be designed for hydrotest pressure, design pressure and temperature with a limit on strain and to include an allowance for the external bending, compression, shear and torsion forces/moments which are derived from the stress analysis. Tees made with two halves and welded had the biggest design failures and in most cases the crotch area weld failed during the burst tests and the design FEA checks.

The finished tees after any heat treatment must maintain the required impact properties (ductility) and the yield strength. All of these makes it difficult to achieve the tees to have the same yield strength that of the matching pipes. So, the tees header and branch thicknesses have to be increased not only for the internal pressure but to compensate for the lower yield strength.

I will appreciate comments to reflect latest developments on this entire subject from the learned community.

 

[TGS4]

You say that the design did not pass a FFS FEA? I don't necessarily believe that. Can you share details?

 

[dapatel]

Well, my consultant and I produced a paper for the ASME conference on the very issue following extensive FEA work. This paper was approved, but I or my consultant could not make it to Canada for its presentation. I may be able to share details privately but not in public. We had to cut out all tees of 24"diameter stainless steel and replace these with new ones designed to B 31.3 reinforcement rules. I did not do this without huge scrutiny from the Owners of the plant! My request here in this thread is for learning on new and latest developments. Thanks for your comments. Looking forward to informative contribution.

 

[TGS4]

Please click on my name to find my contact info.

 

[LittleInch]

So I'm not sure here whether you're telling us something you've found, at what sounds like great expense to the client, or asking for any feedback.

Not quite sure how anyone manged to install 24" tees that hadn't been fully tested and proven, but that sounds like another story.

Without a decent diagram it's difficult to see what your thicknesses and crotch angle / radius etc

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dapatel]

Thanks to those who have commented, I really appreciate it. I am not requesting review of my work in the past. I honestly would like to learn what have been the developments in the last 10 years for the design, testing and manufacture of large diameter 18" and above tees and bends/elbows for both c steel and s steel materials for use in gas and LNG industries. I have summarised my experience in my first post, which goes back to 80s. If one is familiar with the very old specifications by British Gas called BG/PS/T1 and BG/PS/B7, then the picture becomes very clear. These have been adopted by National Grid with a number T/SP/B/12.