Is there a limit on size of branch connection (Tee) ? 3

[markzu3]

This relates to a reinforced Tee - 36" branch on 42" header for a steam pipeline. I wondered whether there was an upper limit as to the size of branch, in relation to the size of the header. I looked through the relevant caluses in B31.1 on branch connection, but I could not see any reference regarding this.

If any one has come accross this issue or had similar experience I would like to hear from you.

 

[1969grad]

There is no code limit on the branch line size assuming you have adequate reinforcement. Depends on how much nerve you have but I have seen full line size hot taps done on Class 150 lines. Made me "shutter."

For new construction always use a fabricated tee per B31.9 as oppossed to a reinforced tee.

 

[markzu3]

I agree with you with regard to the preference of fabricate tee over reinforced ones.Thanks for your comments.

 

[JohnBreen]

Hello,

I believe 1969grad meant to say "....ASME B16.9 Tee..." and I think the largest of these is 24 X 24 inches (that is to say dimensions for sizes above 24 inch are "recommended but not mandatory&quot. You may have to look to MSS SP-75.

I would strongly advise a high degree of caution in the design of very large diameter branch connections and due diligence in the fabrication and testing of the same. I would recommend employing Finite Element Analysis for the analysis of the final design (apply the design by analysis approach shown in the ASME B&PV Code, Section VIII, Division 2). Interestingly, you will find that FEA will show local stresses greater that the Code allowable stresses (even for fabricated branch connections that are designed using B31 "area replacement rules&quot and you will have to reconcile that with your design standard.

If you intend to perform B31 type flexibility and stress analyses, be aware of the limitations on the Stress Intensification Factors provided by the Code (look at the notes in B31.1, Appendix D).

Generally when we refer to "fabricated branch connections" we mean those made by fabrication from straight pipe. We generally use the term "manufactured" to refer to "factory made" fittings. Most of the time that large diameter branch connections are fabricated it is because "manufactured" fittings were not readily available. Pipe with diameter to thickness ratios greater than 100 (some say 70) is generally difficult to use for fabricating branch connections because of the difficulty in keeping it round (it usually exhibits more "out of roundness" as is) and therefore it is harder to get good "fit-up" for making acceptable welds - it may be necessary to build some "jigs" to keep the relative roundness and proximity for fit-up. For this reason alone (notwithstanding internal/external pressure) you may need to go to a greater wall thickness.

Needless to say, more rigor is required in the NDE to be sure you have a sound weldment.

Sometimes reinforcement schemes other than ASME "area replacement" are used for penstocks and large pressurized ducts (see AWWA Manual M-11) but if you have a hot service (e.g., steam) these types of reinforcement may cause temperature gradient related local stresses.

There are several good ASME technical papers that address branch connections by Schneider, Rodabaugh, and Woods that you might be interested in reviewing.

Good luck, John.

 

[StressGuy]

Well, you're not very likely to be able to have a branch size larger than your header size

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.