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Stress Intensification Factors - B31.3

Stress Intensification Factors - B31.3

Stress Intensification Factors - B31.3


Service:  High Pressure Steam
Header:  NPS 36"
Branch connection:  NPS 2" (unrestrained low point drain)
Type of Branch connection:  Weldolet

Background Information

An unrestrained drain is installed at a low point on a steam line.  However, this location is close to an expansion loop and is subject to high bending stresses (Sb) in the header.  Stress calculations performed using AutoPipe show that the computed displacement stress range (SE) exceeds the allowable stress range (SA).  Note that AutoPipe results are as per B31.3 para. 319.4.4 (Flexibility Analysis) and Appendix D (to determine the stress intensification factor, SIF).

A hand check of the equations found in para. 319.4.4 shows that SE is proportional to the SIF when the torsional stress (St) is negligible (i.e. SE=Sb).

However, the SIF taken from Appendix D (Table D300) is independent of the branch to header diameter ratio.  This means that a 10" branch and a 2" branch both have the same SIF (3.3 in this case)!!  

I do not feel it is necessary to use such a high SIF in this situation since my unrestrained drain line is only 2".  


1. Are small vents and drains (i.e. 2", 3") subject to B31.3 para. 319.4.4 (Flexibility Analysis)?

2. Is there a specified branch to header diameter ratio that is not subject to B31.3 para. 319.4.4 (Flexibility Analysis)?

3. If branch connections smaller that NPS 3" are in fact subject to B31.3 para. 319.4.4, can the SIF factor be adjusted to reflect the branch to header diameter ratio?

Here are some reference paragraphs from B31.3 pertaining to small size weldolet branch connections:

para. 304.3.2 (Strength of Branch Connections): ...There are, however, certain branch connections which have adequate pressure strength or reinforcement as constructed. It may be assumed without calculation that a branch connection has adequate strength to sustain the internal and external pressure which will be applied to it if:

(b) the branch connection is made by welding a threaded or socket welding coupling or half coupling directly to the run in accordance with para. 328.5.4, provided the size of the branch does not exceed DN (NPS 2) nor one-fourth the nominal size of the run.

However, according to para. 304.3.5 (Additional Design Considerations): The requirements of paras. 304.3.1 through 304.3.4 are intended to ensure satisfactory performance of a branch connection subject only to pressure. The designer shall also consider the following:

(a) In addition to pressure loadings, external forces and movements are applied to a branch connection by thermal expansion and contraction, dead and live loads, and movement of piping terminals and supports. Special consideration shall be given to the design of a branch connection to withstand these forces and movements.

As I mentioned above, this location is close to an expansion loop which is subject to high bending stresses (Sb) in the header due to large movements.


RE: Stress Intensification Factors - B31.3

        You have posted this in the Piping forum. Try and stick to posting on one forum - it's the norm!! Either it's a general question or a Caesar II question therefore only one post would be sufficient to get the relevant answer.

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