ASME B31.8 831.4.2(c) vs. 831.4.2(j)(3) - Reinforcement members and Pressurized Sleeves, End face thicknesses & fillet welds

[engineer0]

I posted a similar question the other day with no responses so far, but may be able to explain it better in this post.

End face of a reinforcement member in 831.4.2(c) states that it should be tapered to the thickness of the header while 831.4.2(j)(3) seems to indicate for pressurized sleeves (which provide reinforcement but are also pressure containing) that the end face can be left or, if required, tapered to "1.4 times the calculated thickness to meet the maximum hoop stress of the pressurized sleeve". The second requirement seems to not make reference to the carrier pipe and the pressure control fittings can be much thicker than the carrier. Why is there a discrepancy between the two requirements and what are the intentions behind the tapering requirements for each?

Does this have anything to do with in the case of the pressure-containing reinforcement the interaction between the fitting shoulder and the fillet weld? The fillet weld leg is based on the carrier pipe, understood, but does having too much shoulder material cause problems for the fillet weld? Why wouldn't it be based on the carrier pipe? If the carrier pipe is not of concern, then why is the maximum thickness based on a maximum hoop stress? You very rarely, if ever, operate the system at a design point, so at each step down in pressure the "maximum" hoop stress at that moment would, if recalculated, require a lesser wall thickness than the one calculated off design.

I'm quite confused by the requirements and what's driving them. Appreciate any responses!

 

[LittleInch]

Ok,

The first part C relates to non pressurised branch reinforcement.

ASME B 31.4 shows much better this in a number of diagrams in figures 404.3. The basis though is the same as for the sleeve where they are trying to prevent a sudden change of thickness / stiffness and reducing the stress concentration factor at the point where the additional thickness, be it reinforcement or sleeve starts.

The second is where you have a pressurised split tee or saddle where the pressure containment is abruptly moved from the carrier pipe to the sleeve or tee and the fillet weld is being subject to quite a bit of bending. you also don't want to create a hard spot / stiff change of section which can then create a stress concentration at that point.

 

[engineer0]

@LittleInch greatly appreciate your feedback on this.

I will consult ASME B31.4 to hopefully assist my understanding of reinforcement requirements.

Everything you've said makes sense—a sudden change in thickness, as opposed to gradual, along the pipeline will concentrate the stress in an undesirable manner at that point. So it sounds like when looking at reinforcement members, whether pressure-containing or not, the above point applies in all cases.

My question then would be around what B31.8 831.4.2(j)(3) says regarding pressurized sleeve end face thickness being based on the fitting properties (and not the carrier pipe. Yes, fillet weld leg is based on carrier pipe, but not the fitting end face shoulder thickness per that subsection), what "maximum hoop stress" is concerning? Since hoop stress is partly a function of pressure in the line, what pressure is this end face/shoulder thickness based on? Is it the pressure the fitting manufacturer states the fitting is designed to operate at? Is it the MAOP of the segment being welded to? Is it the design pressure of the carrier pipe the fitting is being welded to? It seems like the pressure that would produce a concerning "maximum hoop stress" such that it would stress that joint in undesirable ways is a bit arbitrary.

 

[GD2]

@LittleInch greatly appreciate your feedback on this.

I will consult ASME B31.4 to hopefully assist my understanding of reinforcement requirements.

Everything you've said makes sense—a sudden change in thickness, as opposed to gradual, along the pipeline will concentrate the stress in an undesirable manner at that point. So it sounds like when looking at reinforcement members, whether pressure-containing or not, the above point applies in all cases.

My question then would be around what B31.8 831.4.2(j)(3) says regarding pressurized sleeve end face thickness being based on the fitting properties (and not the carrier pipe. Yes, fillet weld leg is based on carrier pipe, but not the fitting end face shoulder thickness per that subsection), what "maximum hoop stress" is concerning? Since hoop stress is partly a function of pressure in the line, what pressure is this end face/shoulder thickness based on? Is it the pressure the fitting manufacturer states the fitting is designed to operate at? Is it the MAOP of the segment being welded to? Is it the design pressure of the carrier pipe the fitting is being welded to? It seems like the pressure that would produce a concerning "maximum hoop stress" such that it would stress that joint in undesirable ways is a bit arbitrary.

You should treat B31.8 831.4.2(c) and 831.4.2(j)(3) separately for understanding. Whereas 831.4.2(c) talks about a complete encirclement reinforcement type that we encounter in daily life, 831.4.2(j)(3) talks about hot tapping/plugging fittings, where the reinforcing sleeve is pressurised and is thicker than the header pipe wall because the sleeve will be additionally loaded to carry the weight of the hot tapping/plugging equipment. There is some configuration and fillet weld requirement that is illustrated in Mandatory Appendix I, Fig I-3.1. Once you go through Fig I-3.1, 831.4.2(j)(3) will be much easier to apprehend and understand.
What the Code is saying is essentially limiting the face dimension of the sleeve in terms of the hoop stress thickness with a multiplier 1.4 (we almost always use hoop stress for wall thk. calculation) to accommodate the 45 deg chamfer and the fillet weld leg length.