Welded Branch Connections - Parameters of Design

[McDermott1711]

Hi everybody,
I'm amazed with the manner ASME B31.8 treated with welded branch connections. As you can see in appendix F, all of the parameters are geometrical and nothing is related to the forces and moments that would be exerted to the branch point (similar situation in ASME B31.3). Does anybody know about the basis for these equations(non-equations)?

 

[BigInch]

The idea is to duplicate as near the full strength of the underlying pipe as possible. Any additional metal that might be needed for higher loads would simply overstress the underlying pipe components. Why make a connection strong enough to take loads higher than what the underlying pipe would be able to withstand.

 

[McDermott1711]

Thanks BigInch for your reply.
But there is, IMHO, difference between a branch and a pipe with regard to the loadings. There is forces and moments from other (e.g. due to expansion) components that may exert in the branch point, isn't it?

 

[BigInch]

From where forces arise is irrelevant. The geometry requirement just tries to make sure that the connection will not be the weakest link in the chain; i.e. that the surrounding pipe can be expected to fail before the connection.

 

[KevinNZ]

The reinforcing calculation is just for the pressure case. You still need to include an SIF in the piping model for other loads.

 

[McDermott1711]

Thanks KevinNZ for your reply,
What does that mean? If we design reinforcing pad based on the Appendix F and analyze the branch by, say Caesar II, is it possible to have stresses higher than allowable, at the junction node, based on the forces and moments and corresponding SIF?

 

[BigInch]

It's always possible to have forces which cause pipe failure and any non-homogenous material, or structure with irregular geometric properties may develop non-uniform, concentrated stresses. What is hoped for is that the resulting greater thickness will disproportionally lower the stress in relation to the stress concentration factors, i.e. Force/Area and Moment/Section modulus, will actually reduce. The thicker sections of the built up material should extend past the locations of highest stresses.