Help with interpreting branches in EN 13445-3 annex C stress categorisation

[mrmyagi]

Hi, I'm a bit unsure about how to interpret the membrane and bending stresses at the crotch region of a nozzle attached to a cylindrical shell, specifically within the context of stress categorisation according to EN 13445-3, Annex C.

Could you clarify the correct methodology for combining stresses? My current interpretation of the standard is as follows:

1. Perform a stress analysis on the branch including all mechanical loads (internal pressure + external loads), and evaluate the primary membrane stress. Let's assume it's localized (i.e., PL), and checked against the allowable limit of PL<1.5 * f.
2. Perform a second stress analysis without internal pressure, and assess the primary bending stress caused solely by external loads.
3. Combine the results from steps 1 and 2 to assess PL + Pb < 1.5*f.
4. Finally, assess the combined primary and secondary stresses, ensuring they are below the limit of 3 * f.

What confuses me is the exclusion of internal pressure when evaluating the bending stresses in the crotch region. Intuitively, I would expect internal pressure to contribute to these stresses — yet this is how I interpret the standard.

Could you confirm whether this approach is correct, or clarify the correct methodology?