Hello,

NO, this calculation will not provide the correct flexibility factor nor the corrects SIF. If you really must analyze this abomination, you must use finite element methods.

In the component that you describe, much of the natural flexibility of the elbow is lost by changing its geometry. Consequently, the elbow SIF is no longer correct. But arbitrarily using a branch connection SIF is equally incorrect.

Remember SIF’s only apply to beam theory models and the stress analysis that you need to do cannot be approached by beam theory (local membrane bending stresses will be the determining stresses, not the gross section beam bending stresses).

I would not willingly allow such a component in a piping system that I would be responsible for. However, having said that, I think the best approach to analysis here is to model the component (within a comprehensive CAESAR II model) as a series of rigid links with nodes at the tangent points (weld lines) of the elbow and at its mid-bend point (presumably the intersection with the centerline of the branch). There should probably be some mention of local and system stiffness matrices here but let me skip that part. Also place nodes some distance (maybe 5 to 8 pipe diameters) away from the tangent points (use regular "pipe" elements) so that forces and moments will be calculated at those points. Then, make your FEA model. Add enough straight pipe distance to the FEA model (the 5 to 8 diameters) so that the "end effect" will not transfer (will be "damped-out")to the curved section of the model. Apply the forces and moments calculated by CAESAR II at the remote ends of the FEA model (remember the summation of forces and moments must be zero).

Remember that CAESAR II is a beam theory analytical tool. CAESAR II applies the rules provided by the piping Codes. The piping Codes do not attempt to provide rules for every possible (albeit ill-advised) configuration. If bending stresses that are calculated by beam theory are not the limiting stresses (or the configuration does not lend itself to analysis by beam theory), the engineer is responsible to find another approach to the analysis.

Have you decided how you will qualify this component for pressure design? Perhaps you should configure your FEA model so that you can also use it to determine internal pressure stresses. Remember CAESAR II has no way of knowing that this "special" geometry exists - Code equations for straight pipe minimum wall thickness are not applicable here.

Or, replace the elbow with a standard one that is not modified and find another place in the piping system for the branch connection.

Good luck with your projects.

Regards, John.