hello
to calculate a nozzle pipe (for example :header DN 600 mm, branch conexion DN 250mm ) we use nozzle or SIFs Tees????

thanks for advance

The SIF is used as a coarse approximation of the true stress in a fitting, based on tests performed by Markl in the 1950s. Depending on the fitting, the flexibility is used in the FEA solution of the beam element model. The SIF is then used to compute the final stress at the node point.

In many instances, this approximation is not sufficient (i.e. a nozzle on a pressure vessel), so other methods (WRC) are employed.

You as the Engineer have to make the call as to when and where to apply additional methods. Just be aware that many of these "other methods" have their own limits (D/d or t/T), which if you exceed, you're again in uncharted territory.