Comparison of pipe design according to ASME and EN codes

The EN13480 and ASME B31 codes are frequently used for the design piping systems. The rules of these codes are often applied using automated software packages, and the engineer may lose sight of the calculation being performed on the background. In any case, are there significant differences between the fundamental equations and principles in the design rules? How is the allowable design stress calculated? Why should certain load cases be analyzed? Are the material equivalent? Before switching between design codes, the similarities and differences need to evaluated in order to avoid unpleasant surprises.