when designing a system to en 13480 code and using ASME material A 106 B, where does one get the allowable stresses from?

I think.......

Strictly speaking, you can't use ASME materials for en13480 as they have not been harmonised.
but this is bypassed by completing PMA's and ESR's

The stresses will then come from either testing/Manufacturer/Data sheets.


Probably....

But, if you wanted to make the run, and use the "correct" (?) allowables, you would hav eto manually implement the rules of 13480 for determining the allowable stress, using the data for your specific material.