Christine,
From B31.3, 2004, paragraph 304.2.1:
Based upon the classic torus equation
The minimum required thicknes of a bend after bending in its finished form is calculated by:
t-min = (PD) / 2[(SEW/I) + PY]
where at the intrados (inside bend radius)
I = [4(R1/D) - 1] / [4(R1/D) - 2]
and at the extrados (outside the bend radius):
I = [4(R1/D) + 1] / [4(R1/D) + 2]
and at the sidewall of the bend
I = 1.0
and;
Y is the bend angle (e.g., 90 degrees)
R1 is the centerline radius
D is the pipe ouside diameter
P is the internal pressure
You must decide what corrosion allowance is needed and of course you may want to add thickness for groving, erosion, threading, etc.
If you are using a standard B16.9 welding elbow you only need to specify the schedule of the matching pipe.
Regards, John.