When you say "light walled" what is your reference point?
In Alberta, std wall in A 106 Gr B is considered minimum and it is usually easy to find (depends on quantity).
As a rule of thumb, the weld fit-up should be designed the type of weld. Your minimum is not restricted (based on weld type). Your maximum is restricted by weld detail and weld procedures (root gap as per welding specifications).
For those that cannot read B51.
Category E are strainers, filters, seperators, and steam traps. So do all 2 and 3 phase seperators fall under category E fittings or as pressure vessels. What if my steam trap in 4" and < 1.5 cuft? What about my seperator that is 6" and > 1.5 cuft. According to...
Per UG-44(b) the calculation to SA-234 WPB B 16.9 caps which have the same wall thickness and material design strength as SA-106B pipe. See UG 27(c)(1) (tr=PR/SE-0.6P)