Wall Thickness of Pipe Bend

Simply, put the maximum pressure and temperature into the table that comes with the pipe schedule, and order the bends against that schedule. All this stuff has already been done in creating the pipe schedules in the first place, and there is rarely any need to re-invent the proverbial wheel.

If bent from pipe, rather than forged separately, an xD bend on the outside wall would be thinned by a factor of 1/(2x). (And the inside wall similarly thickened.)

A chosen pipe schedule will normally be the next standard thickness up from any calculated requirement. In many cases, this surplus will accommodate the thinning that results from bending. In close cases, you may need to account for it specifically.

Interesting question. It makes me wonder how fittings are fabricated. Are they forged or are they a piece of pipe formed to make an elbow. I don't know, but I think it's an interesting thing to consider. I agree that we don't need to reinvent the wheel, but still have to wonder about what happens to the wall thickness in an elbow.

Separate fittings are mostly forged, and called elbows or ells (and tees, etc.) If bent from pipe, they are called bends. A number such as 3D means that the center-line radius of the bend is 3 x the nominal pipe diameter.

Close-radius bending of pipe thins the outer wall, as already noted; also the cross-section tends to flatten into an oval or sort of D shape. ASME SA-53A (rather than B) pipe resists this flattening, and is (or used to be) used for making serpentine coils.