ASME B16.28

ASME B16.28 standard is about the short radius elbows, includes the ANSI B16.28-wrought steel butt-welding short radius elbows and returns. Information of general scope, design and pressure ratings, sizes, markings, materials, dimensions, end preparations, production test, design proof test and tolerance. 

Any internal misalignment greater than 0.06 in. (1.6 mm) shall be reduced by taper boring at a slope not over 1:3. The minimum length of pipe sections for closures shall be as follows:
(a)one pipe outside diameter for N P S 14 and smaller;
(b) one-half pipe outside diameter for NPS greater than 14.