How about this from L.C. Peng's book - Pipe stress Engineering - page 445 "By doubling the i value, we have increased the [B31.3] stress to the same level of theoretical stress. ... However, since the i values of most common piping components are automatically calculated by most computer programs, we may use the original i and just double the calculated stress." But he goes on to say "The i method used by B31 ... assumes that every inch of piping consists of a girth butt weld without identifying the actual girth butt weld locations".