Thanks BigInch, I have come across the second document. They are useful but still do not answer my question, as what I refer to is distortion due to solidification shrinkage during welding.
The 'distortion' mentioned in the second link (and also in B31.3, 335.1.1(a)) refers to deformation of the pipes to allow a good fit-up, hence a different kind of 'distortion'.
Thanks for your help