Dear MiketheEngineer, I think your first reply is the correct answer(I guess), because I asked this question from someone else before and he replied the same. Another reason is that these two words are used together in ASME sec. IX, but not in the same definition. I don't remember the sentence which these words were used in, but it was like this: "either Pipeline or Piping". This sentence indicates that there is difference between them.