Interestingly enough most mechanical engineers in the US do not learn plumbing design in collage. A few have had one quarter of plumbing design which is hopelessly inadequate.
Even in engineering in the US anyway, its learned on the job and no you never stop learning.
BTW for a good basic engineering book about plumbing design is Alfred Steels "Engineered Plumbing Design" A great book from the design standpoint. ASPE has plumbing databooks too, but I think Steels book(s) are probably a better buy.