What is the difference between a dummy pipe and a trunnion?

A "Dummy Support" is normally a piece of pipe (the same size or one or two sizes smaller) welded onto the back side of an elbow to extend the reach of the line to the next primary pipe support.

A "Trunnion" (or Trunnion Support) is two short stubs of pipe welded to the sides of a vertical line such as to provide a pick-up support for the line that runs vertically down a tall vertical vessel.