In my experience a steam riser is simply a piece of pipe that carries steam vertically.
The horizontal steam main, actually sloping slightly downwards in the direction of steam flow to get condensate and water to flow in the same direction, feeds the individual users of steam via the risers.
The steam riser is always branched off the top of the steam main to keep the steam as dry as possible. You would do this even if the steam user was below the main - i.e. come off the top of the horizontal main, go upwards a metre or so, and then double back down to where the steam is needed.
