To erase the 3D graphics for a pipe run, go to
Ribbon: DM Plumbing->3D-BIM-> 
Erase 3D Pipes
Pulldown Menu: DM Plumbing->3D-BIM->Erase 3D Pipes
You will be prompted to select a pipe in the system from which to erase the 3D graphics.
Select pipe in system to erase 3D from:
The 3D graphics for the selected pipe and any connected pipes will be erased.