I am trying to draw 3D pipe for a customer's model. If I draw it in at 4" nominal size it just shows a single line. If I change that same run to 10" in the propertiesd dialog it shows up as a 3D solid. I'm sure it's just a setting somewhere but I can't find it. It seems to only do it on certain types or size of pipe...
You can find that setting in the System Definition for piping. You can edit the system definitions by going to the Manage tab and then on the Style and Display Panel select the Style Manager dropdown and then select system defintions. You must be in the piping workspace in order for the piping system definitions to be an option. Once the system defintions open up, select the Single Line Graphics tab on the right and either uncheck the options or change the pipe size.
Piping will only display in a single line representation in this manner if you are in the MEP Design Display Representation. If you change to MEP Basic 2-Line you can force the piping into 2 line display. I would recommend reading up on Display Representations in the Help File. It explains the out of the box Display Representations and which ones to use in which situation. MEP Design is set up for printing and MEP Basic 2-Line is set up for modeling.
Hope this helps.