So what if I want to make that 42" pipe look like a 34x53 elliptical pipe? As with many things in Civil 3D, it's all about style. Within the pipe style there is a User Defined option for inner and outer diameter for both plan and profile views.
If I assign my 42" pipe this style, then it will show the Plan view using a 53" dia
And the Profile view using a 34" dia
You'll notice that I also supplied the correct outer diameter based on the wall thickness. If you don't show the outer diameter, then of course this step won't be necessary.
The result: Both of the pipes shown below are 42" pipes. The one on the left uses the elliptical equivalent style while the one on the right uses the true part dimensions to display the pipe. Notice how the pipe on the left is wider in plan view and narrower in profile view than the one on the right.
Now, why in the world would you do this? Well what if you have a custom H&H solution which does not support elliptical pipes. To get it to work, the equivalent diameter must be used. By using the method above, you are able to dump pipe network data from Civil 3D into the custom application without having to hack the pipe diameters on the way.