Steps to Accomplish
Below are the steps to get the Center of Gravity out of a piping model.
1. Open the model in AutoPLANT and make sure Piping is loaded
2. Go to Piping->Piping Tools->Representations
3. Type ACIS in the Layer field.
4. Now, create an ACIS copy of all Piping components in the model.
5. Turn off all layers in the model except ACIS.
6. Type MASSPROP at the command line.
7. The Centroid value of the output is the COG for the piping model.
Note: The MASSPROP command only takes into account objects mass, not the density. This will not give 100% accurate results if you have vastly different materials in the same piping model.