mag,
A bit more specifically, read the CaesarII User Guide (c2ug.pdf) pages 8-6 thru 8-10 plus page 8-44 (pages 218 thru 222, plus 256 of the PDF.)
It's there with desciptions and screenshots of steps and explanations.
Let us know that you understand modal analysis after reading and running the modal analysis of your piping system. See the results for natural frequencies, and then decide if you have problems or not.
_________________________
R Yee