As I am new to dynamic analysis, I would like to clarify certain doubts here:
For the sake of study, I performed Modal analysis on an already executed& commissioned line for which I had already performed static analysis only.
Oxygen removal system: 8” Line connects a deoxy vessel (opr temp = 60deg C and press.=9 bar) and then going to an existing header.
Now when I performed modal analysis on that system for OPE case, and limited the natural frequency to 5 hz, I got the results with many natural frequencies within the range.
The animation window showed the behaviour for each frequency.

My doubt is
1) In real I did not experience any visible vibration at site. So how to correlate the Caesar result and actual behaviour ?
2) Also on choosing SUS and HYD case, still modal analysis showed frequencies and vibration in animation. I suppose there should not be any vibration in sus and hyd case. Am I correct ?
3) I was referring few posts related to same, in which I found “better FIRST mode be GREATER than, say, 5Hz” … By “first mode”, does it mean the first (or lowest) frequency among the list, or frequency of first section of pipe connecting the equipment? Plz clarify.

I know my doubt is too basic.
Thanks in advance.

Hi,
1), 2) Modal analysis calculates only the natural frequencies of given piping under a given condition. It doesn't provide any other parameters that you could correlate in the field e.g., displacement/velocities of vibration. Also for any significant vibration to happen you have to have a resonance at one of these frequencies from external excitation.

3) First modes indicates the lowest mechanical natural frequency.

Thank you MechEngr for your time and quick reply.
My assumptions were completely wrong.

As you said, Modal analysis is for finding out the natural frequency of the system only. If that matches the frequency of external excitation, then resonance happens and failure occurs.

Natural freq.[ Fn = (1/2pi)x sqrt (k/m)] is dependent on mass and stiffness. Under given condition k changes. Animation output doesn’t mean line is vibrating in real.


Aim of modal analysis is to find out the natural freq. and limit it to avoid resonance.

thank you very much mechengr