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Pipeline Vibration Issue

Pipeline Vibration Issue

(OP)
Hey Everyone,

I just entered a new piping engineer roll, and I've had to learn how to use Caesar II quite a bit lately. It's my first time using this program, so I've had to spend some extra time trying to learn this program. The problem I'm looking at is a recycle gas line on one of our vessels that has had some pretty nasty vibration lately. One of our vibration specialists found a connection between the pipe vibration and a downstream compressor's second harmonic (compressor runs at 400 RPM).

Overall, I'm fairly comfortable with modeling a pipeline, but I have some questions that hopefully some of you experts can help me out with to ensure the results I'm providing are adequate:

1. The first 10 or so natural frequencies that Caesar has calculated range from around 1.9 Hz to 13.3 Hz. Its quite clear where some additional pipe supports could be placed to minimize the vibrations. Will a modal analysis be adequate for this kind of problem, or will a harmonic analysis be a better choice? If so, will I know if the solution is sufficient if the natural frequencies are not within the second harmonic range?

2. This line has a couple of dummy pipe supports on it. I'm well aware about the section in the applications guide about how to model horizontal dummy supports. However, the dummy supports in question are more cantilevered than anything, so I use an anchored end instead of a hanger as recommended in the application guide. Are there any issues with modeling them this way? I'm asking because I get quite different stress results if I do it one way or the other. An older model of this line exists in our files, but the model didn't even bother to model the dummy supports at all. Only one degree of freedom was restrained, and I don't think this is the best way to do it either.

3. I've used programs line ANSYS before, and I remember one of the key components is to a mesh refinement. Is this something that I need to do in Caesar II (ie. add more nodes on each pipe span)?

Thanks, in advance, for any help you all can provide. It will be greatly appreciated.

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