Hi,

I have a system where I want to know the natural frequencies of the system. The input is a harmonic load but if I run the system with the harmonic load than I can't find the natural frequencies in the results? If run the system as well as modal as type than I have the natural frequencies but it is without any restraint on the model. Can someone help me how I can restraint into the model?

Kind regards
Koen

Run the Modal Analysis first, and obtain the natural frequencies. Next, go back to the dynamic input and change the analysis type to "Harmonic". You shouldn't have to change the piping model to do this, your restraints should already be in the model.

Kovalgi,

As Richard said, run the modal analysis for natural frequency generation. If that is not clear, you need to read some reference such as Caesar II seminar manual.

Modal is used to generate natural frequencies and Harmonic is to determine the loads and stresses with your excitation frequency and excitation loads. Take out the natural frequencies and compare it to you excitation(comparison defer with you case).

regards,

Hi,

Thank you for your reponse
The natural frequencies are without any restraints? Have restaints no influence on the frequencies?
Other strange thing
If I do a harmonic analysis with a range of frequencies then have I not realistic displacements. I see the restraint in the 3D model but the system move like it have no restraints. I have a displacement of 400 mm by 1Hz where I have a restraint with a gap of 2. Can I give a restraint a gap by a dynamic analysis?

Kind regards
Koen

Dear Kovalogi,

I think you are making some basic mistakes or what you want to communicate to us is not clear. What exactly you mean by natural frequencies are w/o any restraint? W/o any restraint a body is going to have rigid body motion; so would you expect natural frequencies that time? My second question is , how do you know a harmonic analysis is what is required? Harmonic analysis is applicable when the load i..e. the forcing function is harmonic like systems connected to recip. pumps, compressors etc. You should not be using harmonic analysis for a system subjected to random vibration. Most flow induced vibrations are random in nature.

Regards

Here you can learn something about Modal analisys. These webinars are free and are very interesting.

http://coade.typepad.com/coadeinsider/2011/05/dynamic-piping-analysis-in-caesar-ii-part-1.html

Best regards,

Dear Anindya,

I have a model with a pump. The pump create a harmonic load (17Hz and load variation of 50barg). Now I want to know the natural frequencies of the model. Which frequencies the model will be vibrate with restraints. I want that this frequencies far away from the frequencie of the pump. I want also the natural frequencies above 6Hz. If I run the modal analysis than I get a lot of frequencies but they are not realistic because the pipe move where I have restraints. Maybe I make a combinatie that is impossible.

I have no receive a respons for the webinar. Hopefully later

Kind regards

do not use gaps.

Regards,

Non-linear restraints cannot be addressed in dynamics, so CAESAR II linearizes them based on yourreferenced load case. This means the some gaps and/or +Ys could be removed. Check the "ACTIVE BC" report to see this.

Any friction at your pipe supports will require special attention in your model and analysis. You can either add restraints perpendicular to your supports to hold the piping in place for modal analysis. Or, you can include friction (mu) in your initial (static) model and then add a Stiffness Factor for Friction in the dynamic Control Parameters to activate this friction in the modal analysis. I suggest a value of 1000.

Dear Diehl,

I am facing a pipe stress analysis with a centrifugal pump.

Looking at your previous answer, we go for the second posibility, I prefer
model static model and then add a Stiffness Factor for Friction in the dynamic Control Parameters.

Only thing is that, we wonder what the value of 1000 you suggest is based on, perhaps might exist a reference book or the value may be based upon previous experiences.

Thanks a lot in advanced.

That Stiffness Factor for Friction is not a physical property. It's an "artificial" adjustment to the dynamic model to better reflect observed response. Without friction (a damping component) in your dynamic model, sliding supports would slide freely. My guess is that they do not. This stiffness retards that free motion and friction restraints with more friction are retarded (stiffened) more.
I use 1000 but other numbers produce similar system response. It's not a 0 or 1 item. Try a few values on your own.