[Weekly Question] (Issue 7, 2009) — “Discussion on Issues Related to CII Input””
Thread Content
The theme of this issue – “Discussion on Issues Related to CII Input” – builds on the question raised by llu85: 1 How should a ball joint with angular displacement limits be entered? 2 How should the vibration relationship be entered? 3 How to calculate the natural vibration frequency of a piping system. How to adjust? 4 What kind of support structures do we need to model using steel structures in C2? The discussion format can be a textual description or screenshots. I hope everyone will actively participate, discuss together, learn together, and improve together! This post was last edited by fanking on 2009-3-22 17:57]
4 What kind of support structures do we need to model using steel structures in C2? Rigid hanger. I encountered a piping system in which rigid hangers were used for on-site installation; to determine the stress on the nozzles, those rigid hangers were modeled as C2.
The support structures need to be simulated in Cii as well; for pipes with particularly sensitive nozzles, the accuracy of the support simulation has a significant impact on the results.
It is mainly determined by the importance of the bracket to the nozzle. For example, the steel structure near the tower nozzle generally needs to be simulated. Most do not require simulation, and it will definitely take a lot of time.
The common sources of vibration in chemical piping are pumps and compressors; it’s best to use hoses to separate them at the inlet and outlet； A suitable expansion tank should be installed at the outlet of the reciprocating compressor to separate it
For the reboiler mounted on a tower, it is generally best to construct a structure for it; otherwise, the forces involved will be extremely large
It seems there isn’t a standard steel section database in Cii, and personally I think there are very few situations where it’s necessary to create structural models.
To answer the third question: The natural frequency of a structure can be automatically calculated by CAE SAR software; all you need to do is set some parameters. The issue is that after the calculation is done, it is necessary to rely on extensive experience to determine whether the result meets the relevant standards. If anyone has such standards for evaluation, please share them so that we can discuss them together – together, we can achieve better results
The natural frequency can be calculated using the model analysis from dynamic analysis in c2. However, certain processing is required before calculation; the nonlinear constraints should be modified. For example, gaps, friction coefficients, and the load-bearing support + Y should also constitute a Y-directional constraint. A static analysis (not mandatory) can be performed followed by a modal analysis to determine the natural frequencies of the piping system. When designing compressors, it is common to calculate the natural frequency of the piping system, and this is done in order to keep the natural frequency of the piping system away from the excitation frequency of the compressor, thereby avoiding resonance.
Whenever there are large-scale equipment near the pipelines, experienced owners will require modal analysis to be carried out; it’s not necessarily limited to compressors. I don’t quite understand why nonlinear constraints need to be modified ? Is there any evidence? The stiffness of the piping system itself and its boundaries determine the value of the natural frequencies; will modifying them still yield accurate results? More discussion is welcome
It seems like the guy upstairs hasn’t done any modal analysis. If you have done it, you will find that when using software for calculations, if there are nonlinear constraints present, the software is unable to perform those calculations. Because modal analysis is different from static analysis. If you list out the stiffness matrix, you’ll see that the eigenvalues of a stiffness matrix with nonlinear constraints cannot be determined. So it’s not possible to perform the calculation. Also, you said that calculations need to be done for any large equipment located near the pipelines, but I don’t quite understand this. To what is the calculated frequency compared? If the goal is to ensure sufficient stiffness, then the stiffness requirement can be met as long as it remains within the specified span range, since the formula for calculating the span is derived by setting the pipeline’s natural frequency above a certain value. If the span requirements are met, there should be no issue with the natural frequency. At this point, modal calculation seems a bit redundant. Hehe, it's just my personal opinion. It’s just a tentative suggestion; I hope experts can provide an answer.