Flow-Induced vibration

Dear all,
it is possible to perform a flow-induced vibration by Caesar?
If yes, can you explain me or indicate any guidelines to be followed to perform this?
thank you

Hello..

In the design stage, I would suggest to refer to Energy Institute Guidelines. You can use Caesar to calculate piping natural frequency which is an input in the guideline.

If vibration due to FIV is already happening in the field, you can use Caesar to perform Modal & Harmonic analysis to check system response from the vibration. Basically, you tune your Caesar model to match what was observed at site, then you can kill those bad mode shapes to avoid resonance & therefore eliminate, if not minimize vibration to acceptable level.

Any other opinion is greatly appreciated.

Dear Borzky, thank you for the answer.
Unfortunately this is the first time for me with this kind of issue. I have searched the above mentioned guidelines but I can not find It.

In general I agree with Borzki. However, I do not see CAESAR calculated natural frequencies to be a viable input to the EIG, due to relative conservatism of CAESAR versus EIG, if referring to the EIG pipe flexibility classification table.

Although, if you're in a situation where you need something, and something is better than nothing, and using natural frequencies from CAESAR for the EIG are resulting in passing designs despite the conservatism, I can't find fault with something that works.

http://publishing.energyinst.org/feature...rocess-pipework

Hi Michael,

Thanks for your opinion on this matter. Actually, I haven't experience it yet in actual design. I just have some general idea. The previous experience I have is using EI guideline for AIV phenomenon. When I browse today the guideline I've noticed they have some Lspan formula corresponding to the frequency (ranging from 1Hz classified as flexible piping to 16Hz classified as stiff piping).

Hi Aleps,

You need to involve the process engineer since most of the upstream information will be coming from them assuming you are doing FIV in design stage. In my present company one of the client request to put in the line list the AIV & FIV column and process will fill up that column ay "Yes" or "No. If FIV is required, you can develop a spreadsheet showing all parameters required and let the process people fill up for you the process parameters. Basically, the criteria is based on turbulent kinetic energy present in the system (density times square of velocity). And from there, just follow the procedure written in the guideline which gives formula for the calculation of LOF's.

Any other opinion is greatly appreciated.


Cheers!!

My problem with that chart is that it says a line is Stiff around A Hz and above, Medium Stiff around B Hz, Medium around C-D Hz, and Flexible at E Hz or less.

This can be interpreted in many ways. Does this mean that the cut-off for Stiff and Medium Stiff occurs at A? Or half way between A and B?

I've had the EIG tell me a line is Medium Stiff, but Caesar tells me the natural frequency is two levels below, or Flexible.

I think perhaps the EIG's idea of how the pipe acts at the supports is different from the way CAESAR does.

Hi Michael,

I got your point on this matter. I thought previously the EI guideline have no formula for calculating the frequency.

Cheers!!!

Hi,


In my experience, there is not much Piping can do for Flow induced vibration. It's more of Process with the flow conditions, vapor density, flow velocity and most importantly pipe size. Piping input for LOF calculation is the "Support Arrangement" of the piping system. Modifying a piping system to a higher "Support Arrangement" to reduce LOF, hardly works.

Vibration caused by Mechanical excitation should be under piping engineer/ stress engineer's perspective. Required proactive steps should be taken both for the main lines and the connecting small bore pipes & connections.

For a known vibration issue, best would be to get a vibration measurement data, upon which proper corrective actions can be recommended.

I would like to hear more on the subject

Regards,

Hi Harish,

In my 20 years experience, we haven't done FIV on stress analysis in design stage. My opinion is, as long as the process engineer will limit the turbulent kinetic energy (density times square velocity) to an acceptable level (usually they have certain criteria like for flare lines) which is written in their spec, I think should be ok. It's hard to be objective in tackling this issue because you need to do some complicated calculations such as CFD, FEM, etc. to quantify things which is not practical especially if the project schedule is very tight and is a lump sum type of project. Just make sure to review client specs if this a part of their requirement in ITB and inform the relevant people involved (e.g. lead engineer, process engineer, etc.) so it will be properly addressed. Usually, as Haris mentioned process engineer usually take care of this issue in the design stage.

What I remember in my present company, the process engineer will only ask question to stress engineer in cases where the client is asking to increase flow rates for some existing system and it will exceed the process engineer guideline (e.g. density times square velocity criteria).

Any other opinion is greatly appreciated.

Cheers!!!

My experience is that changing the size of the line is not a clear-cut solution, either. You would have to make extraordinary (and uneconomical) size changes to switch the flow regime.

This is because changing the line size affects line speed and pressure losses, but it does nothing to change the relative rates of gas to liquid.

The only acts that control the relative rates is pressure and somehow controlling the mass ratio of gas/liquid. Both of these are inter-related, and boil down to equipment design.

And most process engineers will understandably balk at the notion to allow the pipe to control the design of equipment, as they are also required to design their equipment with their own rules of economics in mind.

Thanks Michael for that very good and realistic opinion.

As far a FIV analysis is concern we have to provide additional piping supports to avoid vibration. In EI guideline size wise support span is given we have to follow the same. Also calculated LOF value.Limit is also given for LOF value in EI guideline. High/LOW/Medium stiff supporting.

Just adding few point for sequential engineering before stress engineer jumps into direct assessment of FIV/FIV. Process discipline to perform initial assessment based on EIG and come up with what all vibration assessments are required (Table T2.1). Some operators have establish functional organization contingency in addition to EIG table T1-1. Process may furnish the support arrangement requirement from flexible to stiff which piping to achieve in their design. Piping to make an effort to keep the LOF less than 1 or within the EIG recommended range based on service and establishment of pipe work if LOF is more than 1 based on type of operating unit project (Table 3-1).

If piping support arrangement is already been set up as stiff and LOF is still above the recommended limit specially above 1 then process discipline can be asked to remove conservatiness from the flow rate provided. even that doesn't help than change in pipe th'k, dia with the acceptance of stakeholder to be implemented if more detailed FEA is not balanced policy option.

LOF more than 1 doesn't guarantee vibration but gives you an alarm for improvement in design.

The problem with multi-discpline problems is convincing multiple disciplines that it's a multi-discipline problem.