Why slug force can not have frequency greater than 4 Hz ?

We find procedures to restrict 1st natural frequency keeping above 4 Hz in slug flow piping. If we design piping with DLF=2 with proper slug force in occasional load, why shall we keep piping stiff in all mode shapes above 4 Hz ?

like seismic event not coming greater than 33Hz and mostly above 4Hz, it looks to be a useless criteria both for slug load and steam/water hammer transient loads where 4Hz has no significance.

What do our forum mebers think on this issue ?

reg,
sam

Hi sam,

I think the 4 Hz is somewhat subjective. Basically, it's only guideline to make piping stiff hoping that the forcing frequency (e.g. slug) will not match the natural frequency of pipe. As we all know it's hard to predict in the design stage if vibration will really happen during operation, if we don't have a well defined forcing frequency data to design for. Unlike reciprocating compressors where the forcing frequency is a function of the machine rpm.

Any other opinion is highly appreciated.

Cheers!!!

4 Hz is the expectation based on experience when using traditional sizing formula for wellhead piping and flowlines.

If you have slug information and you believe it to be accurate for the lifetime of the piping in question, then there's no need to adhere to it.

That 4 Hz point is a screening tool. Systems that have natural frequencies below 4 Hz are more likely to respond (in a negative way) to what may be random, environmental sources of vibration.
LOF - likelihood of failure
We like to deal with hard numbers such as a calculated stress but, as you point out, it is difficult to "predict" response to a ill-defined transient event such as a slug of liquid hitting an elbow in a vapor line. Screening tools can be useful in identifying systems that may require more scrutiny.

I agree with Dave and Michael. In the design stage 4 Hz is a good criteria and it's better than doing nothing at all. Actually, when I was a junior stress engineer I remember our senior stress engineer told us to run a frequency check so we will know if our system has enough guide. It's just a few click in Caesar II and will give you a good idea of how the system behaves.

Any other opinion is appreciated.

Cheers!!!

Yes! I too agree with the LOF concept of screening through minimum acceptable frequency of 4 Hz.

But, we face real problem with support stiffness inputs for modal analysis used for fining out natural frequency of piping.

Unless the primary and secondary supports holding piping are stiff enough and properly represented by support stiffness in analysis, at site we find operational vibration amplified by piping strucural resonance at times!

reg,
sam

A more refined analysis is then required in this case considering the inclusion of structural supports to have a more realistic response. Not sure if the structural modeler in Caesar II is capable of checking the natural frequency of structure also. Or maybe FEA software can be used which is expensive for practical purposes.

Any other opinion is greatly appreciated.

Cheers!!!

Caesar II has enough capbility of modelling beam structure.

What I wanted to stress in my 20/10/17 post is that piping stress comutational jurisdiction does not end at supply of items interface and secondary and primary structures need to be detailed based on stiffness requirement primarily and obviously conforming to stress requirements based on material allowable at temperature.

So, if required mimimum stiffness specified in stress analysis is maintained by structural people, the need is met.

reg,
sam

Hi sam,

Your point is correct. In one of our project involving reciprocating compressor piping system we specifically inform our structural guys to have a natural frequency on their structure greater than 20% of 23 Hz since we design the piping at 23 Hz (compressor second harmonic being 19 Hz), this is as per vendor recommendation. The compressor have performed the final mechanical response analysis. T-post is not used in our case also.

So I agree with you that proper design does not only end at piping side but should be communicated also to structural side in case there is potential vibration issue.

Any other opinion is greatly appreciated.

Today, when piping stress analysis has become commodity, piping structure interfaces are becoming highly vulnerable!

As we are changing from oil based economy to gas based economy nowadays, nuclear one left behind decades ago, we are feeling the need for stiffness based supports of nuclear era!
By specifying costly hydraulic snubbers and rigid struts many times, we are fail to provide right restraint because of flexible structural arrangements.
We still have lessons to learn from nuclear days!
Reg,
Sam

Hi Sam,

In my experience, we have maintained even more than 4 Hz (5 to 7)for slug flow lines (multiphase systems) by keeping axial stops in every change of direction and hold downs in some locations. The piping vibration issue was solved completely and successfully.

Balancing the higher natural frequency with stresses in the allowable limit is important here. When we try to increase the system natural frequencies, the induced stresses will increase which may cause fatigue failure.
Good luck.