Dear all,

Why natural frequency is performed?,

First of all, this question is very basic and I hope you are not doing stress analysis without proper guidance. But your question kind of answers itself. You perform a modal analysis to determine the natural frequencies of the pipe and prevent the system from resonating.

Hi Ltorrado,

Yes. i am doing stress analysis for static conditions. Now i want to learn dynamic analysis.

sorry, my question is at what condition we have to perform Fn (Natural Frequency) analysis. One of my client comment is it applicable for all the conditions like, vessel, pump, exchanger and PSv lines.

At a minimum, a modal analysis should be performed on all systems. You want to try to adjust the boundary conditions such that the lowest natural frequency is above 4hz. This reduces the likelyhood of failure while decreasing the potential to respond to dynamic events.

Dear

I Newly joined traineer Engineer but I would like to explain you as much i know n undertand about natural frequency.

We check natural frequency just to ensure that our piping system is safe from resonance condition.

First of all you have to know about some basic definations like-

Frequency - Its a number of occurance of a repeating events per unit time. & f is praportional to 1/T.

Oscillation- It's a repetitive variation , typically in time.

Resonance- A resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.

So if your amplitude is high means your frequency is low.

So we always check in modal analysis that our pipng system always have frequency greater then equipment frequency (At operating)
which avoid resonante condition of piping system.

Also if you pipe is stiff enough so it will reduce amplitude.

Dear all:

Actually I meet several issues that the air compressor outlet vibrate severely.
During the vibration, The machine's vibration is acceptabe after measuremet. By CAESARII's dynamic analysis, The piping line's first natural frequency is lower 2hz,
So whether the low first natural frequency result in the line's vibration?

Dear Stress Experts,

This is very difficult question to ask when the piping system will resonate?
But we can minimise the chances of resonance. What we normaly do is we design our support span and guide span above 4Hz frequency.
And presume that in normal pressurization-depressurization event in the piping system due to fluid flow will not lead to the vibration which will not cause the entire system to resonate.

However in case of more specified systems such as multiphase flow, slug flow,water hammer,PSv poping systems, reciprocating machinary attached systems pumps or compressors etc are subjected to vibrations due to flow and externaly imposed forces at regular intervals may lead to resonate the system.And the above support span and guide spanning is inadequate.

So we perform modal analysis of the piping system to find out fundamental natural frequency of the system and appropriately adjust support and guide spaning to reduce the risk of resonance.

As a rule, don't make the natural frequency of the piping system within +/-20% of equipment's natural frequency to prevent resonance. If their frequencies are almost same then it will amplify the vibration. Normally we design the piping system much more stiffer that result to higher natural frequency than the equipment.

I have some experience on dynamic analysis of system subjected to earthquake loads.From that background,i would like to put forth following points highlighting significance of natural frequencies on system response.

1)Generally seismic analysis of any system is performed by response spectrum method for which natural frequencies of the system is the primary input apart form response spectrum(frequency vs acceleration plot).
2) Preliminary modal analysis of system gives an idea of the systems's rigidity (flexibility.)
3) based on the natural frequencies results, we will be able to identify where the system lies in the response spectrum(velocity,acceleration,or displacement dominated region)
4) finally in order to satisfy the stress limits,Force & moments limit on nozzle limit ,displacements limits the location and number of supports are decided ,which is going influence the natural frequency of the system.

so natural frequencies of the system have great significance in deciding the dynamic amplification of the system to a given dynamic load.
Kindly read some basic book on vibration or structural dynamics to get an idea of the importance of the natural frequencies.

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Udayakumar
piping stress analysit