Dear SErs
Modal analysis is to find natural frequency with different mode,

but how to understand this result with piping system will vibrate at site or not,

some reading says it natural frequency goes above 15hz then there is possibility of vibration, but the system give more and many natural frequency above 15hz, even the system not connected with rotating equipment it give more and many natural frequency above 15hz at site no vibration then how to predict using modal analysis to find vibration

my intension is to find out the real use of modal analysis in practical usage

some reading says the natural frequency should be kept below 6hz how to keep it below 6hz always many natural frequencies since many modes

regards
pipeman

The rule of thumb for Rotating machine is to keep the frequency below 6-8 Hz, you can do this by adding a support or make your piping system more stiffer. Proper choice of type and spacing of support play a big role too here. See the first natural frequency and mode shape on you CAESAR II output.
And try to use the search button, there are many topics about this has been post before....

T. Syahdilan
Pipe Material & Pipe Stress Engineer PT. PFCE Indonesia

thank you T.Syahdilan

you mean to say that the ceasar ii calculating frequency all should be below 8Hz, correct,

if the cutoff frequency is set to 33hz then how it behave


regards
pipeman

if the vendor give natural frequency for example 47hz then my piping system natural frequency is below 47hz is it ok


regards
pipeman

You can't keep ALL natural frequencies BELOW a certain, low value.

Usually, you do not want very low frequencies as these modes can generate greater induced loads (and significant stress) than the higher modes.

I understand that some design specifications require the FIRST mode be GREATER than, say, 5Hz.

Yes, if your equipment is driving a 47Hz response and the system is "tuned" to 47Hz, you will induce a larger response but I doubt that this (relatively) high frequency will generate a large structural response or bending stress in a beam bending program like CAESAR II.

THANKS FOR YOUR REPLY

(1)that means piping system natural frequency is below natural frequency of the equipment then there is no vibration that could be find using modal analysis


(2)if any vibration in field or during design phase we could find using harmonic analysis

agree


regards
pipeman

For the basics, there are two frequencys:
1. Forcing Frequency (Generated by some external means, like positive displacement action of pumps,Seismic excitation of ground etc.)
2. Natural Frequency of System (It is a inherent property of system depending on its layout, Support config)

The idea is that if forcing frequency matches exactly with natural frequency, we get RESONANCE. Here both the frequency amplitude and energy becomes inline with each other and produce highest loads and amplitudes.This causes the failure of sytem due to vibrations.
Remedy would be to avoid any one -The Forcing or Natural frequency. As Forcing frequency cant be avoided, we change the natural frequency by changing either the piping layout or support functions.

Thumb rule is to avoid each natural frequency below 5 Hz for area near a reciprocating/Dynamic equipment.

Stress admirer, your thumbs are clearly different to Tengku_Syahdilan's, judging by his statement above: quote "The rule of thumb for Rotating machine is to keep the frequency below 6-8 Hz"

Just proves that free advice is worth exactly what you paid for it.

THANK YOU ALL,

After discussion with all of you and Mr.Loren BROWN (COADE)

THE FOLLWING POINTS ARE OBTAINED.

(1) Goal of modal analysis IS to find out the natural frequency of the piping system

(2) It should not never equal to the natural frequency of the equipment,
It should be lower or higher,

(3)It should be higher, is best because no one mode will cross it that means there is never no vibration.

(4)IF the natural frequency is below equipment natrual frequency may be in any one mode it could make resonance, that should be avoid

(5) Best is the first mode natural frequency of piping system should be above natural frequency of equipments, (5HZ, 6HZ, 8HZ are just std thumb rule!!! practice,even in 5HZ possible of vibration)

regards
pipeman

I hope moverz also have some views regarding this topic. Moverz I am expecting your expert comments regarding the topic.

Stress Admirer .... I try to be careful when commenting in this forum to ensure that my comments are correct and helpful. It does annoy me when I see rubbish quoted as a 'rule of thumb' for instance.

Perhaps you could do with a little more knowledge and experience, since you risk misguiding others both in this post, and your comments in another post concerning safety valve reactions, where you have made errors.