Hi.

My Client assign me to solve the vibtation problem on Piping which connected to 2 Blowers in parallel. They also give me a vibration measuring report. It consist of vibration spectrum plot and other result such as vibration accerated rate, velocity and displacement at each point.

Anyway, this system also have expansion joint between blower nozzle and pipe. I understand it use for seperate the vibration between blower and piping system. So vibration is caused of Gas flow inside the pipe.

What dynamic function can I use to analysis this problem and how can we apply the spectrum plot or result into the CII.

In the previouse project, I just compare source frequency and natural frequency of piping and adjust the pipe support. I intend to use the result of vibration report for analysis in this project.

Please kindly advise.

There are, generally 2 sources of vibration in pipes:

1- High Vibrations Caused by Mechanical Resonances
2- High Vibrations Caused by Acoustic Resonances

Piping which has a mechanical resonance can be improved by:
1- reducing or eliminating the energy causing the dynamic shaking forces
2- detuning mechanical resonances to reduce the amplification of the energy
3- strengthening the restraint system to absorb or withstand the dynamic forces while allowing acceptable vibration amplitudes.

For piping systems which have acoustic resonances, it is desirable to determine if the pulsations can be reduced. Listed below are several possible modifications to the system that can be used to reduce steady state pressure pulsations.
1- orifice plates
2- piping configuration change (length, routing)
3- large volumes
4- pulsation bottles (Helmholtz filter)
5- acoustic velocity change (gas composition)

So, first, you should identify the source of vibration then perform either force or displacement harmonic analysis in CAESAR II.

Farhad,

While your response above is technically correct, why you have recommended us of CAESAR II if acoustic resonance is the cause? CAESAR II is not meant for it.Also , pl. noe that even for mechanically incuded vibration ( coincidence of input and natural frequencies of the system ), how would you know if the vibration limits are within acceptable limits? Say, as advised by you, the analyst performs a harmonic analysis, which approach for fatigue should ba taken, stress or strain controlled ? For mechanical vibrations of low magnitude and high no. of cycles, you should not resort to to the ASME fatigue curves which are based on strain controlled approach.

By the way, TH if your concept on acoustic and mechanical vibration is not clear, it is somewhat like this:

Vibration ( and noise)are nothing but wave propagation.Modes are generated due to finite nature of evey structure ie they have ends and the waves reflect at the ends and in turn generate standing waves in the system.This is irrespective of whether the media is solid or fluid.When it is fluid we call it noise or acoustics and when it i solid we call it vibration.The respective modes are acoustic and structural modes respectively.

I would recommend to you that in order to search for a solution, you really cannot do anything if it is acoustic induced ( coincidence of acoustic with structural modes in terms of frequency or wave number) and requires analysis by a vibration specialist. What you can do, since the client has provided you with vibration amplitude and velocity measurement,there is an ASME publication ( in case you don't have it, let me know ) showing acceptable limits for such parameters. If your system values are within acceptable limts, you can try to convince the clients on the same , based on this paper.

Regards

Anindya,

We don't learn to solve these kind of problems in university. In the real world, there's a WAR between you and machines (pumps, turbines, compressors, etc.). If you can't overcome these problems, you'll never be able or dare to solve dynamic problems in your career life time.
As a CAESAR II instructor, during my dynamic sessions, I see fear and lack of undrestanding in trainees' faces and I know that it's very normal. But after training they will know, at least, what's going on when dynamic phenomenon apears.
As you see, our friend "TH Engineer" don't know what to do. What I explained to him was to urge and encourage him to think and study if he likes, otherwise he will face the same problem next times.

Now, dear TH, The FIX is,

- Run modal analysis and see all mode shape plots carefully.
- As a rule of thumb, When flows are greater than 10 meter/sec flow instability can couple with mechanical frequencies to produce shaking forces & displacements. So try to compare the diflected shape of your piping system with mode shapes. As you said, you have all displacements measured before.
- Once you identified the mode shape, you must change the natural frequencies by adding more stiffness (supports).
- Sometimes, you may tweak "Stiffness Factor for Friction" in dynamic's control parameters, to model your real problem.
- For fatigue analysis, you have to calculate shaking forces as well. You can use BOS Fluids software from PAULIN to calculate acoustic natural frequencies and shaking forces.
- Then enter calculated shaking forces in CAESAR II harmonic analysis and perform fatigue analysis.
- Remember, some times pipes don't fails due to fatigue, but pipe supports and flange bolts may fail.

I hope this helps you. Come back to me if you need further assistance.

Regards

As per the CaesarII help documents "Mass participation report" can be used for identifying the modes which are to be suppressed. If the mass participation of a mode is higher in the list, that mode should be avoided (by using additional support or by reducing the gap or changing the routing.)

Dear Mathew,

In modal and harmonic analysis output, there's no "Mass participation" report.

Farhad,

I agree with you that many stress engineers think that dynamics is extremely complicated . The reason is they don't study the basics.However, piping vibration ,particlarly flow induced requres good undrstanding of the basics including acoustic vibrations and not all flow induced vibration can or should be performed using CAESAR II which is a structural analysis program.

Few examples: high frequency acoustic vibrations at pressure reducing stations , medium level acoustic excitations due to flow sparation at elbows etc. All these analyses requires expertise. The dangerous situation is when the novice thinks that every vibration analysis can be done by CAESAR.

Regards

Farhad,

Tell me the guide To study the basics of dynamics, pulsation ,accoustics and the relation between them.

Regards,
sathees

Dynamics:

1)Dynamics of structures: Theory and applictions to earthquake engineering- Anil K Chopra

2)Dynamics of structures-Clough and Penzien

3) Structural dynamics theory and computation- Mario Paz

4) Seismic design using structural dynamics -S.K.Ghosh,Jaehong Kim,Farhad H Shad ( publiction by SKG associates , you can get it from their website www.skghoshassociates.com)

For Acoustics:

1) Fundamentals of noise and vibration analysis for engineers-Norton

2)Vibration handbook -Leo Baranek

You will get lot of papers on the website of Engineering Dynamics. These are very good practical papers.

Hope this helps.

Regards

Thank you anindya.