Hormonic Piping Vibration

[Shofa]

Hi everybody,

I am using Caesar II to solve a piping vibration problem in one of our petrochemical plant. The problem consists of a piping system for chiller water is vibrating due to an external harmonic frequency at 11, 22 and 33 Hz. The max displacement occurs at 11 Hz.

The main problem for me that how can I calculate the exciting harmonic force to excite the model before we can make modifications. The piping system includes reducer from 8” and 6” to 4” and 4” equal tee.

Also, how we can calculate the dynamic force, which is coming from the reducers and branches due to fluid direction change.

Looking for your appreciated help.

Regards,

 

[stanier]

I do not use Caesar but Pipepak from Alogor and would pose a similar question initially to their support service. Thats the value of paying for support. I presume you dont have support.

OK What I would do in this situation would be to look for likely candidates that are exciting the pipe. These may be wind, machines (most likely) or fluid. Determine the exciting frequency, amplitude etc by test or investigation and try and replicate that in your model of the system. It may sound a bit long winded but at least you will understand whats happening.

If you want a quicker answer hire a consultant. Preferably one at COADE then at least you will learn from how they do it. An outside consultant may not want to show you how to do it so next time you have to pay again.

 

[vanstoja]

Why don't you convert your measured displacement at 11Hz into an acceleration at the point of measurement and estimate or determine exactly the weight of your system which is being excited? If I recall correctly, F=ma=Wa/g which should get you the force causing the acceleration at the measurement point. I presume your computer program has a forcing function equation where time multiplies omega =2*pi*f into which you can insert your 11 Hz fundamental frequency for f. That procedure seems to be too simple to explain your state of consternation. What am I missing here?
On the force due to the flow directional changes at the tee, you'll need a flow velocity at the tee to compute the reaction force with an equation I know exists but haven't yet found.

 

[vanstoja]

I found an equation for the pipe reaction force at a bend for an incompressible and a compressible fluid by Sholander,A.M., "Impulse Force Caused by Moving Compressible Fluids", Design News, July 22,1964. For an incompressible fluid it is:
F=(pA+Q*V*rho/g)*(2-2cosAlpha)^0.5 [From Design News 10/30/63] where p=Pressure differential across the bend
A= Pipe area at entry to bend
Q= Pipe flowrate
V= Flow velocity at entry to bend
rho= Fluid density
g= Gravitational constant (for Engl units)
alpha = Bend angle
For a compressible fluid the equation becomes;
F=pA(1+gamma*M^2)*(2-2cosalpha)^0.5 where
gamma = gas parameter
M=Flow Mach number at entry to bend
The gas equation is based on choked flow at an open exit downstream of the bend.

 

[Shofa]

Thanks a lot for the help.

Regards,