Natural frequency of expansion joints

[Vibac]

A general question: How do you estimate the natural frequency of an expansion joint mounted on a pipeline?

For a simpel SDOF approximation I can get the expansion joint's stiffness (spring rate) from the manufacturer, but how about the oscillating mass? Any better idea than summing the mass of "some" pipelengths and the fluid inside? Should you perhaps look at the structural wavelength in the pipe material to estimate an adequate length?

 

[agps]

Hi Vibac!

Did you had any answer for your thread besides this forum?

Can you explain me how do you calculate the mass on a certain point of a structure and how do you get its own frequency.

Tks

 

[Vibac]

No, I didn't post it anywhere else.
What I had in mind was a simple assumtion of two rigid masses separated by a spring. From the top of my head this gives a natural frequency 1/(2pi)*sqrt(k*m1*m2/(m1+m2)). The masses would be the pipesections on each side of the joint,and k the stiffness of the expansion joint.

 

[electricpete]

There are of course a number of different possible modes that any system can vibrate.

I don't have any experience with expansion joints, but I have a guess.

My guess is that the most likely mode of vibration for typical expansion joint is also the simplest: It assumes that the endpoints are fixed and the fluid doesn't move. So the expansion joint could presumably by modeled as an axially vibrating rod with ends constrained to have zero axial vibration. Mass and damping associated with fluid or attached masses would be neglected.


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[MikeHalloran]

Expansion joint spring rates are provided so that you can calculate the forces induced by (thermal) displacement.

You're supposed to 'retain' or 'support' both ends of the joint separately, or put displacement limiters around it, or do something similar, so that the joint is _not_ carrying any of the pipe's weight.



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

Unless, of course, you have designed the joint specifically to carry some pipe weight (with a high lateral spring rate and stout control rods.)

rmw

 

[electricpete]

I agree with Mike's comments.

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[Vibac]

Thanks everybody for your input.