5 replies to this topic

[rowanlim]

Hey all I'm currently sizing a wet gas-gas exchanger using HTRI: my problem is with the vibration analysis, some items exceed a conservative lower limit for vibration-free design. I've tried to do the following:

i) increase tube OD + pitch
ii) increase shell OD
iii) increase tube length

But I can't seem to get a design that doesn't have a negative overdesign % or a high pressure drop (on both tube & shell sides).

My question is: is there a procedure/method to overcome the problems brought by the vibration analysis?

Any advice/feedback would be most appreciated. Attached is the output summary & the vibration analysis report of my design. Thank you!

 STHEXHelp.pdf   229.59KB   209 downloads

Edited by rowanlim, 10 December 2010 - 01:47 AM.

[csp_process]

Hello Rowanlim,
From you query and attached summary I assume you are doing rating of some existing exchanger,

We have also encountered flow induced vibration for few exchangers, so there is this commercial technology-Dimpled tube technology and saddeled tube technology.
They say 'HTRI_Xvibo module' has this option available to reduce vibration (check in attachment 1-Exxon-Mobil_Anti Vibration technology),

You may check other attachments also, nice guide
What i will say is I know the way, but I 'never gone that side, probably someone more exprienced would like to put some more light on the subject.
and yeah I'm not conversant with HTRI -as I use CC-THERM if at all I do.

Attached Files

•  Exxon-Mobil_Anti Vibration technology.pdf   786.96KB   223 downloads
•  DTS_Anti-flow induced Vibration_Exxon.pdf   864.39KB   177 downloads
•  Rating a Heat Exchanger using Chemcad.pdf   537.58KB   175 downloads

[rowanlim]

Thanks for the attachments but I still can't solve my problem; it's either compromising the pressure drop + overdesign % OR the vibration in the HEX.

I've a feeling I lack understanding of how different factors influence these criteria: I've experimented & realized that when I increase the tube OD + thickness (whille keeping other variables constant), the vibration analysis has improved results (which is what I expected) but I got a negative overdesign % & the pressure drop on the shell side exceeds the allowable value.

Any idea what direction I should take?

[mvp]

Rowanlim:
Try Rod Baffles; vibration issues should go away.
Also Check if Phillips has licensed a Fab shop near you to fabricate it.
MVP

[chemsac2]

rowanlim,

Your main problem for vibration is due to large central spacing and hence lower natural frequency. However, you can not decrease baffle spacing as you are already constrained by shellside pressure drop.

For E shells we proceed in following hierarchy:

- Single Segmental baffles, different combinations
- Single Segmental baffles, increased pitch ratio
- Double segmental baffles
- Single segmental baffles NTIW design
- Rod baffles
- ‘X’ shell

However, you are using F shell and U tubes to counter temperature cross. Your options would be limited. I would use following options:

1) Check if NTIW design is possible in F shell. In NTIW design, you can increase baffle spacing to limit pressure drop but add support plates to increase natural frequency of tubes
2) Try 2 E shells in series with NTIW design
3) In your design, decrease baffle spacing to TEMA levels. To take care of shellside DP, increase baffle cut, increase tube pitch with corresponding increase in shell ID to have same number of tubes (subject to limitation of shell ID in your plant)

Generally vibration troubleshooting is handled by concentrating on following:
- Increase damping of tubes (increase baffle thickness, decrease tube-baffle hole clearence etc)
- Increase tube natural frequency (decrease unsupported tube span, increase tube OD/thickness etc)
- Decrease crossflow velocity (use multi-segmental baffles, different shell type etc)
- decrease forcing frequencies (decrease crossflow velocity)

This is a vast topic and hence different combinations of design parameters are needed.

Regards,

Sachin

[rowanlim]

^Thanks Sachin, I'll give it a shot at work tomorrow, I think my supervisor might've used the wrong configuration, because for the same exchanger in a different case (winter, I'm trying out the summer case) used a BEM configuration. I'll try out & get back when I get positive results! Thanks again!