Check the Tubes for buckling as per UHX 14.5.9

[krishsar]

Hi all,

I am facing a problem with the tube buckling check as per UHX14.5.9 (C).

In design condition calculated stress is less than allowable, but on hydotest condition the stress exceeds.

Our vendor is proposing to increase allowable stress by 1.2 times (in test condition) is that correct or anywhere given on code (I could not find any). And moreover is that mandatory to have the buckling check for hydrotest condition or we can avoid it.

Else we should try reducing the unsupported tube length or increase the tubesheet thickness.

Please try to give some suggestion (See attached failed report).

Regards,
KR

 

[SnTMan]

krishsar, obviously, some increase in allowables at hydro must be permitted although the Code does not so state. Part UHX states only that hydrotest cases shall be considered, but does not state anything concerning allowables, other than those used for the design cases.

If per UHX 14, this would appear to be a floating head exchanger. I have never understood how, under Part UHX, tubes in a floating head bundle are subject to meaningful buckling stresses.

So, to your problem, first I would insure your software is using material properties at ambient for the hydrotest cases. Most likely it is. Second, if I could decrease the buckling stress without too much inconvenience, such as by adding a tube support or baffle, I'd do that.

Aside from these measures, I'd probably not increase the allowable without some way to justifiy it, such as another analysis of the tube compressive stress.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[krishsar]

Hi Mike,

Thanks for your reply. As said by you for some equipment´s I have option of introducing additional baffles but for some not due to thermal reasons.

For now I will accept the vendor proposal.

Regards,
KR

 

[chevproe]

Hi Krishsar,
I would also tend to agree with SnTMan that we just can't increase the buckling allowable stress arbitrarily without any sound basis. Your AI may differ and ask you for the technical justification of going for higher allowable.

You may go for multiple trials with decrease in baffle spacing (provided you have sufficient delta-p)..you will need to spend some time playing with the softwares for thermal & mechancial design to reach a balance where your tubes seems to pass the UHX bucking check without any adverse effect on thermal performance ...if it does not work out, consider increasing your tubesheet thickness..
Since the difference between allowable and actual bucking stress is not too high, i think it will be worth exhausting the options before bypassing the code..

 

[krishsar]

Hi Chevpore,

Thanks for the reply. The think is with a particular design we are left out with no other options rather than incerasing the allowable.

Anyway thanks to both but in my case we have no AI approval is required

Regards,
KR