Guys,
I have a tube bundle that works in an upright position as a reboiler with steam on the shell side and methanol on the tube side. Failures are due to caustic corrosion of the tubes over time because of dosing done on the steam system.

This has the effect that wall thinning takes place and eventually leads to failure of the tube. After replacing the tubes now it was noticed that some of the old tubes had developed "indentations" along its length on the side more or less diagonally across from the steam inlet. This has not been seen before and as far as we know the equipment was never been subjected to a vacuum.

It is as if some force applied to the tube from the outside had the effect that it "buckled" to the inside, as if someone had knocked a slot with a blunt chisel and hammer into the tube.

My question is for someone that has seen this before to please explain the cause?! I'm baffled...

Design vs operation: caustic corrosion, steam, methanol...
* which is the tube material? thickness?
* is it compatible with the operation?

identations? knocked a slot with a blunt ...
* any angry or frustated operator releasing his fumes ... ?
* anyone accidentally droped this tube bundle over something diagonally across to the fall and keep his/her mouth shut and installed back into the reboiler?

Ken,
It would help us a lot if you posted some sketches and photos. I have worked with similar exchangers (but not in methanol service) for many years and never come across anything similar before.

Caustic in steam? What sort of steam chemistry control do you have? At the steam inlet nozzle, do you not have a perforated impingement baffle?
Without some data, we can all indulge in guessing games; in any case remote RCA is not practical.

Guys,
Thank you for the replies, i'm actually waiting for the metallurgist report which will have the photos, the tubes were pulled and analyzed while the whole reboiler was retubed and put back into service. I'll post it when i get it.

With regard to the caustic corrosion on the tubes i've found that in some instances you will find in an application like this where the heat transfer results in a condensate level in the bottom part of the vessel that you find a concentration of caustic in a still area, depending on the outlet and inlet nozzles, more or less just off centre of the middle.

Much like a pot of spaghetti on a stove, depending on how the spaghetti lies in the pot you'll get a still area in the middle, usually with some foam on top. In the reboiler this action "concentrates" the caustic in this area.

As the movement is less, the time between chemical and tube is more...

It happens over years though, not overnight.

Guys,
Sorry it took so long but i eventually received the pic's.
Please have a look and give me some opinions, the weld failure and subsequent tube deformation i can explain but the deeper deformation is very smooth and of nearly equal size and shape, affected some tubes and not others...?
I'm looking forward to your replies,
Kencj

Tube_Deformation.doc (679 KB, 140 downloads) Tube Deformation.doc

Amazing. Never seen anything like that.
Is your metallurgist certain this is deformation, and not some kind of deposit?
(from your initial post is sounds like you've seen the outer surface, and it's definitely mechanical deformation). Just confirming.

Ken,
From the pics., it appears that the tubes have buckled inward within the tube sheet thickness, not in the part that is outside the tube sheet. Is this correct? Secondly, the pics are at the top tube sheet; are the tubes OK at the bottom tube sheet?
- Are the tubes seamless or weldded (if so, spiral or longitudinal)? Any correlation of damage with these welds (as against the one with tube sheet)?
Is there a possibility of nascent hydrogen (i.e. as H rather than H2) forming in hot methanol? I am not familiar with its chemistry, hence this qyery.
Were the damaged tubes sectioned and metallurgically examined? Any results? How do they compare with some 'good' tubes sectioned at the same position?

Hi Ken
At joint tube to tube-sheet seem to be strength welded and light expansion.

What about pressure of tube side?
What about pressure of shell side?

I think pressure of shell side greater than pressure of tube side.
The outside of tube surface insufficient pressed to inside surface of tube sheet hole.

Hope this help.
PHOO

-Corrosion started at weld joint and pressure can leak to gab of tube to tube sheet.
-What material of tube and tube sheet, dose any heat treament done on the tube bundle.

Good Luck
Phoo

Ken, this exchanger is used a steam generator?
Ever noticed anormalities in the operation of the equipment, like running dry or something similar? Looks like there have been overheating and overpressure at the steam side (sheet).

The hx is used as a reboiler with methanol inside tube and steam on shell side. I think it's part of the distillation column.

Look like creep at tertiary stage (plastic deformation) due to lack of cooling by steam along the tubesheet. Some hx will have tube inserts to avoid this problem and refractory lining at the inlet tubesheet for high temp application. What is the inlet temperature of the methanol? What is the dia. of the tubesheet?

Is the location of the deformed tubes in the middle of the tubesheet and not all tubes are deformed?

Any inlet temp excussions during the operational period?

This message has been edited. Last edited by: Josh,

Check this:

First: Buckling inside is only happen due to the thin wall (corrosion).

1)
Maybe you have created a vacuum by shutting off the steam and blocking in the exchanger. Steam will condense and create vakuum.

Or

Maybe you have created an abnormal operating overpressure on the shellside by not shutting off the steam and blocking in the exchanger shellside.

Or

Normal higher pressure on shell side.

Is it known for sure that the indentations did not exist when the tube bundle was new?

I vote along with those who say it must have seen a large differential pressure (lower within the tubes). But I'm not a mechanical guy.

May be bi-metallic corrosion has caused the failure of the joint between tube and tube-sheet which resulted in buckling of the tube inwards due to weight of water flowing through it and its. velocity

My observatio is
1.looks like that the temperature had gone high due(tube side).
2.Thinning of tube due to corrossion at elevated temperaure.
3. Something like Gas formation on shell side(may be nascent hydrogen release)
the bucking is inside in all the cases as could be seen in the picture.
4. Has any alalysis been carried on steam quality,Where is the steam inlet in relation to the damage area shown in picture?