Amine Reboiler tube failure

[Rgratton]

I have a 3 year old amine reboiler(kettle type) that experienced a significant tube failure. When the scope of the failure was realized we pulled the bundle for a better look. What we found was on the 5th tube baffle(bundle is approx 20ft long 5 baffles) the tubes were fretted in the baffle holes. The holes in the baffles were originally 1/64" oversized for tube fitment but were now approx 1/4" to 3/8" oversized from whatever mechanism was occuring. We have approx 40% of our tubes damaged.

We persued Flow induced vibration as our operating parameters changed when we switched from DEA to MDEA, but our modelling showed that FIV was not a factor.

Our amine sampling shows our amine is within spec, and that our CO2 levels are less than .002%wt

Possible fabrication errors, Erosion/Corrosion?

Here's some pictures as to what were seeing.

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[Rgratton]

More pics

 

[Rgratton]

more

 

[EdStainless]

What is material of construction?
sure looks like erosion corrosion to me.

Is this the 'hot' end of the exchanger?

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Plymouth Tube

 

[sk10009]

Rgratton!

That there are problems between tubes and baffles is erosion by vibration and if you see the picture 0831, you know that there is also problem(erosion) on the tubes.

I guess the reason they have problems is because you have not operated exchanger with tube bubdle submerged in the service.

If it is not submerged in the servece,tubes and shell over service is easy to get problem like above problems you have.
especially the boundary area is severe.

 

[sk10009]

picture 0831

 

[TomEun]

There is no doubt about erosion due to vibration. Please try to increase the number of baffles with running of HTRI or HTFS. I believe the amine is not a primary corrosion mechanism of the fretting.

Thomas Eun

 

[EdStainless]

This really looks like tubes running dry. What methods are used to make sure that the unit always full of fluid?

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Plymouth Tube

 

[Rgratton]

The tube material is SA-179

The bundle is kept submerged by a wier.

Also the damage was all noted at the final baffle near the U-Bend section of the bundle. There was some damage to the second last baffle similar but not as severe.

 

[EdStainless]

If this is u-tube then the tube side is two pass, is the shell side single pass? Is the outlet on the u-bend end?
These tubes look like they were boiling dry.

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Plymouth Tube

 

[0707]

I think your localized thinning on the last two baffles near the U-Bend section of the bundle is due to process stream velocity and turbulence. Considering that your corrosion problem was due to vibration all the baffles would reveal the same pattern which is not the case.

Please take note of the following:

Corrosion depends on design and operating practices, the type of amine, amine concentration, contaminants, temperature and velocity.

Amine corrosion is very closely tied to the operation of the unit. With a few exceptions, carbon steel is suitable for most components in a properly designed and operated unit. Most problems can be traced to faulty design, poor operating practices or solution contamination.

Corrosion is also dependent on the type of amine used. In general, alkanolamine systems can be rated in order of aggressiveness from most to least as follows: monoethanolamine (MEA), diglycolamine (DGA), diisopropylamine (DIPA), diethanolamine (DEA), and methyldiethanolamine (MDEA).

Lean amine solutions are generally not corrosive because they have either low conductivity and or high pH. However, an excessive accumulation of heat stable amine salts (HSAS) above about 2%, depending on the amine, can significantly increase corrosion rates.

Ammonia, H2S and HCN accelerate corrosion in the regenerator overhead condenser and outlet piping as well as reflux piping, valves and pumps.

Corrosion rates increase with increasing temperature, particularly in rich amine service. Temperatures above about 220oF (104oC) can result in acid gas flashing and severe localized corrosion if the pressure drop is high enough.

Process stream velocity will influence the amine corrosion rate and nature of attack. Corrosion is generally uniform however high velocities and turbulence will cause localized thickness losses. For carbon steel, common velocity limits are generally limited to 3 to 6 fps for rich amine and about 20 fps for lean amine.

The regenerator reboiler and the regenerator are areas where the temperature and turbulence of the amine stream are the highest and can cause significant corrosion problems.

Thinning will be uniform in nature when the process stream velocity is low while it will be localized for high velocities associated with turbulence.

Affected Materials,Primarily carbon steel. 300 Series SS are highly resistant.


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