Tower Metallurgy for MEA Stripper

[Patassa]

I'm not sure if there is an industry standard practice for this but a customer is wanting to replace their MEA tower with some type of stainless. The current tower has 316SS cladding on a carbon steel shell. The cladding was damaged and the MEA + CO2 + H20 = Carbonic Acid has attacked the carbon and cracked the head of the tower.

Estimate of process conditions are
220 F
12 psi

What grade of stainless should the new tower be built out of and roughly what thickness? Is there a standard on this somewhere that I can get my hands on?

Thanks

 

[Ripz]

Amines (MEA) and Carbonic Acid are both aggressive to Carbon Steels which would explain the eventual damage to the Shell. Carbon Steel being susceptible to both Amine Corrosion and Amine Stress Corrosion Cracking, the latter of which is likely your failure mechanism.

However, 300 Series Stainless Steels are extremely suitable for both Amine and Carbonic Acid service. The original damage your client got on their cladding may have been mechanical in nature possibly due to poor construction? Since the Amine and Carbonic acid is unlikely to have caused this. Thus 300 Series Stainless Steels should be suitable for your application. 304 SS or 316 SS.

The thickness would be based on your eventual calculations decided by your design pressure, design temperature and your choice of material type. You'll also need to determine the code of construction you'd like to use. ASME Section VIII Div 1 is widely used.

Also, and possibly most importantly, given the content of your post, I would strongly advise you consult with a mechanical engineer for this job.

 

[Ripz]

Final thoughts. Upgrading to a solid SS vessel, is costly and should have been the last thing your client should have done, as with proper engineer controls these failures could have been prevented.

That being said, what the client wants, the client gets, but if you are interested in knowing more about the failure mechanisms present and their mitigations, have a read of API 571 and API 945.

 

[EdStainless]

Clad construction has served well in these units for decades.
Picking the correct clad alloy and technology is critical.
Some locations in the unit require more protection than others.

You might look to see if there is any operational experience in building these in a duplex stainless.
This would greatly lower the amount of metal required (because they are high strength) and might allow the use of a more corrosion resistant alloy.

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Plymouth Tube

 

[Patassa]

Thanks for the information.
The cladding was damaged during a repair of the outside shell. Unfortunately the client instead of repairing the cladding is considering cutting off roughly half of the vessel from the top down and welding a new section of tower made from 321 SS to the carbon section below it without cladding. (the cladding is only on the top portion of the vessel). However they are also considering building a new tower instead of the dissimilar tower/weld option. I will research the duplex option.

 

[Ripz]

At 12 PSI, i don't think Duplex would prove too much of a cost saving. But the more critical factor is that it's relatively unproven against these two failure mechanisms. It may work, but hasn't been testing much in the industry. 321 should work.

You client did a repair on the outer shell, did they know they were supposed to PWHT the repair to prevent against Amine SCC?

 

[EdStainless]

And if they welded on the outside of an all stainless vessel they could well cause the same failure again.

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Plymouth Tube

 

[Patassa]

I think the issue was the heat from the weld on the outside of the CS shell buckled and cracked the cladding, however this went unnoticed because no internal inspection was done after the patch was installed.

 

[Ripz]

Welding on the outside of a SS vessel would not cause any failures.

The failures occurred by welding the CS which damaged the SS. Thus allowing the MEA to come into contact with the non-PWHT(assumption) CS which eventually resulted in Amine Stress Corrosion Cracking.

 

[EdStainless]

I have seen serious failures in SS from welding on the outside.
Either the inside was not cleaned well enough or the heat input was too high.
Sensitization, carburization, and sulfidation damage are all risks.

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Plymouth Tube

 

[Ripz]

I highly doubt this is mechanically related to the welding. Its an environmentally assisted SCC.