Lean Amine - maximum recommended Chloride concentration to avoid ClSCC on heat exchanger tubes 6

[rscosta1976]

Dear all,

It is known that in an aqueous environment with neutral pH, 304L tubes are susceptible to ClSCC at temperatures above 150 F, even on the presence of a few ppm of chlorides. The risk increase with temperature as expected.

In amine service and considering a high pH (for the solution), what is the actual limit of chlorides on the lean amine solution that is allowable for 304L tubes operating at 220F? Is the limit the same for 316L operating at the same temperate?

How about for 304L and 316L components operating with lean amine solution at 360F (amine reclaimer)? What is the actual limit for lean amine chlorides to effectively prevent ClSCC?

I've been searching on the literature and I found different answers, ranging from a few ppm to 4,000 ppm, so I'm very confused.


Thank you.

 

[EdStainless]

That is because it depends.
If there is any free sulfur, or any other halides, and stresses are high, or there are occasional pH drops, then a few ppm sounds about right.
If temperature cycling and stresses are low, and the pH is always high, and the solution is clean, then pitting will be the limit and not CSCC, and a couple thousand ppm should be realistic.

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P.E. Metallurgy, Plymouth Tube

 

[rscosta1976]

Thank you very much for your answer EdStainless.

I'm analyzing a specific case of an amine (DGA) reclaimer with carbon steel tubes. At the current operating temperature (360-370 F), the bundle is experiencing frequent failures (with in kind retubing on 2012 and 2014) due to severe corrosion on the amine side. Lean amine sampling reveled 200 ppm of chlorides (with a few excursions up to 3,000 ppm) and more than 1%wt HSAS.

Client is asking if it is OK to upgrade the tube bundles to AISI 316L with having the risk of ClSCC. I believe there is some risk of ClSCC for 316L and I was considering to recommend 904L or better (Alloy 28). Please share your thoughts and experience.

Regards,

 

[EdStainless]

How high is the pH? and how low does it drop?
Is this unit used for H2S removal, or CO2?
Was this plant designed to run at this temp? It sure seems high...
Are you within the design gas loading and fluid velocities?
What is the amino-acid (bicine) concentration?

I ask because I suspect that this unit is being operated outside of its design region.
You need a real good picture of what is being done with it today, so that you can make an informed decision.

I have seen 316L used in high velocity regions and 825 used in high temp regions.

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P.E. Metallurgy, Plymouth Tube

 

[rscosta1976]

Thank you for your answer.

The measure pH for lean amine is between 8 and 9, however, I'm concerned that it may drop on the reclaimer due to probable concentration of chlorides, HSAS and other contaminants. Unfortunately, I have no way to confirm this.

Yes the equipment is being operated within the design temperature & pressure and amine circulation rate, however, I have no information about design date for allowable chloride, nor, HSAS, etc.

Regarding the bicine, I'm afraid I don't have an answer. Does it form for DGA? I only know the total HSAS is high than 1%wt (last time it was made - 2014). This analysis is not performed at this plant, since there's no ability to do it at the lab. I don't have the details of it.

Having all these questions without answers, is it possible to have any rule of thumbs for the Chloride limit for 316L in order to be safe?


I apologize for all the question and I truly appreciate your expert thoughts.


Kind regards,

 

[georgeverghese]

If you want some additional assurance, lean or standard duplex 2205 may be more cost effective.

 

[EmmanuelTop]

See page 17 of the document

http://www.amine-gas-treatment.com/dbimg/1142527275.pdf

- it refers specifically to DGA systems and stainless steel.

"Experiments reported by Seubert and Wallace indicate little or no pitting tendencies with 304 SS exposed to DGA solutions containing up to 4000 ppm chloride. Based on these experiments, the maximum acceptable chloride level for DGA plants containing type 304 SS was set at 1000 ppm."

Dejan IVANOVIC
Process Engineer, MSChE

 

[EdStainless]

EmanuelTop, That link goes on to explain why that was the case for this unit. As posted above there are many cases of 100ppm causing serious CSCC issues in similar service.
It is all in the details. I have seen these units rebuilt in 825 just for this reason.
Though in most cases I would think that 2205 would be a very good alternative since it has good pitting and CSCC resistance.

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P.E. Metallurgy, Plymouth Tube

 

[EmmanuelTop]

I have operated 5 and designed 2 Amine units, some of them with extremely high H2S and/or CO2 loading and I don't recall anything more exotic than 304L or 316L (clad only) was used. But again, I fully agree with you - the devil is in the details so the OP should know to which exact conditions his original query refers to.

Dejan IVANOVIC
Process Engineer, MSChE

 

[EdStainless]

I have seen thee units run for decades with 304. I have also see ones that tried 4 different alloys before they managed to get it working correctly.

These days you would pick a lean duplex or 2205 over 316 any day. The times to choose 316 is if you are below -40F or above 600F. When Ni and Mo prices rise again the lean duplex grades will push 316 to the curb.

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P.E. Metallurgy, Plymouth Tube

 

[Chumpes]

dear sanchern
there should be no SCC or pitting of SS 304 and 316 in amine systems up to around 3000ppm chlorides, and chlorides concentration should be controlled with an alert level of 500ppm that includes a safety margin.

 

[rscosta1976]

Dear Chumpes,

I see your point. I believe the 500ppm limit is good for the normal amine pH and I agree that 316L is usually suitable in most amine circuits. However I'm afraid of the conditions on the reclaimer, the pH might drop considerably especially if local evaporation occurs on some tubes. In this case the pH might drop and increase the risk of ClSCC for 304L / 316L, so I was asking if on the particular case of the reclaimer, a higher alloy should be used.



Thanks,

 

[Chumpes]

I am not aware of any failure of 316 in lean/rich amine systems, also I am not aware of any amine unit working with higher metallurgy than 316 for the reclaimer, but EdStainless has just told that 825 has already been used.
I still think that 316 is a good choice for amine systems.

 

[TomEun]

The threshold chloride content against pitting or SCC for stainless steels in amine solutions is higher than that in neutral seawater or acidic environment because the pH values in amine solution is normally 8.5 to 10.5 except the OVHD or reboiler of amine regenerator.

304L SS (chloride< 1000 ppm) or 316L SS (chloride< 1000-4000 ppm) in amine solution may be a good choice in accordance with the amine type, temperature, pH (> 8.0), velocity (< 60 fps), and corrosion allowance in normal operation. However, in the consideration of start-up, shutdown, and upset conditions, when the chloride in the amine solution is over 1000 ppm, 2205 DSS may be a better choice. The stainless steel clad or weld overlay on CS may be acceptable except tubes. Some corrosion allowance of the stainless steel tubes may or may not be required per the severity.

Thomas Eun
Corrosion and Materials Selection/Design Specialist/ P.E.

 

[rscosta1976]

Thank you for your answers. I really appreciate it.

I found some answers to be aligned with my initial thoughts (TomEun and EdStainless). I honestly believe that in the conditions verified in the reclaimer, the pH might drop below 8.0 if you allow evaporation to occur in the bundle as a result of a poor liquid level control. This was the main reason why I was asking the chloride threshold for this specific case. I'm OK with 316L for reboiler, however for reclaimer I'm aiming to have a higher alloy to better resist ClSCC.

Again thank you for your input.

Kind regards,