Alloy Selection for HX tubes in Methyl Chloride Service

[Guest102023]

We are looking at upgrading the metallurgy of a reboiler exchanger. Tubeside process fluid contains 97% Isobutene, 2.5% Isoprene with sprinkings of C2~C6 hydrocarbons (-enes, -ynes, -anes). It also contains methyl chloride (MeCl) at nominally 0.03% and t-butyl chloride at nominally 0.02%. Chloride ion content is 300 ppm. MeCl content can spike up to 10% though. No mention of moisture but we know from experience it is impossible to eradicate. There have been tube fouling issues in the past, but these have been mitigated by addition of chemical treatment, but which has only accelerated corrosion of the carbon steel. Steam is injected on the shell side @ 140°C.

SS317 is being proposed, but I have concerns about SCC, since tube design temperature is smack in the chloride SCC temp range, 80°C.
My initial thoughts are duplex SS, Alloy 20, and Monel; possibly even super-ferritic, although I have little experience with those. Thermal expansion could be an issue if the SS316 shell differs significantly in CTE from the tubes (I have seen severe distortion in small, rigid stainless HXs, even when tube/shell metallurgy was the same, caused by steam injection localized on one side/end).

Looking for thoughts/recommendations from the HX/tubing experts here.

Thanks in advance.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

At the low temp that you run a superferritic.
Here you have good corrosion resistance, good heat transfer, and CTE more similar to steel.
Remember to redesign the wall thickness, account for higher strength and no corrosion allowance.
At 1" OD we make SeaCure tube in walls from 0.020" to 0.083".

My second choice would be a duplex. They aren't immune to CSCC, but they are highly resistant. It might work well, and 2205 would be more available than SeaCure (but we would love to make it for you).

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

Thanks Ed,
I should have noted the shell and T/S are being upgraded to SS316 (as it now stands, but that may change).
The tube size is 1"-12ga, which seems excessively thick for 12 bar design pressure. When I spend good money on a corrosion resistant alloy I expect to be able to apply a corrosion allowance of zero.
I didn't quite comprehend your first sentence.
What about Alloy 20? Having very high Ni it is pretty resistant to CSCC. Client has experience with that; it was misapplied to severe chloride/temperature conditions where it pitted badly. It did not crack but a C alloy is more suited to those conditions.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

Super ferritics have a temperature limit of about 600F, so they would be suitable for your application. This rules them out of many process applications.
Alloy 20 is really useful in sulfuric acid, but not a good option in other service. Lower heat transfer, higher CTE, expensive, and mediocre pitting resistance. Yes I have seen people use 20 when they really needed a 6%Mo superaustenitic or a C alloy.
A 6%Mo alloy (AL-6XN) might give you the corrosion resistance that you need, but it will have high CTE and it will cost more than a superferritic.
We have put SeaCure into many process applications over the last 40 years. This sounds like a good option here.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Duplex stainless steel.

 

[Guest102023]

Thanks guys, useful information.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Guest102023]

Please rank the following alloys according to increasing cost. Product is seamless tube, 1" x 14Ga. My guesstimate is:

1 - 317L
1 - 2205 Duplex (~equal)
3 - Superferritic (Seacure or AL 29-4)
4 - 2507 Superduplex
5 - Monel 400

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

First, there is no reason to use seamless, a good quality welded tube is at least as good if not better.

I would guess that cost would be:
2205
SeaCure ~ 317 (availability of 317 bis a real issue)
2507
400

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

Ed,

There are some alloy tubes I would never buy welded, especially duplex. It depends on the service, though; if the alloy is very resistant to the corrodent then welded tube could be justified.
This week I am working on an unrelated failure investigation where a 304 tube long seam was badly arc welded - it looked perfect from the outside!
(As a welding engineer, I preach that the less welding you can do, the better.)
.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

There is a big difference between tubing welded to size and in-line annealed (unless it is a ferritic, then this works fine) and tubing that was made correctly. Tubing that has been welded, cold drawn, furnace annealed will almost always outperform seamless tubing.
The only two cases that I can think of where seamless is usually superior are HF service and creep limited applications in tubes that were not welded and then cold drawn.

Seamless tubes have a huge variation in wall thickness (often the entire tolerance in a single length) and the collection of ID surface defects in them are frightening.For example we make W&D that gets UTed to tighter specs than any smls tube can meet.
Of course the ferritics and superferritics can't be made seamless, and many alloys (6%Mo SA) are only available as sheet so smls isn't an option.

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

EdS,
Thanks for the useful information from an insider.
Unfortunately there's a wild card - purchasing departments incentivised to cut procurement costs (read: Chinese sourcing).

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[TVP]

brimstoner,

Just to give you a non-biased opinion, I completely agree with Ed regarding properly produced welded and drawn tube vs. seamless. I understand your apprehension regarding low cost tubes with dubious quality, but there is no question that welded & drawn tube will be more consistent dimensionally, and can be inspected using multiple techniques to assure weld quality and overall uniformity. Unless the wall thickness to OD ratio necessitates seamless, I would always go with W&D.

 

[EdStainless]

Brimstoner, That is what QVL's are for. Some items need sourcing restrictions.
I had a PA trying to source something outside of normal channels, I asked his boss if they included the $60k for audit and source inspection in the purchase price. We went back to our usual supplier.

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

I take all your comments on board, because I will be busy specifying CRA tubes in the next little while.
A client of mine a few years back had a fiasco with some 2205 welded but not subsequently heat treated tube that basically unzipped along the seam. My takeaway: the entire tube needs to have the minimum corrosion resistance for the service.
As the saying goes (after translation): buying cheap is buying expensive. On the other hand, paying too much is not sound engineering.
I appreciate all your feedback guys.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"