50# steam and HCN in a 316L reboiler

[myname_earl]

Hello,

I am quite new to API 571 and identifying DMs. I find it hard to put my finger on one exact damage mechanism when several sound relevant to the case.

Currently, we have just opened a reboiler that we run HCN @80F through the tubes and 50# steam on the shell. This is a vertical exchanger that had 3 tubes plugged in 2019. It was during regular operations and needed back in asap so there was not an inspection to identify the failure. It was assumed to be chloride cracking based on findings ~20 years ago on the prior reboiler. The plugged tubes are all local to each other and are located right along the steam inlet.

Reading past threads on chloride cracking it sounds like there is not a minimal amount of chlorides necessary to produce cracking since it can build up in stagnant areas. Our water is a 10-11 pH with low chlorates based on given data (not sure about historical water treatment).

My assumptions before getting NDE results are that we have have a flow impingement causing turbulence related damages or we have a stagnant area in the reboiler that is collecting chlorides in this local area.


Any guidance for damage mechanism expectations and identifying them would be super helpful for a newbie in the nitegrity profession

*EDIT* any recommended literature or training would be very much appreciated.

Mechanical Integrity Specialist (Year 1)

 

[EdStainless]

In impingement areas you have a lot of things happening.
Erosion, higher temps, vibration are the first ones that come to mind.
But since they didn't bother to look who knows.
The leaks in those tubes could be well away from the actual impingement area and could be related to pitting, cracking or some other issue.
At that pH CSCC is unlikely.
NDT if a good idea, but you need to pull a tube.
To both verify what the NDT says (I have stories about this) and to see things that your NDT method didn't pick up.

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P.E. Metallurgy, consulting work welcomed

 

[myname_earl]

Thanks Ed, we'll be performing eddy current here soon which will give us a better insight. We've talked about pulling tubes but apparently, that can be a big pain in the butt. Depending on how much time we can keep it open, I'll likely consider it. I've never seen eddy current readings and I'm not sure what information I can get from it, does it give me location of the defect, size of defect, orientation - things I doubt if it is like UT. Pulling a tube is definitely the best way to get what I need (we'll see after the readings come in) regardless of how painful it'll be.

Mechanical Integrity Specialist (Year 1)

 

[EdStainless]

No, plan ahead to pull a tube.
The call on indication size and depth could easily be off by 60%, either way.
It depends on how closely the artificial indications that they calibrate on match the actual size/shape of the real ones.
The only time that I would give you a pass on this is if all of the indications are directly in line with the inlet.
Then you know that it is impingement damage, and that there are other issues.

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P.E. Metallurgy, consulting work welcomed

 

[myname_earl]

Yep, went ahead after thinking about it and ordered it to be pulled.

You were a huge help and I can't thank you enough!

Mechanical Integrity Specialist (Year 1)

 

[myname_earl]

Ed,

The tubes are actually duplex S31803... I pulled a tube and did not find any OD damage. On the ID we found heavy pitting at the top ~8 inches of the vertical reboiler's tubes. This we believe is the area of the over sized reboiler that the HCN actually vaporizes/polymerizes.

My first assumption is under deposit corrosion because when we opened it, it was HEAVILY fouled. Another consideration was sulfuric acid corrosion. We use SO2 and H2SO4 to dilute the HCN so this sort of makes feasible sense. We send in about a quart of H2SO4 every 10 hours and have readings of 334 ppm of SO2 and 289 ppm of H2SO4 at our sample point which is prior to the distillation tower and reboiler. I do not know if this is enough to even worry about.. I figured if we had a sulfuric acid problem we would see it in the tower or our failure wouldn't take 20 years to develop.

Mechanical Integrity Specialist (Year 1)

 

[EdStainless]

Are they S32205 chemistry? It is richer which would help.
The other issue at the top is intermittent conditions.
I would suppose that the deposits are a big part of this, acid or not.

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P.E. Metallurgy, consulting work welcomed

 

[myname_earl]

kind of what I was thinking... Between the massive oversizing (something close to 3x too large) and our poor cleaning schedule, it's no surprise that we have heavy fouling and pitting. I wasn't able to witness the as found condition of the reboiler, but our assumption is the fouling is relative to the tubes we have pitting in. We actually found a design note from 40 years ago where they took a "pie cut" and covered 3/4 of the tubes due to the oversizing. Why they replaced in kind the size of the reboiler but changed the tubes from 316L to duplex is beyond me. I did find a change order calling out cracking in the tubes which makes sense for the duplex, but I cannot believe they didn't want to downsize this thing.

From the U-1 I can only get SA-789 S31803 so I do not believe so, although I'm unfamiliar with Duplex grades.

Mechanical Integrity Specialist (Year 1)

 

[EdStainless]

If it is that old then the S32205 chemistry wasn't likely in use.
This alloy is a workhorse. Like the name says it is duplex, a mixture of austenite and ferrite.
In well-made material the ferrite will be ~45% (typically speced as 35-60 or 40-55).
This balance optimizes the corrosion resistance of each phase and helps stabilize the structure.
The pitting resistance is easily double that of 316.
And it's CSCC resistance is an order of magnitude better.
You can chloride crack them, but I have only seen it at low pH and very high stresses.
Do not use it above 600F, intermetallic phases will form which hurt ductility and corrosion resistance.
Be careful about cold temperatures, the ferrite phase has a DBTT and I believe that Code limits you to -40.
That said I did some weld quals last year and we did the impacts at -60F just to show that we had margin.
These alloys are very strong compared to austenitics. When replacing 316 they should have reduced the wall thickness by at least 35%, if not more.
The duplex has lower thermal expansion and higher heat transfer than 316.
I have seen oversized heat exchangers choked on the outlet, either the entire outlet or individual tubes with 'washers' welded on the outlet ends.
SS like high flow so any choking usually leads to other issues.

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P.E. Metallurgy, consulting work welcomed

 

[myname_earl]

Hmmm, I'll have to look at the outlet washers/orifices for a fix until we replace the exchanger with a properly sized one.

On the bright side I don't have to change material. I think this is an easy process fix, both for cleaning PMs and correctly sizing the replacement whenever that comes.

Mechanical Integrity Specialist (Year 1)