erosion of stainless steel piping with steam and oxygen 2

[frostrobn]

There is a process in my facility that uses live steam (160 psig saturated) mixed with oxygen which is piped to the process in 316L stainless steel piping. The piping just after the point of oxygen injection in the steam line is eroding rapidly. The line has to be patched every 3 months or less. Does anyone have any experience with this problem? The piping upstream of the oxygen injection point is fine, it's just the pipe after the introduction of oxygen that is troublesome. Other information: The line is under high frequency vibration caused by injecting live steam into the process a few feet after the oxygen injection point. I have pics if anyone is interested. Question: Would 2205 (duplex) stainless steel be a better metallurgy in this situation? Any response would be greatly appreciated. One response I've received pointed at Moly as an alloy in 316L as the culprit due to rapid oxidation due to the oxygen and high temps. Any ideas?

 

[frostrobn]

Corrosion may actually be the issue. Could the presence of molybdenum in 316L present an issue in high temp oxygen and steam?

 

[metengr]

It does in power boiler secondary superheater tubing. There have been cases where the tubing will exfoliate on the ID surface ans result in plugged SH tubes. Preferred choice is either 304H or 347H.

 

[EdStainless]

No, I don't think so. This temp isn't high enough. I suspect that there is some condensation at the injection point and that erosion is the issue.
2205 would help a lot. The other thing that you need to do is to inject the oxygen into the center-line of the pipe and to assure mixing.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[unclesyd]

We had a similar problem except a little hotter 300# steam into a 316 SS System. We lost the line but it was attributable to cavitation and differential thermal fatigue.
If my memory serves me well I think the problem was resolved with the following piece of equipment from Komax.

http://www.komax.com/det-gasliquidcontacting.html

 

[frostrobn]

Is 500 Degf. high enough to cause oxidation? I am not convinced that the desuperheater is working properly. I will verify today by measuring the steam line temp at the oxygen injection point. Now that I have inspected the steam line closer I would say the damage looks like corrosion. In fact the 316L pipe looks like plain carbon steel with heavy scale. I have not seen any areas of "washing". There was a through wall pit in a weld. Does welding concentrate moly in the heat affected zone?

 

[Robertmet]

500 F is not very high for stainless to handle. If the damage is really 'erosion' this is what you find in steam turbines when you don't have dry steam - water droplets erode.

 

[frostrobn]

I am not seeing wet steam erosion. I am trying to establish if the mixture of oxygen and steam at 370-500 Degf. is causing a corrosion problem with 316L. When I first posted this topic I was unsure if it was erosion because I had not seen the interior of the pipe yet. It clearly is not erosion from what I have observed. I'm wondering if the addition of oxygen is the problem with this material.

 

[strider6]

SS could became active if there is a powerful oxydant loose its passive state and corrode, it could became a corrosion-erosion . Maybe this is your case. It would be necessary to make a failure analysis.

S.

http://www.corrosionist.com

 

[EdStainless]

The highly oxidizing conditions would tend to keep the material passive.
I like the thought of thermal fatigue if you are sure that you have no liquid phase.

Your injection/mixing quill should be made from 625 or a similar alloy. And it needs to extend a ways right down the center line of the pipe. A series of small slots will help you get even distribution.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[strider6]

sorry but, if the oxydant "power", concentration of oxydant species, is high enough you will go above the transpassive potential of the SS, SS has an active-passive behavior, and corrode very fast. see for example Fontana-Green "Corrosion Engineering- Effect of Oxidizers".

regards

S.

http://www.corrosionist.com

 

[hacksaw]

okay, was un-aware that the posting got moved from piping

oxygen is not the problem at these temperatures, if anything it helps. were you using carbon steel, different story

one problem that carries over from the piping forum is the possibility of erosion due to incomplete vaporization in the desuperheater

by the way what you are desuperheating with
BFW?

 

[frostrobn]

We are desuperheating with boiler condensate about 30 ft upstream of the oxygen injection point in the line. I am skeptical of the possiblility of erosion since I am not seeing any area of "washing" or areas of clean metal, not even on the outside radius of elbows. The interior of the 316L pipe has the striking appearance of heavily corroded carbon steel. This is only happening after the oxygen injection point in the steam line.

Any other ideas?

Thanks for everyone's responses. It has been very interesting.

 

[frostrobn]

Sorry, correction: The desuperheating water is treated water which could in fact contain some clorides from the sodium hypochlorite used to kill the bugs. I wonder how much residual (ppm) of clorides is too much?

 

[hacksaw]

at that temperature none, but certainly less than 5 ppb

no wonder you are having problems

 

[EdStainless]

Even ppb would be too much Cl, since all of the Cl would be in the very small amount of moisture.
Actually I am a little surprised that this isn't a CSCC thread considering your conditions.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[strider6]

if the steel has the aspect of a corroded carbon steel, it seems like it has lost its passive state, the protective oxyde layer. since it's after the oxygen injection, the only think that i can see is that the oxygen "remove" the protective oxyde layer.

http://www.corrosionist.com

 

[hacksaw]

unlike the exfoliation mentioned earlier in the responses, what you are dealing with is ongoing loss of the oxide layer and its regrowth leading to material loss. High fluid velocity makes it worse.

you need to be using dearated desup water, and definitely with the chorides removed, you need something along the lines of Hast. C

Look carefully at what you are doing process wise. For example, if the chlorides are a part of the process, higher grades materials may be required depending one the reactions going on.

Do you really need to desuperheat? Have you explored alternative desup methods?

 

[frostrobn]

Getting back to you all. I was on vacation. We are going to take the desuperheat off as a trial. All your responses have been very educatonal. Thanks to all.

 

[Metalguy]

"Does welding concentrate moly in the heat affected zone?"

Not at all, but it does concentrate in the ferrite phase of the weld metal itself. That could decrease the oxid. resistance. However, the Cr also partitions to the ferrite, and that's where the oxid. resistance comes from. The aust. phase of the weld metal has less oxid. resistance.

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