Corrosion of stainless steel heat ex tubes in brine 4

[SL1000]

Can anyone explain the basic problem with the above to me and outline methods of prevention. Steam is used to heat saturated brine at around 100'C.
Is another material the answer?

We have experienced severe pitting (clean pin holes with no apparent pattern leaving the rest of the tube in perfect condition)in our tubes, what is confusing is that one set of tubes lasted 2 months and the replacements (same spec) lasted 24hours! Also only a few of the tubes are affected.

And why does velocity matter? i.e. above 3-5m/s less corrosion.
Would decoupling/insulating the unit from the general mass of earth have any positive effect- that is, would the driving potential be reduced?

I think copper nickel tubes with duplex stainless inserts have been used in the past, however we use similar units on potash (slurry) duty and abrasion becomes a problem (I think this is the reason for the duplex inserts).

I welcome your comments in an attempt to improve my understanding of this situation.
Thanks
Simon

 

[hacksaw]

Are you cleaning and passivating the tubes after assembly?

 

[Metalguy]

Which stainless are you using? With high Cl, stagnation can be a killer, and what you describe sounds just like Cl pitting.

I think a 6% Mo stainless (AL6SN or 254SMO) might be able to cope with this. McGuire?

 

[stanier]

Nalco (now part of Ondeo) have some very good books on failure in such environments. These books would give you agreater understanding of your problems.

 

[TEV]

What you are describing sounds like classical chloride pitting corrosion. Your environment is very severe for this effect. Pitting in stainless steel initiates at discrete sites and is self propogating at that site, leaving other areas apparently unaffected. The other areas are actually galvanically noble with respect to the pit.

The reasons for the difference in performance of two different samples could be due to a number of causes:

1. Product cleaning and passivation. If the stainless has residues from manufacture and installation, and if it is not passivated, it will be more subject to chloride pitting. Some tubing also has drawn-in contamination on and adjacent to the surfaces.

2. Composition. Resistance to chloride pitting in austenitic stainlesses (300 series) increases with Molybdenum content. The progression is from poor resistance to good resistance: 304 - 0% Mo, 316 - 2% Mo, 317 - 3% Mo, 904L - 4% Mo, AL6XN or 254SMO - 6% Mo, 654SMO - 7% Mo. A duplex stainless steel such as 2507 may be the real answer to your problem. It has exceptional chloride pitting resistance with higher hardness. See McGuire's write-up on this.

3. Second phases in stainless. Sulfur in stainless forms a precipitate with Managnese; the precipitates are known sites for initiation of pitting corrosion. Tubing generally available in the marketplace is now in the range of 0.005 to 0.017 percent Sulfur. This is high enough to cause enhanced susceptability to pitting. Occasionally you will find stainless with higher or lower Sulfur levels, with consequent lower or higher pitting resistance. The Sulfur level has a profound effect on welding, thus there is an attempt to control it within fairly narrow limits. Other second phases, generally oxides or silicates, may also increase pitting.

My recommendation is to take a serious look at 2507 duplex.

 

[SL1000]

Many thanks for your comments, I believe the tubes used were passivated and that the material was 2205 stainless but I would need to speak with a colleague to confirm this.

We do seem to go through these things at an astonishing rate which really puts our condensate dumping system to the test (not to mention the cost and upset). The reason for heating the slurry- saturated brine (NaCL),insolubles and the KCL is to promote the KCL to the liquid phase prior to a separation and filtration process.

I can mention some of these issue to the tube manufacturer and hopefully have a more useful dialogue to move this forward.

Thanks
Simon

 

[PVRV]

SL1000,

As previously outlined Austenics are not suitable for
chloride envirnoment. Neither is Super Duplex / Duplex
at a high service temperature.

Kindly take a look at nickel alloys. As a potential over kill inconel / Hastelloy C276. (<--- We are considering the latter since we have Brine at high temperature + H2S)

Refer to ASM, Volume 13.

Regards

 

[SL1000]

Thanks PVRV
how would these nickel alloy tubes cope with abrasion compared to the duplex? Is titanium an option on this type of duty?

Our requirements have changed somewhat in terms of the original design with respect to required heat transfer i.e. the rise in brine temperature required is only of the order of 15deg C due to a heat recovery sytem that preheats the brine which was put into operation 4 years ago, we are considering reworking the heat exchanger to half the size in order to half the material costs. I have reservations about this, since if things are bad now, correct me if I am wrong but for the same rise in process temperature a shorter tube bundle with require a higher driving temperature (at a given velocity) and from what I have read in this forum a higher temperature could mean more corrosion ..

Also I have read that stagnation can be a big factor - is this generally associated with the initiation of the pitting effect? Is there an upper velocity at which the passivation film of the tube is affected? If the above is true is there a sensible velocity (and temp)band of operation for a given application?

Simon

 

[PVRV]

SL1000,

Sorry, I did not read your post carefully.


1. Did you consider, improving the pre-heat stage to get
that 15 C ?

What is the pre-heat stage (PHE's? if so are you
utilizing titanim?)


2. What is the steam composition?

3. Correct me please, tube-side steam at what temperature?
, shell-side brine T -->>to 100 C

The tubes are developing severe pitting, Where? inside or the outside ?

4. Whats the shell made of / nozzles, any indications
there ?

5. Any problems with the parition plates (head)?


Regards,

 

[SL1000]

1. Did you consider, improving the pre-heat stage to get
that 15 C ?
What is the pre-heat stage (PHE's? if so are you
utilizing titanim?)

ANS- We have two heat recovery units that utilise exhaust gases to preheat the slurry and we did try to run them in series at one point but experienced a number of problems, hence they are ran in parallel. The slurry is ejected from nozzles down a column that the gases are flowing up.

2. What is the steam composition?
ANS- Not sure what you mean, but it is generated from a 23MW CHP waste heat boiler at 200psig. Upstream of the heat exchanger temperature control valve the steam temp is around 140-145deg C at 45psig. (I think this is superheated?)

3. Correct me please, tube-side steam at what temperature?
, shell-side brine T -->>to 100 C
ANS- As above steam. The exit product temp is controlled at 80deg C or there abouts.

The tubes are developing severe pitting, Where? inside or the outside ?
ANS- I think this is from the inside (brine side) but the tubes are holed right through.

4. Whats the shell made of / nozzles, any indications
there ?
ANS Shell is carbon Steel I think can find out if this is important. No indication, but history tells me that in the past typically pitting at the top is the worst with only a few pits lower down i.e. worse where the steam comes in, and that wear occurs at the bottom where the product is introduced.

5. Any problems with the parition plates (head)?
ANS- Dont think so


Many Thanks
Simon

 

[PVRV]

Simon,

Please examine the questions & your answers carefully, in view of the following:


1. If the pitting is caused by the &quot;brine&quot; wonder why the
shell is not suffering the same.

2. If the pitting corrosion is caused by the &quot;steam&quot;
wonder why the partion plates are not being affected.

Issues to consider are:

a) Avoid the use of dis-similar metals to obviate galvanic
corrosion. Its not easy to change tube/shell material
of an exchanger in service for a specific stream.

b) If the pitting is caused by the steam, perhaps the
manipulating the process else adquate treatment.


Regards

 

[MEKANIKEREN]

If the problem is on the brine side, cathodic protection may be an answer. Check out the January number of NACE Materials Performance on Cathodic Protection.

 

[Metalguy]

The shell doesn't pit much because it isn't stainless. While Cl can corrode carbon steel, you usually don't get the king of sharp pitting that SL 1000 described.

 

[suzuki]

Why not a fluoropolymer tube ???, we design and fabricate Shell&Tube heat exchangers with PVDF, our tubes are 12mm dia. the shell can also be the same fluropolymer material using Dual_laminate construction, have many applications.

http://www.viatec.com

 

[imacid]

thank you for your replay,
is high pressure steam with about 0.08%NaOH able to create a conditions for scc?
could you please suggest some evidences or bibliograhies?


A.NADIFI