Chloride Stress Corrosion Cracking 1

[numberfive]

Experts,

I am being asked by a customer to fabricate two replacement tanks out of either 304 or 316 Gr. SS for a water demineralization skid for a boiler feed water application. The original vessels are constructed of SA-516 Gr. 70 and coated inside and out with a Plasite 7122 coating. These vessels have been in service for 20+ years without failure - it appears that the coating has now begun to fail and they are experiencing internal corrosion. Design pressure will be 125psi. @ 450 deg. F.

I'm not a corrosion engineer, but it seems to me that if we replace these vessels with an austenitic stainless, it is an almost sure bet that they will soon start to experience chloride stress corrosion cracking. I want to suggest to my customer to instead allow us to fabricate two replacement vessels out of the original carbon steel material and apply the coating per the original manufacturer - 20+ years of continuous service isn't bad.

Any input would be greatly appreciated. Agree??? Disagree???

numberfive

 

[metengr]

I would suggest you consider a duplex stainless steel. The higher strength materials will allow you to take advantage of thinner wall thickness. The coated carbon steel will work as you mentioned. However, I would perform a cost comparison for the client between what was installed and duplex stainless steel.

 

[metengr]

Forgot to provide a link to review

http://www.outokumpu.com/SiteCollectionDocuments/Duplex_for_Storage_Tanks-brochure.pdf

 

[numberfive]

metengr,

Thanks for the quick response. I will definitely look at a cost comparison.

 

[Guest102023]

You answered it yourself - "20+ years of continuous service isn't bad." Unless the client is seeking a brick outhouse, I don't see how SS can compete with that, and that is not even considering the chloride risk to the SS.

Perhaps all you need to do is update the coating (they have come a long way in 20 yr) and do periodic out-of-service inspections.


"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]

Do these tanks hold the supply water or the demin water?
If the demin then the chloride levels should be very low.

As much as I like SS I would echo some of what has been said.
Look either at S32205 duplex stainless, or carbons steel with a modern coating inside of it.
There is no guessing which will cost less, or be easier to build. It could be a tossup.


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Plymouth Tube

 

[numberfive]

The print that I have refers to them as "Carbon Filter Vessels". They each have a false bottom on the inside and are partially filled with carbon grains or pebbles. I believe that untreated supply water is injected from the top side, it filters down through the carbon pebbles and is then drawn off the bottom. I'm not sure what the carbon filters out of the water - minerals possibly??? Would filtration through carbon lower the chloride levels of the water? Either way I do believe that these vessels are exposed to untreated supply water.

 

[moltenmetal]

No, carbon won't remove chloride from raw water.

If it is for raw water purification, irrespective of the design temperature, the service temperature is unlikely to be above 60 C and the pressure is likely to be modest. Accordingly your risk of chloride SCC in these tanks is likely quite low, assuming you use good fabrication practice for the 316SS tanks.

That said, it's also an easy service for lined carbon steel, which has been proven in service. Which will be cheaper? Probably the stainless, but that depends on lots of stuff you haven't mentioned.

 

[CoryPad]

Activated carbon is the standard method for chlorine removal from water.

 

[EdStainless]

You need to know the actual service temps for the tanks.
If they are for chlorine removal then they will operate near room temp.

If you go the SS route then making them from 2205 will probably cost less than 316L because they will be ~50% of the wall thickness. Much less metal and much easier to fabricate.
You would also get better corrosion resistance and some reasonable SCC resistance.

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Plymouth Tube

 

[moltenmetal]

Ed- good idea in general, except that these tanks are 125 psig, so unless they're giant, the shells and heads are probably at minimum recommended ASME thickness already. Savings for use of duplex come into play when you have thicker shells, flanges etc. where bulk material cost overwhelms fab cost. I don't build big,heavy stuff, so I've yet to encounter a situation where switching to duplex from 316 has saved money up front. In terms of lifetime costs- that's another matter entirely, as the duplexes are greatly more resistant to many corrosion failure modes.

Carbon is used for all purposes at a temperature as low as possible, because low temperatures increase the sorption capacity of everything that you'd want to adsorb on carbon.

Just in case some are confused, CHLORINE and CHLORIDE are NOT the same thing. Carbon removes CHLORINE, and does NOT remove CHLORIDE whatsoever. Chloride ion will elute with the water. In fact, it is likely that at least some of the chlorine in the feedwater will react with the carbon to produce chloride ion rather than organochlorine species. However, feedwater free chlorine levels are really low- less than 1 ppm- much smaller than the amount of chloride ion naturally found in most feedwaters. The chlorine levels are low enough to not represent much of a worry from a stress corrosion cracking perspective in the raw water itself (eg the OP's feedwater carbon tanks). That chlorine CAN generate a worrisome amount of chloride though if the downstream process used can concentrate that tiny amount of chloride many-fold over time - e.g. boiling with inadequate blowdown- hence the need for removal.

 

[racookpe1978]

Could you keep the original tanks, open them up and sandblast hell out the original-but-failing-liner, and spray-pro (or the equivalent automatic weldin+deposition method) a "new" liner over what's left over from the existing?

Seems like that would save some oney - at the expense of field work working inside a tank.

 

[numberfive]

Thanks for all the great posts!

I discussed the stated concerns with my customer - he much appreciated the insight. We are going to quote two new coated carbon steel vessels and two duplex stainless vessels as another alternative - as always the final decision will come down to $$$.

Thanks again!