Pure Water Corrosion

[lcmechengr]

I have an RO (Reverse Osmosis) skid that will be installed to treat boiler feed water. The water after this treatment skid will be deiozized with a PH of about 5 - 6. What are my piping material options for this service? Is there a threshold for the percentage of deionization were carbon steel can be used?

 

[TVP]

Temperature?

 

[lcmechengr]

The temperature will be 100 deg F to 500 deg F.

 

[EdStainless]

Have you considered either 430 or 2205? You will pick up a lot of Fe from CS. I would worry about external CSCC with a 3xx SS. And from a cost standpoint I can't see using a higher alloy austenitic. I am assume that the oxygen will be very low.
Your pH seems low, is this normal for this service?

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[moltenmetal]

You're feeding a carbon steel boiler, correct?

Is this water that's been deionized but not yet deoxygenated? If so, 316L SS will be fine, provided you insulate and keep chloride away from the surface to mitigate the external CSCC risk EdStainless talks about. Don't forget about the chloride in the insulation...

If it's been deoxygenated and the inhibitors added, carbon steel will either be fine, or your boiler won't be!

 

[lcmechengr]

Thanks for the respones. Do you know of a good reference for metal selection based off of fluid services?

 

[EdStainless]

I have seen cases where the water was deoxygenated, but didn't have inhibitors added yet, that is what I was worried about.

If it is DI, but not deox, then I would still use 2205. It will not cost more than 316L and it will offer CSCC resistance.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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[moltenmetal]

EdStainless: with alloying metals prices as high as they are now, maybe you're right- I don't know. But 316L is still king for both availability and ease of welding to maintain the desired metallurgy. I've never seen small-bore 2205 piping be anything CLOSE to as inexpensive in terms of installed cost as 316L. Usually it's at least double and often greatly more than that, especially once you consider valves and instruments in the mix. More CSCC resistance, true, but likely for more money.

 

[EdStainless]

All that I can tell you is that today the metal cost (base + surcharge) for 316L and 2205 are equal.
You are looking at using lighter wall in 2205 right? The yield strength is double that of 316L. It is so much stronger, more resistant to pitting and crevice corrosion, and it has useful resistance to CSCC.
I would still use 316L for the the valves and instruments.
I have seen many systems installed in welded 2205 and there is no basis for saying that it is more difficult to weld.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[stanweld]

Before deox, 304(L), 316(L), 430 and 2205 have all been used for this system. 430 or 2205 is used when the temperature exceeds about 120 F (depends on specific Engineering organization or Owner preference).

 

[moltenmetal]

Having tried, and failed, to obtain post-weld ferrite contents of an acceptable level in small-bore butt-welded 2205 piping, I beg to differ with you EdStainless. It takes serious SKILL and careful control of the welding process to deposit a 2205 weldment which retains the duplex structure of the parent material. To simply weld pieces of 2205 together so that the weld holds is no big challenge.

In comparison, welding 316L is childsplay.

I'm not saying that the OP shouldn't use 2205 for their application- frankly we still don't have enough detail from them to give them an accurate selection- I'm just pointing out an issue we've experienced with the material. Certainly if you can get away with 316 for the valves etc. the cost impact of using 2205 for the piping would be moderated a fair bit.