coatings/insulation 1

[180]

I need to specify an insulation system for stainless steel pipes. In the past AISI 316 has been used but after some years in service, severe SCC has occured. I have specified the use of either SAF 2205 or 904L as suitable alternatives that are resistant to SCC. The pipes carry water at 85C and the Cl- level is 35ppm. It is clear that SCC is originating from the inside and the outside of the pipes ie I have found both failure modes from metallographic examination. The question is how do you insulate the pipes without providing a water trap for Cl- containing water between the pipe and insulation. Would SAF 2205 resist SCC and other forms of corrosion under this condition. Should the SAF2205 be coated with some kind of paint, if so what about the inevitable scratch on the surface which will remove the paint? I have done some reading about paints and insulation systems and it seems the choice is VAST! Are there any tried and tested answers that can provide certain results?

Thanks so much!
180

 

[Metalguy]

I've never been a fan of 904L for use in Cl. 2205 is better. Make sure your insulation isn't high in Cl, and I think you can leave it bare.

 

[MEKANIKEREN]

Hi 180,

Previos safe limits for SCC on 22Cr duplex has been 90 deg.C. This is now raised to 100 deg.C in the NORSOK specifications. This imply that you can leave SAF 2205 bare.

If you want even more resistance to SCC you should evaluate to use 6MO as (254SMO or AL6XN or 1925HMo).

Nevertheless I would recommend you to use an insulating specification on basis of cellular glass (Foamglas) towards the piping. The cell glass has an open cell structure and moisture will drain by gravity from the pipe wall, thus water does not collect on the piping. If you are using a metal sheeting, make sure to drill some drainage holes in the low points.

Have a look at the following specification:

http://www.nts.no/norsok/r/r00402/r00402.htm

Good luck to you!

 

[180]

Thanks for your response - it has been useful

Does anyone know anything about sodium silicate containing insulation materials. The ASTM standard suggests that sodium silicate be added (somehow) which is a water soluble compound that retards SCC when austentic stainless steel pipe sections are in contact with insulation materials. I thought that this might be an additional safety feature to guard against SCC and pitting of 2205 pipes. Possibly the insulation materials could be tested according to ASTM C692 (test for the influnce of insulation material on SCC suseptability of the base material) regulary to determine the efficacy of the dissolved sodium silicate over time.

Is the addition of sodium silicate a wide spread practise in industry? Are such materials easily available?

180

 

[Metalguy]

IIRC, the use of sodium silicate is *supposed* to help with low-level Cl pitting/SCC. I have a graph somewhere that shows how much is needed for various Cl levels. I don't remember it working at even moderate Cl levels, say over ~500 ppm, but it's been a LONG time since I saw it.

 

[180]

The standard suggested to me also recomends mineral wool that has been processed with a phenol binder. Although the cellular glass really seemd to be the best option, I am struggling to source FOAMGLAS here in South Africa. We do have however the mineral fiber with the phenol binder. will this do? Or is the extra expense associated with the import of FOAMGLAS worthwile in the long run - in short is it that much better than mineral fiber?
Thanks
180

 

[MEKANIKEREN]

The trend here in Norway is that most companies that earlier were using mineral wool is now using cellular glass in areas where the piping may be subject to humidity either from deluge testing or from marine atmisphere. I feel that the temperature in question 85C together with the exposure is so close to the limit that it calls for cellular glass (Foamglas). If you change the piping to 6Mo it is safer to allow partly wet mineral wool fibres as it has a better resistance against SCC, however then the cost of the piping increases significantly.
Some web sites for you:

http://www.unitedperlite.com/cel_glass.htm

http://www.foamglas.com/companies_distributors.htm

http://www.jm.com/insulation/products/sproule/SprouleWR1200_CI192.pdf

http://www.cuf.com/

Regards,
Mekanikeren

 

[PVRV]

What is the pipeline size & lenght?
What is the operating pressure ?

MEKANIKEREN, I am trying to recall where in norsok std does SS require RCP anodes for service temperature above 25 C in chloride environment. I find it very intersting that DSS can go up to 100 C; please provide the reference.

with appreciation