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SS to LTCS flanged connexion

SS to LTCS flanged connexion

SS to LTCS flanged connexion

(OP)
I have an equipment made of SS304L with nozzle flanges I need to connect to LTCS piping, according to project piping class.

I'm guessing some precautions are required to prevent corrosion contamination on my equipment... or maybe not... any advice?

Processed fluid is waste water / sludge.

Thanks for your help.

RE: SS to LTCS flanged connexion

There is the potential for galvanic corrosion if the fluid is conductive.  To eliminate this you would need to electrically isolate the SS flange from the CS flange.  This would require using an "insulating gasket" as well as making sure that there was no other electrical connection between the two materials (e.g., via pipe supports, instrumentation, etc.).

Sometimes people might internally coat for a distance of 10x diameter as this will greatly reduce the current flow through the fluid and reduce the potential for galvanic corrosion - this would be an alternative to the insulating gasket.

The extent of the problem would be dependent on the relative mass of carbon steel vs stainless steel.  If there is a large amount of carbon steel piping then the  galvanic corrosion may not be significant enough to cause a problem but it is better to err on the side of caution.

RE: SS to LTCS flanged connexion

I strongly disagree with the recommendation of rneil. I believe that he is expressing an opinion

Carbon steel and stainless steel do not have a significant galvanic couple between them and are not far enough apartin the galvanic series

An insulating flange kit/design is not required when there is a a bolted connection between carbon steel and stainless steel. This is additional cost that is not necessary.

As a reference, I cite the book "Defects and Failures in Pressure Vessels and Piping" by Helut Thielsh.

http://www.amazon.com/Defects-Failures-Pressure-Vessels-Piping/dp/0318172208

-MJC

What are your references,  

   

RE: SS to LTCS flanged connexion

No argument there,  I expressed an opinion.  The fact is that the potential for galvanic corrosion is dependent on three variables:

1) the difference in galvanic potential between the materials
2) the specific properties of the process fluid (the electrolyte)
3) the difference in relative masses of the metals in contact

i'll leave it to the OP to evaluate these factors and the responses received to decide if they feel action is necessary or not.
 

RE: SS to LTCS flanged connexion

MJC,

If you want references, try:

http://resource.npl.co.uk/docs/science_technology/materials/life_management_of_materials/publications/online_guides/pdf/bimetallic.pdf

BS PD 6484

Quote:

not far enough apart in the galvanic series

For this statement, you may be looking at stainless steel in its ACTIVE state, i.e. it is corroding in the electrolyte anyway.  In its passive state, the stainless steel will have a significantly different electrode potential which can drive bimetallic corrosion in carbon steel.

As rneill points out, the proposed arrangement presents a risk of bimetallic corrosion and it is up to the responsible party to assess that risk and remove or mitigate it if it is deemed necessary to do so.  Mitigation could simply be an 'inspect and replace' policy for the carbon steel piping, for example.

Steve Jones
Materials & Corrosion Engineer
http://www.oilandgaspeople.com/cv/11499664
http://www.linkedin.com/pub/8/83b/b04
 

RE: SS to LTCS flanged connexion

API 571 "Damage Mechanisms Affecting Fixed Equipment in the Refining Industry" has a good discussion on galvanic corrosion including a picture of a carbon steel nipple installed in a stainless steel piping system (admittedly an extreme example due to the small relative mass of carbon steel) that suffered severe galvanic corrosion.

I also wanted to double check ISO 21457, the NORSOK Materials Specifications and a couple of my reference texts, but alas I don't think I'm going to have the time in the next while to dig through this.

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