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Pitfalls of welding SS pipe to CS flange

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KevinNZ

KevinNZ

Mechanical
Jun 12, 2003
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Hi all

What is bad about welding SS piping to CS flanges? As below we are adding a spool between CS and SS piping. The DN900 spool will be SS pipe with SO CS flanges. The plan is to add SS overlay on the CS flange gasket face. (the spool is needed to fill the existing gap)

Screenshot_2024-01-25_130402_sujuay.gif
 
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The welding can be done. High operating temperatures may be a problem. The coefficient of thermal expansion for your typical 300 series stainless is some 50% higher than it is for steel.
 
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Why not just use SS flanges for your new spool? Would likely be cheaper & definitely faster than weld overlaying the faces of CS flanges. Having dissimilar metal flanges at low operating temps is typically a non-issue.
 
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Go with one of the above suggestions and eliminate one of the odd combinations.
The biggest issue is the heightened risk of localized accelerated galvanic corrosion of the CS adjacent to the SS.
The weld itself isn't the issue usually.
Weld overlay works, but just using a SS flange is easier and keeps things cleaner.

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P.E. Metallurgy, consulting work welcomed
 
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Additional information
Pipe DN900 9.52mm wall 316L
Length of spool 1400mm
Flanges class 150 series A (WN MC to Pipe OD)
Max pressure 2 bar @ 120C
We need have two sets of flanges for operations/maintenance
Contents steam but also open atmosphere when there is no flow.
 
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Why not a SS flange one end of spool and CS flange the other - forget any overlay.
You will have dissimilar fillet welds whichever option you choose.
It seems so basic I think I must be missing something here.
 
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It is a minor point here, but I assume "flang-isolation kits" will be used to isolate the bolting hardware ??

MJCronin
Sr. Process Engineer
 
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I believe the purpose of flange isolation kits is greatly misunderstood. Flange isolation kits don't prevent galvanic corrosion. Flange isolation kits do reduce the burden on cathodic protection systems which are seldom installed inside pipelines.
 
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Thanks TBE, most people miss that.
If you need to suppress internal galvanic corrosion coat the ID of the CS.
How far back you need to coat depends on the conductivity of the fluid.

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P.E. Metallurgy, consulting work welcomed
 
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Thanks all.
Based on 10 years of service of a CS flanged welded to SS pipe in the same line, we are going with SO CS flanges welded to the SS pipe. No overlay on the flange face.
 
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