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Flanges 2

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ammp

ammp

Mechanical
Jun 16, 2006
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All,

Is it possible to have two side of flanges, one with Weld overlay (625 alloy) on the groove, the other with no weld overlay?

Both flsnges are typical F65 carboon steel. Previously, we plan to make-up both flanges with weld overlay, but for error during installation causes us to match with wrong flanges without overlay.


Rgds
 
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If the ring joint is 625 and you make up a joint where one flange does not have 625 cladding, you will get an electrolytic action, where the non clad flange will be anodic. A CP system could reduce this if you choose add it. So short answer is no as it will corrode. Hope this helps!
 
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Hi subchrissea,

thanks for the reply mate, really need it.

We are not in position to replace the flange with 625 cladding. However we do have the CP system in place, hopefully it can help control the corrosion.
Can this system survived, or will the corrosion damaged the flange after a while?



rgds
 
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Errr "hopefully it can help control the corrosion"!! And just because you or your company has made an error in manufacture you're willing to presumably drop this thing on the seabed and "hope" no one notices??

I don't know what sort of warranty you will have on this kit, but whatever it is the cost of repair once it goes subsea is so far beyond the cost of fixing it now it doesn't bear thinking about.

This sort of corrosion eats the ring groove and rather rapidly causes leaks around the ring gasket.

I really hope you think again and notify this to the right level so that the correct decision can be made rather than "hope". It might work as a political slogan, but it doesn't work subsea....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way
 
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Littlelnch,

Thanks for the info. Did you mean, even if the cladding is only on the groove, the non-cladded flange will get corroded badly? Fyi, our content is a dey gas and no corrosion is expected. The reason cladding was proposed was to avoid corrosion due to water ingression.

Rgds
 
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That is my interpretation - but what I mean is that the corrosion would be predominant in the external part of the ring groove where it is closest to the galvanic material. I assume because you posted this in offshore structures it was either subsea or in a marine environment with lots of salty water around.

All depends on the amount and penetration of the water. Also no water, no impact of CP.

If this is topsides you might get away with it, subsea probably not.

Either way it is not a good idea to knowingly hide an manufacturing error without getting approval from the client about any mitigation. Sometimes you just need to hold your hands up and admit a mistake rather than compound it by "hoping" it will be ok. That is close to wilful negligence and all sorts of nasty things happen legally when that occurs... There is always an option to replace the flange - it might not be your favoured one and the client might let it go if it will completely stuff up his schedule, but that's his decision to make, not yours.

Sorry if it sounds a bit harsh, but I wasn't going to let that go without a comment - that's what this forum is all about

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way
 
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Littlelnch,

Thanks for the info. This flange is a spool flange, separating between zone 1 and zone 2 pipeline in subsea.
Our field engineer wrongly make-up the flange, hence the situation we're in now. I wanted to be clear, that with current clad to non-clad groove flange and CP system in place, corrosion will definitely happen before I take it to the next level.

BTW, are you talking about crevice corrosion? Pardon me, I have very limited knowledge in material and corrosion.



Rgds

 
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He's talking about galvanic corrosion of the carbon steel flange where it's in contact with the more noble Ni-base alloy. CP will mitigate this as far as the CP will reach. Where CP can't reach, in other words where no clear ionic current path is available between the anode and this carbon steel surface, and water makes it's way to the galvanic couple, corrosion is a possibility. At that point the extent and rate of corrosion will depend on available oxygen or other oxidizing agent that can make it's way to the crevice. This is a sealing surface, and the seal is in many cases made on a very thin line going around the circumference. The last thing you want is corrosion resulting in leakage of this seal. Hence the use of Ni-base alloy overlay.

For subsea applications where risk is often times high (high consequence and/or probability of failure) the customer may want this corrected. As LittleInch stated, the customer is the one that needs to evaluate this risk and determine how to proceed. You never want to hide a mistake like this.
 
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