Stainless flange on carbon piping

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I'm working on a project were we're installing a large (2300 hp) compressor to an offshore oil production facility.One of the piping ISO's (pipe spool drawings) inadvertenly called for 3" carbon steel flanges on a 316 stainless steel 3" sch 80 pipe.  The pipe is a short section of natural gas vent line which never sees any pressure over say 5 psig max.  The exterior of the pipe will be painted.  The interior will see natural gas with some trace water and hydrocarbon condensate.  This pipe has been fabricated and is ready to install.The client is concerned about galvanic corrosion.  Do you guys feel this concern is valid?Miller 211 w/ spool gunMiller Dynasty 200DXLongevity 60i IGBT plasmaO/A w/ crappy chinese torch/gaugesSouth Bend 10K latheGrizzly 4029 10x54 millGrizzly 7x12 hor bandsawangle grnders, bench grnder, bench belt sndr7.5 hp 80gal cmprsor
Reply:YEP!!! In a marine enviroment, absolutely. corrosion is not always a simple matter. Personally, I think you have a serious problem and with out some well establish professional information stating this situation is not an issue, if I was the client, I would not accept it. You might suggest that the fabricator contact a qualified corrosion engineering service and if that firm will provide assurance that there will be no problem then the client might go for it.
Reply:I think that this is not that easy of a call.  Obviously, a stainless flange would be better, but there may be other design considerations that favored the carbon flange.  Galvanic corrosion is generally a concern when you have water in the pipe or a submerged pipe.  In that case, I would expect the carbon flange to get pretty chewed up without water treatment and still be a point of concern with treatment.  The exterior doesn't have a galvanic problem as long as the coating is tough enough to withstand the environment and properly applied (assuming it is not a submerged application).  I don't think that it should be a big deal considering the application that they are using it on.My $.02.Rene
Reply:Thanks for the help guys.Weldinglifer...I see from your profile that you've done some sub-sea work so you obviously know what seawater does to metal.  In this case, the piping is all above the splash zone but still it's an offshore installation.. a "moist salt laden environment" as they say.  It will be painted...sst or not...and the interior will "normally" see only clean fuel gas  (it's the vent line from the gas starter on a 3608 Cat engine).  I say normally because you never know what's going to get into a line given maintenance and rework operations.But wait...it gets even dumber.  I rechecked the ISO and now see that all the fittings are cs and all the pipe is sst.  So it goes like this...cs flange, sst pipe, cs elbow, sst pipe, cs elbow, sst pipe, cs reducer...then all cs to the flare scrubber.  And this spool is only 3 ft long!zerepener, I think you're right that this would probably be ok given the service but the more I think about it the more unprofessional it looks.I'm going to recommend that they cut out this mixed bag and replace w/ all carbon steel pipe (as originally called out in the piping spec and class sheet)Again gentlemen...thank you.Miller 211 w/ spool gunMiller Dynasty 200DXLongevity 60i IGBT plasmaO/A w/ crappy chinese torch/gaugesSouth Bend 10K latheGrizzly 4029 10x54 millGrizzly 7x12 hor bandsawangle grnders, bench grnder, bench belt sndr7.5 hp 80gal cmprsor
Reply:no offense to you, since it's the customers request, but dang!.. that's gonna be stupid lookin.. lol..if you're not livin on the edge, you're takin up too much room..
Reply:Dumb question, is it a weld on flange, or an angle face flange?Disclaimer; "I am just an a$$hole welder, don't take it personally ."
Reply:TossiWelding,Flange is 3" raised face weld neck 150# bored for sch 80.  Elbows are 90 deg LR butt weld sch 80.Dumb question back to you...what's an angle face flange?Miller 211 w/ spool gunMiller Dynasty 200DXLongevity 60i IGBT plasmaO/A w/ crappy chinese torch/gaugesSouth Bend 10K latheGrizzly 4029 10x54 millGrizzly 7x12 hor bandsawangle grnders, bench grnder, bench belt sndr7.5 hp 80gal cmprsor
Reply:You weld a "collar" to the pipe and the "flange" floats. We used to use a "combo" set up, SS collar and a carbon flange. I think they also go by ring flange.Disclaimer; "I am just an a$$hole welder, don't take it personally ."
Reply:We call em lap flanges at work.  As far as all the carbon fittings on stainless pipe, sounds like a typo that needs a revision to me.  I've seen very little carbon to stainless fit-ups here.  We have a whole separate shop across the street for stainless.  Most of the carbon to stainless welds here are carbon supports on stainless, but still very few of those.Tack It!!
Reply:Has to be a typo as this makes no sense. Your QC should check with the design engineer.CWB InspectorRed Seal B PressureArguing with an Inspector is like mudwrestling with a Pig.After a while you realize the Pig likes it.
Reply:Originally Posted by pileobonesHas to be a typo as this makes no sense. Your QC should check with the design engineer.
Reply:TossiWelding,OK,...I got it now.  Angle face flange = lap flange = Van Stone flange (sort of).  Now I see the reason for your question...not dumb by the way, very intuitive.Ya, it was a typo allright.  Like I said, I'm recommending the spool be scraped and rebuilt or at least the sst portions.Miller 211 w/ spool gunMiller Dynasty 200DXLongevity 60i IGBT plasmaO/A w/ crappy chinese torch/gaugesSouth Bend 10K latheGrizzly 4029 10x54 millGrizzly 7x12 hor bandsawangle grnders, bench grnder, bench belt sndr7.5 hp 80gal cmprsor
Reply:Originally Posted by TozziWeldingYou weld a "collar" to the pipe and the "flange" floats. We used to use a "combo" set up, SS collar and a carbon flange. I think they also go by ring flange.
Reply:It sounds like the environment was originally meant for all CS flanges and sections? and now a  SS section has accidentally been specified and installed?If the original specification was all carbon steel, then one assumes whoever designed it didn't think it would of been full of an electrolytic or corrosive substance. I think it would be okay, if I was the engineer on record, I would do a little more research before I signed off on the waiver.  I would be much more concerned about salt water spray, than hydrocarbons.
Reply:I've seen lots of situations where the "wetted surface" was stainless and the flanges were carbon, but they were either slip-on flanges or lap joint flanges.  This is a cost issue.
Reply:i was thinkin  it sounds like a van stone..the fabricator never questioned the drawings???
Reply:One thing to note is that the pipe is relatively thin walled and will corrode through quickly while the flanges are very thick.  The pipe was spec'd as stainless steel because it needs to be resistant to corrosion but the flanges may be carbon steel because they can experience significant corrosion without failure.   The flanged portions of the pipe may also get shrouded in something that protects them from corrosion.  All oil and gas pipe flanges in marine applications are to be shrouded by some means.  Have you seen what a 316SS flange costs vs. carbon steel?
Reply:Interesting following this discussion my workmates recently welded a stainless flange to a carbon steel heat exchanger body with copper tubes and mild steel cap ends using 309l stick rods and an ancient Lincoln stick welder
Reply:Welcome bungleu.30+ yrs Army Infantry & Field Artillery, 25 yrs agoMiller 350LX Tig Runner TA 210, spool gunLincoln 250/250 IdealArcESAB PCM 500i PlasmaKazoo 30"  vert BSKazoo 9x16 horiz BSClausing 12x24 lathe20T Air Press
Reply:Originally Posted by 76GMC1500One thing to note is that the pipe is relatively thin walled and will corrode through quickly while the flanges are very thick.  The pipe was spec'd as stainless steel because it needs to be resistant to corrosion but the flanges may be carbon steel because they can experience significant corrosion without failure.   The flanged portions of the pipe may also get shrouded in something that protects them from corrosion.  All oil and gas pipe flanges in marine applications are to be shrouded by some means.  Have you seen what a 316SS flange costs vs. carbon steel?