Carbon to Stainless Steel Flange Question

[idjit]

Currently on a project installing a high pressure hydraulic system. The majority of the system comprises 316 SS tubing which delivers hydraulic fluid. The However the hydraulic fluid skid is a large reservoir with internal SCH 80/160 carbon steel piping. The skid customer supply and return fluid connections are at the top of the reservoir. These connections are flat face carbon steel SAE Code 61 4-bolt flanges which will mate to customer tubing.

Since all of the non-skid tubing is 316 SS, the initial plan was to use S.S. socket weld SAE Code 61 flanges which would mate up and bolt to the skid flange connections. This would preclude the need for welding dissimilar metals on the tubing side.

http://catalog.anchorfluidpower.com...cket-weld-pipe-4-bolt-stainless-steel-flanges

But now my concern is the interaction of the SS flange and the CS mating flange, particularly in regards to potential for inducing galvanic corrosion.
Unlike common B16.5 flanges, code 61 flanges do not use gasket material between flange faces. One side is a flat face and the other face has an O-Ring groove. The two faces would be butted right up against each other with only a compressed o-ring separating them when assembled. They are however proven for being superior leak proof connections.

Was looking for input into what the true potential is for galvanic corrosion and whether welding CS flanges with SS tubing would be a better option such that the mating bolted flange faces are same CS material
Unfortunately parts are already ordered and project is on a tight deadline.

 

[EdStainless]

Will this connection get wet?
If it stays dry there will be no concern with corrosion.
If it is going to be getting wet then we need to talk about it.

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P.E. Metallurgy, Plymouth Tube

 

[RVAmeche]

I'm not familiar with code 61 flanges, but could you flange to the skid using the same material, and then use a different fitting,union, or B16.5 flange w/ isolation kit, to change material?

 

[idjit]

EdStainless, thanks for your input. If an overhead sprinkler system is activated sure the skid can get wet. That's an unlikely scenario. However this is being installed in a turbine building in the American South. Design conditions are for 100% Rel. Humidity and temperatures up to 130ºF. I think I'm just trying to find the relative likelihood of corrosion occurring. My thought was that since stainless steel is naturally coated, that coating would act as an insulator between the CS flange but maybe my understanding of the chemical process is lacking.

Also it's a question of where the dissimilar metal interaction occurs. Either that would be at the socket weld S.S. tubing to CS flange connection or at the bolted stainless to carbon connection. Which interface has a higher likelihood of corrosion.

RVAmeche B16.5 flanges aren't typically recommended for high pressure hydraulic systems. Unfortunately, adding fittings and flanges in line would also require revisiting a pipe stress analysis that is approved and reviewed. This is a non-nuclear application (turbine building) in a nuke plant but even the smallest change involves a lot of painful paperwork and design changes.

 

[mk3223]

IMO, it can be engineering acceptable to have a CS flange to mate with a SS flange in the hydraulic piping as the system is per design life, say 20 years in term of corrosion allowance.

In the real life, there are some corrosion issues, if not many, of the dissimilar material joints showed up at the end of the design life in the operation plants. It can happen because of the different conditions internally or externally. Often to have a revamping project to replace the section of the CS piping with SS to eliminate the corrosion problem.

 

[awhicker84]

A galvanic / electrical potential chart will show two values for stainless: passivated and unpassivated. This accounts for the protective layer (or the lack of one).

 

[awhicker84]

BTW, if you don't have a WPS / PQR for welding P1 to P8 per ASME IX / VIII then the welding idea won't be possible. Do you have one? Does it require PWHT? I'm assuming you could get away with none considering 316 is annealed condition. Not sure about what is needed on the CS side. ASME will have requirements.

I'd prefer bolted connection > welded connection. 316 SS temperature expansion is greater than carbon steel and martensitic stainless which could cause some issues during welding and during operation.

 

[danschwind]

One adaptation you could make to avoid the contact of dissimilar metals (considering that you will want to avoid it) is to add a hose consisting of a crimped C.S. SAE 61 flange head and the other side of a crimped S.S SAE 61 flange head. Then you proceed to use this hose to isolate the equipment from the tubing.

I actually did something similar on an offshore crane that had all sorts of dissimilar materials. Dunno if it worked but no one complained to this day.

 

[curtis2004]

Idjit,

What size is pipe in question? I think, it should be small bore (<2 1/2" Nominal), based on hydraulic fluid application.
How about addition of a small carbon steel "adaptor spool" with one end SAE flange and the other end with ANSI flange of appropriate class (probably, Class 1500 or even Class 2500)? This will allow you to have CS SAE flange connections in one end, and CS/SS ANSI flange with SS gasket on the other end.
I have seen similar unit in a steam plant, but I think, ANSI Class 1500 flanges were used there.

Regards,
Curtis

 

[moltenmetal]

If wetting is only occasional, or in case of fire, there is NOTHING wrong with connecting carbon and stainless steel flanges to one another. Galvanic corrosion only occurs if the joint between the metals is wetted with an electrolyte either on the inside or outside of the joint.

 

[SJones]

There will be more problems with the dissimilar metal weld than galvanic corrosion in a sheltered atmosphere. Also, don't you want carbon steel out of a hydraulic system for cleanliness reasons?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.