Stainless Steel to Carbon Steel Flanges

[CanuckMiner]

We have an application where we have a stainless steel tank with a flanged outlet (also stainless steel). We are looking to connect a carbon steel valve (flange) to the outlet.

I'm wondering if there are any (code/standards) rules prohibiting or advising against connecting stainless steel flanges to carbon steel flanges. I have looked in our Piping Handbook and ASME B31.3 and even a few catalogues but can't find anything prohibiting us from doing this. Can anyone confirm what the "rules" are for connecting stainless steel to carbon steel flanges? Do we need special gaskets or "extra thick gaskets", etc.? Can anyone point me in the right direction (maybe I missed something in the literature or the Code)?

I do not yet know the grade of stainless. The tank will be stainless because it will be installed in such a manner that there will not be much opportunity for removal in the future, and even "patching" or repairs would difficult.

Fluid is solids-laden water (i.e. essentially light to medium weight mud) with no particular corrosive elements (although the pH could range from mildly acidic to mildly basic). Temperature is ambient (+/- 20 degrees Celsius).

Much obliged for any assistance.

Cheers,
CanuckMiner

 

[BRT549]

can you use a carbon steel slip-on flange over a stainless stub end? Then you wouldn't need to weld at all. Is the vessel going to be a pressure vessel?

 

[SooCS]

I think slip-on flange is worse. The problem is the galvanic corrosion created between stainless steel and carbon steel.

Use insulation gaskets and bolts sleeves instead.

 

[kenvlach]

As a metallurgist, I agree with SooCS.

 

[TBP]

Is this tank under some serious pressure, because the fluid seems pretty innocuous, and low temp. It doesn't sound like you'll fall under one of the ASME piping codes, unless the tank is classed as a pressure vessel. Are you using carbon steel piping in this service now? If you are, how's it standing up?

I think if you get corrosion, it'll likely be dissolved oxygen in the water going after the carbon steel. If you're concerned at all, though, go with the dielectric gasket kit. You get a gasket, sleeves for the bolts, and washers to go under the bolt heads and nuts.

 

[GSDGoBucks]

There is nothing in the code that prevents a 'spec-break' between two different materials (in your case SS to CS). Sounds like a 316SS spiral wound graphoil gasket could be used at the joint between the flange and valve. Consult with a vendor such as a Garlock representative to discuss your issue and they will point you in the right direction.

As a project/maintenance engineer for a major refinery, the question that needs to be discussed is if the product will corrode the CS valve body in such a way to generate a flaw or failure to this valve. Design pressure, temperature, flowrate (potential erosion?), pH level, other contaminants such as H2S, chlorides, ammonia, etc. need to be determined to prevent such an event.

Agree with the dielectric kit at the joint. Especially if the tank needs to be isolated and is cathodically protected.

In summary, it is not uncommon to bolt a SS flange to a CS flange to create a 'spec-break'. Keep in mind, the process conditions need to be determined and verifed so not to generate another problem.

 

[CanuckMiner]

Thank you all for your responses. I'll follow up with gasket suppliers as per the suggestions.

Cheers,
CanuckMiner