Flange Matings Of The Same Rating But Different Materials

I don't think there will be any consequences of mating flanges of the same rating but of different materials for getting the pressure test done with flange joint. The consequences due to different materials of construction is for the fluid to be handled. For example if fluids at cryogenic temperature (like liquid oxygen) is to be handled, only stainless steel is suitable and carbon steel cannot be joined with it.

It is a common situation in boilers, furnaces, heat exchangers ....... When change the temperature gradient, and no longer temperatures requires with specific material, is normally to use to cheaper materials. Is a matter of cost.

Just ensure there is no galvanic corrossion and temperature and other (eg ambient, corrossion conditions) are OK.

If galvanic corrosion is an issue, there are dielectric unions and flanges that isolate the dissimilar metals via barriers of nonconductive materials.

you may what to consider the difference in expantion coeffients if the they're subject to temperature changes.

I agree with the above comments which mentioned about being careful in making sure that both materials are suitable for the service medium.

I do not know which grade is your duplex flange and what is the design temperature of your system. But I would just like to add that different material groups have different pressure temperature ratings, thus, it is also important in making sure that both flanges have an acceptable pressure rating at the design temperature. For e.g. A105 flange has very similar rating as the A182 F51 flange for CL 150. but the F51 flange cannot be use for temperatures above 315 deg C. If they have different ratings, then you will need to check the allowable pressure of that material at your design temperature too.

And one more thing that comes to mind is the pressure test value, if the whole material of construction changes at that flange connection, then you need to make sure that the test pressure for the system is calculated using the piping component which has the highest allowable stress ratio, and that all components in the system can handle that pressure. Unless your connection is between a pipe and a vessel, then paragraph 345.4.3. will apply.

This is common situation in process plants. By the way if galvanic corrosion is an issue, there are PVC sleeves and washers available which can isolate the dissimilar metals via barriers of nonconductive materials. Mechanically there is nothing to worry for the consequencies.

Dear all, are we sure that there will no problem due to different thermal expansion of both cs and ss material. Is the changes in expansion is compensated by the bolting?...this is important especially if it is lethal services.

any opinion or previous experience especially during plant services will be appreciated.

I don't know the correct answer, but I don't think that the difference in thermal expansion at the flanges is enough to loosen the bolts in most services.

I can't say for the other plants, but at our plant, the change in material is always at the flange joints, meaning CS flange joining to SS flange, it is never CS pipe weld with SS flange and then bolt up with another SS flange weld to SS pipe.

A practical experience is the lube oil pipes for the compressor systems, down stream of the strainer is always SS piping to prevent any rust particles and upstream of strainer is CS simply because it can handle the service and is more cost effective.