I want to select material for baffl 5

[ENGRANIA]

I want to select material for baffles cheaper than sa-387 for effluent reactor cooler.
Material of shell sa-204 gr.B
Material if tubes sa-213 TP 304
TEMP. For tube side 430°c
Temp for shell side 410°c

 

[r6155]

@ Christine74
Did you ever see the skid bar during the fabrication of the tube bundle?

Best regards

 

[Christine74]

Same issue with skid bars (which by the way are usually not included unless the tube bundle weighs over 15-20k lbs). If you make the bundle skeleton out of stainless steel, the carbon steel shell would be the anode and any galvanic corrosion would be localized on the pressure boundary. You can move the location of the galvanic couple but you can't make it go away.

Doesn't really matter though, I doubt that galvanic corrosion is even a valid concern for this particular application.


-Christine

 

[ENGRANIA]

Dear SnTMan.
The existing baffles were A-357
The existing tie rods stainless steel without material specs.
Fluid in shell was hydrogen gas
Fluid in tubes was effluent from reactor.

 

[r6155]

@ Christine74
Your post: "Obviously you've never seen an actual heat exchanger before. Tube bundles do not levitate in the shell. Most or all of the baffles would make direct contact with the bottom of the shell cylinder"

I inform you: With SKID BAR tube bundle do not levitate.

Best Regards

 

[EdStainless]

If this is truly dry hydrogen then there is no issue with using CS.
The tie rods are likely SS so that their thermal expansion matches the tubes better.
CS skid bars on SS baffles are common.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Krausen]

Assuming the galvanic corrosion concern is real (does high-temperature hydrocarbon effluent normally have electrolytes?), are you saying that you would rather have the shell acting as the anode than the baffles?

Galvanic corrosion is very real, although it is process-dependent. You may get away with cheap baffles/support plates with alloy tubes if your process is clean. But often times Process Engs won't know exactly how much conductive fluid (moisture, chlorides, etc.) they have inside their shellside process. The most common design error is then to install alloy tubes with carbon steel baffles/support plates inside a shell, where the CS baffles/support plates (and other bundle skeleton components) become sacrificial anodes inadvertently.

Understand the surface area effect in play as well. The relative exposed surface area of each baffle/support plate inside the shell compared the tubes exposed surface area is low, which increases the galvanic corrosion rate in this case. But it's the opposite case for the shell's internal exposed surface area relative to each baffle/support plate. Galvanic corrosion is not a major concern between alloy baffles/support plates & a CS shell due to the surface area effect. See API571 for full explanation of surface area ratio under galvanic corrosion.

The shell is a pressure boundary. Galvanic corrosion on the shell would be very localized so difficult to monitor while the equipment is in service. Maybe non-metallic baffles are worth considering here but I don't know of any non-metallic materials that can be used at that high of a temperature.

Your tubes are a pressure boundary as well, despite the fact you will not lose primary containment if they fail. Corroding out baffles/support plates from using the wrong metallurgy will put your bundle tubes at risk of tube failure. There is no need for non-metallic baffles/support plates, only correctly specified metallurgy.