Corrosion-Resistant U-Tube Heat Exchangers

Yes, provided the galvanic pairing is managed at the tubesheet and the shell-side fluid is one carbon steel tolerates. The noble bundle is the large cathode and the carbon steel beside it the anode, the unfavorable ratio: a small anode feeding a large cathode corrodes fast, so the carbon steel touching the bundle is at risk. The part to engineer is the tubesheet, which is wetted by both fluids and touches both metals. A clad or solid-alloy tubesheet on a carbon steel shell flange is the usual answer, with the shell gasket face kept dry. Where the shell side is seawater or brine the logic reverses, and the shell either moves to an alloy or is lined.

Not by the subcritical heat treatment used on carbon steel, because the temperature range it uses forms embrittling phases in duplex grades. Where sour service calls for hardness control on a duplex bend, the bend zone is solution annealed and quenched instead, which restores the ferrite-austenite balance and removes the cold work together. That is a hotter, faster and more tightly controlled operation than stress relief, and it is followed by hardness testing on the outside of the bend. Ask for the annealing procedure and the hardness map with the quotation.

Tube side, in almost every case, because that confines the alloy to the tubes, the tubesheet face and the channel and leaves the shell in carbon steel. Tube-side flow is also easier to hold at a controlled velocity, which matters for the copper-nickel grades: 90/10 loses its protective film if seawater runs too fast and pits under deposits if it runs too slow, so the velocity window is part of the material decision. Shell-side placement is the exception when the corrosive stream fouls heavily and the bundle must be pulled for cleaning, or when the fluid attacks the bend interior, which cannot be inspected. Then the shell gets the alloy and the bundle is sized around it.

The location and shape of the damage usualy tells you before a lab does. Pits clustered under deposits or at the U-bends point to chloride attack. Branching cracks on the outside of the bend are stress-corrosion cracking. Tubes thinned at the inlet rows directly under the shell nozzle are impingement, which an impingement plate fixes rather than an alloy change. Wear bands at each baffle location are vibration, and a heavier gauge or closer baffle spacing is the remedy. Send photographs of the failed bundle with the inquiry, because a replacement bundle in a better alloy will fail again in the same place if the original cause was mechanical.