We have in operation several waste heat boilers, installed after medium speed Diesel engines. They are water tube boilers and produce 3760 kg/h of saturated steam, @7 barg and @170?C. They were put into operation 15 months ago.

After this period of operation, we have found that the schedule of the tubes (80 mm in diameter) has been reduced drastically, to the point of rupture.

Is there any treatment we can apply to these tubes? Maybe some especial paint or resin? or we should have to change the material of these tubes?

This sounds like a possible sulfidation or reducing atmosphere corrosion mechanism on the OD (gas side) surface of the boiler tubes. Have you confirmed the extent of external wastage or is there internal wastage causing the wall thinning? How much of the boiler heating surface is being effected?

You need to perform wall thickness testing and decide how much of the existing tubing needs to be replaced or can be restored by weld overlays. I presume the tube material is carbon steel. If this is the case, you may need to evaluate the application of a corrosion resistant weld overlay - paints or coatings will not last in this type of service.

Surface treatment like chromizing of replacement carbon steel tubes versus weld overlay of carbon steel tubes may be an option. Right now, you need to confirm the actual corrosion mechanism that is causing the wastage.

The boilers have vertical tubes, 3.20 m long, carbon steel schedule 40     (4 mm thickness), 126 at the entrance of exhaust gases @385?C, and another 126 at the outlet @320?C/Cast Gate vavle.
 
During the periodical cleaning process, the wet soot is partially deposited on the bottom of the tubes plate, because for design reasons it is not possible to eliminate the total amount of soot.

The damage in the tubes (loss in thickness) is almost uniform and is around 50 mm starting in the point were the tubes traverse the tubes plate. As a corrective action, we have eliminated the damaged parts and replaced them by pieces of tube schedule 80 (7.5 mm thickness).

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