Although the use of stainless steel and GRP in scrubbers provides quality, they still require coatings for some areas, for example, discharge nozzle to mitigate corrosion. It is planned to install scrubber systems at about 4.000 ship vessels (most of them are open-loop scrubbers) next year to meet the requirements of the 2020 sulfur cap and ensure air pollution control.
To be more precise, open-loop scrubber systems discharge all washwater overboard. Herewith, less than 2% of the scrubbers are considered to be closed-loop wet scrubber systems, while 20% of them are regarded to be hybrids (both open and closed-loop air treatment systems to comply with washwater discharge regulations in areas where it is forbidden to do).
The thing is that SOx caught in the exhaust gas dissolves in the washwater and produces strong acids resulting in low pH of the washwater. Thus, manufacturers of wet air scrubbers have to pay careful attention to the choice and specification of wet scrubber material and coatings because washwater pipelines cause cracks and leaks in the scrubber.
The acidic washwater flows down the walls of the scrubber tower during the process of air treatment. Therefore, a conventional wet scrubber tower suffers from a high level of acidic corrosion. Additionally, scrubber systems face to potential damage to the anti-biofouling coatings on the hull.
According to IMO regulations, the pH level of scrubber washwater is required to be 6.5 at 4 m from the discharge point when the vessel is stationary. The pH level is far from 6.5 after the neutralization process may damage the binder existing in some biocidal antifouling systems of wet air scrubbers.
Thus, all wet scrubber systems include the following main requirements for the materials: corrosion resistance; acid resistance; and high-temperature performance. The corrosion process can happen both in all seawater pipelines and the water supply to the scrubber tower. “The scrubber tower is an area where an acid attack can be severe”.
Also, the temperature in the wet scrubber tower varies from 50°C to 60°C, while the overboard pipeline temperature can be less than 40°C, that is why the use of appropriate coatings in addition to the proper selection of wet air scrubber materials is highly recommended.
Finally, there is an opinion that a manufacturer of wet scrubber systems will provide the required materials are chosen for the scrubber, and there is a need for coatings just in the overboard discharge section because of the high probability of washwater impact on the hull coating.
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