Corrosion Resistant Design of High Voltage Electrostatic Mist Eliminator Power Supply for Marine Exhaust Desulfurization System
Marine exhaust gas desulfurization systems remove sulfur oxides from ship engine exhaust to comply with increasingly stringent emission regulations. The scrubbing process produces a mist of water droplets and pollutants that must be removed from the cleaned gas stream before discharge to atmosphere. Electrostatic mist eliminators apply high voltage to charge and collect the droplets, providing high efficiency mist removal with low pressure drop. The marine environment and the scrubber operating conditions create a corrosive atmosphere that challenges the reliability of the high voltage power supply.
The electrostatic mist eliminator operates on similar principles to electrostatic precipitators used for particulate control, but is optimized for liquid droplet collection. Discharge electrodes at high potential produce corona discharge that charges the droplets passing through the electric field. The charged droplets migrate to collection electrodes under the influence of the electric field, where they coalesce and drain from the system. The liquid nature of the collected material simplifies removal compared to solid particulate collection.
The corrosive environment in a marine scrubber mist eliminator arises from multiple sources. Seawater used as the scrubbing medium contains chlorides that promote corrosion of metals and conduct electricity. Sulfur compounds removed from the exhaust gas form acidic species in the scrubbing liquid. The high humidity and frequent wetting from the mist create conditions favorable for electrochemical corrosion. The combination of these factors creates an environment more severe than typical industrial precipitator applications.
Materials selection for the power supply enclosure and internal components is critical for corrosion resistance. Stainless steels with appropriate grades resist chloride induced pitting and crevice corrosion in marine atmospheres. Nonmetallic materials including engineering plastics and composites provide excellent corrosion resistance for structural components and enclosures. Conformal coatings on circuit boards protect against moisture and ionic contamination. The materials must maintain their properties throughout the service life in the corrosive environment.
The high voltage components including transformers, capacitors, and output connections face particular exposure to the corrosive environment. The output connection to the discharge electrodes penetrates the enclosure, creating a potential path for corrosive ingress. Sealed feedthroughs with appropriate materials prevent leakage while maintaining electrical insulation. The internal high voltage insulation must resist degradation from any corrosive species that penetrate the enclosure, requiring careful selection of insulation materials.
Enclosure design must prevent ingress of corrosive mist and gases while allowing heat dissipation from the internal electronics. Gasketed enclosures with appropriate ingress protection ratings limit the penetration of external atmosphere. Breather vents with desiccants or corrosion inhibitors can dry the incoming air and protect the internal atmosphere. Positive pressure ventilation with clean air prevents the ingress of corrosive atmosphere, though this requires a source of clean air and increases system complexity.
The discharge electrodes in the mist eliminator operate in direct contact with the corrosive mist. Electrode materials must resist corrosion while maintaining corona discharge characteristics. Corrosion of electrodes changes the electrode geometry and surface condition, affecting the corona onset voltage and the discharge current characteristics. Excessive corrosion can lead to electrode failure and system shutdown. Regular inspection and replacement of electrodes maintains performance, with the replacement interval depending on the corrosion rate.
Power supply cooling in the corrosive environment requires careful design. Forced air cooling draws external atmosphere through the enclosure, potentially introducing corrosive species. Closed loop cooling with internal air circulation and external heat exchangers isolates the electronics from the external atmosphere. Liquid cooling with corrosion resistant coolant provides effective heat removal without exposing electronics to the atmosphere. The cooling approach affects both the reliability and the maintenance requirements of the power supply.
Electrical insulation in the high humidity environment faces increased risk of surface tracking and flashover. Moisture films on insulation surfaces provide conductive paths that can lead to tracking and eventual insulation failure. Creepage distances, the shortest distance along insulation surfaces between conductors, must be increased compared to dry environments to provide adequate insulation margins. Hydrophobic insulation materials that shed water droplets maintain better surface insulation resistance in wet conditions.
Maintenance access for inspection and component replacement must be designed considering the corrosive environment. Sealed enclosures require provisions for resealing after access to maintain the environmental protection. Corroded fasteners can make disassembly difficult, requiring corrosion resistant fastener materials and appropriate coatings. The maintenance procedures must include inspection for corrosion damage and replacement of components approaching the end of their service life in the corrosive environment.
