Author: Daniel G. Teleoaca – Maritime Chief Engineer
Foaming within the Exhaust Gas Recirculation (EGR) system of main engines is a critical yet often overlooked issue that can significantly impact engine performance, emissions control, and long-term reliability. This article provides a comprehensive, professional overview of the causes, effects, and remedies for foaming in EGR systems, with a focus on actionable solutions and best practices for maintenance.
What Is EGR and Why Is It Important?
The EGR system recirculates a portion of exhaust gases back into the engine’s intake, lowering combustion temperatures and reducing nitrogen oxide (NOx) emissions. This process is essential for meeting stringent emission standards and optimizing fuel efficiency in marine diesel engines. The EGR system typically includes an EGR valves, cooler, blower and associated piping.
You can learn more about EGR by reading the article Understanding Exhaust Gas Recirculation in Maritime Vessels.
Causes of Foaming in EGR Systems
Foaming in marine engine EGR systems primarily occurs in the scrubbing water circuit, which treats recirculated exhaust gases to meet IMO emissions standards.
Below are the root causes and supporting evidence from technical sources:
Soot-Oil Emulsions
- Soot from exhaust gases mixes with residual lubricants (e.g., cylinder oil leaks), forming stable emulsions that reduce water surface tension and stabilize foam.
- Marine EGR systems using high-sulfur fuels produce sulfuric acid, which reacts with NaOH dosing agents. Improper pH control (outside 7–9) destabilizes scrubbing water, promoting foam. So, over/under-dosing NaOH fails to neutralize sulfuric acid, leading to acidic foaming or scaling that traps air bubbles.
Mechanical Agitation
- Turbulence in buffer tanks (e.g., from pumps or overflow pipes) aerates scrubbing water, creating foam. Poor tank design (lack of baffle plates) exacerbates this.
Contaminant Ingress
- Oil leaks from EGR blower seals or crankcase ventilation systems introduce hydrocarbons into the scrubbing loop, acting as foam stabilizers. Oil contamination in scrubbing water lowers surface tension, enabling foam formation.
- Particulate matter (e.g., unburned carbon) from exhaust gases nucleates air bubbles, increasing foam persistence.
EGR Cooler Fouling
- Fouled coolers (due to carbon/soot deposits) disrupt water flow, causing localized turbulence and air entrainment. Particulate matter deposition in coolers reduces heat transfer efficiency and increases pressure drops, indirectly promoting foaming by destabilizing water flow.
Effects of Foaming in EGR Systems
Unchecked foaming in the EGR system can have several detrimental effects:
- Reduced Cooling Efficiency: Foam acts as an insulator, decreasing the heat transfer capability of the EGR cooler. This leads to higher exhaust gas and combustion temperatures, undermining NOx reduction and potentially causing engine overheating.
- Loss of Lubrication and Increased Wear: In systems where lubricants are affected, foaming reduces the oil’s lubricating properties, accelerating wear on moving components.
- Coolant Loss and Engine Damage: Persistent foaming can lead to coolant overflow or loss, risking overheating, head gasket failure, or even catastrophic engine damage due to hydro-locking if coolant enters the combustion chamber.
- Erratic Engine Performance: Air pockets and foam in the EGR passages can disrupt exhaust gas flow, causing rough idling, loss of power, increased emissions, and triggering of diagnostic trouble codes.
- Increased Maintenance Costs: Accelerated component wear and repeated overheating events drive up repair frequency and operational costs.
Remedies and Preventive Solutions
Marine EGR systems face unique challenges due to high-sulfur fuels, seawater cooling, and stringent IMO Tier III emissions standards. Below are targeted solutions derived from industry guidelines and technical sources.
Process Water Management and Foaming Control
Marine EGR systems rely on scrubbing water (not engine coolant) to cool recirculated exhaust gases.
- NaOH Dosing Optimization: Maintain scrubbing water pH between 7–9 to neutralize sulfuric acid formed from sulfur in fuel. Overdosing causes scaling in EGR coolers, while under-dosing leads to acidic foaming and corrosion.
- Bleed-Off Water Treatment: Treat bleed-off water to <15 ppm oil content before discharge. Use oil content meters (OCM) and integrate multi-stage filters (e.g., hollow fiber membranes) to remove soot-oil emulsions.
Soot and Carbon Build-Up Mitigation
- EGR Cooler Cleaning: Clean EGR coolers every 500–1,000 hours using ultrasonic or chemical methods to remove carbon deposits. Blocked coolers reduce heat exchange efficiency, increasing exhaust temperatures and NOx.
- Hollow Fiber Membrane Filters: Clean these “spaghetti filters” weekly to prevent soot and oil from re-entering the scrubbing loop, which destabilizes process water.
Sensor Calibration and System Testing
- O₂ Sensor Maintenance: Calibrate O₂ sensors monthly to ensure accurate EGR ratio control. Faulty sensors can cause over-recirculation, increasing soot load and foaming risk.
- Regular EGR System Testing: Conduct function tests on EGR bypass (EGRBP) and turbocharger cut-out (EGRTC) systems weekly to prevent valve sticking or blower failures during prolonged idle periods.
Lubricant and Combustion Quality
- High-BN Cylinder Oils: Use lubricants with a BN (Base Number) of 100+ to neutralize acidic byproducts from sulfur. Monitor soot levels in oil (<3%) and replace filters every 250 hours.
- Fuel Sulfur Compliance: For low-sulfur EGR systems, ensure fuel sulfur content aligns with scrubber capabilities. High-sulfur fuels require robust NaOH dosing to prevent acid formation.
Mechanical Integrity Checks
- EGR Valve and Blower Inspections: Inspect EGR valves for carbon buildup every 1,000 hours and lubricate actuator arms. Clean EGR blower impellers to prevent soot-induced imbalances.
- Leak Detection: Pressure-test EGR piping and coolers annually. Leaks introduce air into the process water loop, promoting foaming and reducing scrubbing efficiency
Advanced System Upgrades
- Demister Pads in Scrubbers: Retrofit scrubbers with multi-stage demister pads to separate entrained water and reduce foam carryover into the engine.
- Redundant Sensors: Install backup pH and O₂ sensors to avoid overdependence on single-point data, ensuring stable EGR operation during long voyages
In conclusion, foaming in main engine EGR systems is a multifaceted issue with roots in mechanical integrity, fluid quality, and operational practices. By understanding the causes and implementing targeted remedies-ranging from regular cleaning and fluid management to prompt repair of leaks-engineers and technicians can safeguard engine performance, reduce emissions, and minimize costly downtime. Proactive maintenance and adherence to manufacturer guidelines remain the most effective strategies for preventing and resolving foaming-related problems in EGR systems.
Learn more by following the link about “Marine MARPOL Convention on Air”.
Find more by following the link about “Marine Clean Air Act and Inspections”.
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