Combating Corrosion and Transitioning from VLSFO to LSMGO

The implementation of stringent emission regulations (MARPOL Annex VI) has forced the maritime industry to adapt to new fuel types. Operating in designated Sulfur Emission Control Areas (SECA) requires switching from Very Low Sulfur Fuel Oil (VLSFO – max 0.5% S) to Low Sulfur Marine Gas Oil (LSMGO – max 0.1% S). This transition process, if not managed correctly, poses severe risks to the ship’s machinery, including thermal shock, loss of propulsion, and accelerated high-temperature corrosion. This article details the engineering best practices for fuel changeovers.

The Hazards of Fuel Changeovers

The primary challenge lies in the drastic difference in physical properties between VLSFO and LSMGO. VLSFO requires heating (typically to 110°C – 130°C) to achieve the optimal injection viscosity (usually 10-15 cSt). Conversely, LSMGO is a distillate fuel that is already thin at ambient temperatures. If LSMGO is introduced into a hot fuel system, its viscosity can drop dangerously low (below 2 cSt).

Thermal Shock and Pump Seizure

If hot VLSFO is replaced too quickly by cold LSMGO, the rapid temperature change causes the fuel pump barrel to contract faster than the plunger. This leads to a loss of clearance, resulting in the plunger seizing or scoring (thermal shock). To prevent this, the temperature change rate during the changeover must be strictly controlled, typically not exceeding 2°C per minute.

More information on Maintenance of ALCAP Heavy Fuel Oil Separators

High-Temperature Corrosion (Hot Corrosion)

While the focus is often on low-temperature (cold) corrosion due to sulfuric acid, high-temperature corrosion is a significant threat when operating on blended VLSFOs. These fuels can contain high levels of catalytic fines (cat fines), vanadium, and sodium.

During combustion, vanadium and sodium react to form low-melting-point compounds (vanadium pentoxide). These molten salts adhere to exhaust valves, piston crowns, and turbocharger blades, aggressively stripping away the protective oxide layers and causing severe pitting and hot corrosion. The best defense is rigorous fuel purification (running separators in parallel, minimum throughput) and strict temperature monitoring of the exhaust system.

Step-by-Step Changeover Procedure (VLSFO to LSMGO)

A standardized procedure is vital for a safe transition before entering a SECA:

1. Preparation: Ensure the LSMGO service tank is full and settled. Verify that the MGO cooler is operational (to maintain LSMGO viscosity above 2 cSt).
2. Reduce Heating: Gradually reduce the steam supply to the VLSFO heaters. Monitor the viscometer closely. Allow the fuel temperature to drop slowly.
3. Switch Valves: Once the VLSFO temperature has dropped to a safe level (usually around 60-70°C, but specific to engine manufacturer guidelines), switch the three-way changeover valve to draw from the LSMGO tank.
4. Cooling: Engage the MGO cooler if necessary to ensure the fuel at the engine inlet remains at the correct viscosity and temperature (typically ambient to 40°C max).
5. Monitor Leaks: Because LSMGO lacks the lubricity and thickness of VLSFO, check fuel pumps and high-pressure pipes for internal and external leaks.

A meticulous approach to fuel management not only ensures regulatory compliance but also extends the operational life of the vessel’s critical power plants.