Pure Fuels, Efficient Operation
Motor ships accounted for about 98% of the global fleet around the year 2000. Rudolf Diesel invented diesel engines in 1892. The first four-stroke marine diesel engine operated successfully twenty years later. By 1930, two-stroke diesels emerged favorites for the then popular larger and faster ships.
Water, microbes, and sediments are the primary contaminants of bunker tanks. Mixing two incompatible fuels makes the blend unstable. Deterioration of stored fuel degrades and corrodes the engine and other ship components while increasing harmful ship emissions.
Bunker Fuels: Types & Specifications
Any fuel used by ship is bunker fuel. Types include:
- Distillate Fuels / Gas Oil / Marine Gas Oil: obtained from crude oil distillation
- Residual Fuels / Marine Fuel Oil / Residual Fuel Oil: left behind as residue after distillation
- Intermediate Fuels / Marine Diesel Fuel / Intermediate Fuel Oil (IFO): mixture of the above two
- Residual and intermediate fuels made up 75% of global bunker fuel consumption in 2013 with IFO380 being the most popular.
ISO 8217:2012 is the international standard defining the requirements of petroleum fuels used for marine diesel engines. The most important specifications for marine diesel are:
- Maximum Density Limit: for efficient operation of purifiers and for the proper ignition of low-viscosity fuels
- Maximum Al+Si Limit: as aluminum silicate particles cause abrasive engine damage
- Maximum Total Potential Sediment Limit: indicates the asphaltenes content in bunker fuels determines the percentage of sulphur, nitrogen, nickel, vanadium, and hydrogen – elements that boost emissions from the crude oil
Flow of Fuel Oil in Ships
Fuel oil flows from storage tanks to settling tanks through fuel transfer pump and suction strainer. From service tanks, it moves through the purification system via controls, supply pumps, and heaters to the engine.
Purification system consists of a series or parallel arrangement of the purifier-purifier, purifier-clarifier, or clarifier-clarifier combination. From here, fuel oil enters service tanks directly or through an additional duplex filter.
Settling tanks first separate water and solid sediments by sedimentation due to gravity. Large vessel movements stir up dirt and sediments that collect over time at the bottom of the bunker and settling tank.
Centrifugal pumps are the foundation of bunker fuel cleaning for only they separate water and solid sediments completely. This is purification or centrifugal cleaning and is based on the increasing difference in density between fuel and water with increasing temperature.
Conventional purifiers do not accept fuel oils with density above 991kg/m3 at 150C. Second separators called clarifiers are often used downstream of purifiers for better cleaning. Filters remove particles that purifiers and clarifiers cannot.
Advanced Computer Driven Fuel Cleaning Systems operate as clarifiers for fuel oils with densities above 991kg/m3 at 150C. Separated water and sludge accumulates at the periphery and is discharged periodically while clean oil is discharged continuously.
Contaminants & Solutions
Following factors contaminate bunker fuels:
- Excessive Water that mixes with fuel oil during transport and from condensation of atmospheric moisture in the fuel tank
- Catalyst Fines enter the fuel via aluminum silicate used as catalyst in the catalytic cracking process and increases engine wear
- Microbes such as bacteria and fungi thrive in the presence of water and 15-400C temperatures. These creatures consume organic-inorganic fuel molecules for nutrition
Foul smelling water-bottom, sludge, slime, and corrosion in filters, lines, and tanks indicates microbe presence. Microbes alter the properties of the fuel oil that results in:
- – choked filters
- – low engine performance
- – generation of corrosion-causing hydrogen sulphide that is fatal in high concentration
- – engine wear and corrosion
- Commingling Incompatible Fuels makes the final blend unstable. Fuels with similar densities and same viscosity grades are usually compatible
Ships built after July 1, 1998 have separate bunker, settling, and service tanks in order to switch to low-sulphur fuel in Sulphur Emission Control Areas (ECAs). Ships without separate tanks have to mix fuels
- Heating Bunker and Settling Tanks prevents water condensation. Heating these beyond the fuel’s flash point temperature however may cause explosion and thermal currents will interfere with sedimentation in settling tanks
- Eliminating Water in Fuel eliminates bacterial contamination. Good water-housekeeping, frequent draining-cleaning of bunker tanks, and installing close-able de-aerated openings helps check contamination by water
- Water-Soluble and Oil-Soluble Biocides stamp out microbe contamination
- Avoid Fuel Commingling. If unavoidable, mix them only after checking compatibility. And if compatibility check is not possible, minimize the quantity of one fuel in the mixture
- Efficient Supply Chain Management minimizes water condensation and microbial contamination during all stages of fuel transport
- Proper Operation and Maintenance of Purification and Filtration Systems
Bunkering fuel contamination is a grave issue confronting the global shipping industry. Like most challenges, this pitfall is better handled through cooperation across the supply chain and proactive prevention.