Based on international definitions, heavy fuel oil is characterized by a kinematic viscosity of more than 180 mm²/s at 50°C.
Understanding Heavy Fuel Oil Viscosity
Viscosity is a crucial property of fuel oils, representing their resistance to flow. Higher viscosity means the oil is thicker and flows more slowly. Heavy fuel oil, also known as HFO or bunker fuel, is a residual fuel, meaning it's a byproduct of the crude oil distillation process after lighter fractions like gasoline and diesel have been removed.
Official Definition According to MARPOL
The MARPOL Marine Convention of 1973 provides a specific definition for heavy fuel oil to regulate its use and transport due to environmental considerations. This definition is based on two physical properties: density or kinematic viscosity.
According to MARPOL:
- Heavy fuel oil is defined by a density of greater than 900 kg/m³ at 15°C.
- Alternatively, it is defined by a kinematic viscosity of more than 180 mm²/s at 50°C.
Therefore, any fuel oil meeting either of these criteria is considered heavy fuel oil under this convention. The viscosity measurement at 50°C is particularly relevant because HFO often needs to be heated to be pumped and handled effectively, especially in marine engines.
Why Viscosity Matters for Heavy Fuel Oil
The high viscosity of heavy fuel oil is due to its composition. As the reference notes, heavy fuel oils contain:
- Large percentages of heavy molecules.
- Components such as long-chain hydrocarbons.
- Aromatics with long-branched side chains.
These complex and large molecules contribute to the fuel's thickness and resistance to flow. This high viscosity requires heavy fuel oil to be heated before use in engines to reduce its viscosity to a level suitable for pumping, purification, and injection. Proper temperature management is essential for using HFO efficiently and reliably.
Understanding the viscosity definition, especially the threshold of more than 180 mm²/s at 50°C as set by regulations like MARPOL, is key for anyone dealing with this type of fuel.
