Viscosity is an important quality specification for residual fuel oil. Specifically, it is a measure of a liquid's resistance to flow. High-viscosity fuel oil is more difficult to pump and is therefore less desirable as a product.
Refiners can manage fuel oil viscosity either through processing or through blending:
- Visbreaking- The visbreaker is a conversion process designed primarily to reduce the viscosity of vacuum resid by thermally cracking it
- Blending - The viscosity of resid can be reduced by blending it with a lower viscosity "cutter stock." This is typically either cycle oil, FCC slurry, or straight run kerosene