Overview - ISO/TR 18588:2023, viscosity-gravity constant for marine fuels
ISO/TR 18588:2023 provides best practices for characterizing fossil residual marine fuels (excluding FAME as defined in ISO 8217) using the viscosity‑gravity constant (VGC) and the Bureau of Mines correlation index (BMCI/CI). The Technical Report documents formulas, algorithms (Annex A) and sample data analysis to indicate whether a supplied residual marine fuel (VLSFO / HSFO) is predominantly paraffinic, naphthenic or aromatic in nature. It responds to changes in the marine fuel market following MARPOL Annex VI sulfur limits and the resulting diversity of blend formulations.
Key topics and technical requirements
- Scope: characterization of fossil residual marine fuels not containing FAME; no normative references.
- Two characterization approaches:
- BMCI (CI) - detailed correlation index using density, averaged boiling point and viscosity conversions. BMCI classification: Paraffinic (CI < 29.8), Naphthenic (CI < 57.0), Aromatic (CI > 75.0).
- VGC - empirical indicator derived from density and kinematic viscosity (ASTM D2501 approach). Easier to apply to routine test data.
- Important formulas and conversions:
- Algorithms to convert kinematic viscosity at 50 °C to viscosities at 38 °C, 99 °C and 100 °C (used in BMCI and VGC calculations).
- VGC formula using density at 15 °C and viscosity at 100 °C.
- Indicative thresholds from the report and literature:
- VGC ≈ 0.80 or below → paraffinic; VGC > 0.92 → aromatic. Speight ranges: paraffinic 0.74–0.75, naphthenic 0.89–0.94, aromatic 0.95–1.13.
- Empirical mapping (from sample analysis): paraffinic fuels ~VGC 0.82 / density ~888 kg·m⁻3; aromatic fuels ~VGC 0.92 / density ~994 kg·m⁻3.
Practical applications and users
- Who benefits:
- Fuel testing laboratories - apply VGC and BMCI algorithms from routine density and viscosity tests.
- Ship operators & technical managers - evaluate fuel character for combustion performance, engine tuning and compatibility.
- Fuel suppliers and blenders - classify products (VLSFO, HSFO) to support matching and quality claims.
- Classification societies, regulators and purchasers - use indicators to compare fuels and assess supply consistency.
- Practical uses:
- Rapid screening of whether a residual marine fuel is more paraffinic or aromatic.
- Supporting fuel selection, blending decisions and compatibility assessments.
- Complementary input to fuel handling, storage and engine performance risk assessments.
Related standards
- ISO 8217 - specification for marine fuels (defines FAME exclusion).
- ASTM D2501 - viscosity‑gravity constant calculation reference and historical methodology.
- BMCI/CI references used in petroleum characterization literature (Riazi, Speight).
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