Overview
ISO 6578:2017 - Refrigerated hydrocarbon liquids - Static measurement - Calculation procedure - defines the calculation steps to convert measured volumes of refrigerated hydrocarbon liquids (primarily LPG and LNG) at the time of static tank measurement to equivalent volumes, mass or energy at standard conditions (15 °C and 101.325 kPaA). The standard applies to liquids stored or transferred under static storage conditions (e.g., marine carriers and shore tanks). It does not cover pressurized gases or products predominantly composed of ethane/ethylene or partially refrigerated liquids at substantially elevated pressures.
Key topics and requirements
- Volume-to-standard conversion: Procedures to convert measured liquid volume at observed temperature and vapour pressure to volume at the standard reference condition (15 °C, 101.325 kPaA).
- Mass calculation: Methods to compute liquid mass from corrected standard-volume and density; includes corrections for vapour-phase contributions and air-vacuum effects.
- Energy (calorific) content: Calculation of gross calorific (energy) content from composition, mass and density-used for energy accounting and custody transfer.
- Density from composition: Procedures to estimate liquid density based on chemical composition for both LPG and LNG.
- Real vs. ideal vapour behavior: Distinguishes volumetric calculations on ideal and real (compressible) gas bases using compression factors.
- Normative and informative annexes: Includes data and procedures in Annexes A–G such as molar volumes, correction factors for LNG mixtures, gross calorific values, molar masses and boiling points-integral to the calculation workflow.
- Scope limitations: Explicit exclusion of pressurized gases and certain hydrocarbon-dominant or high-pressure conditions.
Applications and users
ISO 6578:2017 is intended for practical use in:
- Custody transfer of LPG/LNG on marine carriers and shore tanks (static measurements).
- Terminal and storage operators, ship operators and marine surveyors performing inventory and transfer calculations.
- Metering and measurement engineers, metrology labs and technicians responsible for accurate mass/energy reconciliation.
- Contracting parties in fuel trading who require standardized volume/mass/energy conversion for commercial settlements.
Practical benefits include consistent custody transfer accounting, reduced dispute risk, and reproducible conversion of measured volumes to mass or energy for regulatory, commercial and safety documentation.
Related standards
- ISO 91 - Temperature and pressure volume correction factors (referenced for LPG volume corrections).
- ISO 6976 - Gas calorific values and compression factor definitions (referenced terms).
- ISO 3993 - Pressure hydrometer guidance (referenced context for density measurement).
Keywords: ISO 6578:2017, refrigerated hydrocarbon liquids, LPG, LNG, static measurement, calculation procedure, volume correction, mass, calorific content, density, custody transfer.