Overview
ISO 8973:1997 specifies a standardized calculation method for determining the density and vapour pressure of liquefied petroleum gases (LPG) based on their compositional data. Developed by ISO Technical Committee 28 on Petroleum products and lubricants, this international standard provides a practical approach for LPG quality specification. The method uses compositional analysis through gas chromatography combined with empirical density and vapour pressure factors of individual LPG components.
This standard is essential for industries involved in producing, handling, or certifying LPG products. It offers a reliable calculation technique to estimate key physical properties of LPG mixtures at specified temperatures, facilitating quality control and product specification without direct custody transfer measurement responsibilities.
Key Topics
- LPG Definition: A gas stored and handled as liquid under moderate pressure and ambient temperature, mainly composed of C3 and C4 hydrocarbons, with vapour pressure below approximately 1600 kPa at 40 °C.
- Calculation Basis: Uses molar composition from gas chromatography (ISO 7941) combined with density and vapour pressure factors for common LPG components like propane, butane, ethane, and their isomers.
- Density Factor: The density of each LPG component in kg/m³ at 15 °C under its own vapour pressure.
- Vapour Pressure Factor: Absolute vapour pressure of LPG components at 37.8 °C, 40 °C, 50 °C, or 70 °C.
- Calculation Procedure: Involves determining mass fractions, followed by summation of component densities weighted by mass fraction, and vapor pressures weighted by mole fraction. Final vapour pressure is expressed as absolute and gauge pressure.
- Reference Standards: Relies on ISO 7941 for gas chromatography and references ISO 6578 for custody transfer measurement, clarifying this standard is not intended for quantity measurement.
- Reporting: Results expressed with precision to 0.1 kg/m³ for density and 1 kPa for vapour pressure.
Applications
ISO 8973:1997 is applicable in the petroleum and LPG industries for:
- Product Quality Specification: Ensuring LPG products meet defined density and vapour pressure criteria for commercial or technical standards.
- Material Certification: Providing documented evidence of LPG physical properties for compliance, shipment, or blending.
- Process Control: Verifying LPG batch properties based on composition without performing extensive direct physical measurements.
- Research and Development: Supporting formulation of LPG blends and assessment of vapor-liquid equilibrium parameters for new products.
- Safety and Storage Management: Predicting vapour pressures to safely design storage and transportation conditions.
It is particularly useful for laboratories and quality control departments that require standardized calculation methods to supplement physical testing.
Related Standards
ISO 8973:1997 interacts with and complements other standards critical for LPG handling:
- ISO 7941:1988 - Commercial propane and butane: Specifies gas chromatographic analysis essential for compositional data input.
- ISO 6578:1991 - Refrigerated hydrocarbon liquids static measurement: Governs custody transfer calculation procedures which differ from the simplified method in ISO 8973.
- ISO 3993:1984 - LPG density by pressure hydrometer: Provides alternative density measurement techniques related to the subject.
- ISO 4256:1996 - Determination of LPG gauge vapour pressure: Defines physical test methods for vapour pressure for comparison with calculated values.
Summary
ISO 8973:1997 facilitates a standardized, reliable calculation method for key physical properties of liquefied petroleum gases, enabling accurate specification and quality control. By leveraging compositional analysis alongside established density and vapour pressure factors, this standard supports the petroleum industry in maintaining consistent LPG product quality, optimizing safety, and enhancing operational efficiency.
