Overview
ISO 3993:1984 is an international standard developed by the International Organization for Standardization (ISO) that specifies the pressure hydrometer method for the determination of density or relative density of liquefied petroleum gas (LPG) and light hydrocarbons. This method is crucial for precise physical testing of petroleum products, particularly for custody transfer and fiscal purposes where exact density measurements are essential.
The standard covers testing procedures, apparatus specifications, calibration methods, and safety considerations relevant to materials with vapor pressures up to 1.4 MPa (absolute vapor pressure 1.5 MPa). It emphasizes the use of a certified hydrometer for reliable results, while acknowledging alternative calibration procedures for other applications.
Key Topics
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Scope and Application
The method is designed exclusively for LPG and light hydrocarbons having vapor pressures not exceeding 1.4 MPa gauge pressure. Use of the pressure hydrometer method is limited to materials within this pressure range to ensure safety and accuracy.
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Definitions
- Density: Mass per unit volume of the liquid, expressed in kilograms per cubic meter (kg/m³) or grams per millilitre (g/ml).
- Relative Density: Ratio of the liquid’s density to that of pure water at specified temperatures.
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Apparatus
- Certified glass hydrometers calibrated to appropriate density ranges.
- Hydrometer cylinder made from glass or transparent plastics, designed for pressurized testing.
- Thermometers compliant with ISO standards for accurate temperature measurement.
- Water baths controlled to maintain consistent temperatures (typically 15 °C, 20 °C, or 60 °F).
- Pressure gauges and needle valves for precise control of sample pressure during testing.
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Calibration
Calibration can be performed using certified hydrometers or alternative methods involving reference liquids like pure propane and butane with known densities.
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Sampling and Test Procedure
The standard prescribes careful sample collection, filling, and purging procedures to mitigate contamination or pressure-related hazards. The test involves equilibrating the sample temperature, reading the floating hydrometer scale, and applying necessary corrections.
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Safety Precautions
Strict adherence to national and local safety codes is emphasized due to the hazardous nature of LPG and hydrocarbons under pressure. Cylinder cleaning restrictions and the use of protective shields are recommended.
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Precision and Accuracy
Statistical repeatability and reproducibility criteria ensure the method’s reliability in laboratory and field applications.
Applications
ISO 3993:1984 finds practical use in several critical areas:
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Custody Transfer and Fiscal Measurements
Accurate density determination using the pressure hydrometer method enables fair trade and taxation based on volumetric and mass quantities of LPG and light hydrocarbons.
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Quality Control in Petroleum Industry
Facilities producing or distributing LPG and hydrocarbons rely on this standard to verify product specifications and comply with regulatory frameworks.
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Field Testing
Although laboratory conditions are preferred, the standard allows usage of the apparatus in the field, with noted limitations on precision.
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Calibration of Analytical Instruments
Reference densities generated by this method assist in calibrating gas chromatographs and other density measurement devices.
Related Standards
- ISO 5024 – Petroleum and natural gas products-Standard reference temperature for physical properties and density measurements.
- ISO 6578 – Method for calculation of density of petroleum gases by gas chromatographic analysis (alternative approach).
- ISO 911 – Petroleum measurement tables for density and volume corrections.
- ISO/R 653 – Specifications for thermometer calibration relevant to standard density testing.
- ASTM D1250 – Standard Guide for Petroleum Measurement Tables, often referenced for temperature corrections.
- API 2540 – Density correction tables used alongside ISO 3993 for legal measurement adjustments.
This standard plays a pivotal role in the petroleum sector by providing a robust and internationally recognized method for the determination of density or relative density of LP gases and light hydrocarbons. Its comprehensive approach ensures safety, accuracy, and traceability for various industrial and commercial applications.
Open preview (PDF)Liquefied petroleum gas and light hydrocarbons — Determination of density or relative density — Pressure hydrometer method