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API MPMS 11.3.3.2 1974 (2012): Relative Density and Specific Gravity for Miscellaneous Hydrocarbons and Products
A Comprehensive Guide to the Manual of Petroleum Measurement Standards Chapter 11.3.3.2 for Density Data and Custody Transfer Applications
Scope and Application
API Manual of Petroleum Measurement Standards (MPMS) Chapter 11.3.3.2, originally published in 1974 and reaffirmed in 2012, provides authoritative relative density (60/60°F) and specific gravity (60/60°F) values for a wide range of miscellaneous hydrocarbons and petroleum products. The scanned version of this standard remains a key reference for density data covering pure hydrocarbons, intermediate streams, specialty solvents, and complex petroleum blends not addressed in other MPMS sections.
The standard applies primarily to custody transfer operations, inventory control, pipeline measurement, and refinery process calculations where accurate density correlations are essential. By providing standardized density values at 60°F, it enables consistent conversion between measured volumes and mass, supporting both domestic and international trade agreements. Table 1 lists the principal hydrocarbon categories covered in the standard.
| Category | Examples | Relative Density (60/60°F) Range | Typical Application |
|---|---|---|---|
| Paraffinic Hydrocarbons | n-Pentane, n-Hexane, n-Heptane | 0.626 – 0.684 | Solvents, gasoline blending |
| Cycloparaffins (Naphthenes) | Cyclopentane, Cyclohexane, Methylcyclohexane | 0.745 – 0.779 | Chemical feedstocks, reformer feeds |
| Aromatic Hydrocarbons | Benzene, Toluene, Xylene (BTX) | 0.865 – 0.880 | Petrochemical production, octane enhancement |
| Olefins (Alkenes) | Propylene, Butenes, Hexenes | 0.519 – 0.673 | Polymer feed, alkylation |
| Specialty Products | Isoprene, Dicyclopentadiene, Naphthalene | 0.681 – 1.025 | Rubber, resins, specialty chemicals |
Tip: When applying API MPMS 11.3.3.2 to a product not explicitly listed, use the nearest hydrocarbon family and consult the standard’s interpolation guidance. Always verify the measured relative density at 60°F against the tabulated value for consistency.
Technical Requirements
Data Tables and Density Correlations
The core of API MPMS 11.3.3.2 comprises tabulated relative density values at 60/60°F and the corresponding specific gravity entries. Each compound or product is listed with a unique identification, its molecular formula (where applicable), and the density values determined at the reference temperature of 60°F (15.56°C). The standard uses the older notation of relative density (60/60°F) interchangeably with specific gravity, though modern practice generally draws a distinction between the dimensionless quantity and the mass-density ratio.
The relative density (RD) is defined as the ratio of the density of the hydrocarbon at 60°F to the density of water at 60°F. The specific gravity (SG) in this context is numerically identical when using the same reference water density (0.999016 g/cm³ at 60°F per older tables). The standard also includes guidelines for converting these values to density in mass per unit volume (e.g., kg/m³ or lb/ft³) using the corresponding water density.
Important: The 2012 reaffirmation does not change the data values from the original 1974 publication. However, users must ensure they apply the correct reference water density for the units being used. Some modern conversions may rely on the latest water density standard (0.999020 g/cm³ at 15.56°C per RIPP 2015). Minor differences may affect high-accuracy fiscal metering.
Temperature Correction Methodology
Because hydrocarbon density varies significantly with temperature, API MPMS 11.3.3.2 provides temperature correction coefficients (CTL) for each miscellaneous hydrocarbon category. These coefficients are derived from experimental data and allow the user to calculate the relative density at any standard temperature (e.g., 60°F) from a measurement at an observed temperature, or vice versa. The correction methodology is consistent with the broader MPMS Chapter 11.3 series, ensuring compatibility with other commonly used tables (e.g., Table 5, Table 6).
The general form of the correction is:
RD at 60/60°F = RD at t × Kt
where Kt is the volume correction factor for the hydrocarbon type at temperature t.
For pure components, Kt is given as a polynomial equation fitted to the tabulated data.
| Compound | RD (60/60°F) | CTL at 60°F (per °F) | Applicable Temperature Range (°F) |
|---|---|---|---|
| Benzene | 0.8790 | 0.00058 | –20 to 120 |
| Toluene | 0.8665 | 0.00052 | –20 to 130 |
| Cyclohexane | 0.7791 | 0.00050 | –20 to 110 |
| n-Heptane | 0.6840 | 0.00053 | –20 to 140 |
Quality Control and Verification
The standard outlines procedures for verifying the integrity of the tabulated data during field implementation. Users should regularly cross-check measured relative densities against the tabulated values under controlled laboratory conditions (ASTM D4052 or D1298). For mixtures not explicitly listed, the standard permits the use of blending rules (e.g., additive volume) but warns that large deviations may occur for non-ideal mixtures. A tolerance band of ±0.001 in relative density is considered acceptable for most custody transfer applications when using the standard density values.
Implementation Highlights
Integrating API MPMS 11.3.3.2 into measurement systems requires careful attention to the following implementation aspects:
- Data Retrieval: The scanned document is not machine-searchable; operators should prepare a digitized lookup table or integrate the data into electronic flow computers (EFCs) for rapid retrieval.
- Temperature Measurement: Accurate relative density correction relies on precise temperature readings. Install RTDs or PRTs with a calibration traceable to NIST and with an uncertainty better than ±0.2°F.
- Sample Handling: Follow ASTM D4057 (manual sampling) or D4177 (automatic sampling) to obtain representative hydrocarbon samples. Volatile samples require pressurization to prevent evaporation losses that distort density.
- Blended Streams: For mixed hydrocarbon streams that are not near-ideal, use the blending correlations in Section 11.3.5 of MPMS or consider performing a custom density measurement campaign.
Best Practice: When implementing API MPMS 11.3.3.2 in a custody transfer metering system, pair the relative density data with the volume correction factors (VCF) from MPMS Chapter 11.3.1 (Table 5) for crude oil or Chapter 11.3.2 for refined products. This ensures seamless volume-to-mass conversion across the entire product range.
Compliance and Best Practices
Although API MPMS standards are voluntary, they are widely adopted by international regulatory bodies and contractually required in most petroleum sale-and-purchase agreements. Adherence to API MPMS 11.3.3.2 demonstrates due diligence and helps minimize measurement disputes. Key compliance considerations include:
- Reaffirmation Status: The 2012 reaffirmation confirms the data remains technically valid. No substantive changes were made in 2012; users may rely on the 1974 values as long as they are applied consistently.
- Audit Trails: Maintain records of the specific physical copies or digitizations used. For fiscal reporting, reference “API MPMS 11.3.3.2 (1974, reaffirmed 2012)” as the data source.
- Updating: Monitor API for potential future revisions (e.g., new experimental data for pyrolysis gasoline, renewable fuels). Currently, the standard is static, but alternative chapters (Chapter 11.3.4) may supplement data for bio-blendstocks.
- Training: Ensure operators and metering technicians understand the difference between this standard and the more familiar crude oil or product tables. The miscellaneous hydrocarbon tables have narrower applicability but are critical for specialty streams.
Warning: Do not substitute data from other MPMS chapters (e.g., Table 6 or Table 24) for the miscellaneous hydrocarbons listed in API MPMS 11.3.3.2. The density correction coefficients are specific to the chemical composition of each compound family. Using the wrong table can introduce errors exceeding 0.5% by volume, which is unacceptable for fiscal metering.
In summary, API MPMS 11.3.3.2 1974 (2012) remains a foundational reference for the measurement of widely traded hydrocarbons and niche products. Its detailed tables, temperature correction methods, and quality procedures enable accurate and consistent volume and mass determination across the petroleum industry. Maintaining strict compliance with the standard’s requirements, while staying alert to future updates, will ensure reliable measurement integrity.
Q: What is the difference between relative density (60/60°F) and specific gravity (60/60°F) in API MPMS 11.3.3.2?
A: In the context of this 1974 standard, the two terms are used interchangeably. Both refer to the ratio of the density of the hydrocarbon at 60°F to the density of water at the same temperature. Modern usage distinguishes “relative density” (dimensionless ratio) from “specific gravity” (often with a defined reference density). Users should confirm the water density value employed for mass conversion.
A: In the context of this 1974 standard, the two terms are used interchangeably. Both refer to the ratio of the density of the hydrocarbon at 60°F to the density of water at the same temperature. Modern usage distinguishes “relative density” (dimensionless ratio) from “specific gravity” (often with a defined reference density). Users should confirm the water density value employed for mass conversion.
Q: Can I use API MPMS 11.3.3.2 for renewable hydrocarbons such as biodiesel or renewable diesel?
A: The standard was published before the widespread introduction of renewable fuels. Biodiesel (FAME) and hydroprocessed esters and fatty acids (HEFA) are not explicitly listed. These materials may exhibit different thermal expansion behavior. For bio-blendstocks, refer to experimental data or API MPMS 11.3.4 where available, or perform a custom determination.
A: The standard was published before the widespread introduction of renewable fuels. Biodiesel (FAME) and hydroprocessed esters and fatty acids (HEFA) are not explicitly listed. These materials may exhibit different thermal expansion behavior. For bio-blendstocks, refer to experimental data or API MPMS 11.3.4 where available, or perform a custom determination.
Q: Why is the temperature correction coefficient (CTL) expressed as a single value in some tables and as a polynomial in others?
A: Simpler tables use a linear CTL over a narrow temperature range, while more accurate polynomial expressions are provided for the full range of expected operating temperatures. Always select the polynomial if available for best accuracy. The linear approximation is acceptable only when temperature variations do not exceed ±15°F.
A: Simpler tables use a linear CTL over a narrow temperature range, while more accurate polynomial expressions are provided for the full range of expected operating temperatures. Always select the polynomial if available for best accuracy. The linear approximation is acceptable only when temperature variations do not exceed ±15°F.
Q: How often should I verify the tabulated density values against my own measurements?
A: API recommends annual verification for critical custody transfer streams and quarterly verification for specialty chemicals where density may drift due to aging or contamination. Any discrepancy greater than ±0.001 in relative density warrants investigation. Always calibrate the testing hydrometer or densitometer using certified reference standards.
A: API recommends annual verification for critical custody transfer streams and quarterly verification for specialty chemicals where density may drift due to aging or contamination. Any discrepancy greater than ±0.001 in relative density warrants investigation. Always calibrate the testing hydrometer or densitometer using certified reference standards.