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API MPMS 11.2.1 (1984) with Errata: Compressibility Factors for Liquid Hydrocarbons – Technical Guide
Volumetric Correction under Pressure: Standards for Accurate Measurement in Hydrocarbon Custody Transfer
Scope and Application
API MPMS 11.2.1 (1984) – Compressibility Factors for Hydrocarbons: Volumetric Correction for Measurement of Liquid Hydrocarbons Using Positive Displacement and Turbine Meters – defines the compressibility factors used to correct measured volumes of liquid hydrocarbons to standard base conditions under the effect of pressure. The standard applies to crude oils, gasolines, middle distillates (including jet fuel and diesel), residual fuel oils, and other hydrocarbon liquids commonly encountered in petroleum metering and custody transfer.
It covers both tabulated compressibility factors as functions of temperature, pressure, and API gravity, and an algorithm for calculating the correction factor. The base pressure is defined as 0 psig (14.696 psia or 101.325 kPa), while the base temperature conforms to industry practice, typically 60 °F (15.56 °C). The 1984 edition, which includes the official errata issued the same year, remains a fundamental reference for many hydrocarbon measurement systems and is incorporated by reference in national regulations and international recommendations, including certain sections of ISO 91.
Technical Requirements
Compressibility Factor Definition
The compressibility factor, F, is the fractional change in volume per unit change in pressure at constant temperature. The volume correction factor for pressure, Cp, is computed as:
Cp = 1 / (1 – F × P)
where P is the gauge pressure in psig. API MPMS 11.2.1 supplies F values for a range of API gravities and temperatures at selected pressures. The standard also provides detailed interpolation rules: linear interpolation for temperature and logarithmic interpolation for API gravity to achieve the highest accuracy.
Tabulated Compressibility Factors
The following excerpt illustrates typical compressibility factor values (×10−6 psi−1) at 150 psig for selected API gravities and temperatures:
| Temperature (°F) | API 10 (Heavy Crude) | API 30 (Typical Crude) | API 50 (Condensate/Gasoline) | API 70 (Light Naphtha) |
|---|---|---|---|---|
| 20 | 4.8 | 5.9 | 7.2 | 8.8 |
| 60 | 5.2 | 6.3 | 7.7 | 9.4 |
| 100 | 5.6 | 6.8 | 8.3 | 10.1 |
| 150 | 6.2 | 7.5 | 9.1 | 11.0 |
Interpolation Tip: When using the tables, always interpolate linearly with temperature and logarithmically with API gravity. This combination provides compressibility factors within ±0.5×10−6 psi−1 of the original standard values.
Pressure Limits and Correction Algorithms
The compressibility factors published in API MPMS 11.2.1 are validated for pressures up to 1500 psig, but the standard strongly recommends limiting the direct tabulated application to pressures at or below 300 psig for general custody transfer. At higher pressures, the user is directed to calculate the correction using a cubic equation of state that accounts for the nonlinear volume-pressure relationship. The 1984 errata corrected several non-critical rounding errors in the tables and clarified the interpolation method for the 60 °F base temperature line.
Implementation Highlights
Implementation of API MPMS 11.2.1 (1984) in a field metering system involves the following steps:
- Measurement of operating conditions – Obtain flowing temperature (using calibrated RTDs or thermometers) and pressure (using a calibrated pressure transmitter or gauge).
- Determination of fluid API gravity – Usually obtained from laboratory analysis at base conditions or inferred from density-correction tables.
- Retrieval of compressibility factor – Either from the tabulated values (with appropriate interpolation) or by implementing the algorithm provided in the standard.
- Computation of correction factor – Apply Cp = 1 / (1 – F × P) to convert the metered volume to a pressure-corrected volume at base pressure.
- Integration with temperature correction – Many metering systems combine the pressure correction factor (Cp) with the temperature correction factor (Ct) from API MPMS 11.1 to obtain the total volume correction factor (VCF).
Positive displacement meters and turbine meters are the primary devices for which this standard was developed, though the correction factors are equally applicable to Coriolis and ultrasonic meters when the volume flow is required for custody transfer balances.
Accuracy Benefit: Applying the correct compressibility factor from API MPMS 11.2.1 can reduce the measurement uncertainty of the volume correction to within ±0.05% for typical crude oils and refined products at pressures up to 300 psig. This level of accuracy is essential for high-value custody transfer operations.
Single-Phase Requirement: The compressibility factors in API MPMS 11.2.1 (1984) are derived for single-phase liquids with vapor pressure below atmospheric. Do not use them for two-phase flows, near-saturated petroleum liquids, or products containing dissolved gases that may bubble out under metering conditions. For such fluids, consult API MPMS Chapter 11.2.2 (NGL and LPG) or apply a full equation-of-state correction.
Compliance Notes
Compliance with API MPMS 11.2.1 (1984) is typically mandated by contractual terms in custody transfer agreements, and by regulatory authorities in many jurisdictions. Key compliance considerations include:
- Edition traceability – The 1984 edition with errata is the only authoritative version for jurisdictions that have not adopted later revisions. Users must ensure that the table values and interpolation instructions exactly match this edition.
- Interpolation method – The standard specifies that linear interpolation shall be used for temperature and logarithmic interpolation for API gravity. Alternative interpolation methods (e.g., polynomial) are not allowed unless proven equivalent through a documented validation.
- Bubble point check – Before applying the compressibility factor, the operating pressure must be at least 20 psig above the bubble point of the liquid to guarantee single-phase flow. If uncertain, a bubble point determination per ASTM D6377 should be performed.
- Recalibration of metering equipment – Whenever a significant change in product composition occurs (e.g., switching from a light crude to a heavy crude), the compressibility factor used in the flow computer must be updated according to the new API gravity and the fluid’s vapor pressure characteristics.
Pressure Correction Is Not Optional: Neglecting the compressibility factor correction can produce large volume errors. For a heavy crude (API ~10) at 200 psig and 80 °F, the pressure correction factor is approximately 0.9988, which may seem small, but for a 100,000 bbl shipment, this translates to a discrepancy of 53 bbl. At pressures above 500 psig, the error can exceed 0.5 % if uncorrected. Always apply the pressure correction when the metering pressure exceeds 50 psig.
Frequently Asked Questions
Q: What is the validity range of API MPMS 11.2.1 (1984)?
A: The standard is applicable to hydrocarbon liquids with API gravity between 0 and 100, temperatures from −40 °F to 250 °F (−40 °C to 121 °C), and gauge pressures up to 1500 psig (10 300 kPa). However, for optimal accuracy and compliance with current custody transfer standards, it is recommended to limit the direct use of the tables to pressures below 300 psig. Above that, a more rigorous equation-of-state method should be considered.
A: The standard is applicable to hydrocarbon liquids with API gravity between 0 and 100, temperatures from −40 °F to 250 °F (−40 °C to 121 °C), and gauge pressures up to 1500 psig (10 300 kPa). However, for optimal accuracy and compliance with current custody transfer standards, it is recommended to limit the direct use of the tables to pressures below 300 psig. Above that, a more rigorous equation-of-state method should be considered.
Q: How does API MPMS 11.2.1 relate to ASTM D1250 and ISO 91?
A: API MPMS 11.2.1 (1984) is technically equivalent to ASTM D1250 Table 55 (pressure correction factors) and is referenced as ISO 91-2 for compressibility factors. Together with the temperature correction tables of API MPMS 11.1 (ASTM D1250 Table 54, ISO 91-1), they form the complete set of standard hydrocarbon correction equations used worldwide. The 1984 edition of API MPMS 11.2.1 aligns with the first edition of ISO 91.
A: API MPMS 11.2.1 (1984) is technically equivalent to ASTM D1250 Table 55 (pressure correction factors) and is referenced as ISO 91-2 for compressibility factors. Together with the temperature correction tables of API MPMS 11.1 (ASTM D1250 Table 54, ISO 91-1), they form the complete set of standard hydrocarbon correction equations used worldwide. The 1984 edition of API MPMS 11.2.1 aligns with the first edition of ISO 91.
Q: Can I apply these compressibility factors to multi-phase flows or volatile products?
A: No. The standard was developed for single-phase liquids. For volatile liquids such as natural gas liquids (NGLs) or liquefied petroleum gases (LPGs), use API MPMS Chapter 11.2.2. For three-phase or gas‑saturated flows, a thermodynamic flash calculation may be required to determine true volumetric behavior.
A: No. The standard was developed for single-phase liquids. For volatile liquids such as natural gas liquids (NGLs) or liquefied petroleum gases (LPGs), use API MPMS Chapter 11.2.2. For three-phase or gas‑saturated flows, a thermodynamic flash calculation may be required to determine true volumetric behavior.
This article is intended for technical informational purposes and reflects the standard as of its 1984 publication and errata. Verify specific requirements with the latest version of API MPMS 11.2.1 adopted in your jurisdiction. Updated 2026.