API MPMS 11.2.4 (2007, Reaffirmed 2012): Temperature Correction for NGL and LPG Volume Tables 23E and 24E

Scope and Application

API MPMS 11.2.4, part of the Manual of Petroleum Measurement Standards (MPMS), provides standardized methods for correcting the volume of liquefied petroleum gases (LPG) and natural gas liquids (NGL) to a base temperature of 60°F (15.562°C). This standard specifically covers the use of Table 23E for NGL and Table 24E for LPG, which are generated from the thermal expansion coefficients consistent with the International Temperature Scale of 1990 (ITS-90). The standard applies to products in the liquid phase within specific temperature and relative density ranges typical for commercial custody transfer and inventory control.

The standard is applicable for observed temperatures between –40°F and +140°F (–40°C to +60°C) and for relative densities (60°F/60°F) from 0.350 to 0.800. It is essential for any operation involving volume measurement of LPG (such as propane, butane, and mixtures) and NGL (including ethane–propane blends) where accurate temperature correction is required for billing, regulatory compliance, and operational accounting.

Technical Requirements

Correction Factors and Generation of Tables

Tables 23E and 24E are derived from the standardized thermal expansion coefficient (γ) which varies with temperature and density. The volume correction factor (VCF) is defined as:

VCF = exp[ –γ · ΔT · (1 + 0.8 · γ · ΔT) ]

where ΔT is the difference between the observed temperature and 60°F, and γ is obtained from polynomials fitted to experimental data for LPG and NGL. The standard provides the exact polynomials for each product group, along with certified table values that serve as interpolation references.

Note: Values above are illustrative. Always reference the official API tables for custody transfer calculations.

Measurement and Calculation Process

The standard requires the following inputs:

• Observed volume at flowing temperature (V_obs)
• Observed temperature (T_obs) to ±0.25°F repeatability
• Relative density (or density at 60°F) determined per API MPMS Chapter 9

The volume corrected to base temperature (V₆₀) is then:

V₆₀ = V_obs × VCF

Interpolation between table entries is performed using linear interpolation, as the granularity of the published tables is sufficient for most operational needs. However, for automated calculations, the standard endorses the direct use of the underlying polynomials to avoid rounding errors.

Implementation Highlights

When implementing API MPMS 11.2.4, operators should integrate it within the broader MPMS framework—especially Chapter 11 (Physical Properties Data) and Chapter 12 (Calculation of Petroleum Quantities). Key implementation aspects include:

• Temperature Measurement: Use validated temperature sensors (RTDs or thermocouples) with traceability to ITS-90. For LPG and NGL, the temperature sensor must be immersed in the liquid stream or in a thermowell that provides a representative sample.
• Density Determination: Measure relative density at 60°F using a hydrometer (per API MPMS 9.1) or a digital density meter (per API MPMS 9.2). The density must be corrected for glass expansion if hydrometers are used.
• Software Validation: Any calculation engine that implements the Table 23E or 24E algorithms must be validated against the official API tables at 20 test points per product range, with a maximum allowable deviation of ±0.02% for VCF.
• Custody Transfer: For fiscal metering, the standard should be used in conjunction with API MPMS 12.2 (Calculation of Liquid Petroleum Quantities Measured by Turbine or Displacement Meters) or API MPMS 14 (Natural Gas Fluids Measurement) as appropriate.

Compliance and Verification

API MPMS 11.2.4 was reaffirmed in 2012, confirming its continued technical relevance and alignment with the International Temperature Scale of 1990 (ITS-90). Compliance involves:

• Periodic Verification: Compare correction factors obtained from in-house calculations with the published tables at least quarterly. Document any deviations and recalibrate instruments if systematic errors exceed 0.03%.
• Traceability: Ensure all temperature and density measurements are traceable to national standards (NIST in the U.S. or equivalent).
• Audit Trail: Maintain records of observed volumes, temperatures, densities, and the VCF used for at least five years, per typical custody transfer requirements.
• Product Sampling: For non‑typical LPG/NGL streams (e.g., with high non‑hydrocarbon content), verify that the product falls within the scope of the standard. If not, apply appropriate engineering corrections or consult specialized literature.

Frequently Asked Questions