Overview
EN ISO 12213-3:2009 - Natural gas: Calculation of compression factor - Part 3: Calculation using physical properties (ISO 12213-3:2006) specifies a practical method to calculate the compression factor (Z) of natural gas and similar gaseous mixtures using measurable physical properties rather than a full molar composition. The method is based on the SGERG‑88 virial equation and is intended mainly for pipeline quality gas over normal transmission and distribution pressure/temperature ranges. Typical reported uncertainty for pipeline applications is about ±0.1%.
Key Topics
- Purpose: Compute the compression (real‑gas) factor Z from physical inputs rather than full composition.
- Primary input variables (preferred set A):
- Superior calorific value (Hs, volumetric basis)
- Relative density (d, at 101.325 kPa and 0 °C)
- Carbon dioxide mole fraction (xCO2)
- Hydrogen mole fraction (xH2) when present
- Pressure (p) and temperature (T)
- Calculation basis: SGERG‑88 virial equation derived from the GERG‑88 family (master and standard forms). The method internally represents the gas as an equivalent five‑component mixture (hydrocarbon equivalent, N2, CO2, H2, CO).
- Procedure: Iterative reconstruction of an equivalent composition from physical inputs, calculation of virial coefficients B and C, then numerical solution of Z and molar density ρ.
- Ranges of applicability (pipeline quality):
- Pressure: 0 to 12 MPa
- Temperature: 263 K to 338 K (computer implementations may use 250 K lower bound)
- CO2 up to 20% mole fraction; H2 up to 10% mole fraction
- Superior calorific value ~30–45 MJ·m−3; relative density 0.55–0.80
- Performance: Uncertainty increases outside pipeline ranges (see Annex F). The standard includes normative annexes with symbols, method description, worked examples, conversion factors and a Fortran subroutine (SGERG.FOR).
Applications
- Custody transfer, billing and energy accounting where accurate volumetric-to-mass/energy conversions are needed.
- Pipeline transmission and distribution operations for volumetric flow corrections and metering.
- Gas quality monitoring and blending operations (including gases with synthetic admixtures).
- Software developers and instrument manufacturers implementing real‑gas calculations in SCADA, flow computers and gas analysis tools.
- Regulatory bodies and laboratories seeking a standardized, repeatable method for Z‑factor computation.
Who uses it: pipeline engineers, gas metering specialists, custody transfer auditors, gas analysts, flow computer vendors, and standards bodies.
Related standards
- ISO 12213‑1 - Introduction and guidelines
- ISO 12213‑2 - Calculation using molar‑composition analysis
- ISO 6976 - Calculation of calorific values, density and Wobbe index from composition
- GERG technical references (SGERG‑88 / MGERG‑88) used as the thermodynamic basis
Keywords: EN ISO 12213-3:2009, natural gas, compression factor, SGERG-88, calculation using physical properties, pipeline quality gas, superior calorific value, relative density, carbon dioxide, hydrogen.
