Overview
EN ISO 20765-1:2018 (ISO 20765-1:2005 adopted by CEN) defines a standardized method to calculate thermodynamic properties of natural gas in the gas phase for transmission and distribution applications. The standard is intended for pipeline‑quality gases and sets out equations and procedures to compute volumetric properties (compression factor, density) and caloric properties (enthalpy, heat capacity, Joule–Thomson coefficient, speed of sound, etc.) for mixtures that remain in the single‑gas phase. The method is based on a fundamental Helmholtz free energy formulation and leverages the structure of the AGA8 approach to provide analytical derivatives for reliable implementation.
Key topics and requirements
- Scope: Gas‑phase thermodynamic properties for conditions typical of gas transmission and distribution; not for two‑phase or dense‑fluid ranges.
- Fundamental basis: Equation of state expressed as Helmholtz free energy (ideal + residual parts), enabling calculation of both volumetric and caloric properties from analytical derivatives.
- Composition input: Detailed molar (component) analysis - a 21‑component representation is specified; components with mole fraction > 0.00005 should normally be included.
- State variables: Temperature and pressure are typical inputs; the standard gives conversion procedures to density where required.
- Accuracy / uncertainty:
- Volumetric properties (compression factor, density): ≈ ±0.1 % (95 % confidence).
- Caloric properties: generally larger uncertainty; the standard documents the expected accuracy and how input uncertainty propagates.
- Implementation guidance: Analytical formulas and normative annexes provide symbols, equations, documentation and implementation notes to avoid numerical differentiation.
- Normative and informative annexes: Include the Helmholtz ideal gas formulation, detailed equation parameters, implementation examples and trace component assignment guidance.
Applications
- Pipeline and distribution network modeling: Accurate density and compressibility for flow and hydraulic calculations.
- Custody transfer and metering: Compression factor and density calculations for volume/energy conversion in billing.
- Energy and process simulations: Enthalpy and heat capacity data for thermodynamic modeling, pressure‑letdown and heating/cooling calculations.
- Instrumentation and software: Embedded in gas property libraries, flow computers, energy calculators and SCADA analytics.
- Regulatory and standards compliance: Harmonizes thermodynamic property calculations across operators and jurisdictions.
Who should use this standard
- Gas transmission and distribution engineers
- Metering and custody transfer specialists
- Energy accountants and auditors
- Software developers of gas property libraries and flow computers
- Laboratories performing gas analysis and property calculations
- Regulators and standards bodies specifying calculation methods
Related standards (examples)
Keywords: EN ISO 20765-1:2018, ISO 20765-1, natural gas thermodynamic properties, Helmholtz free energy, compression factor, gas density, enthalpy, heat capacity, AGA8, pipeline‑quality gas.