ISO 20765-2:2015

Overview

ISO 20765-2:2015 defines a rigorous method to calculate thermodynamic properties of natural gas and similar mixtures in the homogeneous (single‑phase) regions: gas, liquid and supercritical (dense‑fluid). The standard provides an equation‑of‑state framework based on the Helmholtz free energy to compute volumetric and caloric properties over extended temperature, pressure and composition ranges beyond transmission/distribution conditions.

Keywords: ISO 20765-2, natural gas, thermodynamic properties, single‑phase, equation of state, Helmholtz free energy.

Key topics

• Thermodynamic basis: Fundamental equation formulated from the reduced Helmholtz free energy, including ideal‑gas and residual contributions, and reducing functions for density and temperature.
• Properties calculated: Volumetric (compression factor, density) and caloric (enthalpy, heat capacity, Joule‑Thomson coefficient, speed of sound) properties.
• Method of calculation: Input variables, conversion from pressure to reduced density, numerical implementation details for homogeneous regions.
• Ranges of applicability: Extended composition limits (e.g., methane down to 0.30 mole fraction; nitrogen up to 0.55; CO2 up to 0.30; ethane up to 0.25; propane up to 0.14) and support for hydrogen‑rich and CO2‑rich streams used in sequestration scenarios.
• Uncertainty assessment: Expanded uncertainties (95% confidence) documented - e.g., volumetric properties maintained at ≤ 0.1% and speed of sound generally within 0.1%; liquid and dense‑fluid uncertainties quantified where data permit.
• Supporting material: Normative annexes with symbols/units, critical parameters and molar masses, formulation of residual Helmholtz terms, reducing functions, examples and guidance for trace components.

Applications and users

ISO 20765-2 is intended for engineers, simulation and metering software developers, pipeline operators, gas analysts, LNG designers and researchers who need accurate property calculations for:

• Metering and custody transfer where dense or nonstandard gas compositions occur
• Pipeline integrity and hydraulics involving high‑pressure or low‑temperature conditions
• LNG design and cold‑end process modelling (saturated liquid density accuracy for 100–140 K)
• Carbon dioxide sequestration and CO2‑rich gas mixtures
• Thermodynamic modelling tools and process simulators requiring a high‑accuracy equation of state

Practical benefits include consistent property reporting, reduced measurement uncertainty, and improved modelling of nonstandard or multi‑component natural gas streams.

Related standards

• ISO 20765-1 - Gas phase properties for transmission and distribution applications (complementary)
• ISO 7504, ISO 14532 - Gas analysis and natural gas vocabulary
• ISO 80000-5:2007 - Quantities and units - Thermodynamics

For implementers, the standard’s annexes provide parameter tables, example calculations and guidance on uncertainty and reporting, making ISO 20765-2 a robust reference for advanced natural gas thermophysical property calculations.