ISO 20765-5:2022

Overview

ISO 20765-5:2022 - Natural gas - Calculation of thermodynamic properties - Part 5 provides simplified, practical methods to calculate dynamic viscosity, the Joule–Thomson coefficient, the isentropic exponent, and speed of sound (excluding density) for use in natural gas flow metering. The methods are intended for gas-phase conditions in the temperature range −20 °C to 40 °C and absolute pressures up to 10 MPa. The standard emphasizes straightforward formulas suitable for metering applications (for example, orifice plate flow measurement) where computational simplicity and fit‑for‑purpose accuracy are required.

Key topics and requirements

• Scope: Methods are specified for calculating viscosity, Joule–Thomson coefficient, isentropic exponent, and speed of sound for metering natural gas flow. Density calculation is excluded and should be obtained using high‑accuracy methods (e.g., GERG‑2008 / ISO 20765‑2).
• Viscosity methods:
  • Lohrenz–Bray–Clark (LBC) method is recommended when gas composition is known. LBC is widely implemented, requires basic component data and molar density (which can be obtained via GERG‑2008). Estimated uncertainty of LBC is about 4% (95% CI) for the documented data set.
  • A simpler empirical formula is provided for cases where only temperature and mass density are available; its estimated uncertainty is about 5% (95% CI).
• Other properties:
  • Simple formulae for Joule–Thomson coefficient, isentropic exponent (κ), and speed of sound are given. These are less precise than full equations of state but are judged “fit for purpose” in metering contexts.
• Uncertainty guidance: The document discusses how uncertainties in these properties propagate into mass flowrate calculations (per ISO 5167) and provides target uncertainty levels to meet common metering accuracy goals.
• Implementation aids: Annexes include symbols/units, example code for the LBC viscosity function, routines to infer equivalent component mixtures when only bulk properties are available, and example calculations.

Practical applications and users

This standard is targeted at professionals involved in natural gas flow measurement and metering:

• Flow measurement and metering engineers (orifice plate, differential pressure meters)
• Pipeline operators and custody transfer teams
• Gas metrology and instrumentation manufacturers
• Software developers implementing gas property libraries and flow calculators
• Laboratories performing flow verification and uncertainty analysis

ISO 20765-5:2022 is especially useful where rapid, reproducible property estimates are needed within the metering workflow and where full equation‑of‑state calculations may be impractical.

Related standards (for context)

• ISO 20765 series (thermodynamic property calculations for natural gas)
• ISO 20765-2 - GERG‑2008 equation of state (recommended for accurate density)
• ISO 5167 - Measurement of fluid flow by pressure‑differential devices (orifice meter calculations)

Keywords: ISO 20765-5:2022, natural gas, viscosity, Joule–Thomson coefficient, isentropic exponent, speed of sound, thermodynamic properties, metering, LBC method, GERG‑2008, ISO 5167.