Overview
EN ISO 20765-5:2022 - Natural gas: Calculation of thermodynamic properties - Part 5 specifies simplified, practical methods to calculate dynamic viscosity, Joule–Thomson coefficient, isentropic exponent, and speed of sound for natural gas metering. The standard targets gas-phase conditions in the temperature range −20 °C to 40 °C and absolute pressures up to 10 MPa. Density is excluded from the scope (density is normally obtained from an equation of state such as GERG‑2008 / ISO 20765-2), but accurate density is required as input for several calculations.
Key topics and technical requirements
- Recommended calculation methods:
- Lohrenz–Bray–Clark (LBC) method is recommended for gas-phase viscosity because it is simple, widely implemented, and needs minimal component data. LBC requires gas composition and molar density as inputs.
- Use of GERG‑2008 (ISO 20765-2) is recommended to provide molar density when composition is known.
- Simplified formulae for Joule–Thomson coefficient, isentropic exponent, and speed of sound are provided; these are intentionally simple and intended to be fit for metering purposes (though less accurate than full equations of state).
- Uncertainty guidance:
- The standard gives uncertainty assessments showing density must be calculated with high accuracy for custody metering, while viscosity, isentropic exponent, and Joule–Thomson coefficient may have substantially larger allowable uncertainties. A target uncertainty of about ≤25% for these secondary properties is considered acceptable to achieve a typical 0.1% flowrate uncertainty (k = 2) when density is accurate.
- Informative annexes include example code (Visual Basic) for LBC implementation, component parameter guidance, and methane viscosity examples.
Applications and who uses it
- Primary users:
- Flow computer developers, custody transfer metering engineers, pipeline operators, gas distribution companies, and laboratory analysts.
- Practical applications:
- Orifice-plate and differential-pressure metering calculations (ISO 5167), flowrate correction and compensation, metering system verification, and on-line flow measurement where compact, computationally efficient thermophysical property models are required.
- Benefits:
- Enables robust, standardized calculation of thermodynamic parameters needed in high-pressure natural gas metering and helps ensure consistent measurement uncertainty across systems.
Related standards
- ISO 20765-2: GERG‑2008 equation of state for density calculations
- ISO 20765 series (other parts addressing thermodynamic properties)
- ISO 5167 (measurement of fluid flow by pressure-differential devices / orifice metering)
Keywords: natural gas, viscosity calculation, Lohrenz‑Bray‑Clark, Joule‑Thomson coefficient, isentropic exponent, GERG‑2008, metering, flow measurement, ISO 20765-5.