Overview
EN ISO 6974-5:2014 (ISO 6974-5:2014) specifies an isothermal gas chromatographic method for the quantitative determination of nitrogen (N2), carbon dioxide (CO2), methane to pentanes (C1–C5) and a composite C6+ hydrocarbon pseudo‑component in natural gas. Intended for both online process analysers and laboratory instruments, the method supports composition-based calculations such as calorific value (CV), density and Wobbe index.
Key topics and requirements
- Scope and analytes: Measurement of N2, CO2, CH4, C2–C5 isomers and a composite C6+ representing all hydrocarbons ≥ C6. Working ranges are provided for each component (see Table 1 of the standard).
- Isothermal column switching: A single injection with automated column switching and accelerated backflush of the C6+ fraction. Lighter components are retained while heavier hydrocarbons are eluted and then separated.
- Detector: Thermal Conductivity Detector (TCD) is specified for component detection.
- Composite C6+ handling: C6+ is measured as one “pseudo‑component” peak and calibrated as a composite; detailed extended analyses are used to calculate properties of the C6+ fraction if needed.
- Assumptions & limitations:
- Oxygen is not expected in source gas; any O2 (air contamination) will be measured with nitrogen.
- Helium and argon are assumed negligible and not routinely analysed.
- Samples must be free of liquid hydrocarbon condensate and water.
- Quality control & uncertainty:
- Establish repeatability through repeated analysis of a reference/test gas and maintain control charts (mean ± standard deviations).
- Uncertainty treatment aligns with ISO 6974‑1 and ISO 6974‑2 procedures for compositional uncertainty reporting.
- Documentation: Clauses cover apparatus, procedure, control of the apparatus, expression of results and test reporting.
Applications
- Routine online process monitoring of natural gas composition in transmission and distribution systems.
- Laboratory compositional analysis for billing, custody transfer, and quality assurance.
- Input to thermophysical calculations (CV, density, Wobbe index) and to legal or commercial contracts.
- Configuration and performance verification of gas chromatographs used in gas industry operations.
Who should use this standard
- Gas analysis laboratories and technicians
- Pipeline and transmission operators
- Gas utilities and trading companies
- Instrument manufacturers and integrators of gas chromatographs
- Quality managers responsible for composition-based commercial calculations
Related standards
- ISO 6974‑1: General guidelines and calculation of composition
- ISO 6974‑2: Uncertainty calculations
- Other parts of ISO 6974 (Parts 3, 4, 6) for alternative column/analysis configurations
- ISO 6976 for calorific value calculation
Keywords: natural gas, gas chromatography, ISO 6974-5, isothermal method, nitrogen, carbon dioxide, C1–C5 hydrocarbons, C6+ hydrocarbons, calorific value, Wobbe index, thermal conductivity detector, uncertainty, online analysis.
