Overview - ISO 6974-5:2014 (Natural gas, isothermal GC method)
ISO 6974-5:2014 specifies an isothermal gas chromatographic (GC) method for the quantitative determination of nitrogen (N2), carbon dioxide (CO2), methane (C1) through pentane (C5) individually, and a composite C6+ pseudo‑component representing all hydrocarbons with carbon number 6 and above in natural gas. Intended primarily for online process analysers but applicable to laboratory instruments, the method supports calculation of gas properties such as calorific value, density and Wobbe index (see ISO 6976).
Key topics and technical requirements
- Analytical principle: single injection with a column‑switching/backflush arrangement. Heavier hydrocarbons (C6+) are retained and backflushed as one composite peak; lighter components are separated subsequently.
- Detector: Thermal Conductivity Detector (TCD) with time constant ≤ 0.1 s.
- Columns and valves: multi‑column set-up (boiling‑point column split into short/long sections, plus a porous polymer column) using six‑port (or 10‑port) switching valves. Example packing and column dimensions are provided in the standard.
- Isothermal operation: column and valve ovens controlled typically in the 70 °C–105 °C range and maintained to ±0.1 °C.
- Carrier and auxiliary gases: carrier gas Helium (He) ≥ 99.995 %; compressed air or carrier gas for valve actuation.
- Performance and quality control: use of reference gases, repeatability assessment through control charts, and procedures for setting valve timings and restrictor settings (see Annexes).
- Working ranges: the standard lists component mole‑fraction working ranges (e.g., methane typically dominates natural gas composition; nitrogen, CO2 and C1–C5 ranges are provided for guidance).
- Uncertainty: compositional uncertainty and reporting follow ISO 6974‑1 and ISO 6974‑2 guidance.
Applications - who uses ISO 6974-5
- Online process analysts and control engineers implementing continuous composition monitoring in gas processing plants.
- Laboratory analysts performing routine compositional analysis of pipeline or sample gases.
- Gas quality teams and instrument manufacturers validating GC configurations for natural gas streams.
- Use cases include calculation of calorific value, gas trade quality assessment, process control and gas property monitoring where a fast, reliable isothermal GC method is required.
Related standards
- ISO 6974‑1 - General guidelines and calculation of composition
- ISO 6974‑2 - Uncertainty calculations
- ISO 6976 - Calculation of calorific values, density and Wobbe index
- Other parts of ISO 6974 (Parts 3, 4, 6) cover alternate GC arrangements and component sets.
Keywords: ISO 6974-5:2014, natural gas analysis, gas chromatography, isothermal GC, nitrogen, carbon dioxide, C1–C5, C6+ hydrocarbons, Thermal Conductivity Detector, backflush, column switching, gas composition uncertainty.
Open preview (PDF)Natural gas — Determination of composition and associated uncertainty by gas chromatography — Part 5: Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons and C6+ hydrocarbons