Overview
EN ISO 6974-4:2001 specifies a gas‑chromatographic method for the quantitative determination of natural gas composition - with defined uncertainty - using a two‑column system. The method covers nitrogen, carbon dioxide, C1–C5 hydrocarbons and a C6+ composite for both laboratory and on‑line measuring systems. Results are suitable for calculating calorific value, relative density and the Wobbe index.
Key topics and technical requirements
- Scope & application ranges: Mole fraction ranges are defined (examples):
- Nitrogen: 0.001–15%
- Carbon dioxide: 0.001–10%
- Methane: 75–100%
- Ethane: 0.001–10%
- Propane: 0.001–3%
- Butanes/pentanes and C6+: specified down to 0.001%, C6+ up to 0.2%
- Analytical principle: Two packed metal columns in series (short + long) with a backflush arrangement; all analytes detected by a thermal conductivity detector (TCD).
- Columns & packing: Packed with 28% DC‑200 on Chromosorb PAW; example dimensions given (column 1: 0.45 m, column 2: 9 m, ID 4.75 mm).
- Carrier gas & reference materials:
- Helium carrier gas, purity > 99.99%
- Working reference gas mixtures (WRM) prepared gravimetrically per ISO 6142 and validated per ISO 6143
- Operating controls:
- Oven temperature stability ±0.1 °C over 70–150 °C
- TCD time constant ≤ 0.1 s
- Multi‑port injection/valve system for injections and backflush sequencing
- Uncertainty, precision & reporting: Designed to be used with ISO 6974‑1 (tailored analysis) and ISO 6974‑2 (measuring‑system characteristics & data treatment) to establish measurement uncertainty and precision.
Practical applications and users
- Typical users:
- Natural gas laboratories and QA/QC teams
- Pipeline and distribution operators for custody transfer and gas quality monitoring
- Metering station and energy traders requiring accurate calorific value/Wobbe index
- Instrument manufacturers and integrators of on‑line GC systems
- Practical uses:
- Routine composition analysis for billing, trading and regulatory compliance
- On‑line process control and leak/contamination detection (e.g., CO2, N2 levels)
- Method validation and calibration using WRMs for traceable results
Related standards
- ISO 6974 series (Parts 1, 2, 3, 5, 6) - guidance, statistics and alternate column configurations
- ISO 6142, ISO 6143 - preparation and validation of calibration gas mixtures
- ISO 7504 - gas analysis vocabulary
Keywords: EN ISO 6974-4:2001, natural gas composition, gas chromatography, two columns, nitrogen, carbon dioxide, C1–C5, C6+, thermal conductivity detector, WRM, measurement uncertainty.