ISO 6974-4:2000

Overview

ISO 6974-4:2000 specifies a gas‑chromatographic method for determining the composition of natural gas with defined uncertainty, suitable for both laboratory and on‑line measuring systems. The method uses a two‑column GC arrangement with backflushing and detection by a thermal conductivity detector (TCD) to quantify nitrogen, carbon dioxide, methane (C1) to pentanes (C5) and a combined C6+ hydrocarbon fraction. Results from this analysis are used to calculate calorific value, relative density and the Wobbe index.

Key topics and technical requirements

• Two‑column, backflush principle: Short (≈0.45 m) and long (≈9 m) packed columns in series (28% DC‑200 on Chromosorb PAW) permit backflushing of heavy components and full detection of all peaks by the TCD.
• Carrier gas and references: Helium carrier gas (>99.99% purity). Use gravimetrically prepared or certified working‑reference gas mixtures per ISO 6142 / ISO 6143.
• Operating conditions (examples from standard):
  • Oven temperature: 70–150 °C (typical example 110 °C)
  • TCD temperature: 120–200 °C (typical example 150 °C), time constant ≤ 0.1 s
  • Carrier flow: average linear velocity for N2 of 5–10 cm/s (≈30–60 ml/min); typical flow 40 ml/min
  • Sample loop: 1 ml; typical analysis time ~20 min
• Performance and quality: Peak resolution and detector/column performance must meet criteria (referenced to ISO 7504) so components are quantified with minimal interference. Precision and accuracy statements and test reporting formats are specified.
• Scope limits: Applicable mole fraction ranges (examples): N2 0.001–15%, CO2 0.001–10%, CH4 75–100%, other C2–C5 components specified; C6+ as a summed fraction.

Practical applications

• Routine compositional analysis of pipeline and distribution natural gas for custody transfer, quality control and billing.
• On‑line process monitoring in gas plants and treatment units where continuous composition data are required.
• Laboratory confirmation testing and instrument calibration/validation using working reference mixtures.
• Calculation inputs for energy content, gas quality indices (Wobbe index), and gas property modeling.

Who should use this standard

• Gas laboratory analysts and metrology labs
• On‑line analyzer manufacturers and integrators
• Gas transmission and distribution operators
• Process engineers and QA/QC specialists involved in gas composition and energy measurement

Related standards

• ISO 6974 parts 1, 2, 3, 5, 6 (series on natural gas composition by GC)
• ISO 6142, ISO 6143 (calibration gas mixtures)
• ISO 7504 (gas analysis vocabulary)
• ISO 6568 (related GC methods)

Keywords: ISO 6974-4, natural gas composition, gas chromatography, two-column backflush, TCD, C1–C5, C6+, nitrogen, carbon dioxide, on-line measuring system.