ISO 11541:1997

Overview

ISO 11541:1997 specifies a laboratory method for the determination of water content in natural gas at high pressure (greater than 1 MPa). The standard covers both sweet and sour natural gas (containing hydrogen sulfide) with water concentrations of 10 mg/m3 or more, and defines sampling, apparatus, reagents, test procedure, calculation and reporting requirements for accurate high‑pressure moisture measurement.

Key topics and technical requirements

• Principle: A known volume of pressurised gas is passed through an absorption tube filled with phosphorus pentoxide (P2O5). Water is absorbed as phosphoric acid and the mass increase of the tube equals the water mass captured.
• Reagents & safety: Granulated P2O5 on a solid support and silica wool are specified. Strong safety warnings for P2O5 (corrosive) and silica wool handling are included.
• Apparatus: Pressure vessel rated for test pressure, absorption tube, filter tube, stainless steel couplings, needle valves, manometer, gas flow meter (max 10 m3/h), thermometer, barometer, vent line and optional heater. Materials must be compatible with sour-gas service where applicable.
• Sampling & procedure:
  • Pre-weigh absorption and filter tubes.
  • Assemble tubes inside pressure vessel and pressurise from the sample probe.
  • Flow rate: typically 2–3 m3/h; transmitted volume 1.5–3 m3 (do not exceed half the tube capacity).
  • Reweigh tubes after sampling and calculate water content from mass gain and corrected gas volume.
  • Note: testing at pipeline pressure is preferred to reduce sampling time and increase partial pressure of water vapor.
• Interferences: Alcohols, mercaptans, glycol and H2S may react with P2O5 and affect results - the standard flags these as potential interferences.
• Performance: Estimated uncertainty ±5% of measured value (not better than 5 mg/m3); detection limit ~10 mg/m3. Greater gas volume can improve results. Method can be used below 1 MPa if flow rate is reduced.
• Parallel sampling: Recommends asymmetric manifold and duplicate apparatus to detect contamination/heterogeneity; results differing by more than twice measurement uncertainty require retest.
• Reporting: Results reported at normal conditions (288.15 K, 101.325 kPa) to nearest 1 mg/m3, with sample location, pressure, temperature and any deviations.

Applications

• Quality control and moisture monitoring in transmission pipelines, gas processing plants, storage and LNG operations
• Routine laboratory analysis for pipeline custody transfer and compliance with moisture specifications
• Field sampling at compressor stations, wellheads and sniffing points when high‑pressure conditions are present
• Use by gas analysis laboratories, QA/QC engineers, pipeline operators and regulatory inspectors

Related standards

• ISO 10715 (referenced for natural gas sampling guidelines)
• Prepared by ISO/TC 193 (Natural gas) - consult latest editions of referenced standards for current practice.

Keywords: ISO 11541:1997, natural gas, water content, high pressure, phosphorus pentoxide, P2O5, moisture determination, high‑pressure sampling, sour gas, hydrogen sulfide.