Hydrocarbon Dew-Point Measurement in Lean Natural Gas

Gas tap with pipeline system at natural gas station

Why measure hydrocarbon dew point?

Hydrocarbon dew point (HCDP) is a vital measurement in natural gas to ensure the gas quality and safe operating conditions and to maintain the efficiency of the processing and distribution systems.

Sometimes referred to as ‘hydrocarbon liquid drop-out’, HCDP indicates the temperature at which natural gas moves from fully gas phase into mixed phase when hydrocarbon condensates start to form. This parameter is influenced by both pressure and composition, so higher proportions of heavier hydrocarbons in the gas mix will raise the HCDP. Each gas composition has its own unique HCDP temperature and pressure relationship, which can be depicted as a dew-line curve on a phase envelope (see below). The cricondentherm is the highest HCDP for that composition found at pressure x. When natural-gas processing maintains cricondentherm below the minimum pipeline operating temperature, then the gas flow will be free of all liquid HC content, irrespective of any pressure changes.

Typical natural gas phase envelope

Figure 1: Typical natural gas phase envelope

If these heavier hydrocarbon components are allowed to condense into liquids at any point in the process, they can create several problems: 

  • Corrosion of pipelines and distribution systems as liquids become a breeding ground for bacterial corrosion.  
  • Damage to compressor blades due to high-velocity impact.
  • Damage to gas turbine burners and blades as HC condensates cause a hotter, longer flame.
  • Risk of the gas failing to meet the contracted quality, which may result in a ‘shut-in’.
  • All the above can have huge financial implications, from production/distribution downtimes to equipment failure.

    What is lean natural gas?

    Natural gas is described as lean when it is primarily made up of methane or ethane and does not contain the normal concentrations of heavier hydrocarbons that are found in rich natural gas. For this reason, it has a lower calorific value and a significantly lower HCDP.

    A low HCDP reduces the risk of hydrocarbon liquids forming in the system, but the HCDP can rise during process fault conditions, so measurement is still an important requirement.

    Typical lean natural gas phase envelope

    Figure 2: Typical lean natural gas phase envelope

    Why is a HCDP measurement difficult in lean natural gas?

    The primary instrument for measuring HCDP is the chilled mirror. The device works by reducing the temperature of the mirror until hydrocarbon liquids start to form on it. The HCDP is determined either visually by a skilled operator or by automated optics such as the PST-Michell Condumax II.

    Measuring a very low HCDP presents a unique challenge: getting the mirror temperature low enough. When the instrument reaches its maximum cooling capacity without any hydrocarbon condensation forming on the mirror, it becomes impossible to get a measurement.

    The PST solution

    The Condumax II has a maximum mirror temperature depression of around 55°C, but there is a measurement option. It is important to monitor the HCDP in case the process goes into a fault condition and heavier hydrocarbons appear in the composition, so the Condumax II can be used as an alarm only.

    Under normal conditions, when the measurement phase is initiated, the mirror temperature will be driven down to the minimum cool temperature without the trip point being reached. The message “HCdp below cooling limit” will display, indicating that the gas is in its normal condition. By setting a minimum cool limit on the mirror temperature, the Peltier heat pump won’t be working at capacity during each measurement cycle, thus extending its lifespan.

    We are the world’s leading experts in moisture monitoring and dew-point measurement.  We have eight different technologies covering all moisture applications, backed by unrivalled technical and customer support.  To learn more, talk to one of our application specialists today. 

    Related Information

    Natural Gas

    Related Blogs

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    What is the Ideal Way to Measure Hydrocarbon Dew Point?

    Hydrocarbon Dew-Point Measurement in Natural Gas

    Best Practices for Measuring Dew Point in Transmission Gas

    Moisture Measurement in Biomethane

    Related Categories

    Hydrocarbon Dew-Point Analyzers for Natural Gas


    Author: James Coveney, Technical Support Engineer




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    Related Products

    Portable Dew-Point Tester – Michell CDP301
    Hydrocarbon Dew-Point Analyzer - Michell Condumax II


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