Measuring Hydrocarbon Dew Point with Gas Chromatographs

One of the challenges in measuring hydrocarbon gases is the precipitation condensation of liquids, which can impact measurement accuracy and create maintenance issues. Emerson’s Shane Hale authors an article, Hydrocarbon Dew Point Measurement Using Gas Chromatographs in Pipeline & Gas Journal magazine.

Much like the dew that forms on a lawn when the temperature drops below the dew point, the hydrocarbon dew point (HCDP), is the temperature at a defined pressure at which hydrocarbon liquids begin to form. Shane highlights the challenges these liquids in the gas pose:

Hydrocarbon liquids in the gas stream can cause hydrate formation, increase compression costs, cause issues with pressure regulator freezing, and lead to damage of gas turbines and other end-user equipment. Additionally, when the hydrocarbon liquids entering the transmission network are not measured through gas metering stations, or the high energy heavy hydrocarbons drop out as condensate (or “drip”) in the transmission network, the energy content of the gas leaving the network is less than the energy of the gas entering the network, resulting in increases in lost and unaccounted for (LAUF) energy.

Because of the lost and unaccounted for energy, Shane notes that custody transfer agreements are increasingly specifying HCDP limits. The traditional way to measure HCDP is to use a chilled-mirror device, much like fogging a mirror with your breath. It works by reducing:

…the temperature of a mirror in a measurement chamber filled with the natural gas until enough hydrocarbon mist condenses on the mirror to be detected. Other dedicated HCDP analyzers using different measurement techniques are also available; however, they all provide a HCDP only at a single pressure and are dedicated analyzers that provide a single measurement.

Gas chromatographs, such as Rosemount Analytical 700XA Process Gas Chromatographs, have been used in custody transfer applications for measurements such as:

…energy content, compressibility, density, and other physical properties.

Shane describes how this same gas chromatograph can be used to help determine the dew point for a mixture of hydrocarbons by [hyperlinks added to quote]:

…using an equation of state (EOS) to calculate the hydrocarbon dew point at any pressure from the composition obtained from a gas chromatograph (GC). By entering the composition of the natural gas into a recognized equation of state, the theoretical HCDP can be calculated for any pressure as well as the cricondentherm (the highest dew point temperature at any pressure). The validity of the calculated value depends on the accuracy of the composition used especially for the higher carbon number hydrocarbons (C6 to C9).

Source: Pipeline & Gas Journal

Knowing the dew point at any pressure is valuable as an:

…operational diagnostic that can be used to avoid two-phase flow, and therefore incorrect measurement. In normal single-phase gas flow conditions, the HCDP of the gas is less than the flowing temperature and all of the hydrocarbons are in the gas phase… By calculating the HCDP at the current pipeline pressure and comparing it to the current pipeline temperature, the operator can determine if there is two-phase flow. Even better, by setting an alarm so that if the HCDP temperature is within a certain range (say 10°F) of the flowing temperature, the operator can have an early warning that two-phase flow (and thus, inaccurate flow measurement) is about to occur…

Shane provides the requirements you should consider in applying gas chromatographs in hydrocarbon dew point applications. He concludes:

Maintaining single-phase flow through the flow meter and thus maintaining flow measurement accuracy can result in significant improvements in lost and unaccounted for gas, directly improving profitability. As this functionality is an extension of the capabilities of the gas chromatograph that is already needed for other functions in the custody transfer application and well understood by metering personnel, it provides an easy method for pipeline operators to protect their gas pipeline systems from hydrocarbon liquids without significant additional capital expenditure or engineering and operational effort.

If hydrocarbon custody transfer is part of your process, the article should be well worth your time to read.

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Hydrocarbon Dew Point Measurement Using Gas ChromatographsUpdate and bump: The article is now available as a printable PDF file.

Update 2: I received a note from a reader that condensation more accurately reflects what I’m try to say than precipitation. I must agree and have updated the first sentence.