Practical Hydrocarbon Dew Point Specification for Natural Gas Transmission Lines

Hydrocarbon liquid dropout can cause a number of problems in gas transmission lines, including increased pressure drop, reduced line capacity, and equipment problems such as compressor damage. To avoid liquid dropout, most current operating specifications for gas transmission lines require that the lines be operated above the hydrocarbon dew point (HDP) or cricondentherm hydrocarbon dew point (CHDP). The HDP may be determined either by direct measurement such as the Bureau of Mines chilled mirror method or by calculation using an equation of state (EOS) with a measured composition. This project (GPA Project No. 081) was undertaken to determine a practical hydrocarbon dew point specification allowing small amounts of liquids that have no significant impact on operations. Results from the project show that 0.002 gallons of liquid per thousand standard cubic feet of gas (GPM) has a negligible effect on pressure drop and should not disrupt pipeline operations. Calculation of an accurate HDP from a GC analysis such as typically available at a custody transfer point may be useful but is highly dependent on the characterization of the heavy fraction. An extended analysis of the heavy fraction is best. However, an empirical method has been developed to predict the C6, C7, C8, C9 and heavier composition when only a lumped C6+ fraction characterization is available.