The Fundamentals Of Pipeline Gas Chromatographs

By Shane Hale, Emerson Process Management | July 2011, Vol. 238 No. 7

Figure 1: The typical sample system components for a pipeline natural gas measurement.

Gas chromatographs (GCs) are installed all over natural gas pipeline networks, providing an analysis of the flowing gas and calculating the physical properties used for the flow calculations and for custody transfer. However, a clear understanding of just how the GC works and the considerations that need to be made for the installation and operation of the GC are often lacking in the industry. This article discusses the major components of the GC and provides an understanding of the theory and practice of gas chromatography in the pipeline industry.

Natural gas in a pipeline is a mixture of hydrocarbons and diluents such as nitrogen and carbon dioxide. The composition of the mixture varies considerably and therefore the physical properties of the gas, such as the energy content (Btu or kilojoules) and density (lbs/cf or kg/m3), also vary. The job of the GC is to measure the concentration of each of the components so the physical properties can be calculated. The GC does this job in a three-step process: the first step is to take a fixed volume sample of the gas; next is to separate the mixture into the individual components; and finally the individual components are measured. The time taken to do these three steps depends on the type of GC and the degree of detail of the analysis, but typical C6+ GCs perform this cycle every three to four minutes.

Taking The Sample
The maxim “garbage in – garbage out” applies to taking a sample for a GC analysis. The world’s best chromatograph will still give incorrect readings and cause errors in the custody- transfer calculations if the gas it is analyzing is not representative of the flowing gas in the pipe. To take a sample from the flowing gas stream, a sample probe will take a representative sample away from the liquid and solid contaminants usually found on the pipe wall. Any solid (down to at least two-micron) or liquid contaminants should be removed from the sample, and the pressure reduced to between 15-30 psig (100-200 kPaG).

When the pressure is reduced, and the sample is transported to the GC through the sample lines (typically 1/8-inch stainless steel tube), the temperature of the sample must not drop below the hydrocarbon dew point (HCDP) as this will result in the heavy components (also the high energy content components) dropping out into the liquid phase (forming condensate or “drip”) and the gas sample will no longer be the same as what is in the pipeline. To avoid dropping out any of the components from the sample, the API 14.1 standard recommends all the sample components should be heated to at least 30°F (16.6°C) above the expected hydrocarbon dew point (API 2006) using heated regulators and heat-traced sample lines. Figure 1 shows how these components fit together on the typical pipeline natural gas application.

When the sample reaches the GC, it will go through the local sample handling panel that provides the “last line of defense” particulate and liquid filtration and the stream selection valves. The sample fills a fixed volume sample loop that is injected into the chromatograph. To ensure the sample size is not affected by variations in the sample pressures of different streams, the sample loop is equalized to atmospheric pressure and the sample is injected into the columns.