This work illustrates the importance of parameters for natural gases being transported by pipeline. An algorithm is proposed that allows for estimating pressure drops and flow conditions along single phase gas and low liquid content two-phase flow natural gas pipelines. The Masjed-I-Soleiman (MIS) to Mahshahr gas pipeline, located in the southwest of Iran, was used to demonstrate the impact of different design variables on liquid formation in the line and on line pressure drop.
Nearly all natural gas contains a heavy fraction that is reported in the gas analysis as a C6+ or C7+ fraction. The fraction normally contains a mixture of compounds and it frequently is the determining factor in fixing the dew point of the gas. Liquid formation in a natural gas pipeline changes the line pressure drop and flow characteristics. A seemingly inconsequential amount of liquid can cause significant changes in line pressure drop and/or capacity. The liquid will accumulate in low spots and immediately before pipeline elevation increases, and this can cause additional problems in line operation. In extreme cases lines designed for gas flow may be incapable of operating when liquid forms and the line must transport a two-phase mixture. For many tasks in gas processing, selecting and using an arbitrary normal paraffin (such as nC7 or nC8) will produce calculation results of sufficient accuracy. For predicting gas dew points and trace liquid formation, however, more detailed information on the fraction is required. To incorporate a C6+ or C7+ fraction in an equation of state calculation, information must be available about the density, normal boiling point, critical temperature, critical pressure, and acentric factor for the component. In most natural gas pipelines both temperature and pressure vary along the length of the line, and liquid may form at any point. In addition, retrograde condensation may also occur, even at pressures of 1000 psia or less, Maddox and Erbar . Consideration of all these possibilities requires that a rather extensive algorithm be developed to properly organize the calculations and make certain they are completed in the correct sequence. 1 Moshfeghian is on sabbatical leave from University of Shiraz, Shiraz, Iran 2 The gas analysis used as the basis for design of the MIS-Mahshahr pipeline is shown in Table 1. There is a small amount of C6 (hexane) in the analysis for which the specific gravity is shown to be 0.7. This is larger than the reported  gravity of hexane (0.6) and even larger than the reported specific gravity of n-heptane (0.69) . Calculations using the heaviest component as hexane (C6) show the stream to be well above its dewpoint at all points on the pipeline. Using the heaviest component as heptane (C7) shows the gas above its dewpoint at pipeline inlet, but liquid formation down the line. Using the heaviest component as octane (C8) shows liquid present at the inlet and throughout the length of the line.