by Jeff Sines, Senior Product Engineer
The chemical composition of "natural gas" can vary widely depending on where the gas is extracted, resulting in a wide variation of fluid properties that are needed to calculate the flow rate and pressure drop when designing, sizing, analyzing, or troubleshooting a natural gas piping system. Often the exact composition and fluid properties of a particular natural gas are not known, so the properties of methane (usually being the largest constituent), is used to approximate the behavior of the gas in a system.
Previous versions of PIPE-FLO® Professional did not have natural gas in the fluid table library, but customers could modify the methane fluid table by adjusting the relative molecular mass based on the percent volume of each constituent as described below. With the addition of the NIST REFPROP to PIPE-FLO Professional 15.0, several fluid tables were added to calculate the properties of natural gases from various regions of the United States. This provides the user with the ability to model their natural gas piping systems with greater accuracy in the calculated results.
Table 1 shows the various natural gases available and the difference in the fluid properties calculated at 60 °F & 50 psig and 60 °F & 100 psig using the NIST REFPROP fluid library in PIPE-FLO Professional v15+. Table 1 also shows the properties of methane and modified methane at these pressures and temperatures.
Note that the relative molecular mass varies from 16.04 for Methane to 19.83 for High CO2 and High N2 natural gas, with density at 50 psig varying from 0.1877 lb/ft3 to 0.2324 lb/ft3 for these two gases. The average relative molecular mass of the NIST natural gases (excluding methane and the modified methane) is about 18, with an average density of 0.2125 lb/ft3. This shows that using the modified methane properties is closer to the average of the actual natural gases than using just methane properties.
Figure 1: Various natural gases modeling in PIPE-FLO Professional to show effect of fluid properties on calculated flow rates and velocities.
Note that at 50 psig inlet pressure, the calculated flow rates vary from 1066 acfm (4740 scfm) for the gas with the highest relative molecular mass, to 1185 acfm (5264 scfm) for the gas with the lowest relative molecular mass. The average flow rate at this pressure (excluding methane and modified methane) is 1116 acfm (4961 scfm). Again, the modified methane properties result in flow rates closer to the average than using just the methane fluid properties.
Evaluating the results at 100 psig, the flow rates vary from 789.9 acfm (6349 scfm) to 889.2 acfm (7047 scfm) for the gases with the highest and lowest relative molecular mass, respectively. For a particular gas, the flow rate in scfm has increased going from 50 psig to 100 psig for the same 5 psid pressure drop, indicating that the mass flow rate has increased (by about 34%), mainly due to the increase in gas density by about 79%.
However, the flow rate in units of acfm has decreased with increasing pressure. This may appear to be contradictory, but the higher density gas has a lower velocity even though the mass flow rate has increased. For each gas, the velocity has decreased by about 25%, resulting in a 25% decrease in acfm going from 50 psig to 100 psig.
Calculating Relative Molecular Mass of Natural Gas Based on Chemical Composition by Percent Volume
To calculate the relative molecular mass of a mixture of gases, the percent volume and relative molecular mass of each of the constituents must be known. A weighted average can then be calculated as shown in Table 3.
|Calculating the Relative Molecular Mass of a Gas Mixture|
Rel. Mol. Mass
|Carbon Dioxide (CO2)||0.1%||x||44.009||=||0.044|
Since the chemical composition and fluid properties of natural gas varies widely depending on where the gas is extracted from, the calculation of flow rate and pressure drop will vary from gas to gas. With the addition of fluid property calculations using the NIST REFPROP in PIPE-FLO Professional 15+, several fluid tables are now available to determine the fluid properties of various natural gases for more accurate results when modeling a system in PIPE-FLO.
Methane is typically the largest constituent of most natural gases and the fluid properties of methane could be used as a first approximation for calculations. Modifying the methane fluid table using a weighted average relative molecular mass for the natural gas mixture will yield more accurate results. However, the most accurate results will be achieved when the fluid properties of the actual natural gas are used.
For more information on NIST REFPROP, visit: https://www.nist.gov/srd/refprop