Figure 56  General Gas 02 – Methane Pipeline

There are many different formulas that can be used to calculate the flow and pressure loss of compressible fluids in pipelines. Flow through long pipes can usually be considered isothermal and many calculation methods assume isothermal flow.

When using the software to model gas systems there are options to model isothermal flow using various formulas such as: the General Fundamental Isothermal Flow Equation, the Complete Isothermal Flow Equation (used for general compressible fluid systems),  the AGA Isothermal Flow equation, the Weymouth Isothermal Flow Equation, the Panhandle A & Panhandle B Isothermal Flow equations and the IGT Isothermal Flow equation. All of these equations assume a constant gas temperature.

If there are changes in temperature then this will affect the gas density and viscosity. These changes are not automatically accounted for in isothermal flow calculations. When major changes to gas temperatures are encountered, additional fluid zones should be used in the model, with properties set at the appropriate temperature condition.

Methane at 70 deg F is being pumped through a 24" diameter pipeline. Gas flow units of MMSCFD have been selected (mass flow units can also be used for compressible fluid systems). The gas pressure falls from 50.3 psi.g (65 psia) to 10.3 psi.g (25 psia) over the 24 mile length of the pipeline. 

Note: psi.g = psi gauge, psi.a = psi absolute.

The pipeline has been split into a number of sections so that the gas pressure at various points can be established.  It can be seen that as the pressure in the pipeline reduces, the density of the gas reduces & the velocity of the gas increases (the mass flow rate remains the same).