# Gas Pressure Loss Calculations

For accurately calculating pressure losses in gas pipes there is no simple formula, as is the case for liquids. This is because the specific fluid physical properties of density, temperature and enthalpy do not remain constant over the pipe length. The FluidFlow gas calculation routines are rigorous calculations which allow for the increase in kinetic energy and changing physical properties that occur as gas accelerates with pressure loss. The calculations allow for the changing non-ideal behavior of the gas as it flows by using an equation of state to describe the changes in gas physical properties. There are three equations of state (EOS) available within the software (Benedict Webb Rubin with Hans Starling modifications, Lee Kesler, and Peng Robinson), it is possible to select the most appropriate EOS for each physical property. Using the EOS the gas thermophysical properties such as enthalpy and density are calculated as the gas accelerates. An analytical solution to the EOS, energy and momentum equations is not possible and FluidFlow solves these equations numerically. FluidFlow makes no assumptions of gas ideality or adiabatic flowing conditions. Our algorithm dynamically splits the pipe into segments based on an incremental density change. For each segment, upstream conditions of flow, static pressure, static density and static enthalpy are known and these are used to calculate downstream static temperature and density. From these values it is possible to backsolve the EOS to obtain downstream static pressure. The energy equation and momentum equations can now be solved for each segment. The method is a development of a paper originally published in The Chemical Engineer - Relief Line Sizing for Gases Part1 and 2. Dec 1979 - HA Duxbury. For pressure loss calculations across other fluid equipment items refer to the liquids section.