# Gas Volume Definitions

Generally the density of a gas flowing in any piping system is not constant. This means that it is not meaningful to specify gas flowrate in actual volume flow units without providing the temperature and pressure base at which the actual volume is referred to. It is common practice to refer to gas volume flow against a specified basis. There are many bases available for referencing gas volumes. Today the most universally accepted reference is metric Standard Conditions (STP) which are defined by ISO 5024 at 15°C and 101325 Pascals. The American Gas Association and the International Gas Union both support the use of this reference. Normal conditions (NTP) defined as 0°C and 101325 Pascals are still used throughout Europe. FluidFlow can use either basis. The FluidFlow gas calculation routines are rigorous calculations which allow for the increase in kinetic energy 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. There are three equations of state available (Benedict Webb Rubin with HS modifications; Lee Kesler; and Peng Robinson), it is also 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. These intensive calculations mean a longer solution time than that experienced in earlier versions of the software. FluidFlow makes no assumptions of gas ideality or adiabatic flowing conditions the most accurate solution for real gases is made. Sonic Velocity - This is the maximum velocity a gas can reach in a pipe of constant cross-section. The sonic velocity is a thermophysical property and is a function of pressure and temperature. As a gas accelerates the density, pressure and temperature decrease which makes the accurate estimation of sonic velocity difficult. FluidFlow does not use a constant sonic velocity in its algorithms, but estimates this value for each density increment as Sqrt(pressure change / density change). The accurate estimation of sonic velocity is important because it determines where sonic choking occurs.