When entering gas data, we recommend defining the gas properties for a pressure condition that is on the Ideal Gas Law line. Normally the easiest approach is to always define the gas properties for the specific temperature within your system and at the 0 bar.g pressure (atmospheric condition), even if your system generally operates at a completely different pressure.


The software will automatically account for and calculate the density of the gas as the pressures within your model. If you have a system that operates at a very high-pressure condition where the ideal gas law does not predict the true real density of the gas, then a Z compressibility factor can also be defined.


A Compressibility Factor can be specified on the Calculations Tab. The Pipe Flow Expert software can use the Z compressibility factor to calculate the REAL density of the gas at a high-pressure condition. It does this by calculating the density based on the ideal gas law and then it applies the Z compressibility factor to arrive at the real gas density. 


For example, if your system is operating at some high-pressure condition, where the real gas density is different from the one as calculated by the ideal gas law, then you can specify a Z Compressibility Factor on the Calculations Tab (under Config Options in Pipe Flow Expert). The compressibility factor will be applied as the software solves the model, by first calculating the density of the gas at the high-pressure nodes using the ideal gas law and secondly by then applying the Z compressibility factor to arrive at the real density within your model during calculations.


Using the above approach, where the fluid density is defined for the standard atmospheric pressure condition (0 bar.g) at some specific temperature condition, when the standard flow rate (volume of flow at the defined standard condition) is reported, the Pipe Flow Expert software will convert the mass flow to the standard volume using the correct fluid density for the standard pressure condition (even if a Z compressibility factor was specified for use in calculating the real-density at the nodes within a high-pressure system). 


i.e. the mass flow rate, even in a system with high pressure conditions, where a z compressibility factor is applied, will convert back to give the correct standard flow volume (the correct SCMH or SCFM value for example).