When entering gas data, we recommend defining the gas properties for a pressure condition using data on the Ideal Gas Law line and then applying a compressibility factor if required. 


Normally the easiest approach is to 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 at 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 in Config Options 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 (divides by) the Z compressibility factor to arrive at the real gas density. 


The standard condition used for reporting is set in Config Options → Units and is independent of the temperature at which you define the fluid density; the software converts between the two.


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 is reported (volume of flow at the selected standard condition), the Pipe Flow Expert software will convert the mass flow to the standard volume using the correct fluid density for the selected 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 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).



Important Note: 


If the real gas density is defined at the pressurized condition, then the z-compressibility factor must be left at 1.0 so that it is not applied during the calculations (this would change the already given real density). Provided the system operates at similar pressure conditions then the calculated mass flow results will be correct however when the volume flow at standard conditions is calculated with the compressibility factor left at 1.0, it will not convert to the correct volume at standard temperature and pressure conditions (because there is no compressibility factor to use in adjusting back from the compressed real density). In this case, for Z Compressibility of 0.9 the reported Standard Volume would be 10% too low, hence we always recommend defining your gas density on the ideal gas line and using an appropriate compressibility factor.


If the gas density is defined at the pressurized condition by prediction from the ideal gas law, and an appropriate compressibility factor is used to correct to the real gas density at the pressurized condition, then the calculated mass flow results will be correct, and when the Standard Volume flow is calculated, the compressibility factor will be accounted for, to give the correct volume at the selected standard temperature and pressure conditions.