The gas calculator helper within the Fluid Database can calculate and display the compressibility factor of certain gases at different temperature and pressure conditions, and it will show the real density of the gas at the specified condition.


The user should choose to either:


  1. Define the gas properties at a condition on the ideal gas law line (normally say 0 bar.g) and then specify on the Options screen an appropriate compressibility factor for the pressure condition at the start of pipe.


OR


  1. Define the gas properties at the start pressure condition using the real gas density at this pressure (if it differs from the predicted ideal gas law density at this condition) and then specify the compressibility ‘Z=1.0’.


This ensures that the calculation proceeds to use the correct real gas density at the start pressure condition.


i.e. 


  1. If the gas properties are specified on the ideal gas law line then the predicted gas density at the start pressure condition will then be adjusted according to the compressibility factor that has been specified, in order to end up with the real gas density at the start condition. 


OR


  1. If the gas properties are defined at a high-pressure condition, specifying the real gas density, then a compressibility factor of ‘Z=1.0’ should be used. The calculated density at the start pressure condition will then be derived from the real gas density (at the high-pressure condition) by applying the ideal gas law equation. This will normally give a good result in terms of predicting the real gas density at the start of the pipe, provided that the defined high-pressure condition and the start pressure are not too different.


This means that the gas properties should generally be specified on the ideal gas law line and a compressibility factor can be specified as appropriate, or if the real gas density is specified for the gas at a high-pressure condition at the start of the pipe then the compressibility factor should be set to 1.0.