Need to find the gas pressure drop thru each segment of the system. Low pressure, Natural gas, pipe sizes from 2" to 6"? What will be the most acurate metod?
If your pressure drop is less than 10% of your absolute pressure then you can safely use incompressible models e.g. Darcy-Weisbach. But there are plenty of online calculators for gas flow out there, so it is hardly worth making any simplifying assumptions. Google is your friend.
