For gas flow in a real pipes, the preponderance of cases are well below 0.6 Mach where the incompressible-flow assumptions work pretty well. I have never come across a situation where I got more representative data by going to the compressible-flow arithmetic. I took a class on compressible flow in grad school and that discussion was almost entirely about high-speed body in effectively still air with the occasional reference to flows accelerated in convergent/divergent nozzles within pipes.
For low pressures, I mostly use the isothermal gas flow equation (incompressible), but I break the pipe up into small enough chunks to keep downstream density greater than 90% of upstream density and rigorously recalculate compressibility, density, Reynolds Number, and Fanning Friction Factor for each step (I love MathCAD for this). I find that with modern pipes I'm never in the fully turbulent region at low pressures so friction factor changes noticeably from step to step.
As you might expect, in Oil & Gas we deal with vertical flows quite a bit. For small vertical changes (say less than 1000 ft), I use the horizontal calcs without much deviation from observed data. For greater distances the industry has developed empirical closed-form equations that explicitly include hydrostatic head from a gas column. If you find yourself calculating flows from 20,000 ft under the sea floor in 6,000 ft of water, then you probably want to use them. At 3000 ft onshore, including the hydrostatic head in the calculations gives you an answer that is within about +/-5% of ignoring it.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
