Real Gas and Compressible Flow Networks: Engineering Workflow
Engineering context
Compressible gas networks behave differently from liquid systems because gas density changes continuously with pressure and temperature. As gas flows and pressure drops, density falls, velocity rises, and the actual volumetric flow increases — only mass flow (and standard volumetric flow) is conserved at junctions. Because of this coupling, results must be read across the whole connected network.
Gas flow is solved using real-gas equation-of-state methods rather than simplified ideal-gas, purely isothermal, or purely adiabatic assumptions, so capacity, flow distribution, velocity, and choking are evaluated against the real connected system.
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Free TrainingEngineering workflow
- Define the analysis objective and operating cases — decide whether you are asking a capacity question (how much the network can deliver) or a distribution question (how a set throughput splits across branches).
- Choose the inlet boundary condition to match the analysis objective — Known Pressure for capacity, Known Flow for distribution. Where choking can occur, avoid over-constraining the model.
- Define the boundary conditions — select the gas from FluidFlow’s fluid database, where its properties are modeled using an equation of state (such as Peng-Robinson, Lee-Kesler, or BWRHS). Set operating conditions at each boundary: pressure and temperature or flow and temperature at the inlet; pressure or flow at the outlet. Choose the pressure model correctly — stagnation for vessels, static for pipes.
- Build and connect the network — add equipment (compressors, blowers, fans, valves, restrictions, and fittings) and connect with pipes to establish topology.
- Set the calculation basis — choose one standard-volume reference (STP or NTP) and keep it consistent across the model.
- Inspect the model for choking issues — after solving, review Mach numbers and choked-flow warnings (endpoint, restriction, or expansion choking) to locate where sonic conditions are reached.
- Analyse the model as a normal pipe system — pressure drop, velocity (including noise-velocity screening), flow distribution across branches, and along-pipe property behaviour (pressure, temperature, density, and velocity).
Why the full system matters
Gas density changes continuously along every pipe in the network, so the actual volumetric flow, velocity, and pressure gradient are all coupled. A simplified isothermal or ideal-gas calculation misrepresents the density change and produces incorrect velocity and pressure drop results. Choking — where flow reaches the speed of sound at a restriction — can prevent the model from converging if it is not anticipated and addressed. These behaviours must be solved simultaneously across the whole network.
How FluidFlow helps
FluidFlow analyses compressible gas networks within its steady-state pipe network solver, using equation-of-state property methods and the Duxbury method to capture changing density, velocity, and friction along each pipe. It reports mass, actual, and standard volumetric flow, along-pipe properties, Mach numbers, and choked-flow warnings, and scales from single lines to branched headers and parallel paths.
