Friction loss for a compressible fluid such as a gas needs to account for the change in density of the gas as it moves along the pipe and loss of pressure occurs. When using the compressible flow calculation engine Pipe Flow Expert can be set to use one of the following compressible flow equations:

  • General Fundamental Isothermal Flow Equation
  • Complete Isothermal Flow Equation
  • AGA Isothermal Flow Equation 
  • Panhandle A Isothermal Flow Equation
  • Panhandle B Isothermal Flow Equation 
  • IGT Isothermal Flow Equation 
  • Weymouth Isothermal Flow Equation 


The formula for each of the above equations is detailed in a separate PDF document named ‘Compressible Flow Equations’. This PDF is available from the ‘Documentation’ menu inside the Pipe Flow Expert software.


The compressible flow equations calculate a gas flow rate for a difference in pressure between two points. The Pipe Flow Expert software first solves the compressible network using non-compressible flow equations to find an approximate solution and then it employs the compressible flow engine to converge this to an accurate solution. 


The non-compressible calculation engine operates by adjusting flows within the network to achieve a pressure balance, whilst the switch to the compressible flow calculation engine operates by adjusting node pressures to achieve a flow balance within the pipe network.


The compressible calculation engine takes account of friction loss through fittings at the start and end of a pipe, where a lower pressure but higher velocity at the end of pipe will results in a higher pressure loss through the fitting then if the same fitting occurred at the start of the pipe.


In addition the compressible flow calculation engine handles complexities that do not occur for non-compressible calculations, such as when considering a fan or blower with a flow versus head curve, where the actual operating point (flow and head value) has to balance out even when taking account of the compression and then expansion of the gas through the pipe.