The Pipe Flow Expert software comes with its own fluid database of common liquids and gases. It also contains functionality that allows users to calculate new gas data for specific temperature and pressure conditions.

If your pipe system contains a fluid that is not present in the fluid database then you can simply add your fluid and its properties (density and viscosity, for a specific temperature and pressure condition), and this data can then be used by the program.

Pipe Flow Expert allows up to 20 different fluid zones to be defined across a pipe system and the fluid for each zone can be individually specified. Once a fluid is selected from the fluid database the software will then automatically use the associated properties of the fluid when performing the system flow and pressure drop calculations.

For Liquids:
Pipe Flow Expert performs pressure drop calculations using the Darcy-Weisbach equation to calculate friction loss, where the friction factor is calculated using the Colebrook-White equation. The Darcy-Weisbach equation provides very accurate calculation results for non-compressible Newtonian fluids, since the density of these fluids can be considered to be constant.

For Gases:
Pipe Flow Expert performs pressure drop calculations using a specialist Compressible Isothermal Gas Flow Calculation Engine that allows a choice of compressible equations. The gas flow calculation engine automatically takes account of any compression or expansion of the fluid that may occur due to pressure changes within the pipe network.

More information on 'Working with compressible fluids' is provided in the Pipe Flow Expert User Guide. It is up to the pipe systems engineer to be familiar with good engineering practice and to use their own judgment in regard to the accuracy tolerance of the calculated flow rate and pressure drop results for a system that contains a compressible fluid.