## How does Pipe Flow Expert calculate different gas densities within a pipe network?

**Calculating gas densities within a pipe system**

When defining gas data for use within the Pipe Flow Expert software the following applies:

**When using the Compressible Calculation Engine**

Calculation of density at pressures other than the defined pressure condition will:

- calculate a new density using the ideal gas law
- calculate and adjust this density using any specified Z compressibility factor

Therefore when using the Compressible Calculation Engine (recommended):

Define the gas density based on the ideal gas law
and specify any required Z compressibility factor separately
on the Config Options -> Calculations Tab.

This will ensure that the correct density is used at the pressurized condition in your pipe network, while also ensuring that the results are converted back to a correct value when displaying Standard Flow Volume (Flow at Standard Conditions)

Gas properties (density, viscosity, specific heat ratio) should be defined at
the required temperature conditions since the software assumes isothermal flow
with a consistent temperature.

It is common to specify gas properties at the 0 bar g pressure condition
(atmospheric pressure conditions) since changes in density with changes in pressure in your pipe network
will automatically be calculated and accounted for as described above.

Viscosity varies mainly with changes in temperature and varies much less with changes
in pressure (often no significant change unless under high pressures above 35 bar g).
For high pressure systems the gas properties can be defined at a pressurised condition (but again on the ideal gas law line).

**When using the Non-Compressible Calculation Engine (Not Recommended)**

Density will not be recalculated at other pressure conditions within the system.

Non-Compressible calculations assume a constant density:

- this is valid for low velocity, low pressure, gas systems
- fluid zones can be used to adjust gas properties within the system
- in general, results using the non-compressible engine should only be relied upon when the pressure loss in the system is less than 10% of the absolute highest pressure condtion, and where the gas density has been defined for this condition.

**We always recommend using the Compressible Flow Calculation Engine when working with gas systems.**