The Pipe Flow Expert software is intended for general use with either liquids or gases (subject to certain criteria for gas systems).
Pipe Flow Expert uses the Colebrook-White equation to calculate friction factors. Pipe Flow Expert uses the Darcy-Weisbach equation to calculate friction loss in a pipe.
These equations assume a constant fluid density & viscosity and they provide a very accurate solution when working with none compressible fluids (liquids).
When using the software to model gas systems there are certain criteria that the model must operate within to ensure the solution is of acceptable accuracy.
Changes in pressure and temperature will affect the gas density and viscosity. These property changes affect the actual pressure drop and are not automatically accounted for in the Darcy- Weisbach equation, therefore some limitations must be applied to ensure that the calculated results are within an acceptable accuracy.
Generally systems which involve gases fall into two categories:
Low pressure loss systems:
Where the pressure loss is less than 10% of the highest absolute pressure in the system, if the pressure drop is calculated using the entering fluid density then good reliability of the results can be expected.
High pressure loss systems:
Where the pressure loss is more than 10% but less than 40% of the highest absolute pressure in the system, if the pressure drop is calculated using the average fluid density then good reliability of the results can be expected.
the system, it is necessary to model the system using different fluid zones. Up to 20 fluid zones can be used in a Pipe Flow Expert model.
Using Fluid Zones:
Where a system has been split into a number of separate fluid zones, the fluid density and the fluid viscosity for each fluid zone should be set independently.
The density of each fluid zone in the system must reflect the average density of the compressed fluid condition in that fluid zone.
When a system is solved if the fluid density is not within 5% of the average fluid density for a
particular zone, a warning will be issued in the results log. Suggestions to Update Fluid Zone Data to have a particular density based on the average pressure within the fluid zone will be issued in the results log.
General Suggestions:
For systems that contain compressible fluids the following should be noted
The mass flow rates entering the system and the mass flow rates leaving the system must be balanced. Normally In-Flow or Out-Flow values are entered using a mass flow rate units, such as lb/sec, lb/min, lb/hour, kg/sec, kg/min or kg/hour, for a system which involve compressible fluids. Where volumetric In-Flow rates entering the system have to be used these values must be entered as the ‘actual’ flow rate of the fluid for that particular fluid zone applicable to the pipe from which the fluid will enter, i.e. the flow rate of the fluid must be based on the density of the fluid zone (and not on the uncompressed volumetric flow rate of the fluid).
Where volumetric Out-Flow rates leaving the system have to be used these values must be entered as the ‘actual’ flow rate of the fluid for that particular fluid zone applicable to the pipe from which the fluid will leave, i.e. the flow rate of the fluid must be based on the density of the fluid zone (and not on the uncompressed volumetric flow rate of the fluid).
The pressure at all In-Flow points for an individual fluid zone must be the same to the degree that the pressure does not change the volume and density of the fluid zone significantly.
Devices which change the volume/density of the fluid should not be included as part of the system analysis.
Pipe Flow Expert uses a constant value for the compressible fluid density throughout each individual fluid zone in the pipeline system. Where volumetric flow rates are used to specify the In-Flows and Out-Flows to the system, the individual density for each fluid zone is used to convert from volumetric flow rate units to the mass flow rate units used internally by Pipe Flow Expert. The calculations are performed using the mass flow rates to achieve mass flow rate continuity and balance within the pipeline system. The results are then converted back to the appropriate volumetric units for each pipe, based on the fluid zone data associated with the pipe.
The effects of pressure changes and temperature changes on the fluid density are not modeled. Note 1:
The Fluid density at the compressed fluid condition can be calculated using the normal density of the compressible fluid and the fluid pressure.
Compressed fluid density =
Example: If a volume of 10 m³ of air at normal temperature and pressure is compressed to 6 bar g The Fluid density would be:
1.2047 x (6.000 + 1.01325) / 1.01325 = 8.3384 kg/m³
Note 2:
The Actual flow rate of the fluid at the compressed fluid conditioncan be calculated using the uncompressed volume of the fluid and the fluid pressure.
Actual flow rate =
(Uncompressed fluid volume x Atmospheric pressure) / (Fluid pressure + Atmospheric pressure)
Example: If a flow of10 m³/s of air at normal temperature and pressure is compressed to 6 bar g the Actual flow rate would be: