Pipe Model


The size section of the Pipe Model tab specifies the pipe geometry.

  • Pipe Material

    • User Specified - If selected, the geometry of the pipe must be directly specified.

    • Library Material - Many common pipe materials are included in the standard pipe material library. These and any other accessible library materials can be selected. When a library material is selected, the parameters defining the geometry cannot be modified.

  • Pipe Geometry - Several pipe geometries can be modeled. The critical value for flow calculations is hydraulic diameter, based on cross-sectional area Ac and wetted perimeter Pw:

    • Cylindrical Annulus - Flow in an annulus or "ring" pattern. This requires both an outer and inner diameter to calculate hydraulic diameter.

    • Cylindrical Pipe (default) - A typical pipe. The hydraulic diameter and inner diameter are identical

    • Helical Tube - Requires hydraulic diameter and coil diameter. Specification of this geometry affects the friction model pressure loss.

    • Noncylindrical Pipe - Requires flow area (cross-sectional area) and wetted perimeter to calculate hydraulic diameter.

    • Rectangular Duct - Requires width and height to calculate hydraulic diameter.

  • Size - Select from a list of defined nominal sizes for the selected Pipe Material. Only available for library materials.

  • Type - Select from a list of defined types (classes/schedules) for the selected Pipe Material. Only available for library materials.

  • Geometric Specification - Several required fields that change depending on the selected Pipe Geometry. Only editable for User Specified Pipe Material.

  • ID Reduction (Scaling) - Reduction in hydraulic diameter due to scaling can be accounted for by entering a percent reduction in this field. Zero represents no reduction.


The Length is defined as the distance between the two connecting junctions. This length has no relationship to the visual length of the pipe drawn on the Workspace.

Model As Zero-length Connector can be checked to turn the pipe into a lossless connection between two junctions, such as for two components that are directly welded together. Zero-Length Connectors (ZLCs) are denoted by a red C icon next to the pipe name on the Workspace. Intermediate branching flow is not allowed between two ZLCs.

ZLCs are hidden by default in the Model Data and Output. Generally the output for ZLCs is not of interest because they will have the same inlet and outlet properties.

You can enable ZLCs to be shown in the Model Data by opening the Model Data Control, going to the Show Selected Pipes/Junctions tab, and selecting the Show Zero-Length Connectors checkbox under the list of pipes. The same checkbox is present in the Output Control on the Show Pipes/Jcts tab. You will then need to click on the pipe numbers in the list to make them visible in the Model Data or Output, or you can use the All button to select all pipes in the model.

Friction Model

Multiple friction data sets can be compiled for each library pipe material. These different data sets are then made available in the Data Set drop down list. For AFT standard materials, the friction data set is generally "Standard."

The Friction Model can be modified for any individual pipe, and the default friction model can be set in Parameter Options. The Friction Model for a library material can be modified by selecting the User Specified radio button in Data Set, and must be directly specified for User Specified materials.

  • Absolute roughness (default) - Absolute average roughness height. Values of pipe roughness can be found in many pipe handbooks or from manufacturer's data.

  • Relative roughness - Relative roughness is the absolute roughness divided by the pipe diameter.

  • Hydraulically smooth - Hydraulically smooth implies that roughness is negligible. This is not the same as frictionless.

  • Explicit Friction Factor - Friction factor for the pipe, you can enter the value explicitly.

  • Frictionless - For troubleshooting purposes, it is occasionally useful to model a pipe as having no friction. There are limitations to where such a pipe can be located in your model.

  • Weymouth equation - A model for natural gas pipelines under fully turbulent flow, using pipe diameter to characterize pressure loss.

  • Panhandle equation - A model for natural gas pipelines, using Reynold's Number to characterize pressure loss.

For detailed information on how these Friction Models are defined, see Pressure Drop in Pipes - Detailed Discussion.