The Internal Fluid component is used to specify the properties of an internal fluid for a pipeline (the pipeline can be defined using a Line component or simply a Pipe Section component).
An internal fluid is characterised by its mass density, internal pressure (above hydrostatic) and a level above the mudline. These are specified via a Properties dialog on the Internal Fluid component.
You associate an internal fluid with a particular pipeline when you add the pipeline to a model via the Model component. The Model component Add Line dialog is shown below to illustrate this.
Add Line Dialog
You will note the drop-down list for Internal Fluid near the bottom of the dialog, this allows you to choose one of the Internal Fluid component currently defined in the project. The specification of internal fluid is of course optional; pipelines are empty (air-filled) by default. If you associate an internal fluid with a Line component, the whole line is assumed to be flooded, and the internal fluid is associated with all pipe sections within the line.
Additional articles describe the Internal Fluid Data Inputs and Internal Fluid Best Practice.