The model has many pipes that can be combined

Users often model losses in AFT xStream using junctions with no transient behavior, such as a bend, static valve, or orifice, between two pipes of identical diameter. While this may help show where these losses are occurring in the overall pipe system, it can result in extra pipes which can have a detrimental effect on pipe sectioning and run time. Combining pipes and modeling the losses within the pipe itself can improve sectioning and run time while maintaining accuracy.

How are pipes that can be combined contributing to long run times?

To run a transient simulation, AFT xStream sections pipes to apply the Method of Characteristics. In any system there will be a single controlling pipe. The controlling pipe is the pipe with the fewest sections - by default, 10. AFT xStream uses the length of a section in the controlling pipe to divide all pipes in the model into sections and determine a uniform time step for the transient simulation.

Each pipe in a model must be sectioned individually, meaning section count must increase with pipe count. The run time is proportional to the square of the number of sections, meaning many sections in a model will often cause extremely long run times. Reducing the number of pipes in a model is often an effective way to reduce run time.

Consider a model that includes a pipeline that is 100 meters long, and has 9 bends in it. If those 9 bends are modeled as unique junctions, the pipeline must be modeled as 10 individual pipes. Each pipe has 10 sections by default, meaning the pipeline will have 100 sections. If the pipeline is modeled as a single pipe, and the 9 bends are modeled as additional pipe friction using the Fittings & Losses tab in the Pipe Properties window, the number of sections in the pipeline could be reduced by a factor of 10, in turn reducing the run time by a factor of 100.

What can the user do?

The Fittings & Losses tab in the Pipe Properties window can be used to add losses directly to pipes to replace individual junctions on the workspace. This process can be done manually, or automatically using the Join Selected Pipes/Jcts tool in AFT xStream.

Select the pipes to be combined and the junctions between those pipes on the Workspace, and choose the Join Pipes/Junctions from the Edit menu. This option will combine the selected pipes into one pipe, add the losses from the junctions into the Fittings & Losses tab.