Every point of resistance to flow in a piping system is a source of wave reflections. Without any resistance in a system there would be no reflections or pressure loss.

It is beneficial to include all of the resistances in a system during a transient analysis, to capture the pressure losses that occur in both steady-state and transient conditions.

However, the magnitude of a reflection from a minor loss such as a locked open valve or an elbow is generally small enough to be ignored. Including every minor loss as a component requires numerous Pipe Sections, which has the potential to exponentially increase run time.

Rather than ignoring the component entirely, and thus ignoring the pressure loss, it is possible to effectively ignore only the minor reflection by lumping the loss of the component into a pipe. By increasing the overall resistance of a pipe, the additional pressure loss is accounted for and run times are significantly improved, with usually only minor changes in results.

Note: It can be difficult to tell what components can be lumped into pipe resistance, and what reflections are important. Generally, components that do not change position or behavior during a transient, and have low steady-state pressure loss can be lumped. If it is not clear that a certain component has no significant reflection it is best to include this component in the analysis. Comparing analyses with a component explicitly modeled versus lumped gives an indication of the impact this simplification has.

AFT Impulse provides an easy way to lump these resistances with Fittings & Losses, by allowing the user to define common components directly "within" a pipe.

Automatically Combine Pipes

The Combine Pipes window is opened from the Section Pipes window and will automatically combine adjacent pipes of the same diameter separated by a static junction. When pipes are combined, all length, elevation, pipe K factors, intermediate junction loss, and visual segment data is preserved into the resulting pipe.