Infinite pipe is a special feature enabled with a Transient Special Condition in the Assigned Flow and Assigned Pressure junctions. This feature prevents the junction from reflecting pressure waves.

This is useful in modeling the part of a pipe system which is so long that its communication time prevents wave reflection interactions with the rest of the system during the simulation. During steady-state the flow or pressure is fixed, but during transient flow both the pressure and flow change in a manner consistent with the fact that no waves are reflected.

The infinite pipe feature can significantly reduce model run time. Any pipe long enough that it does not reflect any waves during a meaningful simulation time will also require a large number of sections. This may dramatically increase the number of calculations required, even if the behavior in that particular pipe is not of interest. Using an infinite pipe is a way of avoiding significant computations for the pipe but still maintaining its effect on the system.

Note: The connected pipe must have at least two sections.

The theory for an infinite pipe is given in Infinite Pipe Transient Theory.  (Wylie et al.Wylie, E.B., V.L. Streeter & L. Suo, Fluid Transients in Systems, Prentice Hall, Englewood Hills, New Jersey, 1993., 1993, pp. 121-122)

Example

Gasoline flows from a refinery to a tank farm 30 miles away. We want to know what the effect of closing the storage tank valve will be on the tank farm equipment.

The wavespeed of gasoline in these pipes is about 3500 ft/s - meaning that the communication time in P1 is about 90 seconds. If the transient simulation is less than this time, there is no need to model the entire pipe. Furthermore, P101 will require at least one section as the controlling pipe - this means that P1 will require almost 800 sections.

Alternatively, we can model P1 as only 400 feet in length (infinite pipes must be long enough to contain 2 sections) and apply the infinite pipe feature. This reduces the total number of sections in the model to about 5. For the same time step, this would mean that the number of calculations required is reduced by a factor of more than 100.

The results from the second run will be identical as long as the simulation time is less than the communication time of P1.