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Pipe fills tank modeled like reservoir from top and shows vacuum in Hammer simulation?


I have system, consisted by pump, rising main and in the end reservoir (according to the examples, it is modeled with reservoir, not tank). The reservoir fills from top, or I thing that I have modeled like that.

Elevation - 721 m; Elevation (inlet/outlet) - 721,5 m.

After simulation in Hammer with no protection there is a vacuum at the end of rising main. The strange thing in my opinion is that there is a vacuum in the rising main near the reservoir. Is it the software take 

in account that the pipe ends with contact with the atmosphere (it flows above water level) and may be in practice it is some kind of big air valve, or in other words the end is one big inflow orifice?