you has explained how an Air Valve works as condition boundary in a system has pump. But I did not find in your Wikis how to prevent negative pressures in a pipe in WaterGEMS. is it possible to install an air valve at high points and it let air to enter the system to prevent pressure becomes negative. this will be a gravity pipe system. Can it be simulated in WaterGEMS?
OpenFlows | Water Infrastructure Forum - Air Valve Functionality in WaterGEMS Only
Harati,
You can simulate this in WaterGEMS, but the profile just won't show exactly what would happen - take a look at the article that Sushma linked to. It explains how to use the air valve element to do this, and how to interpret the results. If part-full flow occurs in the downstream pipe, it will appear as the HGL being below the pipe elevation as Tom explained. The pump operating point and other results will be accurate, but your profile appearance just won't be ideal.
If you want to see the profile looking "accurate" (depth of part-full flow when it occurs), you can model the system in SewerGEMS using the Implicit or Explicit dynamic solver. An example of this can be seen in this article.