SewerCad Model Calculation Problem

Question

Dear All, 
 Is it possible that I will still have a flow on a pipe even though the junction before that pipe has already negative pressure? The scenario was that, the pipe and the junction I'm talking about was on the highpoint. 
 I am studying an existing piping system, and I am wondering why do that pipe still has flow? On my point of view (which I don't know If I am correct) the flow should be zero (0) as the pump cant deliver the flow on that highpoint? Am I correct? 
 Any help are highly appreciated. 
 Thanks

Akshaya Niraula · Accepted Answer

June, 
 What comes in must goes out at a node. From the hydraulics we know at any node, Area1 x Velocity1 = Area2 x Velocity2. Assuming pipe diameter are same on both ends, the equation becomes, Velocity1 = Velocity2. The only way that the right hand side can be zero is if velocity on left hand side is zero. If you are seeing some flow on coming pipe [that means you have some velocity (V=Q/A)] then same amount of flow has to be there on the pipe going out of that node. 
 If you like test this then please open up C:\Program Files (x86)\Bentley\SewerCAD8\Samples\Sample2.stsw, run the model, observe the model results for pressure pipe (flow). Now, place a pressure junction some where in the pressure pipe and let it split. Change the elevation to 2000 (some very high number) and run the model. Pressure at this node will drop to negative yet the flow in the pipe remains the same. 
 Practically there will not be any water as pump cannot deliver that much head. Now software is telling you, if "somehow" there is x amount of flow flowing through this pipe then there will be y pressure which happens to be a crazy negative pressure. 
 HTH

Jesse Dringoli · Answer

Also see Tom's comment in the below post: 
 communities.bentley.com/.../100094.aspx 
 A pump can sometimes still deliver flow when there is a negative pressure, as it will operate as a siphon. This is usually not desirable. If there is an air valve at the high point, you can use that in SewerCAD to model the effect discussed in the above thread.