Problem

Significant flow is seen through a pump or valve during the transient simulation, shortly after the pump starts up or the valve is opened, but that flow is not seen when looking at Time Histories (graphs) at the downstream end of the system. Is the flow being lost?

Solution

Assuming there are no demands, tanks, reservoirs or other outflow points between the pump and then downstream observation point, this may be due to significant line packing effects due to the slightly compressible nature of water. If there is a significant volume of water in the pipeline (for example a very long length of pipe, with large diameters) then this can be noticeable and cause the symptom described above. Water molecules "pack" against each other as the valve opens or pump starts, and the water column continues to move (seen as a positive flow). For a long pipeline, it can take a significant amount of time for the entire column of water to compress; during which a positive flow can be observed on the upstream side. This effect diminishes as you move your observation point downstream.