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As a tank in WaterGEMS reached its top level/elevation, the model assumes that there is an altitude valve at the tank and the tank is shut off from the system until outflow exceeds inflow. However, some tanks do not have an altitude valve and the tank simply overflows.
To simulate an overflow, the user must install a reservoir in the model at the elevation of the overflow pipe invert, connected to the tank by a pipe with a check valve that permits flow from the tank to the overflow reservoir but not back. This simulates the overflow pipe.
Such a tank is show below, where:
T-1 is tankP-1 is inflow pipeP-2 is outflow pipeP-3 is the overflow pipeR-1 is the reservoir that receives the overflow water
The results of a model run for that model are shown below, where the blue line is the tank water surface elevation, the green line is the tank inflow, the red line is the outflow and the magenta line is the overflow. At hour 6, the tank water level exceeds the reservoir level (18 ft) and the overflow begins and continues until hour 12 when the outflow exceeds the inflow and the overflow stops.
Such a model can show instability where the water level reaches the top of the tank within a time step and the tank is disconnected. Such instability can be corrected by increasing the size of the overflow pipe, increasing the distance between the overflow and the top of the tank, decreasing the net inflow to the tank, increasing tank diameter or decreasing the model time step.
A copy of the example model illustrated above can be downloaded below. Note that this model can only be opened in version 10.03.XX.XX and greater (not yet available as of October, 2019), and you must be signed in in order to see the download button.
Tank overflow example model
What happens when a tank becomes empty or full? (altitude valve)