Reservoirs and tanks are defined by different properties when used in a hydraulic network model. Elevation is a parameter required for both. However, it has a different meaning. In the case of tanks, elevation refers to the Z coordinate of the tank bottom. On the other hand, elevation of reservoirs refers to the Z coordinate at the water surface. In contrast to tanks we use a different term to enter the elevation values for reservoirs. We define it as total head (see Figure 1.3.1: Enter reservoir properties”). Theoretically, a reservoir can be considered as a tank with infinite capacity for receiving or providing water without changing the water level. The following table summarizes the total head of a reservoir.
|Node ID||Total head(m)|
The properties of the reservoir can also be modified using the property grid as previously shown for pipes. There, the property grid can be opened by double-clicking the element on the drawing space or by a double click on the element ID in the data browser window. Note to select the element type first in the data browser window. Please, then proceed to enter the property values for the reservoir as displayed in the Table 1.3.1: Reservoir Total Head.
The water level in a tank varies depending on the amount of water incoming and exiting. It means it might get empty or fill up at some point. This is the reason why we need to set a minimum and maximum water level for a tank. Additionally, we can set an overflow option for it in its property box by selecting the field “Can overflow” and choosing “Yes” as a value (see Figure 1.3.3: Enter tank properties). Whenever this option is enabled and the maximum level reached, the set value is going to be maintained by overflowing the water volume surplus. However, in case the option is disabled and the maximum level reached the tank will not receive any flow.
It is also important to know how the volume of water stored in the tank relates to the water level. For this purpose EPANET offers two options: The first and simpler one is to assume the tank is equivalent to a cylinder. In this case the user should introduce the diameter of the cylinder. The second option is to create a curve of volume vs a water level. For simplicity, in this workshop we are going to assume that each tank is equivalent to a cylinder. The next table comprises the elevation and complementary parameters for tanks.
|Node ID||Elevation (m)||Initial Level (m)||Min Level (m)||Max Level (m)||Diameter (m)|
It is essential to highlight that the simulation is performed by considering that reservoirs and tanks are filled from the bottom. If in the modeled system tanks and reservoirs are filled from the top, an adjustment can be made as shown in the figure below to avoid errors. Hereby a Pressure Sustaining Valve (PSV, see workshop 2 to learn in detail how to configure valves) is installed at the inlet of the tank, with the pressure setting equal to zero. Set the elevation of the downstream node for the PSV equal to the elevation in which the pipe discharges the flow to the tank. A small pipe with a large diameter (i.e. with negligible headloss) has to be created to connect this node to the tank. Finally, use a check valve in the outlet pipe of the tank. That way only the filling from the top is allowed.