Bentley WaterGEMS CONNECT Edition Help


Tanks are a type of Storage Node. A Storage Node is a special type of node where a free water surface exists, and the hydraulic head is the elevation of the water surface above some datum (usually sea level). The water surface elevation of a tank will change as water flows into or out of it during an extended period simulation.

Water Level/Elevation

The user can choose either Elevation or Level as the Operating Range Type. The water level in a tank can be described based on either the hydraulic grade line elevation (Elevation) or the water level above the base elevation (Level).

Applying a Zone to a Tank

You can optionally group elements together by any desired criteria through the use of zones. A Zone can contain any number of elements and can include a combination of any or all element types. For more information on zones and their use, see Zones.

To Apply a Previously Created Zone to a Tank

  1. Select the tank in the Drawing View.
  2. In the Properties window, click the menu in the Zone field and select the zone you want.

Active Topology

By default a tank is active in a model. A tank can be made inactive (not used in calculations) by changing the Is active? property to False. If a tank is made inactive, any connective pipes should also be made inactive as otherwise this will give an error.

Defining the Cross Section of a Variable Area Tank

By default, tanks are treated as having a circular shape with a constant cross section described by its diameter. If the tank has a constant cross section that is not circular, the user can select Non-circular and specify the cross sectional area. If the user selects Variable Area, it is necessary to provide a depth to volume table.

In a variable area tank, the cross-sectional geometry varies between the minimum and maximum operating elevations. A depth-to-volume ratio table is used to define the cross sectional geometry of the tank.

To Define the Cross Section of a Variable Area Tank

  1. Select the tank in the Drawing View.
  2. In the Properties window, click the Section menu and select the Variable Area section type.
  3. Click the ellipsis button (...) in the Cross-Section Curve field.
  4. In the Cross-Section Curve dialog that appears, enter a series of points describing the storage characteristics of the tank. For example, at 0.1 of the total depth (depth ratio = 0.1) the tank stores 0.028 of the total active volume (volume ratio = 0.028). At 0.2 of the total depth the tank stores 0. 014 of the total active volume (0.2, 0.014), and so on.

Setting High and Low Level Alarms

You can specify upper and lower tank levels at which user notification messages will be generated during calculation.

To set a High Level Alarm

  1. Double-click a tank element to open the associated Properties editor.
  2. In the Operating Range section, change the Use High Alarm? value to True.
  3. In the Elevation (High Alarm) field, enter the high alarm elevation value. A high alarm user notification message will be generated for each time step during which the tank elevation exceeds this value.

To set a Low Level Alarm

  1. Double-click a tank element to open the associated Properties editor.
  2. In the Operating Range section, change the Use Low Alarm? value to True.
  3. In the Elevation (Low Alarm) field, enter the low alarm elevation value. A low alarm user notification message will be generated for each time step during which the tank elevation goes below this value.

Inlet Type

In general, tank inlet and outlet piping are treated as being connected to the tank at the bottom and have only a single altitude valve that shuts the tank off from the rest of the system when the tank reaches its maximum level or elevation. However, some tanks are filled from the top or have altitude valves (sometimes called a "Float Valve") that gradually throttle before they shut. This can be controlled by setting the Has Separate Inlet? Property to True. The user must pick which of the pipes connected to the tank is the inlet pipe which is controlled or top fill. (If there is a valve vault at the tank with a altitude valve on the fill line and a check valve on the outlet, these should be treated as two pipes from the tank even if there is a single pipe from the tank to the vault.)

If the tank is a top filled tank (which may refer to a side inflow tank above the bottom but below the top), the user should set Tank Fills From Top? To true and set the invert level (relative to the base) of the inflow pipe at its highest point. Water will not flow into the tank through that pipe unless the hydraulic grade is above that elevation.

If the inlet valve throttles the flow as it nears full, the user should set "Inlet Valve Throttles?" to True. The user must then enter the discharge coefficient for the valve when it is fully open, the level at which the valve begins to close and the level at which it is fully closed. These levels must be below the top level and any pumps controlled by the valve should not be set to operate at levels above the fully closed level. The closure characteristics are determined by the Valve Type which the user selects from a drop down menu.

When the tank is described as having a separate inlet, additional results properties are calculated beyond the usual values of tank levels (elevations) and flow. The user can also obtain the relative closure of the inlet valve, the calculated discharge coefficient, the head loss across the valve, and the inlet and outlet hydraulic grade of the valve and finally the inlet valve status.

Water Quality (Tanks)

If the user is performing a water quality analysis, it is necessary to specify the initial value for Age, Concentration or Trace depending on the type of run. If the tank is a source for some water quality constituent concentration, the user should set "Is Constituent Source?" to True and specify the constituent source type. See the Constituent Alternatives help topic.

If this analysis is a constituent analysis, the user may specify the bulk reaction rate in the tank by setting "Specify local bulk rate?" to True and setting the "Bulk reaction rate (Local)" value.

Tank Mixing Models

Real water distribution tanks cannot be exactly described as plug flow or completely mixed but these are reasonable approximations to fluid behavior in tanks. WaterGEMS CONNECT supports four types of tank mixing models which the user selects in the drop down menu of Tank Mixing Models.

The Complete Mixing model assumes that all water that enters a tank is instantaneously and completely mixed with the water already in the tank. It applies well to a large number of facilities that operate in filland-draw fashion with the exception of tall standpipes.

The Two-Compartment Mixing model divides the available storage volume in a tank into two compartments, both of which are assumed completely mixed. The inlet/outlet pipes of the tank are assumed to be located in the first compartment. New water that enters the tank mixes with the water in the first compartment. If this compartment is full, then it sends its overflow to the second ompartment where it completely mixes with the water already stored there. When water leaves the tank, it exits from the first compartment, which if full, receives an equivalent amount of water from the second compartment to make up the difference. The first compartment is capable of simulating short-circuiting between inflow and outflow while the second compartment can represent dead zones. The user must supply a single parameter, which is the fraction of the total tank volume devoted to the first compartment. This value canbe determined during calibration if this model is selected.

The FIFO Plug Flow model assumes that there is no mixing of water at all during its residence time in a tank. Water parcels move through the tank in a segregated fashion where the first parcel to enter is also the first to leave. Physically speaking, this model is most appropriate for baffled tanks that operate with simultaneous inflow and outflow such as ideal clear wells at water treatment plants. There are no additional parameters needed to describe this mixing model.

The LIFO Plug Flow model also assumes that there is no mixing between parcels of water that enter a tank. However in contrast to FIFO Plug Flow, the water parcels stack up one on top of another, where water enters and leaves the tank on the bottom. This type of model might apply to a tall, narrow standpipe with an inlet/outlet pipe at the bottom and a low momentum inflow. It requires no additional parameters be provided.