What happens when a tank becomes empty, full, overflows?

Applies To
Product(s): WaterGEMS, WaterCAD, HAMMER
Version(s): V8i, CONNECT Edition
Area: Modeling
Original Author: Mark Pachlhofer, Bentley Technical Support Group



What happens when a tank becomes empty or full?

How is an altitude valve handled?

Tank Full or Empty in Extended Period Simulation (EPS)

When the water level in the tank reaches the minimum or the maximum (as specified in the properties) during an extended period simulation (EPS), a built-in altitude valve will close the adjacent pipe. An empty tank will close the downstream pipe, since it cannot drain any more and a full tank will close the upstream pipe, since it can't fill any more. Once the opposite conditions occur in the system, the pipe(s) will automatically open back up. Note: This built-in altitude valve apples to full or empty tanks in versions and earlier, as well as if "Allow Overflow?" is set to False in version 10.04.XX.XX and later.

This can present a problem in some systems, because for example, as soon as the pipe closes for an empty tank, it may instantly be able to fill again from another pipe, triggering the pipe to reopen. As soon as the pipe reopens, it drains to empty, closing the pipe again. This can cause excessive intermediate timesteps and rapid oscillations in the system. So, it is suggested that you configure your controls (typically pump controls) such that the tanks never become full or empty. (Example:  If your tank fills at an elevation of 90 ft. you might want to set your pump to shut down at 89.9 ft or if your tank empties at a level of 80 ft. you might set your pump control to shut down at 80.5 ft.) The controls would look like this:

IF tank T-1 Elevation >= 89.9 ft. THEN pump P-1 STATUS = OFF

then you would have another control for turning the pump back on

IF tank T-1 Elevation <= 80.5 ft. THEN pump P-1 STATUS = ON


If the altitude valve is based on some other logic, you could create controls to open and close the adjacent pipe based on that logic. For example if a reduction in pressure causes the altitude valve to reopen some time after it closes, you could use something like the below. Make sure that the pressure setpoint always indicates a draining condition, otherwise the control to close the tank will engage immediately when the control to open it triggers (they will fight against each other) You also may need to set a higher priority (ex: 5) on the control to open the pipe, as the tank elevation condition may still be true.

IF Tank T-1 Elevation >= 89.9 ft. THEN pipe P-100 STATUS = Closed PRIORITY 1

IF Junction J-1 Pressure <= 35.4 psi THEN pipe P-100 STATUS = Open PRIORITY 5

Tank Overflow option in Extended Period Simulation (EPS)

Starting the CONNECT Edition Update 4 release of WaterGEMS and WaterCAD, there is now an option that allows to the tank to overflow. 

To use this feature, open the tank Properties grid and set the property field “Allow Overflow?” to True. With this setting, the tank can overflow during the simulation.

If “Allow Overflow?” is set to True and the tank is full, you will see a user notification that the tank is overflowing. In addition, if “Allow Overflow?” is set to True, the built-in tank altitude valve is not used. This means that there will continue to be flow to the tank if the tank is full.

Controls to keep the tank from overflowing would still optimal to avoid water loss in the system. 

If "Allow tank overflow?" is set to False, the tank will operate with the built in altitude valve. These feature is available if the Engine Compatibility is set to WaterGEMS 2.2, WaterGEMS 2.00.12, WaterGEMS 2.00.10, or EPANET 2.2.

Solution - Transient simulation (HAMMER)

Empty tank

An automatic altitude valve is not simulated in tanks or surge tanks during the transient simulation. HAMMER assumes tanks are large enough so as not to become empty during a transient simulation. The water level will be allowed to drop below the Minimum level entered in the tank properties and outflow will continue. If the hydraulic grade reaches the tank base elevation, air inflow will occur (and may be shown as "vapor" volume in some versions). Such air pockets are limited to the location of the tank, so the air-liquid interface is not tracked and you won't see the effect of the water level moving down the adjacent pipe, draining the pipe. See: Assumptions and limitations of tracking air or vapor pockets in HAMMER

Also, when viewing a graph of flow downstream of an empty tank, you may see a positive flow continue to be reported. This is the movement of the water column away from the tank as the air pocket continues to expand (as the pipe continues to drain out).

If you have a situation where a tank empties during the transient simulation, you may need to consider whether the impact of the drained tank is of interest to the overall transient results, or if you only need to focus on the transient that occurs just after your event (valve closure, pump shutdown, etc), in which case you could either accept the results you see from the limitation mentioned above, or omit/simplify the area around the tank.

Note that Hydropneumatic tanks assume that the engineer does not want the tank to become empty and does not have the capability of modeling what happens when air in the tank enters into the pipeline. See: Hydropneumatic tank user notifications: "gas volume exceeds volume of tank" or "fills its containing tank"

Full tank

If a surge tank or regular tank becomes full (water level reaches the Maximum) during a transient simulation, overflow is simulated via the weir settings (with the circular or non-circular section type). If your tank has a closed top and you want to model surcharging, consider using the variable area option with the "chimney" mentioned at the bottom of this article: Modeling surcharging of an enclosed storage.

See Also

Understanding Tank Operating Range

Creating Controls - Conditions, Actions, and Control Sets - WaterGEMS and WaterCAD SELECTseries 6 and CONNECT Edition

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