Two Tanks, One High, One Low

Question

Hello, 
 I have a situation with two tanks in my model (WaterCAD, standalone). 
 The current set up is thus: 
 The model - The larger of the two tanks is at a higher elevation, as are the piping and junctions for the larger tank. The smaller of the two tanks is at a lower elevation, as are the piping and junctions for the smaller tank. 
 Here are the current operating parameters of the two tanks: 
 T-1 
 Elevation (ft) 553.46 
 Elevation (Base) (ft) 553.46 
 Level (Minimum) (ft) 17 
 Level (Initial) (ft) 18.5 
 Level (Maximum) (ft) 19.5 
 
 T-2 
 Elevation (ft) 550.52 
 Elevation (Base) (ft) 550.52 
 Level (Minimum) (ft) 17 
 Level (Initial) (ft) 18.5 
 Level (Maximum) (ft) 19.5 
 The issue - The larger of the two tanks is flowing into the smaller tank according to the model (makes sense if the system is gravity fed). I need the two tanks to run in parallel because that is the current operation. 
 The question - If the system is completely gravity fed, what valve or valves do I need to use to prevent the larger tank from flowing into the smaller tank, and have both run in parallel operation? I need both tanks to contribute to the system. I assumed a check valve would work to prevent the larger tank from flowing into the smaller tank, but I have no experience using them, so I am hesitant to. 
 
 Thank you for reading all of this, 
 
 Clint Taylor, EIT

Jesse Dringoli · Accepted Answer

Hi Clinton, 
 I am not entirely sure I understand your system properly but if you have two tanks that are close together but both drain at the same time and you want to not see any flow in a pipe that connects between them, you have encountered the "hydraulically close tanks" situation described here . In short, water will always flow from higher to lower HGL during a given steady state timestep, but you can get results more even by using a smaller timestep increment during an EPS. If this is a steady state and you just want to assume that the tanks are in a situation where they are draining evenly together at the same time (and the flow is zero in the pipe between them), then setting their initial hydraulic grade the same should do this. The "tank is closed" message simply means that the tank is neither filling nor draining, which may be expected in your case but I am not entirely sure since it sounds as if you may have another pipe leaving each tank where you want water to drain (at the same time). 
 If this does not help, it may be best to provide a sketch so we can better understand your system and how it should operate, plus a copy of the model for review.

Yashodhan Joshi · Answer

Hello Clinton, 
 In addition to Tom's response, can you tell us how both the tanks are filled? Are they filled through single source or separate pumps? 
 Typically for a system with multiple tanks the higher tanks or tanks having higher HGL will fill the tanks with the lower HGL's first until an equilibrium is reached and then start supplying. In this case if the lower tank is continuously supplying the higher tank will continuously supply the lower tank to try to reach an equilibrium until its empty. 
 In this case you can check your actual study and compare how both the tanks are emptying in the current operation. If there are any valves present on the pipeline or part of network connecting these tanks, you would need to study them too. 
 For this case you can try something like a "back-flow preventer" type valve: How can I model a backflow preventer? 
 However, it would be beneficial to use the same Elevation (Initial) for both the tanks to ensure they are operating simultaneously with the same HGL.

Two Tanks, One High, One Low

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