Modeling Presure/Siphon/Gravity Pipe

[billleber]

Does anyone have suggestions for modeling a pressure line between a ground storage tank and an open reservoir? I'm trying to get a gauge on the flowrate range because the flow will have to be regulated through the use of a valve based on the level in the tank, the reservoir and/or the flow in the pipe.

I've modeled the line in WaterCAD and XP-SWMM. WaterCAD shows a small range of flow, which makes sense if there is a suction/siphon effect in the force main. However, XP-SWMM does not show this effect and as the tank empties, flow in the pipe goes to a gravity-flow scenario.

The difficulty comes in determining if I should use a larger pipe diameter to convey a larger flow when the tank is nearing empty, or if a smaller diameter is preferable to maintain cleansing velocity.

Any insight is appreciated.

 

[KRSServices]

Fundamentally you have to determine your liquid level in order to obtain the HGL. Are you emptying a full tank? 1/2 full?

Next, you mentioned the pipe is a pressure line, then a force main and then siphon. What exatly are you trying to model? From your last sentance I think you want to know know how to model the final flow characteristics, particularly the velocity, with minimal head. Am I correct? Is the liquid being assisted by a pump or only the hydraulic head from the tank?

Assuming the liquid is water and the head is calculated soley from the liquid level in the tank, you will want to set the the scenario where the tank is nearly empty and the reservoir full, or nearly full. This will provide you with an idea of the velocities encountered at that liquid level.

Assuming of course that the pipe is graded, once the bottom of the tank is drained, then at that point, the flow becomes gravity. The model will not reflect this, however a good nomogragh can provide a reasonable estimate of the velocity at that point.

If it is pumped, you will have to correctly enter the pump data, because if not, the iteration resultss will be erroneous.

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