Forum Post: Simple gravity-fed water distribution system

Hello –

I am trying to model an existing gravity-fed water distribution system for a small rural community using WaterGEMS V8i. The existing system is as follows: There is a ~13,000 L storage tank atop a hill with a single outlet; this outlet is connected to a pipe which runs downhill and branches to serve four tap stands. The tank will be filled by a nearby well, to be constructed—we would like to determine whether the existing piping architecture is sufficient or will need to be replaced.

When the tank was fed by a previous well (soon to be decommissioned due to damage), the community would need to refill the tank roughly every three days; I would therefore like to model the scenario where the tank is emptying through its outlet to the community (without continuously being refilled), either in the case where all the spigots are open, all are closed, or some are open and some are closed. Specifically, I would like to know whether the pressure in the system with be acceptable (in both the high and low pressure cases), and whether the diameters of the existing pipes will allow adequate flow rate to be delivered to the tap stands by gravity.

I have taken an initial stab at modeling the system (attached), however I am unable to produce a system which both shows any plausible dynamic behavior and which does not produce "Network unbalanced" errors. Two specific points of confusion:

• Modeling of the tap stands – This seems like a very basic question, but how should I model the tap stands? I have been modeling them as junctions attached to Discharge to Atmosphere elements. I am not sure whether or not I should assign demand to the junctions—ideally I would like to answer the question "what would happen if went to the community, filled up the tank, and opened all the spigots?", e.g. not specify a demand and have the "demand" calculated based on the pressure gradient created by the Discharge to Atmosphere elements. However, creating junctions with no demand produces a system which doesn't do anything (no flow is demanded, etc.). I also tried to model the system using Pressure-dependent demand (which sounds, by its title, like the right approach), but was unsure how to produce a pressure-dependent demand curve (e.g. I have no idea at what to use as a reference pressure—at what pressure the demand is completely satisfied). So I ended up just picking what seemed like a reasonable demand (0.2 L/s) for each of the junctions and running the simulation. However, this produces numerous "Network unbalanced" errors and eventually warns me about disconnected demand nodes. 
• Modeling the tank – Likewise, I am not sure if I'm modeling the tank properly. Currently in my model the tank has only one inlet/outlet pipe. In reality, the tank is a top-feed/bottom gravity discharge tank. The instructions for modeling this type of tank (http://docs.bentley.com/en/HMWaterCAD/Bentley_WaterGEMS_Help-11-89.html) suggest combining a pump with a PSV and a reservoir; there are two problems with this: 1. I am trying to model a case where there is no flow into the tank—where the tank is only emptying, and 2. I'm not sure what type of pump definition to use when modeling the tank in this way.