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:
Hi,
WaterGEMS is a good choice for this calculation. I have a couple of suggestions:
- You can use either a demand or a 'discharge to atmosphere'. Demand is easier to set up and you can assign a pattern if you need to. With a 'discharge to atmosphere' flow will vary depending on the pressure available, but you need to enter additional data - specifically a 'typical' flow and pressure drop. Leaving these values set to zero causes the unbalanced network errors in your model. But you have to choose decent values to use. Alternatively you can use an 'emitter coefficient' and I suggest you read the help topic called "Estimating Hydrant Discharge Using Flow Emitters" about this. That might help you.
- The tank empties in about 2 hours. That's what causes the disconnected node warnings (the nodes are disconnected from a water source, because the only source in your model - the tank - is empty). If you just want to know how long it would take to drain the tank with all the taps open (and no inflow into the tank), then you can leave the model as-is and just accept that those errors will appear.
Regards,
Mal
Also regarding your question on the top feed tank - you can model a top-filling tank (see this related thread) but that isn't necessary in this case since you aren't modeling the upstream system. If you are only interested in the downstream system and want to assume no inflow into the tank, you don't need to worry about the top-filling aspect.
If you decide that you need to model it at a later point, you'll need to add the upstream system, then choose "true" for "has separate inlet?" and "tank fills from top" and enter the level (distance above tank bottom) of the invert of the upstream pipe.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Hi Mal and Jesse –
Thanks for the helpful responses. Mal, a few follow-up questions:
The 'Discharge to Atmosphere' data I mentioned is entered on the Properties of the discharge to atmosphere element itself. See the 'Flow (typical)' and 'Pressure Drop (typical) fields. If you can figure out the right values to use for these fields, then this approach will work well (and the discharge will vary with pressure).
To prevent information overload I didn't mention the Pressure Dependent Demand alternatives, functions, calculation options etc. in my last response. But, in short, you can use those things to treat a "regular" Demand (added to a junction) - which doesn't vary with pressure - as a demand that does vary with pressure. As you can see though, it takes extra data input...and in your simple model there are easier ways to set your model up. So you should probably ignore that Pressure Dependent Demand stuff for now.
An emitter coefficient is property of a Junction. If you can figure out the right value to use for this fields, then this approach will also work well (and the discharge will vary with pressure). Remember that you would use an Emitter coefficient on a junction OR a "regular" Demand on a junction OR a Discharge to Atmosphere - not all three.
So there are several ways to do what you want, but try not to overcomplicate it.
You could go out and measure the flow when the tap is open, or you could get out a calculator (or our FlowMaster product) and use the orifice equation (you would have to pick an equivalent orifice size to approximate each tap). Or, you could use your sensible approximation of 0.2 L/s as the typical flow and a pressure drop equal to the static pressure (approx. 180 kPa) on a Discharge to Atmosphere.
Personally I would probably stick with the simplest option - a "regular" Demand on each tap node of, say, 0.2 L/s (though I might do one run with demands = 1L/s to see how that looks).
If you do that you will see that the pipes have plenty of capacity (almost zero headloss) but the tank empties in around 2 hours. So if anything, you could probably use a bigger tank.
Also, regarding "changing the demand subject to the tank level" for a gravity system - if you used one of the methods that relates outflow (demand) to pressure (PDD, Discharge to Atmosphere or emitter coefficient), the demand will drop as the tank level drops.
However if the tank bottom elevation is higher than the elevations of the demands, their demand/outflow will not drop all the way to zero by the time the tank is drained, so you'll still have the same problem with the tank becoming empty. You could do things like demands patterns that drop to zero at a certain time, but like Mal said, it may be overcomplicating things.