Disconnected Demand Nodes user notification when computing model

Product(s): WaterCAD, WaterGEMS
Version(s): CONNECT Edition, V8i
Area:  Modeling

Problem

When computing a model, the following user notification is generated:

"Disconnected demand nodes detected; hydraulic results are likely to be compromised." 

When this happens, you may notice flow passing through valves or pipes, even if they are closed.

Solution

This user notification means that there are junctions in the model that have a demand applied to them but are disconnected from a source, such as a tank or reservoir.

In this condition, flow will pass through the closed valve. The reason is because when you enter a demand at a node, you are telling the model that it must satisfy that demand no matter what. So, by closing off a demand, those two things are at odds with each other. The demand is still satisfied, flow goes through the closed valve/pipe and you see this notification. This can also occur if the pump is reversed i.e. the incorrect downstream pipe is selected. You can read more about resolving this here.

The cause of this can be from any number of settings in the model. For instance, a pump or valve that is closed will stop flow coming from upstream. If there is no other potential source of flow in the system, the demands will not be satisfied.

Other areas that can be checked include tank settings. If a tank is empty, it will not be able to supply flow. You can read more about what happens when a tank becomes empty or full here.

Make sure that the orientation of your pumps and/or valves is correct and that the pumps' "Status (Initial)" is set to "On" (or have controls to turn them on). If you have the incorrect "downstream link" selected (particularly for a pumps) this will inhibit flow from the element.

If you have a hydropneumatic tank in the model as a source, make sure the property field "Treat as Junction" is set to False. If it is set to True, the hydropneumatic tank will act like a junction instead of a potential source of flow.

Another item to check is the status and connectivity of pipes in the system. If a pipe is set to be closed, it will cut off flow from upstream. It is also possible that a pipe is not connected to the rest of the system. For instance, a pipe may be overlaying another pipe rather than be connected to it. These types of connectivity issues can be difficult to spot visually, but there is a tool called Network Navigator that can be used to find disconnected segments of a model. See the TechNote on Using Network Navigator for more information.

If you intentionally cut off the demands by closing the valve or pipe and wanted to see the flow drop to zero, you will need to use the Pressure Dependent Demand (PDD) feature. With PDD, if the pressure drops to zero when closing the valve, the demand will drop to zero. Alternatively, you could simply remove the demands in conjunction with closing the pipe (you could use a new scenario with new demand alternative in this case, to preserve the original demands). Also, if you want to look at the effect on the system of closing off isolation valves, consider the Criticality tool.

See Also

Node is not connected to, or is isolated from, a boundary (reservoir or tank) user notification

How do you change the direction of the downstream pipe for valves and pumps?

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