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How do I model a system where there is intermittent demand or where demand is being cut off from it's water source at some point by closing a value?
There are two ways to simulate this situation.
1) Set up a hydraulic pattern on your demands nodes and at the point the demands are being cut off from the system give the pattern a 0 multiplier. See screen shot below for an example.
2) If you are using a throttle control valve (TCV) to cut off the flow from the system by closing the valve using a relative closure pattern then be sure to apply pressure dependent demands to the area of your model where the demand is dropping to 0. You can find information for how to setup pressure dependent demands from this wiki:
The attached model will help convey the setup for each of these situations.
Once the model file is downloaded you can read over the project properties (File > Project Properties...) for some details about each scenario.