System Disconnection at Air Valve

Question

I have a water force main in WaterCAD that I am modelling, and I have a high point along the main. To get the pump to recognize the high point, I have placed an air valve right before this junction at the same elevation with the 'Treat as Junction' field set to False. However, running the model causes the system to disconnect. The pump seems to have the head capability to pump to this high point. I am not sure what is causing this error. 
 Would a possible issue be that my tank is before this high point? So it is: Pump > Tank > High Point > Demand Junction. In reality, the tank is being used as a storage for a different system entirely, and does not serve any of the demands in my currently modelled system.

Jesse Dringoli · Accepted Answer

Hi Kristy, 
 Thanks for sending the model. The problem is the nearby tank - the pump is providing the head necessary to "lift" the water to the tank elevation, but the air valve in question is downstream of the tank and 30 feet higher. The hydraulic grade at the air valve is controlled by the elevation of the tank boundary condition, and would need an additional pump (between the tank and the air valve) in order to prevent negative pressure at the air valve. 
 There is a path around the tank, but there are only fixed demands downstream of the air valve so the pump can only provide a fixed amount of flow through that path, while also needing to maintain the boundary condition at the tank (initial elevation). There is essentially no operating point on the pump curve that satisfies all of this (known tank HGL, fixed demand, and positive pressure at the air valve) 
 Kristy Chang said: In reality, the tank is being used as a storage for a different system entirely, and does not serve any of the demands in my currently modelled system. 
 The tank is currently connected to your system, so if it does not serve your system, you will need to modify the model to correctly match the real conditions (close the pipe? Make the tank inactive? Connect it to what it actually connects to?), as a tank imposes a hydraulic boundary (known HGL) that the model solves around. 
 I have added some notes based on this conversation, to the "Troubleshooting" section of the related wiki article: 
 Modeling Air Valves At High Points in WaterCAD or WaterGEMS

Yashodhan Joshi · Answer

Hi Kristy, 
 There are two ways you can go about this; 
 1. Make the tank inactive / delete it and directly pump till the air valve location. This would however need modification in the pump curve (higher head). If this is acceptable then you can go ahead. 
 2. Another option is to have separate pumps configured between the tank and the air-valve. Since you mentioned that the tank is currently not being used for your system (only being used as a storage), you can configure additional pumps. The tank would also then act as a source for pumps pumping towards the air-valve location. If the tank is "only" supposed to be used to satisfy demands to some other location then this would violate that condition. Also, for the sake of designing the system as close to your conditions as possible consider adding the demand on the tank. This will simulate the demands the tank is supposed to satisfy but not configured in your system currently. 
 3. The third option is a bit tricky in which you have to configure the pump to supply both the tank and the air-valve location separately. This would take care of the condition that the tank is only to be used as a storage for some demands not currently in the network and that branch would terminate there. A second pipe would be connecting your pump to your air-valve location. The only challenge here would be to define a pump curve which can simultaneously fill the tank as well as maintain head at the air-valve location. 
 Hope this helps.

System Disconnection at Air Valve

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