8156.Bentley doubt 19-03-21.zip
When computed, the program says the tank is empty and I want to solve the pressures problem aswell. WATERGEMS CONNECT EDITION.
Hello,
This is the same issue as past models you have sent. You have only one tank in the model, with no reservoir to refill it. Once the demands in the system draw the tank down, it becomes empty and you get disconnected demand node messages.
You will need to do one of two things. First, you can morph the tank into a reservoir. If you do this, you should not see the same issues, since reservoir cannot become empty; they are an infinite source of flow. Second, you can include a reservoir that refills the tank.
Regards,
Scott
Thank you, Scott. Actually I have 3 reservoirs in the upper part of the model which are connected to the tank to fill it and then redistribute into another pipe.
Can you resend the model? The model provided above does not include any reservoirs.
31-03-21.rar
Thanks for sending the model. For the tank becoming empty, this is likely because of the elevations of the tank in relation to the reservoir. Before going further, I wanted to point out that the Operating Range Type for the tank in your model is set to Level, but the levels entered for the tank range appear to be elevations. An operating range type of Level means that the level is relative to the base elevation of the tank. More information on this can be found here: Understanding tank operating range. You will want to make these changes.
I reset the level values for the tank before computing, but the tank still became empty. This is because of the elevations of the reservoirs and the tanks. The tank will refill from a reservoir based on the hydraulic grade of the elements. A reservoir would need to have a higher elevation than the tank. Of the five reservoirs in the model, only two have higher elevations. Those two are only 5 and 10 meters higher than the minimum elevation of the tank. With headlosses in the flows out of the reservoirs, this will mean that the hydraulic grade will end up being below the bottom of the tank. You can see this by looking at the hydraulic grade results at junction J-362. This junction is on the path between the tank and the reservoirs. The hydraulic grade here is 2,231.88 meters, which is below the minimum elevation of the tank. This is why the tank is not being refilled by the reservoirs.
Another factor to review would be the settings on the valves in the system. There is a PRV between reservoirs and the tanks with an initial pressure setting of zero. This should be adjusted, though unless the elevations of the reservoirs and/or tanks is changed, this settings will not really have an impact, as there would still be no flow between the reservoirs and tank. This valve may be contributing to disconnected demand nodes near the tank when the tank is empty though. The PRV is closed based on the pressures in the system (see this link for details), meaning that flow from the reservoirs can't satisfy demands near the tank.
So, some way of adding more head to the system may be needed. This would be with pumps or with adjusting elevations of the reservoirs.
For the pressure results, this is also mainly because of the elevations in the system. The elevations at junctions in the system are all pretty uniform and are pretty close to the elevations of the reservoirs and tanks. Many junctions appear to have an elevation in the range of 2230 meters. As the flow moves away from the reservoirs (and the tank, while there is flow from it), there are headlosses introduced in the pipes. These my be relatively small, but this will decrease the hydraulic grade as you move away from the reservoirs. You will notice that many of the junctions with negative pressure are the furthest from the reservoirs. Adding head to the system will likely help with this as well.
Answer Verified By: Sushma Choure
Thank you so much for this answer, Scott. It really helped to understand and complete the model.