I have an area of my model that has normal pressures when I run a Steady State analysis, and even at time step 0 in an EPS analysis. But starting at time step 0.75 hours and beyond in the EPS, the pressures in this area all become negative. I am not sure what is causing it. It could possibly have to do with the pump station and tank in the area (see Downing Ridge PS, Tunbridge PS, and Tunbridge Storage Tank), but I'm not sure. Could someone take a look at my model and help me out? It is uploaded onto the sharefile: DMWA System Model_updated. The problem area is highlighted in red in the screenshot below:
First, just look at the tank levels (Right click > Graph) and see if the tank has drained. If this happens, you need to control your water sources so they can keep water in the tank.
Hello Dominic,
I looked at the model and as Dr. Walski suggested, the issue appears to be the tank Tunbridge PS ST becoming empty. Because of that, water needs to come from further away to satisfy demands in this part of the system. Keeping water is the tank should help.
More information on negative pressures can be found here: Troubleshooting negative pressures at pumps, junctions, and other node elements.
Regards,
Scott
Hello Dominick,,
Is the pump in that location? It is generally recommended to model the actual conditions, so you don't know to place the pump someplace it isn't. If the pump isn't upstream of the tank, the issue would be modeling conditions that would keep water in the tank, which I imagine occurs in the actual system.
If the pump is properly placed, this link has information on the pump message. Negative pressures on the suction side of the pump can occur in some cases, as mentioned here.
The pump is downstream of the tank in the actual system. I am trying to get the model to reflect actual conditions. In the actual system, there are not negative pressures and the problems I am seeing in this area of the model are not experienced in the actual system. This leads me to believe something is not set up correctly in the model, but I can't figure out what.
Hello Dominick,
One thing I noticed is there is a hydropneumatic tank near the pump in question. This is currently set as a junction (Treat as junction=True). If this should be operating as a tank in the calculation, this should be set as False. I did this in the original model you provided, but it didn't help with the issue. If this should be operating as a tank, you can try changing this to see if it helps. More information on modeling hydropneumatic tanks in WaterGEMS and WaterCAD can be found here: Modeling hydropneumatic tanks in WaterGEMS and WaterCAD.
If this doesn't help, it would be useful to see an updated copy of the model files.
Scott,
Nothing in the model has changed since I last uploaded a copy of the model files. I've confirmed that the model layout is the same as actual field conditions. I also set the hydropneumatic tank "Treat as junction" setting to False and it did not help.
In that case, what fills the tank in the actual system? Or if the demands are satisfied from another source besides the tank (or the hydropneumatic tank) is there some way to increase the head so the pressures are not so low?
The tank is becoming empty to satisfy demands, but isn't able to be refilled. Once the tank is empty, the downstream demands need to be satisfied by sources further downstream, which is causing lower hydraulic grades due to headlosses.