Low Pressure Sanitary Sewer - Negative Pressures

Question

I'm modeling a low pressure sanitary sewer. The goal is to see how the system reacts as homes/grinder pumps come online. The issue I'm having is with only a single grinder pump operating to start in a steady state analysis, the system HGL goes negative part way through the system. See engineering profile attached. When I model the system with 3 grinder pumps online at the same time, there is enough flow so that the HGL is positive. It's as if the the single pump is trying to fill the system and cannot. Any insight into why SewerCAD reacts this way would be helpful. I have also attached the model. Currently the additional pumps controls are set so the on/off HGL settings are the same so that they don't come online. 
 PDF Low Pressure Model.zip

Scott Kampa · Answer

Hello Ben, 
 If you set the air valve property field "Treat as junction" to False, this may lead to an improvement in the results. The partly full pipes that Dr. Walski mentions will also likely be displayed. The following link has more information on modeling air valves at high points in SewerCAD: Modeling Force Mains with Air Valves . 
 If that doesn't help, it might be useful to see a copy of the model, so we can get a better idea of what else might be happening in the system. This link has details on sending the model files to us. 
 Regards, Scott

Scott Kampa · Answer

Hello Ben, 
 Should the other pumps be on as well? The controls are set up such that the pumps may be off when they should be on. For instance, PMP-3 is initially off, but turns off because of a control, even though the pump wet well is full (the "off" elevation is the max elevation of the wet well). In addition, the pump is set to turn on when the wet well elevation is greater than 794.84 ft. However, since that elevation is the maximum elevation allowed, the pump would never turn on based on the control. Typically, the pump will be on at some high value and off at some low value. This link has further details. Note that W-2 is also full and the pump is off, so this pump may need to be on as well. Reviewing the controls to make sure they are accurate would be recommended here as well. 
 If PMP-3 is on, the results are improved. In addition, the air valves are being treated as a junction in the model. I would recommend setting the "Treat as Junction" property to False so that it is treated as an air valve. I would also recommend replacing J-36 with an air valve with the same elevations. While negative pressures still occurring, the results were improved. Some additional adjustments might lead to further improvements in the results. 
 Regards, 
 Scott