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.
Profile Report_Engineering Profile - Profile - 1 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdfLow Pressure Model.zip
Tom, this lower pressure sanitary sewer is discharging to a gravity sewer manhole modeled as an outfall in a section of pressure sewer that is constructed to the north of the first section from about STA 15+00. The first profile is not discharging far to the right, but flow heads to the north at J-25. I have attached the additional engineering profile for that section & a larger version of the first profile.
The notification I get that is of concern is "Pressures below the lowest physically possible pressure in system at one or more time steps".
There is an air valve in the first profile near where the pressure goes negative, which I'm currently modeling as a junction.
In a modeled low pressure sanitary sewer with pressure pipes that have high points/air valves, when you say "if there are downhill slopes with an air valve at the preceding high point, the downhill section will drain and will flow partly full." - how does the model show that? Is it represented by what I'm seeing?
Profile Report_Engineering Profile - Profile - 2 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdf3718.Profile Report_Engineering Profile - Profile - 1 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdf
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.
Scott, I did provide a zip of the model in my first post above, "Low Pressure Model.zip", but here it is again attached. I believe all the associated files are there.2330.Low Pressure Model.zip
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.