Model is negative at the water plant and I don't exactly know what is causing it.

Question

When I run my contour map my model is showing negative pressure at the water plant. 
 WaterGems

Jesse Dringoli · Answer

Hello Andrea, 
 Do you have a junction near the "water plant" that has an elevation set too high? Take a look at a profile of the area to help visualize the hydraulic grade versus elevation. Negative pressure will occur when the hydraulic grade is below the physical elevation. See more here: Troubleshooting negative pressures at pumps, junctions, & other node elements 
 If this does not help, please provide a copy of the model for review along with a screenshot of what you are seeing: Sharing Hydraulic Model Files on the OpenFlows Forum

Jesse Dringoli · Answer

Yes I received the model. 
 If you graph pressure at P-1 or P-3 you will see that it becomes negative after around 13 hours. This happens because tank WP-1 GST becomes empty. See the following article for more information on that: What happens when a tank becomes empty or full? (altitude valve) 
 Your pump controls for this tank seem a bit odd. The pump is on the upstream side (filling the tank), but the controls are set to turn the pump off if the elevation drops. Did you mean to use ">=" for the pump off control and "<=" for the pump on control? If I switch them, the tank does not become empty or full and negative pressure no longer occurs. See more: Troubleshooting Controls in WaterCAD and WaterGEMS 
 In the calculation options, you have set to report only every 2 hours, so you miss out on seeing a lot of details of the results that happen between. I would suggest changing the Reporting Timestep to "all". This will also reveal that the network becomes unbalanced at times, likely related to the empty tank problem. 
 I also noticed you have two hydraulically close hydropneumatic tanks downstream of the pump station. I would recommend combining these into one single hydraulically equivalent tank. See more here: 
 Rapid flow oscillation between hydraulically close tanks 
 Modeling hydropneumatic tanks in an EPS in WaterGEMS and WaterCAD

Jesse Dringoli · Answer

Andrea, 
 The 2022 scenario is configured to run as EPS so the flow changes over time. I assume you are referring to time zero. The reason why the flow in P-1 is zero in that scenario is because the pump "WP-1 Well Pump" is off. That pump is off because your controls are telling it to turn off (there are two controls and both of their conditions are true at time zero because the tank level is exactly 25, and the pump off control takes precedence). 
 In the "2022 Modified" scenario, the flow is also zero for me for the same reasons (and for all timesteps). I am looking at the model you had previously sent, without making any changes.

Jesse Dringoli · Answer

I right clicked on the pipe, choose "Graph" and selected the two scenarios. A line graph is the default so most likely the advanced chart options have been modified and saved as the new default. Try resetting them to factory defaults using the gear icon at the top - choose "Restore factory default chart options". See the section at the bottom of this article: Change graph and profile appearance using Chart Options

Jesse Dringoli · Answer

Andrea, right click in the middle of the graph, choose Series Options and select the other scenario on the left side. See: Viewing graph data for multiple scenarios

Jesse Dringoli · Answer

Can you clarify what you mean by "running through"? Are you encountering a specific problem? Please clarify the steps to reproduce and clarify what you are seeing. 
 If you are referring to the scenario named "Peak Demand", I took a general look at the demands: 
 
 Most of the demand is from Unit Demands, consisting of a varying number of units for each junction, for a unit demand called "unit demand - 1" which has a base flow of 1 gpm. The pattern is set to fixed for all unit demands and there is no pattern defined. 
 Many demand nodes also have a base flow demand consisting of 1 gpm with a fixed pattern, except for " MUD36_Nodes-81" which is set to 1200 gpm. Perhaps this represents a fire? Because the pattern is fixed, the same 1200 gpm applies for the entire EPS duration. If this represents a fire that occurs for a certain duration, a pattern should be assigned. See: Running a fire flow analysis in EPS (Extended Period Simulation) 
 In your calculation options, you have set to multiply the base demand by 1.3 for all nodes in the selection set called "Elements added via ModelBuilder..." . The 1200 gpm demand node is included in this selection set so you will see a calculated demand of 1,560 gpm in the Results section of the properties. 
 You will notice in the Calculation Summary the flow demanded column shows a value of 2127 gpm at all times. This is because all demand patterns are fixed. This model may not be set up for an EPS yet as you would typically have a diurnal pattern assigned.

Jesse Dringoli · Answer

Hi Andrea, the best approach may depend on what you need the model to tell you about your fire. 
 Do you need to see what the instantaneous pressures are in the system when 1200 gpm is flowing at a hydrant nearby that junction? If so, you could run a steady state simulation (instead of the current EPS) with that fixed demand added to that node. To ensure that the 1200 gpm demand does not appear in other non-fire scenarios, you would need to ensure that they use separate Demand alternatives. 
 If you need to assess the impact the fire has on your tanks, you may need to run an EPS like you have it now, but configure a pattern so that the fire only lasts for a certain period of time. This is what the article I mentioned in my previous reply explains: Running a fire flow analysis in EPS (Extended Period Simulation) 
 If you need to assess exactly how much fire flow is available at one or more hydrant locations based on constraints like minimum residual and minimum zone pressure, you would use the automated fireflow feature. In this case you would model the junction demands as the normal demands (remove the 1200 gpm fixed demand) and use the Fireflow alternative instead of a new Demand alternative. You mentioned that you could not get your fire flow alternative to work - if you attempted to use automated fireflow and encountered a problem, please review the information in the following article and if it does not help, provide more details: Understanding Automated Fire Flow Results

Jesse Dringoli · Answer

To see the pressure when the "fire" happens, look at the "Results" section of the properties of the junction after computing a steady state simulation with the fire demand added to the node. You can also use the hydrant flow curve option to see a range of demand and resulting residual pressure (remove the 1200 gpm demand from the node first).

Regards,

Jesse Dringoli
Technical Support Manager, OpenFlows
Bentley Communities Site Administrator
Bentley Systems, Inc.

Jesse Dringoli · Answer

Andrea, is this a new model revision? Is the negative pressure on a pump suction side? Is the elevation value correct? Try creating a profile of the area to see the hydraulic grade with respect to the elevations. 
 Generally speaking increased demands result in increased headloss which results in lower pressure. if this is a scenario has a demand multiplier resulting in higher demands than the other scenarios, then lower pressure may be expected. 
 The following article has further guidance on troubleshooting negative pressure: Troubleshooting negative pressures at pumps, junctions, & other node elements 
 If this does not help, please provide a copy of the updated model.

Sushma Choure · Answer

Hello Andrea, 
 There appears to be some confusion here, what Jesse meant is setting the time analysis type to "Steady State" under calculation options. As you have set the active demand adjustments in calc options, to multiply demands by 1.3 and have not defined any hydraulic pattern.

Sushma Choure · Answer

Steady state run is helpful when you are only concerned about peak flow analysis, EPS is useful when demands vary as per time, by assigning patterns, tank modeling over time can be done, pump operating over time can be observed etc. 
 Patterns are not created in your model, so running steady state for all the scenarios will be logical, else you can define patterns and the run EPS. 
 Here is information on running EPS analysis. 
 Run an Extended Period Simulation (EPS)

Scott Kampa · Answer

Hello Andrea, 
 Demand patterns can be entered using the Patterns manager (Home > Patterns or Components > Patterns). Depending on the format, you could also use ModelBuilder to import demand patterns . Once the patterns are added, they can be applied to junction demands in the Demand Control Center. 
 Regards, 
 Scott