Sim, It looks to me like all nodes have a HGL of 10m. Based on that, would you conclude that the "HGL of a Junction that has a positive Demand is equal to HGL of lowest Tank or Reservoir of the network"? Obviously not. So it looks like there is some other cause here. Why not try adding a negative demand to another model to see what happens? Or, why not try making the negative demand in your model big enough to cause some significant velocity in your pipes. Then you might get a better understanding of this. Regards, Mal
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Forum Post: RE: Negative Demand
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Forum Post: RE: Pump Selecting
Inserting a 'dummy' reservoir is likely to give incorrect results. Better to use 'pressure dependent demands' and then create a system head curve at the pump that you are trying to design. See the link that Jesse provided earlier for more details. Regards, Mal
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Forum Post: RE: Help With setting up a VSP or VFD with 4 pumps.
It could be that you have a conflict with another control statement. To make sure that this control statement will be used, set it to a higher priority than any other statement for this pump. (The default priority is essentially 0 and you should use a higher number, up to 5.)
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Forum Post: RE: Help With setting up a VSP or VFD with 4 pumps.
Also make sure the controls are included in the control set you're using in the scenario in question (if you are using one). If this doesn't help, you can zip and attach the model files here, or upload privately using the process in the link below: communities.bentley.com/.../bentleysecurefilesupload.aspx
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Forum Post: what is the difference between ''pressure'' and ''pressure head''
Hello, what is the difference between ''pressure'' and ''pressure head'', according WaterCAD Best regards
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Forum Post: RE: what is the difference between ''pressure'' and ''pressure head''
Vanfa, The pressure head is the hydraulic grade - the physical elevation of the junction and the pressure is the same, but also takes into account the specific gravity of water which is 0.998. Therefore, 56.79 * 0.998 = 56.67. Regards, Mark
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Forum Post: RE: what is the difference between ''pressure'' and ''pressure head''
I'll add that the specific gravity can be changed in the calculation options (View Calculation Options) depending on the exact fluid and temperature.
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Forum Post: RE: what is the difference between ''pressure'' and ''pressure head''
Even more general Pressure head refers to the energy from pressure expressed in length units (m, ft) and is calculated as the difference between the HGL and elevation of the point at which it is measured. Pressure is actually force per unit area and is usually expressed in newtons per sq m (Pa), kPA, pounds per sq ft, psi, bar, but sometimes it can be expressed as height (m, ft) of a liquid column that can produce that pressure at the base.
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Forum Post: Fire flow in dedicated fire main
I am new to WaterCAD. A hydrant test was performed 25' from the entrance to a proposed community. This community will have a dedicated fire main as well as a water main for daily demands. The fire main will have a total of 7 hydrants. I need to have 1000 gpm at the furthest hydrant. How would I model this up in WaterCAD?
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Wiki Page: What is the difference between pressure head and pressure in WaterCAD?
Applies To Product(s): Bentley WaterCAD, Bentley WaterGEMS, Bentley HAMMER Version(s): 08.11.XX.XX Environment: N\A Area: Output and Reporting Subarea: N\A Original Author: Mark Pachlhofer, Bentley Technical Support Group Problem Description What is the difference between pressure head and pressure in WaterCAD? Steps to Resolve Pressure head is the hydraulic grade minus the physical elevation at a junction or the weight of a given height of water column at its bottom end.It is independent of the amount of water: the pressure head of a 10-foot high or deep 2-inch diameter pipe is the same as that of the 2-foot diameter pipe of the same height or depth (business dictionary.com, 2014). The pressure is hydraulic grade minus the physical elevation at a junction, but also takes into account the specific gravity of water, which is 0.998. For example, if you have a pressure head of 56.70ft your pressure would be 56.79ft * 0.998 = 56.67 ft of H20.
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Wiki: Hydraulics and Hydrology - Wiki
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Forum Post: StormCAD Model Results and Maximum Network Traversals
Hi, I have a StormCAD model. The model was working fine when I was using StormCAD Series 2. I had to go back and revise the model, and when I changed pipe sizes, the model suddenly would not converge. I tried increasing the maximum network traversals to 1000, and the model still would not converge. At that point, I saved the file and re-opened the file in StormCAD Series 3. The model converged and worked with the revised pipe size (frustrating program error). However, I found a peculiar issue. The model results is different when the maximum network traversals is 1000 as opposed to the default 5. As long as the model converges, shouldn’t the results be the same regardless of the maximum network traversals? Anyone know what’s going on? Thanks.
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Forum Post: RE: Pump Selecting
Dear Mall Thank you very much for your good responses. I read below Wikis carefully: communities.bentley.com/.../15371.system-head-curves-with-no-downstream-storage-solution-500000056093.aspx communities.bentley.com/.../16879.estimating-a-pump-curve-for-a-model.aspx Now I know how to draw System Head Curve for any network (Closed or non-closed). But my first question is unanswered yet: Assume there is a network with just one Reservoir which has lower Elevation against Junction Elevations. Now I want to select a sufficient Pump for entire network. It needs at least one head and one flow to use ‘Design Point’ Type or so. How can I get these two points? I did what Wiki communities.bentley.com/.../16879.estimating-a-pump-curve-for-a-model.aspx mentions, but for my closed-system, it does not run, because there is no any Boundary in the network and ‘Node is not connected to a boundary’ error occures. Either PDD has been defined or not, this error message occures, I insert a dummy Reservoir to remove this error. You says it causes incorrect results. Please guide me how I must do to solve the problem with defining and selecting a good Pump for this condition. All what you and others say is related to AFTER selecting pump. Graphing System Head Curve (PDD or so) is related to AFTER selecting Pump and my question is about SELECTING Pump itself. Sim
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Forum Post: RE: Negative Demand
Dear Mall Please have a look at attached model. You are right. After running the model there will no any eproblem occur and every thing go well. A question remains is that if ‘assigning negative demand to the junction’ works like a sufficient pump? After inserting a sufficient pump, you would see the HGL of junctions are similar to HGL of junctions before inserting pump. What does ‘Negative Demand’ work and how it affect on other junctions’s pressures? Sim
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Forum Post: RE: StormCAD Model Results and Maximum Network Traversals
Hello, The reason is because there is an enhancement in SELECTseries 3 (and greater) which is only invoked when setting the Maximum Network Traversals to a value of 30 or greater. I've created the below Support Solution to explain this in more detail: Different results when changing Maximum Network Traversals
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Forum Post: RE: Pump Selecting
Once you have the system head curve (which is independent of the pump curve), you can overlay some candidate pump curves to select the one that intersects the system head curve near optimal efficiency. Here are some sections of the Advanced Water Distribution Modeling and Management book that you should check out: Chapter 8.6: Developing System Head Curves for Pump Selection/Evaluation Page 312: Pumping into a Closed System with No Pressure Control Valve Chapter 10.8: Power Consumption Page 438: Determining Pump Operating Points
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Forum Post: RE: Negative Demand
With a pump, it will shift on its curve based on the system conditions. When using a negative demand on a junction, it will always inject that flow rate regardless of the system conditions. A negative is like a known in the equation, similar to fixed positive demands and fixed boundary conditions - you tell the program the flow you want to leave (positive demands) the flow you want to inject (negative demands) and the hydraulic grade at reservoirs and tanks. The program then solves the rest of the results around those "knowns" in the equation, physical properties and controls. Let's say you have a model like this: Negative demand pipe reservoir at 100 m. If the negative demand that you entered results in a headloss in that given pipe of 2.0 meters, then the model will solve the hydraulic grade at the negative demand location as 102 m. This is the HGL is needs to be in order to satisfy those "knowns" (reservoir at 100 m, the negative demand you entered, pipe size/roughness).
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Forum Post: RE: StormCAD Model Results and Maximum Network Traversals
Thanks!
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Forum Post: Convergence NOT Achieved
Hi, I have a StormCAD model that was created using V8i Version 2. There are four outfalls in the model. Each storm drain system leading to the outfall is mostly surcharged because the starting outfall tailwater is higher than most of the top of pipes. Everything about the model was working fine. Then, when I upsized an outfall from 60” to 66”, convergence was not achieved. As a test, I then upsized the outfall to 72” (much too large for what I need), and convergence was achieved. I downsized the pipe size back 60” to the original model. At another outfall, I downsized the outfall from 72” to 66”. Convergence was also not achieved. As a test, I downsized the pipe to 60”, and convergence was achieved. I upsized the pipe back to 72” to the original model. In both situations when convergence was not achieved, I increased the Maximum Network Traversals to 1000, and convergence was still not achieved. At this point, I re-opened the model in StormCAD V8i Version 3, revised the two outfalls to be 66”, and convergence was achieved using a Maximum Network Traversal of 5 (the fact that two models give two different results in the same program family is disconcerting). I thought I was in the clear going forward. For one of the outfalls, I adjusted upstream pipe inverts, and convergence was not achieved. I increased the Maximum Network Traversal to 1000, and convergence was achieved. However, the results are now wildly different for the entire model than when the Maximum Network Traversal of 5 for each of the storm drain systems leading to outfalls. Also, why would changing pipe inverts for pipes that are surcharged suddenly have the storm drain system not converge when the Maximum Network Traversal is only 5? I really don’t trust the program or the model results that. What’s going on?
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Forum Post: RE: Convergence NOT Achieved
Hello, Please see my post below that links to an article that explains what is going on behind the scenes when you set the max traversals above 30 and why this change was implemented: communities.bentley.com/.../101443.aspx Basically you should only set the max traversals to a high value in cases where lower values do not help, as the logic explained in the Support Solution article is used, which could produce different results for particularly challenging models. I would suggested starting with a smaller value for max traversals, such as 6-10. If that doesn't help, try something a little higher like 20-25. Also check your data input for any problems such as typos in the roughness coefficient, bad inverts, etc. Regarding why a model might suddenly not converge when changing the invert - surcharging pipes are particularly difficult, as explained in the Support Solution article I created. They can be sensitive to input changes, especially if they're very close to not converging already. Have you considered using the automated design feature to size the pipes? Have you considered upgrading to the latest version? (SELECTseries 4, 08.11.04.54) See below links for more on the new features: What's new in StormCAD V8i SELECTseries 4 (Technote) What's new in SewerGEMS, SewerCAD, CivilStorm and StormCAD V8i (SELECTseries 4) (Webinar) If this doesn't help, please send your model for review. You can either attach to the forum thread (using the Advanced Reply Editor) or use the Secure Sharefile method .
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