Hello,
Myself and my firm are pretty new to WaterCAD. I have built a model with 75 elements and 81 pipes, a medium sized system for a school campus (non residential). We have two new 10" water service laterals from our private loop system that tie into the same 12" main in the street. The City water department has given us flow data generated by their hydraulic model at two points along that main, each point is roughly 300'-400' away from either of our new lateral connections.
The flow data given was the following:
Location A:
Static - 87 psi , Static Pressure Range - 85-100 psi, Flow - 4000 gpm (expected @ 20 psi), Residual - 83 psi
Location B:
Static - 86 psi , Static Pressure Range - 85-99 psi, Flow - 4000 gpm (expected @ 20 psi), Residual - 81 psi
Since my knowledge of the City water system is limited, I can't confidently say that each lateral will be pulling those demands and pressures simultaneously during the event of a fire onsite. Therefore, I have opted to model the source in the main at a single location (Location B above) with a reservoir and a pump. Essentially, in the street, there are 3 key nodes: the source and each of the two new "tee" connections to the 12" main. So I have modeled the existing 12" DIP that connects these nodes. I back calc'd a flow rate from the info above that the system would have given at the residual pressure, that way, I could model a pump curve for my pump with 3 points - static, residual and @ 20 psi. We have about 20 hydrants on site, but the selection set I created only looks at 6 key hydrants for our worst case scenario at which each of the 6 hydrants will need to supply 480 gpm to a single building, while limiting pipe velocities to 15 fps.
My problems/questions are the following:
1. All elements have been assigned elevations based on finished grade. How do I determine the water service elevation in the reservoir? I thought this might be equal to the static pressure at the source converted to feet of water, but when I run the analysis this way, my pressures onsite are in range of 150 psi, much higher than static pressure in the main?
2. Is my general set up correct? I have also heard modeling the two test locations with nodes and connecting those two nodes to a single reservoir off at some distance to calibrate the system demand maybe better than setting up a pump at the source. Calibration - adding a demand at each test location node (one at a time) equal to the back-calc'd flow at residual pressure to make sure the pressure at that node actually equals the expected residual pressure. This process would be iterative to play with the reservoir distance, connecting pipe properties, etc to get this right.
I have uploaded my model for reference. Thanks!
also, forgot to ask:
3. When I go to Pipe flextables to check what my pipe velocities were, flows and velocities are all 0 throughout the system?
Hello Noah,
It seems you have shared only the .WTG file for the model. For the model to open and run we would require the .WTG and the .WTG.SQLITE (database) file. The .SQLITE file stores the model information and is required to open and analyze the model.
In the location you have stored the model there will be number of files created. Just share the .WTG and .WTG.SQLITE files to check on your model.
Here is the link to share model files on the Forum which also explains which files are required to open the model;
Sharing Hydraulic Model Files on the Haestad Forum
What version of WaterCAD are you currently working on? Go to File > Help > About and look for the version number on the bottom left corner.
Since you are a new user of WaterCAD here are some links which you may find useful;
Learning Resource Guide for new users of Bentley WaterCAD
Get to know WaterCAD CONNECT Edition
WaterCAD TechNotes and FAQs
Hope this helps.
Regards,
Yashodhan Joshi
1) It sounds like you're using the standard method of approximating a connection to an existing system. You can read more about setting up the pump and reservoir based on your static and residual flow tests here: Modeling a Connection to an Existing System
However, it is usually not valid to do this in more than one location and it sounds like you have two locations. You can read about that in the above article.
2) As for modeling the connection to the existing system, the pump+reservoir approach should be better than the single reservoir approach, because it will simulate the reduction in pressure at the source when the inflow increases. However, having two connection points can skew the results if they are in close proximity as explained above. The ideal setup would be to model at least a skeletonized version all the way back to the real source. You can read about that in the above article.
As for your item on "calibration" - if you have flow and pressure results from the current system, but you're adding an expansion that ties in, then it seems to me that the flow and pressure data you were given is irrespective of the demands. If you are adding a new, proposed expansion, then the demands do not exist yet and there would be nothing to calibrate. You could estimate/project the demands of the expansion and add them to the new junctions. You can read more about demand modeling and estimation in our book Advanced Water Distribution Modeling and Management.
3) If the results are showing zero and this is an EPS (extended period simulation), you might be looking at the wrong timestep. By default it will be showing results for time zero. If this is a steady state, then the zero flow could be due to a lack of demands or tanks. It's hard to say unless we had a copy of the model. Did you add your demands yet?
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Hi Yashodhan & Jesse,
Thanks for your responses. I have re-uploaded the model including the sqlite file. Since my original post yesterday, I tinkered with the model a little more. I am now able to get flows, velocities and pressures that makes sense now. The difference was I did not apply a base demand of 480 gpm at each of the 6 hydrants. I simply relied upon the base fire flow dialogue box to apply the fire base demands at each hydrant since I had them selected as a selection set and "replaced the baseline demand" with the 480 gpm I was prescribing in the dialogue box. My updated question list is below:
1. How do I determine the water service elevation in the reservoir? I have arbitrarily set it at elevation 22.0, higher than any point on site since the water main is lower than the site by a few feet. I'm guessing it doesn't really matter since the pump controls the pressures and flows in the main are set by the pump curve, but wanted to be sure I wasn't adding any unnecessary head.
2. I will review the links you have provided, but why did the analysis not populate the hydrants with the base fire demand of 480 gpm that I prescribed through the base fire flow dialogue box? Having to physically go and double click each of those hydrants already in the selection set to set a regular base demand of 480 gpm, then go to the base fire flow dialogue box and 'replace the base demand' seems like a complete waste of time? I made sure each of those 6 hydrants were 'open' before running the analysis both ways.
Thanks again for the help!
I will answer your queries serially;
Noah Ramos said:1. How do I determine the water service elevation in the reservoir? I have arbitrarily set it at elevation 22.0, higher than any point on site since the water main is lower than the site by a few feet. I'm guessing it doesn't really matter since the pump controls the pressures and flows in the main are set by the pump curve, but wanted to be sure I wasn't adding any unnecessary head.
This depends on what pressures you require at your downstream nodes. Local authorities generally prescribe certain standards to maintain a minimum pressure at the nodes. You can check if the pressures remain above the prescribed values during your simulation. Ideally you should also look at how much head your pump is adding as its operation will govern the pressures and flows downstream. If the pressures are within the design limits it should not be a problem.
Another thing to note is that the "Reservoir" element in WaterCAD is considered as an ideal source with infinite volume. Hence the "Elevation" you prescribe at the reservoir is assumed to remain constant during your analysis. Hence the service water level can be input as the "Elevation" of the reservoir if you are analyzing under ideal conditions (i.e. the HGL / service level in the reservoir is not fluctuating).
Noah Ramos said:2. I will review the links you have provided, but why did the analysis not populate the hydrants with the base fire demand of 480 gpm that I prescribed through the base fire flow dialogue box? Having to physically go and double click each of those hydrants already in the selection set to set a regular base demand of 480 gpm, then go to the base fire flow dialogue box and 'replace the base demand' seems like a complete waste of time? I made sure each of those 6 hydrants were 'open' before running the analysis both ways.
Make sure you have set the Calculation Type as "Fire Flow" in the Calculation Options. Once done, the Fire Flow (Needed) will be added to your base demands (if you have selected that option) and run the analysis to check which of the fire nodes are failing. You can check the results under Analysis > Analysis Tools Tab > Fire Flow Results.
To understand how a Fire Flow Analysis is setup in WaterCAD, please refer the "Quick Start Lessons" for a chapter on "Automated Fire Flow Analysis".
Here are some article which can help you;
How does the Automated Fire Flow analysis work?
Understanding Automated Fire Flow Results
Understanding the "Fire Flow (Needed)" and "Fire Flow (Upper Limit)" Fields for a Fire Flow Analysis
Answer Verified By: Sushma Choure