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Tank in Series with Known Demands Issue

Hi,

I'm trying to setup a scenario where a supply (Reservoir 1) fills Tank 17 at a specified flowrate. A FCV is used on the line between the reservoir and Tank 17 to set the Flow, 168-GPM. Tank 17 fills from the bottom and has a separate pipe for the outflow. The Tank 17 outflow pipe tees splitting flow into two separate directions. The first is to Tank 12 (16,000-ft away). Tank 12 has a separate inflow and outflow pipe. The Tank 12 outflow pipe has a known demand (86-GPM). The second branch for water exiting Tank 17 tees to a junction with a known demand, called Tank 22 Pumped Demand, 71-GPM.

The summary graphs for Tank 17 and Tank 12 show that they appear to fill in a logical time period. Tank 17 is the first tank online and fills in 15-hrs. The graph shows it oscillates between filling and emptying until Tank 12 is full, 29-hrs. Everything beyond that generates user notifications which create concern that flow is not actually making it out of Tank 17. Numerous system disconnected errors and pressures below physically possible pressure in system as well as notification that FCV can't deliver flow. The notifications show Tank 17 and 12 both fill but the pipe segments between show no flow in the properties dialog boxes.

A scenario was created to have two demands setup on the system as mentioned above. The results are not realistic. The hydraulic grade for the system appears to be completely off and it also shows Tank 12 actually flowing back toward the other junction with a known demand. 

Using the network navigator to trace downstream, starting from reservoir 1 the highlighted route stops at Tank 17. Are we missing something to have flow coming out of each Tank?

Should a scenario like this run as a steady state or EPS.

Is there a better way to model Tank 17 filling at a specified rate. Other forums discussed the PSV and the FCV which has been modeled. 

Any sage advice would be greatly appreciated. We also tried creating a smaller model but run into similar issues. Below is an image of the scenario, but slightly modified in the actual model.

Here is a link to Onedrive with the actual waterCAD file if anyone has time to review.

https://eesportal-my.sharepoint.com/:f:/g/personal/brett_schlanger_ees_us_com/Eqj2gbBnATRMmyuUQ0k39kcBeBH9o_YpjKi29n4I3uXq4A?e=UfUjwe

Thank you,

Brett

Parents
  • Hello Brett, 

    Thank you for uploading model files, I see that you have PSV status as active, with pressure setting to maintain 0, you may want to recheck that to make sure you enter correct value of pressure to be maintained. 

    As seen in the graph above in base scenario, you will see that, when tank T-17 becomes full at 14.89 hours, it cannot take anymore flow, so the flow from reservoir stops and hence FCV cannot deliver flow as tank is full, which ultimately generates other user notifications. 

    In the base scenario, there is zero demand, so tank does not empty at all, as there is no flow demand in the network and hence FCV cannot deliver flow as tank is full. 

    For Scenario - Tank 12 demand

    There is demand of total 157 GPM, which is not fulfilled continuously. As tank 17 becomes full and FCV stops working, you should put control such that tank 17 never becomes fully empty of 100 % full, based on pipe P-62. E.g. When tank percent full >90%, close the pipe, else OPEN. 

    Creating Controls - Conditions, Actions, and Control Sets (CONNECT Edition and V8i SELECTseries 6) 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Hi Sushma. Thank you for reviewing and getting back to me. I created a simplified model of the system based on some of the other forums I read for modeling a limited source for storage and not using the FCV or PSV. This utilizes the source at a junction as a negative demand with a reservoir for the overflow. Source is -100,000-GPD. The demand downstream of Tank 17 is 30,000-GPD on the system itself. I created a control set as you mentioned for the Tank 17, for the downstream pipe to be closed when Tank is <5% full and >95% full. The results are still concerning. One of the issues I see is the starting point for the hydraulic grade line for the pipes downstream of the tank, P-10 and P-9. The hydraulic grade starting position appears way off. The other is the graph for Tank 17. It doesn't show Tank 17 really ever fills. The 100,000-GPD should exceed the 30,000-GPD demand and allow Tank 17 to fill but the model doesn't depict this. Increasing the source to 300,000-GPD doesn't impact the Tank 17 capacity as well. Any suggestions are appreciated. Thank you!

    Simplified Model.wtg.dwh.01.zip

  • Hello Brett, 

    In the simplified model, you have setup control on pipe P-10, which should have been on pipe P-11, logic being pipe P-11 closes when percent full is > 95%, so no more water enter into tank from reservoir. 

    I believe you are referring to this wiki, about modeling fixed inflow. 

    Modeling Fixed Inflow from a Marginal Source 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Hi. Thanks again, but still running into issues. I made the edits you mentioned and this certainly corrected the hydraulic grade issue but still seeing issues with outflow from the tank. In this simplified model I included a second downstream tank 12 to model the current scenario were working on.

    Basically there is a source supplying Tank 17. Tank 17 then gravity feeds to Tank 12. There is a demand on the system after Tank 12. The scenario were looking to model should reflect filling the tanks and watching the demand draw off the tanks. We need to take into consideration how fast the tanks fill vs the required demand however the model doesn't appear to be setup right to reflect this condition. Can you please advise.

    The attached model doesn't show flow leaving Tank 17. Originally we had set the controls as you suggested plus added T<5% close the downstream pipe. The graph for Tank 17 doesn't make much sense as it shows Tank 17 taking over 72-hours to fill while the source is 100,000-GPD. Even with the controls removed the downstream Tank 12 doesn't now fill. Can you advise? Thank you.

    Simplified Model.wtg_1-17-19.zip  

  • Hello Brett,

    The tank T-12, which feeds the demand at junction J-5 DEMAND is empty at the start of the simulation. With no water in the tank, there is nothing available to satisfy the demand at the junction. You will want the initial elevation for the tank to be higher than the minimum elevation of the tank.

    In addition, I would include controls on the tank so that it doesn't become empty again during the run. If that happens, and no other source of flow, the "disconnected demand node" message will occur at other time steps.

    Regards,

    Scott

  • Thanks. I thought that Tank 17 would flow into Tank 12 via gravity. Is that not the case?

Reply Children
  • Hello Brett,

    The controls on the tank seem to be hindering that. Instead of using the IF/THEN/ELSE statement, I created two separate controls for each of the two pipes. For instance, for P-10, I have a control that opens the pipe if tank T-17 is more than 5% full and another that closes it if it is less than five percent full. (I did the same for the controls on P-11, though these never really came into play, as the tank never got above 5% full.)

    With this setup, there will be some flow into T-12. However, as soon as there was any flow out of T-17, that dropped the percent full value for the tank below 5%, thus closing the pipe again. In the mean time, T-12 will become empty again, thus making it so that the demands can't be satisfied. This happened less frequently, but it still occurred.

    Assuming the controls are accurate, one thing to consider having a higher initial elevation in both tanks. T-12 particularly should not become empty, or start empty, because of the demand downstream. 

    The other thing to review is the flow going into the reservoir compared to what is going toward the demand. You have an inflow of 100000 gpd at node SOURCE. Three-quarters of the flow goes to the reservoir, while only about 25000 gpd of flow moves in the direction of the demand, which is 80000 gpd. This will mean that most of the demand will need to come from the water stored in the tanks. Even if you start the tanks at a higher elevation, you may still see the tanks become empty and see the disconnected demand node message.

    Regards,

    Scott

    Answer Verified By: Sushma Choure