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Problem setting up a booster pump station with a storage tank control node.

I have a booster pump station with 3 VSP installed. Its system pressure dependent, however, the pressure transducer is inside the storage tank. The storage tank has separate inlet and outlet pipes and when i try to run the model, it gives me an error "Cannot run VSP with a control node that has separate inlet and outlet pipes". I can run the model if I change those pumps to run as constant flow pumps. The booster pump station is installed directly next to the storage tank and the primary function is to ensure the pressure in the distribution system stays at its required level. 

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  • Hello Jesse,

    I uploaded another set of files. I changed the control node to J-19. However, im unsure if this is an accurate place to put the control node. How I want it to work is this:

    The booster pumps will turn on when the hydraulic grade in the tank is at 344.0 ft (82 ft above the floor of the tank). The well pumps will be called to operate when the hydraulic grade in the tank is 339 ft (77 ft above the floor of the tank) and they will shut off when the hydraulic grade is 342 ft above the floor of the tank. Im getting an error now that is calling out the booster pumps and telling me "Parallel VSPs are not allowed to have different pump curves". This is odd as there is only 1 b.pump being modeled. I have the curves added to eachother (flows increase and head stays the same). The control node for the wells is also j-19, however in the control sets i have Booster pumps and the 2 wells references a pressure inside the tank. if the controls reference the tank and the control node is not the tank is that a problem?
    Another bit of information is im using MDD_2009 as a daily demand scenario that was older, i havent made many adjustments to is and therefore am not sure how accurate the information is, I.E. the different alternatives that are being called upon. I havent finished my calculations for the current MDD so im using this old information as the current go to. I also have H-1 with a demand collection of 1000gpm to simulate a fire flow at that node. 
    I updated the inlet for the tank to be at level 83.5 ft like you advised. I added an overflow reservoir and put its elevation at the overflow invert elevation. 
    You mentioned you didnt understand where the regulated pressure needed to be, the pressure needs to remain above a specific value across the entire system, not just one particular node. 
    I also have a question about pump station capacity. The wells that i have added have a total allowable capacity dictated but water rights. Is there a way to make sure those pumps operate at a specific flow? 

    The purpose of the booster pumps is to ensure pressure remains higher in the entire system therefore in the actual application (out in the field) the pumps are run on a "constant pressure" mode.

    There is a set of scenarios named "static_2002" those are old. The ones I created are under "Sk*** Base". I believe those are only set up to mimic an automated fire flow analysis. I'm just trying to make sure what I have here is setup to the current conditions and allowable for me to start scheduling hydrant calibration tests. 

    Thanks for your continued help.

  • I uploaded another set of files.

    I see the new files but I am not clear on the steps to reproduce. The scenario "TW Fire Base Physical" is set as steady state and is using a control set with no controls enabled.

    The booster pumps will turn on when the hydraulic grade in the tank is at 344.0 ft (82 ft above the floor of the tank). The well pumps will be called to operate when the hydraulic grade in the tank is 339 ft (77 ft above the floor of the tank) and they will shut off when the hydraulic grade is 342 ft above the floor of the tank

    Can you confirm that "booster pumps" refers to the single pump called "B. Pumps"? It is only a single pump, so I assume it is using a pump curve representing the combination of multiple pumps. If you need to perform energy studies, you'll want to ensure to explicitly model all individual pumps.

    Is the "Well pumps" the single pump called "T***** Pump"? 

    Do you mean that the tank will supply demands but when it gets low, the "booster pump" will turn on to fill it, but if the tank continues to drop, the "well pump" will turn on? The problem I see with the setup is that when the booster pump is on, it can fill the tank via the inlet, but the tank will not be able to supply water through the outlet at the same time, because the outlet is exposed to the downstream side of the pump. So, I think the pump would be supplying both the tank and the downstream demands. Similarly if the well pump was called to turn on, it may be "fighting" with the other pump and not be able to fill the tank because of the head already provided by the booster pumps. (or maybe you just want the well pump to supply demands in that case and not to fill the tank). I may have misunderstood.

    Im getting an error now that is calling out the booster pumps and telling me "Parallel VSPs are not allowed to have different pump curves". This is odd as there is only 1 b.pump being modeled.

    I do see this message when setting both pumps to the same control node. This is because when you set two VSPs to the same control node, the program thinks you are trying to model VSPs in parallel with the logic mentioned in this article about lead/lag pumps turning on and off automatically. As seen in the article, the pump curve needs to be the same in this case.

    If you were indeed trying to set both pumps as VSPs, this would not be possible because they are both discharging into the same pressure zone (see note in this article). Even if you were to "trick" the software into working, they would essentially "fight" against each other and the hydraulics would likely not be able to solve.

    in the control sets i have Booster pumps and the 2 wells references a pressure inside the tank. if the controls reference the tank and the control node is not the tank is that a problem?

    Having controls and a control node for a pump is not a problem and the elements can be different. The problem is that the validation does an initial check of whether the VSPs are in parallel and if the required criteria is met as explained further above. 

    Also note that your pressure-based controls would need to have a band. Currently you have them set to turn on/off based on the same setpoint of 36 psi. This could cause rapid oscillation as the pressure may immediately drop as soon as the pump turns off, causing the opposite control to immediately turn back on. See this and more here: Troubleshooting Controls in WaterCAD and WaterGEMS

    I added an overflow reservoir and put its elevation at the overflow invert elevation. 

    I see that the reservoir elevation is set equal to the tank top but that would mean that water would never flow from the tank to the reservoir. The elevation would need to be above the top in order for flow to travel to the lower elevation reservoir, but when the elevation reaches the top of a tank, the built-in altitude valve will close the pipe. To model the overflow you would need to set the reservoir elevation below the tank top. See more here: Modeling a Tank with an Overflow

    You mentioned you didnt understand where the regulated pressure needed to be, the pressure needs to remain above a specific value across the entire system, not just one particular node. 

    VSPs can be used to maintain a fixed pressure at a particular location, but you can only have one VSP or a station of VSPs in parallel pumping into a pressure zone, as they need to have direct control over that pressure (and not "fight" against another VSP for example). With the pressure maintained at a particular node, pressure elsewhere might still be below or above that value, due to differences in elevation and pipe headloss. If you simply have a constraint on your design that calls for a minimum pressure to be maintained, maybe you do not necessarily need to try to force the pressure via things like a VSP, but rather size pipes and use storage and control schemes to accomplish this?

    I also have a question about pump station capacity. The wells that i have added have a total allowable capacity dictated but water rights. Is there a way to make sure those pumps operate at a specific flow? 

    There are ways to force particular flow like using a negative demand or a FCV, but that is not advisable. It is always best to model the system as it actually operates. If the well has an "allowable capacity", you'll need to determine exactly how that is enforced and then model that. For example if there is an actual valve that throttles to keep the flow below a certain value, you could model it as a FCV but that is unlikely as seen in the blog post at the bottom of this article. If there isn't anything in place to keep the flow below a certain value, then that might mean that you need to design the rest of the system in such a way that the flow from the well does not exceed that value.

    The purpose of the booster pumps is to ensure pressure remains higher in the entire system therefore in the actual application (out in the field) the pumps are run on a "constant pressure" mode.

    So, is only the booster pump working as a VSP? What about the well pump? See previous comments regarding the ability of a VSP to maintain pressure. Even if the well pump is not set as a VSP and the booster pump is, if the well pump is on at the same time as the VSP, then the VSP would no longer have direct influence on the target pressure and the hydraulics would likely not be able to solve.


    Regards,

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

  • Downloading the connect version right now.

    How does the pump achieve this specific flow rate? If it is not changing its speed to maintain this, perhaps you are actually looking at a calibration exercise (getting the pump flow result to match the measured flow rate).

    The well pumps are indeed vfd pumps so in order to achieve a specific flow rate they would need to change their frequency. However, being a VFD pump they don't change the RPM (if i'm not mistaken) they just change the frequency that is supplied to the pump. These 3 pumps have a maximum allowable flow due to water rights, that flow being 540 gpm. 

    EDIT: I made a mistake with VFD explanation. The VFD does change the RPM however it does so differently than a VSP. 

  • Hi Nick, I am not sure about a VFD that achieves a flow based on a "frequency" and will ask around.

    You mentioned that the pumps have a "maximum allowable" flow though, which may suggest that the pumps operate at a constant speed with a flow control valve that throttles to keep the flow below the target value...? You you tried the approach of using a constant speed pump with a FCV? FCVs are not used often and in this situation would seem to waste energy though so I recommend reading the following:

    Using a Flow Control Valve (FCV)

    (Blog) Death To Flow Control Valves


    Regards,

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

  • I just saw your edit:

    The VFD does change the RPM however it does so differently than a VSP. 

    Can you elaborate on how it does this differently, and how it maintains a maximum flow and not a constant flow?


    Regards,

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

  • If im not mistaken, a variable speed pump changes the speed of the motor by changing the input amps and voltage, where a variable frequency drive pump changes the speed of the motor by changing the voltage and frequency. 

    The maximum allowable flow is not based on the pumps maximum flow rate. For example, one pump has a maximum flow rate of 500 gpm, however its run at 300 gpm. Now one of the other pumps has a maximum flow of 300 gpm but its run at "a sweet spot" of 150 because it begins to cavitate and anything higher than that.

    As a side thought, and please correct me if my thinking is wrong, but it sounds like the pumps are being run at what the PUD believes is the pumps best setting while also maintaining the flow below the allowable water rights. 

  • Hi Nick,

    Thanks for the clarification.

    When you say the pump "runs at", do you mean that is the fixed flow value, and the pump changes its speed to meet that specific flow rate? This would indeed be modeled by setting the pump as a VSP with the fixed flow option, but I am still concerned about what appears to be these two different VSPs discharging into the same pressure zone. If that does prove to be problematic (rule out other problems first) perhaps you could simplify the fixed flow pump station as a fixed negative demand (inflow) but then you would not be able to introduce any on/off logic or look at pump performance studies.

    You mentioned one pump runs at 300 gpm and the other at 150 - are these two pumps in parallel? Parallel VSPs requires the same target value and I seem to recall the model had only a single pump in the station. Or, are you referring to to the two separate pumps on opposite ends of the system? (I seem to recall one was a fixed head).


    Regards,

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

Reply
  • Hi Nick,

    Thanks for the clarification.

    When you say the pump "runs at", do you mean that is the fixed flow value, and the pump changes its speed to meet that specific flow rate? This would indeed be modeled by setting the pump as a VSP with the fixed flow option, but I am still concerned about what appears to be these two different VSPs discharging into the same pressure zone. If that does prove to be problematic (rule out other problems first) perhaps you could simplify the fixed flow pump station as a fixed negative demand (inflow) but then you would not be able to introduce any on/off logic or look at pump performance studies.

    You mentioned one pump runs at 300 gpm and the other at 150 - are these two pumps in parallel? Parallel VSPs requires the same target value and I seem to recall the model had only a single pump in the station. Or, are you referring to to the two separate pumps on opposite ends of the system? (I seem to recall one was a fixed head).


    Regards,

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

Children
  • Jesse,

    I made some adjustments to the model over the last week. I have a question about "tricking" the model. I assume the booster pumps will raise the distribution pressure to a certain hydraulic grade. If I put a tank, that has a very large diameter (so when water flows out the water level doesnt change ever) can i ignore putting the booster pumps in? I'm not running an assessment on if the booster pumps will work or not, they have worked for 10+ years. I want to know fire flow conditions and if the system will meet those parameters. If I put an "Infinite tank" in the system to mimic the constant hydraulic grade, those booster pumps are set to maintain, can i safely assume this is a way to make the system a little less complicated? I'm still having trouble getting everything to work properly and if I can simplify the system in any way and have it still accurately model what is going on, that would be beneficial.

    What are the potential drawbacks of modeling my system this way? 

    After spending this last week really diving deep into the system I have a much better understanding how the system needs to operate. The booster pumps essentially turn on at specific pressure and they are set to maintain that pressure throughout the system. The 3 well pumps are set to turn on when the water level in the tank drops to a certain level. The well pumps need to be level controlled and the booster pumps need to be pressure controlled. If the well pumps are pressure controlled they will never turn on as they will see a system pressure higher than their minimum level. 

    I have sent a model over for clarification. The one parameter im still unsure of is the G. Well and G pump elevations. However, that reservoir is close to the C. Well and C. Pump I am going to assume the levels are similar. Aside from the pump being unknown, if the pump begins to cavitate I will worry more. 

    thanks

  • I made some adjustments to the model over the last week. I have a question about "tricking" the model. I assume the booster pumps will raise the distribution pressure to a certain hydraulic grade. If I put a tank, that has a very large diameter (so when water flows out the water level doesnt change ever) can i ignore putting the booster pumps in? I'm not running an assessment on if the booster pumps will work or not, they have worked for 10+ years. I want to know fire flow conditions and if the system will meet those parameters. If I put an "Infinite tank" in the system to mimic the constant hydraulic grade, those booster pumps are set to maintain, can i safely assume this is a way to make the system a little less complicated? I'm still having trouble getting everything to work properly and if I can simplify the system in any way and have it still accurately model what is going on, that would be beneficial.

    Using a reservoir would be the same as using a tank with a huge diameter - it would model a constant hydraulic grade where flow can both enter and leave. 

    If you have pumps that you know will produce a constant downstream hydraulic grade regardless of system conditions, then you could try using a reservoir set to that elevation, replacing all the pumps and upstream piping. This would produce that constant hydraulic grade. You would need to use a check valve on the exiting pipe if you need to model the pump check valve that would prevent reverse flow through the pump in cases where the downstream hydraulic grade is higher. If there are cases where the pump station needs to be turned off, you could have controls to close the pipe instead.

    What are the potential drawbacks of modeling my system this way? 

    The main drawback of using a reservoir is that it assumes the pumps really would always produce that exact hydraulic grade, which may not necessarily be the case during all conditions. In other words, you risk potentially missing out on problems that would prevent the pumps from producing the constant hydraulic grade.

    Also you would not be able to assess pump energy cost and other details of the performance.


    Regards,

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

  • Jesse,

    How should I model the well pumps? They are set to turn on when the water level in the tank drops down to 77 feet above the floor and to turn off when level raises above 80 feet. However, I cant set them to have a control node as a tank with separate inlet and outlet. As well as I cant have them control a singular node because the 3 pumps are different sizes and have different pump curves (cant run them in parallel). 

    My initial thought was to pick arbitrary nodes for each pump and give them simple control schemes that say when the tank hydraulic grade drops turn on and raises turn off. 

    How my brain is comprehending this, please correct me if I'm wrong, but my understanding is that: The pumps are set to control the parameter (hydraulic grade/pressure) at a given control node but will only operate when the control scheme is met (tank level drops) etc. However, I need the well pumps to turn on regardless of if the pressure at a specific node is higher or lower than their control node parameter. Essentially, the well pumps will raise the distribution pressure to a point where its above the inlet invert of the tank so then the tank will begin to refill. If the pumps never turn on the tank will never refill. 

    Now, moving onto the booster pumps...I dont believe a constant reservoir will work for this situation as its more complicated than I originally figured. Im not necessarily concerned with energy cost and performance, but I want those booster pumps to maintain a system pressure that is ~36 psi. Those booster pumps work independently of the well pumps. The booster pumps turn on when a system pressure drops and turn off when the system pressure rises again. The well pumps turn on when the water level in the tank drops. However, the booster pumps have the pressure transducer that is at the bottom of the tank (I believe) and the well pumps are all controlled by 1 set of floats (I believe). The well pumps WILL raise the distribution system pressure because they are adding water to the system. 

    I'm currently struggling with wrapping my brain around how to exactly model this and how each of the scenarios I have currently set up work. 

    Thanks in advance.

  • Hello Nick,

    Can you confirm what scenario you are looking at for this setup. The active scenario in the model provided on Tuesday appears to be a fire flow run. This will help us make sure we are looking at the correct setup for your questions above.

    Regards,

    Scott

  • Hi Nick,

    Some general comments while we wait for clarification about the model:

    How should I model the well pumps? They are set to turn on when the water level in the tank drops down to 77 feet above the floor and to turn off when level raises above 80 feet. However, I cant set them to have a control node as a tank with separate inlet and outlet. As well as I cant have them control a singular node because the 3 pumps are different sizes and have different pump curves (cant run them in parallel). 

    The limitation with the top fill tank and different pump curves is in regards to a variable speed pump which, which set to control a tank, will attempt to keep it at a constant water level. If you want to set controls to turn a pump on and off based on high and low levels, you would instead set the pump as a constant speed (is variable speed = false) and use logical controls to accomplish this. You can use the control "wizard" to create the same on/off controls based on tank level. See: Using Controls, Conditions, Actions and Control Sets in WaterGEMS and WaterCAD

    You can also see an example of this setup in the Example1.wtg model included in the Samples folder within the installation folder:

    Now, moving onto the booster pumps...I dont believe a constant reservoir will work for this situation as its more complicated than I originally figured. Im not necessarily concerned with energy cost and performance, but I want those booster pumps to maintain a system pressure that is ~36 psi. Those booster pumps work independently of the well pumps. The booster pumps turn on when a system pressure drops and turn off when the system pressure rises again. The well pumps turn on when the water level in the tank drops. However, the booster pumps have the pressure transducer that is at the bottom of the tank (I believe) and the well pumps are all controlled by 1 set of floats (I believe). The well pumps WILL raise the distribution system pressure because they are adding water to the system. 

    The reservoir approach would produce a constant pressure for the entire simulation, but it sounds like these pumps actually turn on and off, and when they are on, they produce the 36 psi. Is that correct? If so, you could still model it with controls to open and close the pipe leaving the reservoir (to essentially turn the "pump" on and off). I may not be correctly understanding how the system operates though so further clarification on the logic may help.


    Regards,

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