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Design of dipping tube tank for horizontal or slopping downward pumping reach (Hammar)?

Hello Everyone,

I am protecting a horizontal sewage line with a dipping tube and I have some questions

1) Should the dipping tube tank base be on the ground level or can it be lowered sometimes for some reason?

2) when I tried to put it in the ground, I found during the simulation that the tank got empty -because the tank base is higher than the delivery point-and the gas volume continues increasing, is that acceptable?

3) the assigned initial gas volume--> should I calculate it using gas law, or it is assumed any value to get the pipeline protected?

I calculate it using P1V1=P2 V2

these are my calculations: 

assume total volume== 5m3

compression chamber= 3 m3

P1V1= 10.33 *3

P2= HGL-pipe invert level+10.33 

get Vi (initial volume)

assumed tank base = pipe invert +3

------------

4)when I lowered the tank-->  I found the gas in tank is under negative pressure during simulation what could be the reason?

thanks in advance

Parents
  • 1) Should the dipping tube tank base be on the ground level or can it be lowered sometimes for some reason?

    The location of the tank is based on your engineering judgement and design constraints. If you want to check the performance of a tank that is placed below ground level (if that is possible in the real system) then you can certainly adjust the tank base elevation accordingly.

    2) when I tried to put it in the ground, I found during the simulation that the tank got empty -because the tank base is higher than the delivery point-and the gas volume continues increasing, is that acceptable?

    What is the "delivery point"? Is it an upstream reservoir, or a pump? If you have a pump upstream, it should be raising the hydraulic grade at the downstream hydropneumatic tank so that is is pressurized in the initial conditions. Are you modeling an emergency shutdown of that pump? If so, once the pump is no longer adding head to the system, if the upstream reservoir and other downstream boundary conditions are lower than the hydropneumatic tank, then indeed the tank will eventually drain as long as that pump remains off. This situation is described in more detail in the following article in our Wiki: Negative pressures occur during a transient simulation no matter what protection is used

    Also, the following article explains what happens when a hydropneumatic tank becomes empty and how a pump stop+start can be used to simulate the length of time that the pump remains off. You will want to adjust your system so that the tank does not become empty. - Hydropneumatic tank user notifications: "gas volume exceeds volume of tank" or "fills its containing tank"

    3) the assigned initial gas volume--> should I calculate it using gas law, or it is assumed any value to get the pipeline protected?

    The initial gas volume is again based on your engineering judgement and design requirements. It is not necessarily something that you need to calculate - you could simply assume for example that the gas in your tank is compressed to a specific initial volume in your initial conditions, and enter that value. The program will then take that initial volume along with the initial conditions pressure. You can read more about how this works in the following article: Modeling Reference - Hydropneumatic Tanks (see "Initial conditions behavior" and "Transient Simulation Behavior" sections)

    4)when I lowered the tank-->  I found the gas in tank is under negative pressure during simulation what could be the reason?

    Where exactly are you seeing a negative gas pressure? There are many reasons why the pressure may still be low/negative - see my comment further above about tanks becoming empty. If you have not done so already, create a profile of the system and animate it in the Transient Results Viewer to better understand the sequence of events that occur during a transient simulation leading to the negative pressure: Using Transient Results Viewer animations for visualizing a transient simulation


    Regards,

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

    Answer Verified By: hadeel abdelhamid 

  • Thanks a lot for the quick reply

    For the last question, i mean that i found that gas in the tank came to negative pressure with Water level above the bottom of dipping tube.. i changed many parameters with no effect on that.. what may be the reason for that?

  • Hello Hadeel, 

    As mentioned above as long as pump stays off and HGL at upstream reservoir and other downstream boundary conditions are below hydro tank, tank will drain out, till the pump restarts and add more head. So changing the parameters of hydro tank, wont help in that case. 

    It is possible for the tank to not be completely empty but experience a negative pressure. See the section of the hydropneumatic tank article called "A note on pressure at an empty condition", which explains this situation in more detail.

    Which means that when the hydraulic grade drops to the bottom of the tank, the gas volume is still greater than the full tank size and the tank is not yet empty. The HGL then drops below the bottom and the tank is still not quite empty of water. This basically means that there is a negative pressure in the almost-empty tank - imagine it as the water column "pulling" at gas pocket inside the tank.

    What user notifications do you receive regarding hydropneumatic tank after computing the model? 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

Reply
  • Hello Hadeel, 

    As mentioned above as long as pump stays off and HGL at upstream reservoir and other downstream boundary conditions are below hydro tank, tank will drain out, till the pump restarts and add more head. So changing the parameters of hydro tank, wont help in that case. 

    It is possible for the tank to not be completely empty but experience a negative pressure. See the section of the hydropneumatic tank article called "A note on pressure at an empty condition", which explains this situation in more detail.

    Which means that when the hydraulic grade drops to the bottom of the tank, the gas volume is still greater than the full tank size and the tank is not yet empty. The HGL then drops below the bottom and the tank is still not quite empty of water. This basically means that there is a negative pressure in the almost-empty tank - imagine it as the water column "pulling" at gas pocket inside the tank.

    What user notifications do you receive regarding hydropneumatic tank after computing the model? 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

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