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Discharge to atmosphere

Good afternoon,

I'm modeling a situation with a discharge to atmosphere, but even after read the article "Modeling Reference - Discharge To Atmosphere [TN]" I have some doubts.

Concerning the attached project, at the beginning I know that in normal conditions the debit required is 20l/s (typical flow), but how can I know the pressure drop if the pressure at the final pipe, in steady state, depends of the selected value for pressure drop? I used to consider a very low pressure drop (0,001bar) because I thought that in two close points the pressure would be similar, now I understand that it is not correct.

Please, considering the attached file, can someone explain me how to proceed to estimate these values (flow (typical) and drop pressure (typical)) correctly? d2a.rar

  • Hello Joao,

    There is a section in the TechNote you reference that talks about what you can do. If you look at item 2 under the section "Common applications of the D2A acting as an Orifice", you can see a couple of ways of estimating the pressure drop for a given flow. One such method is to use the orifice equation to find the pressure for a given flow.

    Regards,
    Scott
  • Hello Scott,

    First of all, thank you for your reply. The situation that I want to simulate is the topic 2 of "Common applications of the D2A acting as an Orifice". What I do not understand is how can I calculate the drop pressure with the orifice equation (V=C*(2g*headloss)^0.5) after calculate headloss.
    supposing: V - velocity, C - discharge coef. (also unknown value)

    Again, thank you for your reply

    Best Regards
    Joao Henriques
  • Hello Joao,

    You can try estimating the C value to find the headloss. There are a number of sources available that you can use to estimate the discharge coefficient. I have documented a couple of those in the D2A TechNote. The velocity will come from the flow and the area of the orifice (V = Q/A).

    I have updated the Discharge to Atmosphere TechNote to reflect this information. You can find the updated portion of that in the item 2 of the section "Common applications of the D2A acting as an Orifice". It includes a brief example when you know the flow through the orifice. You can find the TechNote at the following link: communities.bentley.com/.../3443.modeling-reference-discharge-to-atmosphere-tn

    Regards,
    Scott

    Answer Verified By: Sushma Choure 

  • Thank you very much for your answer Scott,

    Just one thing missing in my mind; the head loss units should not be "ft" and not "ft/s"?
    And if the "drop pressure" has to be in pressure units, how can I convert the head loss (ft) to pressure units (ex bars)?
    Because I can not consider the head loss units "ftH2O" right?

    Thank you very much for your help
    Best Regards

    Joao Henriques

  • Hello Joao,

    The typo with the units has been fixed.

    For the pressure drop, the headloss calculated from the orifice equation is basically equivalent to the pressure drop. In the example presented in the D2A TechNote, you would enter the typical pressure drop as 7 ft H2O. Make sure the pressure units are set correctly before you enter the value. If you want to change the units to something like bars, you can right-click on the units and select Units and Formatting. When you select a different pressure unit, the program will automatically convert the pressure value to the correct unit value.

    Regards,
    Scott