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Non-zero flow in closed pipes next to hydropneumatic tanks

Ref: http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/123522

Thank you for the response.  I was able to get the booster pump to turn on and prevent the hydropneumatic tanks from emptying completely.  All four tanks are connected to the main waterline by individual pipes (one pipe for each tank) that connect to a common junction on the main line.  With the EPS I've been running, the four tanks should be inactive during the day (with one or two booster pumps providing all the needed flows and pressures during the day), and there should therefore be no flow in the pipes that connect to the tanks.  However, whenever I graph the flow for any of these four pipes, I get the plot shown below.  These spikes and drops in flow are causing the tanks to experience abrupt changes in pressure and liquid volume when these parameters should remain constant during the day (due to the pipe being closed).  I have controls set up so that the pipes are set to be closed, so I'm not sure why these spikes and drops in flow are occurring in the first place.  Do you know why they may be occurring?

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  • Robert,

    In this case it will probably help us to see a copy of your model files. As suggested by Scott above, there are two options for sharing your model on Communities, whichever you choose please be sure to zip your files first. The first option is to attach the zip file containing your model to your reply on the forum using the Advanced Reply Editor (you'll find the link below and to the right of the reply box). If your data is confidential please use the instructions in the link below to send it via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley employees. Please be sure to reply on this thread with the name of the file after it has been uploaded.

    communities.bentley.com/.../7079.be-communities-secure-file-upload

    If you upload the files to Sharefile, please post here so we know it is available.

    Regards,
    Mark

    Mark

  • I've uploaded the folder to Sharefile. It's called Water Model_RJT.zip.

    Another issue I've been having has to do with the pressures and liquid volumes in the hydropneumatic tanks. Each tank is supposed to have an operating pressure range of 40 - 60 psig. A volume of 31.3 gallons of water is supposed to be discharged from each tank when dropping in pressure from 60 psig to 40 psig. However, the graphs keep showing a liquid volume range of 10 - 15 gallons only. The pressures in the tanks also don't correlate with the operating pressure range.

    Each tank has a total volume of 115.9 gallons and a maximum drawdown of 70.5 gallons. If you know of a way to correct these issues as well, I would greatly appreciate it.

    Thanks,
    Rob Tejano
  • For the tank settings, what does the effective volume represent? Is that the maximum volume of liquid that can be stored in the tank? Also, my model never allows me to enter values in the "Volume of Gas (Initial)" field. Do you know what I should change to allow me to enter values in this column? Does the "Pressure (Gas-Preset)" field indicate the initial air pressure inside the tank? I was also wondering if you think changing the inlet orifice diameter or gas law exponent would help with getting the pressures and liquid volumes to line up with each other correctly.

    Thanks,
    Rob Tejano
  • Robert,

    The following information can be found in the HAMMER help documentation for the topic titled, 'Gas Law vs. Constant Area Approximation' and 'Initial Conditions Attributes': The effective volume represents the difference in volume between the HGL 'On' and HGL 'Off'. The HGL 'On' and HGL 'Off' fields represent the maximum and minimum hydraulic grade lines within the hydropneumatic tank (i.e. when an associated booster pump would turn on or off). An approximate diameter is computed based on the effective volume of the hydropneumatic tank so that the tank cross sectional area multiplied by the distance between HGL on and HGL off gives the same volume as the hydropneumatic tank.

    In order to help you out with the 'volume of Gas (Initial)' question I'll need to know what version of WaterCAD, WaterGEMS, or HAMMER are you using? The version number can be found by going to HELP > About > <ProductName> inside your program and locating the number in the lower left hand corner and may be in brackets. It should start like this 08.11.XX.XX where X are some numbers. I don't see a reason why you shouldn't be able to enter this value in the field.

    The 'pressure (gas-preset)' field is used if there is a bladder in the tank . It's the pressure of the gas prior to exposing the tank to pipeline pressure and should be otherwise, this should be omitted as it is ignored.

    To test your theory on changing the inlet orifice diameter and gas law exponent you could perform a sensitivity analysis as outlined in the wiki below.

    communities.bentley.com/.../22932.how-do-i-perform-a-sensitivity-analysis

    Regards,
    Mark

    Mark

  • Robert,

    To add to what I posted yesterday, it's difficult to tell whether you're using WaterCAD, WaterGEMS, or HAMMER for this model. It seems like you're using WaterCAD or WaterGEMS because you were referring to running an EPS simulation. If you are using WaterCAD or WaterGEMS it's not necessary to input any information like the 'pressure (gas-preset)' or gas law exponent under the 'Transient (physical)' section of the hydro tank properties because that information is only used during a transient simulation for HAMMER.

    For more information on hydropneumatic tanks see the wiki solution below. The topic is extensive and covers most of what you need to know about the element.

    communities.bentley.com/.../modeling-reference-_2D00_-hydropneumatic-tanks

    Regards,
    Mark

    Mark

  • I'm using WaterCAD V8i (SELECTseries 5). The version number is 08.11.05.61.
  • Robert,

    I agree with Mark - the properties of the hydropneumatic tank that you asked about in your previous reply are all related to transient simulations in Bentley HAMMER. HAMMER uses the same file format as WaterCAD and WaterGEMS, so a steady state or EPS model can be later opened in HAMMER for a transient simulation. For a steady state or EPS, the transient attributes are not used.

    Regarding the "effective volume" - that only applies to the "constant area approximation" option in the "tank calculation model" attribute. It is a legacy option, mainly for compatibility with the method by which you would model a hydropneumatic tank in versions of the program before the hydropneumatic tank element was introduced. So, in almost all cases, you would use the "gas law" option, in which case the effective volume does not come into play. This is explained further in the "Initial conditions" sections of the hydropneumatic tank technote that Mark linked to.


    Regards,

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

Reply
  • Robert,

    I agree with Mark - the properties of the hydropneumatic tank that you asked about in your previous reply are all related to transient simulations in Bentley HAMMER. HAMMER uses the same file format as WaterCAD and WaterGEMS, so a steady state or EPS model can be later opened in HAMMER for a transient simulation. For a steady state or EPS, the transient attributes are not used.

    Regarding the "effective volume" - that only applies to the "constant area approximation" option in the "tank calculation model" attribute. It is a legacy option, mainly for compatibility with the method by which you would model a hydropneumatic tank in versions of the program before the hydropneumatic tank element was introduced. So, in almost all cases, you would use the "gas law" option, in which case the effective volume does not come into play. This is explained further in the "Initial conditions" sections of the hydropneumatic tank technote that Mark linked to.


    Regards,

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

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