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Water hammer analysis of sewer Force Mains in HAMMERV8i

Hello.. I am analyzing a sewer Force Main in HAMMERV8i for pump start up conditions. After doing all the procedure i validated the loop and "no problem" was found. Then i went to compute tab & run it and a list of errors appeared on screen. kindly guide me how to resolve these problems. For assistance i have attached below the images of Error box and flex table of pipes.  Thanks

  • Hello Waqas,

    There seems to be an issue with initial conditions of the model itself, please compute for initial conditions first. The cause of this error differs from model to model. 

    Please go through the below mentioned technote, which explains about this error in detail for pump startup condition.

    User Notification "Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect" 

    If this doesn't help you then please upload the model files for our review. 

    Sharing model files 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Final waqas ali.rar 

    Thank You Shushma .. but i am unable to solve the problem. I have attached my model file for your review. kinldy suggest me some better solution for it

  • Hello Waqas,

    As Sushma mentioned, this issue is related to the initial conditions calculation. You are modeling a pump startup event, but you have included a demand at the downstream end of the model with no other source. This is causing an error related to disconnected demands. Basically, the initial conditions results are invalid, which is why you are getting the error message.

    There are a couple of things you can try. First, if there is some other source for this demand when the pump is off, you can model that source. However, since this is a force main that you are modeling, there are a couple of different options you can try that may work better. 

    It is possible that the demand you entered is not a demand like you see in a water system, but the expected flow. I am not sure how you derived the value, but one option you could try is to model the node with a demand as a discharge to atmosphere element set as an orifice. As mentioned in the link above, a D2A set as an orifice is common when modeling the free discharge of a pressure sewer system. There is some information on setting this up in the section called "Common applications of the D2A acting as an Orifice."

    The second option would be to model the startup slightly differently. Instead of starting with the pump off and then having it startup, you would model a pump shutdown and pump start-up in the same scenario. The advantage to this is that you will be able to keep your demand at the junction. Demands in the transient run act as pressure-dependent demands, meaning that when the pump is off, you won't see the disconnected demand node issue you are currently seeing. You can mind more information on modeling pump shutdowns and startups in the same scenario at the following link: Modeling a pump startup and shutdown transient event in the same simulation.

    Regards,

    Scott