Hammer model setup/errors

Hello Fadi,

2) After inserting the first branch pipe and vacuum breaker, the model ran with the previous warnings (wave speed/length  adjustment, negative pressure). But when I added the second vacuum breaker, I got the error message "Invalid results. Please check the input data for the for the connected elements". The highlighted element is a pipe in the network, of which one end is a junction that is connected to a 1m branch pipe and a vacuum breaker. The vacuum breaker elevation is 1m higher that the junction on the main network, and the pipe is 1m long and of 100mm dia. I don't see any wrong inputs here. What is the solution?

I may need to see the model to look into this second issue, but there are a few things you can try. First, you can check to see if a smaller time step, such as the manual time step of 0.01 seconds mentioned in the earlier post, helps at all. This will have some impact on fields such as pipe length and may offer better results.

Aside from property fields like elevation, you should also review other properties of the air valve. For instance, make sure that the orifice diameter or air flow curve is correct. Check the setting for "Treat as Junction" as well. If this is set to False, try setting it to True to see if that helps.

After computing the initial conditions, you can also run a validation (Analysis > Validate). As long as the initial conditions are computed, the validation procedure will review the transient settings of the elements in the model in case there are any obvious data entry issues that need to be fixed.

If that doesn't help, I will need to see a copy of the model in order to look into this. There are two options for sharing your model files on BE Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley.   http://communities.bentley.com/content/bentleysecurefilesupload.aspx

Regards,

Scott Kampa

Bentley Technical Support

Hi Scott,

Thanks for the quick responses.

I tried 0.01 s time step. The simulation ran fine but it took approx. 5 minutes. I then tried 0.02s, but I got the same error message. I can not afford 5  minutes simulation as there are multiple scenarios I want to run for this network, and there is a similar network that I also need to analyse after this one.

Is there a better way to setup the model to avoid this siuation? E.g. if I breakdown each pipe into equal segments by inserting multiple junctions, would that help?

I've uploaded the mdoel file for you to look at.

Thanks and Regards,

Fadi Sirrieh

Hello,

We have looked at the model, trying a few things to see what we could find out. In computing the active scenario (Surge_All Pumps Fail_NO CV), the error message regarding invalid results was generated. We changed the time steps to a custom time step of 0.01 seconds. In doing this, it is clear there is a severe surge after the pump shuts down, with causes vapor pockets to form along the path. With a lot of transient events occurring, this can make the model sensitive to instability. This may be what is happening in your modeling case.

One top of this, you are using the Extended CAV method to analyze the air valves. This does add a level of complexity to the model calculations as well. I will note that when Extended CAV is not used, the model computes without any errors about invalid results.

The model is probably failing to compute because of a combination of the complicated interaction between the air pockets at the air valve switching between extended and concentration mode and the many vapor pockets that form and collapse in the rest of the system.

The best solution for your case is to set "Use Extended CAV" (found in the transient calculation options) to False. Extended CAV allows the user to see some tracking of the air pocket in the pipes adjacent to the air valve. However, since your air valves are connected to a short pipe, the benefit of this method is minimal, especially given the overall instability of the results in general.

I will also note that you may want to look into using a smaller time step than the one generated automatically by HAMMER. The length adjustment from this relatively large time step is very large. A smaller time step will allow for less adjustment. I would still recommend doing a sensitivity analysis. For the short pipes near the air valves, a timestep 0.0009 seconds (of 1.0 m / 1100 m/s = 0.0009 sec) would be needed to prevent adjustment, but that would make for an unreasonably long run duration. However, if you look at different time steps and see no appreciable change in results, you can then make the case for a smaller time step.

For more information on air valves and how they operate in HAMMER, please see the air valve TechNote found at the following link: communities.bentley.com/.../modeling-reference-_2d00_-air-valves.aspx

There are another thing worth looking at in the model. First, the pumps have a control valve setting of 2 seconds, while the pump shutdown occurs after 5 seconds. The control valve is a valve that operates from the start of the transient simulation. With a setting of 2 seconds, that basically means that the valve starts closing at the start of the simulation and fully closes after 2 seconds, i.e. the pump is already closed before it shuts down. If there is no valve on the pump to inhibit close, such as a check valve, it is common to set the control valve time to a large value, like 99,999 seconds. While this doesn't resolve the issues in the model, we thought it worth checking out.

Please let us know if you have any additional questions.

Regards,

Scott Kampa

Bentley Technical Support

Hello,

We have looked at the model, trying a few things to see what we could find out. In computing the active scenario (Surge_All Pumps Fail_NO CV), the error message regarding invalid results was generated. We changed the time steps to a custom time step of 0.01 seconds. In doing this, it is clear there is a severe surge after the pump shuts down, with causes vapor pockets to form along the path. With a lot of transient events occurring, this can make the model sensitive to instability. This may be what is happening in your modeling case.

One top of this, you are using the Extended CAV method to analyze the air valves. This does add a level of complexity to the model calculations as well. I will note that when Extended CAV is not used, the model computes without any errors about invalid results.

The model is probably failing to compute because of a combination of the complicated interaction between the air pockets at the air valve switching between extended and concentration mode and the many vapor pockets that form and collapse in the rest of the system.

The best solution for your case is to set "Use Extended CAV" (found in the transient calculation options) to False. Extended CAV allows the user to see some tracking of the air pocket in the pipes adjacent to the air valve. However, since your air valves are connected to a short pipe, the benefit of this method is minimal, especially given the overall instability of the results in general.

I will also note that you may want to look into using a smaller time step than the one generated automatically by HAMMER. The length adjustment from this relatively large time step is very large. A smaller time step will allow for less adjustment. I would still recommend doing a sensitivity analysis. For the short pipes near the air valves, a timestep 0.0009 seconds (of 1.0 m / 1100 m/s = 0.0009 sec) would be needed to prevent adjustment, but that would make for an unreasonably long run duration. However, if you look at different time steps and see no appreciable change in results, you can then make the case for a smaller time step.

For more information on air valves and how they operate in HAMMER, please see the air valve TechNote found at the following link: communities.bentley.com/.../modeling-reference-_2d00_-air-valves.aspx

There are another thing worth looking at in the model. First, the pumps have a control valve setting of 2 seconds, while the pump shutdown occurs after 5 seconds. The control valve is a valve that operates from the start of the transient simulation. With a setting of 2 seconds, that basically means that the valve starts closing at the start of the simulation and fully closes after 2 seconds, i.e. the pump is already closed before it shuts down. If there is no valve on the pump to inhibit close, such as a check valve, it is common to set the control valve time to a large value, like 99,999 seconds. While this doesn't resolve the issues in the model, we thought it worth checking out.

Please let us know if you have any additional questions.

Regards,

Scott Kampa

Bentley Technical Support