How to use air valves on SewerCad.

The basic answer is: don't worry about it. The model is fairly robust. However, there are some instances where control valves (especially when they are close together) can interact and cause poor convergence. When that happens, get in touch with tech support.

If you do have convergence issues, usually you can set the lower air valves to "Treat as Junction" because they are unlikely to open.

How we will know if there is a poor convergence?

Here is the Air Valve technote in question:

communities.bentley.com/.../2644.modeling-force-mains-with-air-valves-tn.aspx

To answer your first question about why and when to use air valves, take a look at the background near the beginning of the article, under the section called "The trouble with high points". Basically they are useful in a SewerCAD model in cases where you need to make sure that an upstream pump recognizes a particular high point (and adds enough head to overcome it) that would otherwise experience negative pressure. In these cases the air valve would essentially be open and you'd have part-full flow downstream of it.

Here is the paragraph about the need to set certain air valves to be treated as a junction:

"Because air valves have the possibility to switch status, they can lead to instability in the model especially if there are many air valves in the system. To improve the stability of the model, it is desirable to force some of the valves closed. This can be done by setting the property "Treat air valve as junction" to True for those valves that are expected to be closed anyway."

So, to answer the question of which ones to set as junction - as the technote suggests, it would normally be the ones that you'd expect to be closed anyways. Basically because of the way that the air valve works behind the scenes, multiple air valves in series can be problematic, making it difficult for the numerical solver to converge on a balanced solution. Only the air valve nodes with the "treat as junction?" property set to "false" will act as air valve (pushing up the HGL as needed and simulating part full flow) when the situation warrants (HGL dropping down to the physical elevation). Therefore you only need to set this property to "false" for the locations that you actually need to use this feature. You can still keep other air valves in the model if you'd like (for display purposes or completeness for example) with the "treat as junction?" setting set to "true". Keeping too many air valves in the model, all set to "false" for "treat as junction?" will add unnecessary complication to the model, leading to potential convergence issues.

When the run completes and you get a green dot at the start of each line in the Calculation Summary that appears, everything is fine with convergence. If you get a yellow dot, look at the number of trials and see if it is near the maximum from the calculation options. If you get a red dot, then it did not converge and you should address it.

For the most part, this won't be an issue unless you have a crazy number of air valves. See also, Jesse's note.

To answer your question about how to know if there are convergence problems - symptoms would be:

1) Odd, unexpected results in the model such as hydraulic grade very low or very high where it should not be.

2) A message in the calculation summary stating that convergence was not achieved for a particular subnetwork

3) A "network unbalanced" message in both your User notifications and the pressure subnetwork details (in the Pressure Summary tab of the calculation summary)

We'll look into improving the technote with some of these details.