Modeling Backflow Preventer

Question

Dear Scott 
 1- Regarding your explanation in this Wiki , Please have a look at BA type of valve in this paper . and let us know how a Reduced Pressure Backflow Preventer is simulated via WaterGEMS elements? 
 2- Another question of mine: I have a model of HAMMER default examples and made some changes on it. Initial Condition ends with HGL=64.41ft (for downstream of TCV) and the first step of Transient Run shows HGl=64.41ft too, but at next step it drops to 62.22ft. Nothing has been changed during first step and second step and operating Rule is Fixed during analysis. I mean Initial Closure percent is equal to initial closure percent at operating rule. What do you think about HGL decreasing between two first steps? Model attached. 
 8741.New folder (3).rar

Jesse Dringoli · Accepted Answer

Regarding #1 - this appears to be an RPZ device. See below article and related links within: 
 
 communities.bentley.com/.../19837.how-can-i-model-an-rpz-device-reduced-pressure-principle-backflow-preventer-solution-500000067195 
 
Regarding #2 - the model you sent appears to be the "TCV_Closure_Example" model included with HAMMER. The change seen in the first timestep can be attributed to numerical noise exaggerated by the HGL scale in the network. If you look at the profile in the Transient Results Viewer, you can’t even see that change occur – the HGL difference between the upstream reservoir and downstream reservoir is almost 300 feet. So from a practical sense, it doesn’t matter. 
 
That said, I have proposed some changes to this example model so as to be more realistic. This will be considered for either the upcoming CONNECT Edition release, or a future release. If you lower the upstream reservoir elevation to 120 ft, the results will be more reasonable and you won't see that small change in the first timestep.