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Water modeling - flows passing through a closed pipe

I am attempting to model a connection an existing distribution system.

The goal is to supply two houses with drinking water and fill tanks for fire flows.  What I have done is brought the water to the housing complex, then one line proceeds to the houses where it splits and services the houses.  another segment diverges and heads up hill approx. 50 feet to a tank.  I have controls set that close the line to the houses and open the line to the tank when the line from the tank to a fire hydrant is opened, I accomplished this by setting simple controls that tell them to open or close based on the level within the tank.  The tank should be stagnant and only drained when the hyrdant line is opened.  This works fine when I set the simple controls to trigger when the level in the tank goes below 4 feet (max height is 6 feet).  There are also simple controls set to open the fire hydrant line at 7 AM and close at 8 AM with it's initial status as closed.  Problems arise when I change the simple control height to anything above 4 feet.  The line from the hydrant will open at various times throughout a 24 hour period even with the controls telling it to only be opened between 7 and 8 AM. 

So, my question is; how is the link from the tank to the hydrant overriding the controls and opening at other times?

My guess is it has something to do with the in ability of the gravity system to to fill the tanks beyond 4 feet.  There is no pump on the line from the distribution line to the tanks, so I am relying on the head in the line to be able to refill the tanks.  But still why this would cause the line to the hydrant to open at random times I am not sure. 

Also, if anyone could help me figure out how to find out what the maximum pressure and flow I could achieve at the hydrant that would be great.  currently it tells me I have 640 or so gpm but zero pressure, which might be true considering I have less than 1 psi at the tanks and only a decrease in elevation of 47 feet from the tanks to the hydrant. 

One more, I want to know at what pressure if any the hydrant could achieve 500 gpm.  Do I set the base demand at 500?  How would my pattern look for this if I just wanted the hydrant to flow for 1 hour.  Currently I have the multiplier set to 1 between 7 and 8 AM and 0's for all other hours of the day.  the base demand is set at 500.  The patterns do not seem very intuitive to me.

I have never used a water modeling program prior to this and have taught myself from the manual.

Any help would be great, even "you are in way over you head!"

Thanks.