Reservoir drainage wet well vacuum relief

[bellmouth]

I am designing for a vacuum relief pipe for a drainage inlet wet well. A couple of 48" dia pipes from a reservoir drain into the wet well, which in turn is piped via a 72" dia pipe to a pump station. The design flow of the pump station is 160 MGD. The vacuum relief design is in anticipation of the either the reservoir inlet valves or pumps misfuctioning so that the pumps are pulling vacuum, requiring inlet air from the top of the wet well to prevent pump or wet well damage. The wet well is 90 ft tall and 14 ft dia. What factors would be used to calculate the size of the vacuum relief pipe?

 

[JedClampett]

I would call it a vent, but that's just semantics. For steel tanks, a good rule of thumb is to provide the outlet pipe diameter for the vent. This makes the air pressure at 40 fps about 1/2 inches and doesn't exceed the roof live load. The 40 fps would be caused by some kind of pipe break.
Make sure the vent can work if frosted over and that it is screened to keep critters out. The open area of the screens should exceed the pipe area.
Take a look at AWWA D100 for some suggestions and details.

 

[bimr]

You have to determine the worst case scenario for opeations. If the liquid will overflow through the vacuum valve, the valve would have to be sized for the maximum liquid rate. If the valve will only have to relieve air, then the valve is typically sized for a higher air flow velocity.