# Flow From a Draining Water Tank



## jeb6294 (Aug 23, 2010)

Silly me, I came all the way to Afghanistan and forgot to bring my CERM.

I'm writing up a request for a design/build proposal for a drainage system for one of the water treatment plants we're building over here. I know the flow of the waste water from the treatment plant is 4 liters/second, but they are also going to need to figure in the water coming out from the three storage tanks if they ever overflow or need to be drained. The only reference I can see in the specs is 16 L/s for overflow per tank, but that seems a little low for a 211k gallon tank with 3.5 meters of head and an 8-inch outlet. It's going to be piped, but I figured on just using the orifice calculation.

An answer would be great, but I'll also need the equation because I still have to check the actual valve to verify the size...can't get good afghan help. Thanks.


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## Chucktown PE (Aug 23, 2010)

V = C (2gh)^0.5


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## Chucktown PE (Aug 23, 2010)

C is going to be 0.98


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## Chucktown PE (Aug 23, 2010)

V = 8.1 m/s, for an 8 inch pipe that is 0.2625 m^3/s = 262.5 L/s.


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## IlPadrino (Aug 24, 2010)

I'm not sure I follow...

Chuck gave you an answer that assumes the storage tank is draining all from an 8" outlet at the bottom of the tank. To drain, I assume that's where you'd connect to the drainage system but wouldn't you be able to throttle the flow?

What's causing overflow and how does this connect to the drainage system?


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## jeb6294 (Aug 25, 2010)

IlPadrino said:


> I'm not sure I follow...
> Chuck gave you an answer that assumes the storage tank is draining all from an 8" outlet at the bottom of the tank. To drain, I assume that's where you'd connect to the drainage system but wouldn't you be able to throttle the flow?
> 
> What's causing overflow and how does this connect to the drainage system?


Each tank has an overflow that is required to flow 16 liters/second. In addition to that, there's a 200mm (8in) drainage valve in case of emergency. Each plant has three tanks and each one of them is 211k gallons so, while the flow wouldn't be constant, the designer needs to figure he's going to be getting a big rush of water if there's ever an emergency and they need to drain those tanks in a hurry. Unfortunately they are doing things a little backwards here so we had to make the plants fit where they would fit...one plant would flood a village and the other would flood one of the DFAC's (dining facilities) if there's nowhere for the water to go. They'll probably have to end up having to dump everything into an underground tank and then pump it across the base somewhere. This is all for a request for a proposal from the contractor so this number is just a heads up to them that, however they do it, they need to remember to account for those tanks in addition to the waste water from the treatment plant.


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