Potable Water Storage Tank Vent Sizing 1

[LoYo]

Dear all,
I have a problem calculating vent sizing for potable water storage tank (concrete), the tank is designed to operate at atmospheric pressure. The tank is 10.5 m Width , 14.5m Length and 4.5m height. Maximum level of water is 3.5m.

Maximum flow rate @ inlet = 350 US GPM and Maximum flow rate @ outlet 1000 US GPM.

I can not find any reference regarding this matter, all I can found is for tanks storing flammable fluids. Also in case of concrete tanks, will the breathing in & out of tank can really cause failure to structure?

Thanks

Amr Salah
Plumbing & Fire Protection Engineer

 

[LittleInch]

So long as the vent is short, work on the basis of at least equal or bigger sized pipes for the air compared to the water out and you won't go wrong. Air flows much easier than water for the same volume rate so your internal pressure won't increase or decrease enough to make any difference. Yes if you create a partial vacuum or partial overpressure and your tank is not designed for either, forces can be very large and you would either crack your tank on over pressure or possibly crush it or have the roof fall in on vacuum.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LoYo]

Thanks LittleInch for your reply,
I need to submit calculation for this matter so I can not just put a larger pipe (although it would be sufficient). I need data to support my choice.

Thanks

Amr Salah
Plumbing & Fire Protection Engineer

 

[LittleInch]

Then work out your max volumetric flowrate and look up a few online catalogues from PVRV (pressure and vacuum relief valve) vendors which will give you sizes. However to do this you need to know, or state your max pressure / vacuum the tank can take. Something like this

http://www.cheresources.com/invision/topic/20303-sizing-of-pvrv-on-a-storage-tank/

or search on this site.

Then just put ones in the same size or bigger than the water outlet pipe....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JStephen]

If you need to calculate the required flow rate and need a reference to do so, use API-2000. It'll basically tell you that air rate in or out has to match water rate in or out.

If you know the flow rate and need calculations for the vent, simplest solution is to use a commercially available "free vent" with flow data available. Varec has their flow data online, some of the others don't. Note that sizing a vent this way can give you a very small vent.

You can use duct-flow information from ASHRAE to estimate vent flow rates. They have data for screens, for exhaust hoods, for ells, etc., it's just a matter of hunting the information up and working out the details.

 

[77JQX]

That 1000 gpm outflow rate seems small. What happens during a main break?