Steel Vent Calculation

[nicoga3000]

I'm looking at a vent that will be used to prevent a tank from collapsing due to vacuum pressures in a pipe burst/free drain scenario. The situation would be a container is filled with water when an inlet or an outlet pipe bursts. The water flowing out would create some pressure in the tank that would need to be balanced by some vent(s) at the top of the tank.

So here's an equation that I have on file:

Av = Q/(4005*Ce*sqrt(pressure differential))

It's not a matter of if I can plug and chug my way through this - it's a matter of finding out WHERE this equation comes from. Nobody I've spoken to has any idea, nobody around my office seems to know, and really, it's not OK to use an equation without knowing it's validity nor it's origin.

Does anyone have ANY clue if this equation will suffice or information regarding it's origin? I need some sort of legitimate reference, but I just can't find a darn thing. Much appreciation!

 

[BigInch]

I think it would be easier to develop a new equation based on gravity flow out a drain of a given diameter when the tank is at maximum fluid level. Don't forget to include the discharge coefficient of the nozzle. You'll find that problem in any elementary fluid dynamics textbook.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[JStephen]

I would assume that pressure in the tank remained atmospheric, and calculate liquid flow rates based on that assumption. Then use the flow rate you calculate and assume you need to size the air vent for that rate accordingly.

The equation you post may be based on Bernoullii's equation as applied to an orifice and a reservoir. Assume out in the tank, you have some negative pressure, zero velocity. At the orifice, you have zero pressure and can then solve for velocity.

API-2000 gives an equation for sizing emergency-relief vents of the lifting manway type, and that may be another source for the equation in question.

For standard mushroom and gooseneck type vents, you can find some similar items in the duct flow tables of the ASHRAE handbooks, including screen factors.

There are also commercially available "free vents" available from Tyco/Varec, Shand & Jurs, and others, that have flow data available for them.

Typical water tank practice is to oversize vents considerably.