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Natural Draft Vents On Storage Tank


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#1 Lavi

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Posted 28 September 2016 - 08:29 AM

We have a storage tank with a single Central Vent (with goosenecks) and 8 peripheral smaller vents (with goosenecks) on the roof of a storage tank.

 

The idea is to have natural recirculation; The hot vapor from inside the tank shall be discharged through the bigger central vent and the cooler air from atmosphere is sucked in through the smaller peripheral vents.

However, this is not happening. Hot fumes are being discharged through the peripheral vents also.

 

Hydraulics of these vent lines were checked and a third party assessment of line hydraulics has also been performed. There is no problem with the hydraulics of these lines.

 

What can be the reason behind this unexpected circulation of air?



#2 proinwv

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Posted 28 September 2016 - 09:45 AM

Higher pressure in the tank at the orifices than outside same.



#3 Saml

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Posted 28 September 2016 - 05:50 PM

I can think about three items:

 
1) You need that the "column weight" of air outside the tank is bigger than the the same height of vapor inside. Basically that air density outside should be higher than the gas density inside. So take (density differential between tank vapor) by g  by height differential between the perispheral opening and the top goose neck and you've got the driving force. 
 
If the MW of the fumes is higher than air, and the temperature does not compensate for that difference, the flow won't happen as you intend. 
 
2) That the pressure drop of the fumes leaving the tank through the top goose neck is lower than the above driving force.

 

3) At the beginning you should consider that the top goose neck is full of air and cold until a circulation patter is established. Your driving force to start the circulation should be calculated at the base of the goose neck. This is similar to a wood fireplace stack that send the smoke inside the house until the chimney is hot.
 

As my first mentors told me: fluids flow by pressure gradient, not by design intent, So look why the pressure gradient is not what you expected.


Edited by Saml, 28 September 2016 - 05:51 PM.


#4 Art Montemayor

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Posted 29 September 2016 - 01:08 PM

Lavi:

 

Your intentions are noble.  However, as Paul Ostand points out, the pressure at both the central and peripheral nozzles is essentially the same.  Therefore, there is no reasonable driving force to favor either set of nozzles - as Saml notes.  The hot vapors will exit through both sets.  All nozzles have an inherent pressure drop due to the resistance they lend. 

 

This is what happens in almost all storage tanks, in my experience, unless you can create a driving force - like a fan to aspirate the hot vapors from the central nozzle, creating a slightly lower pressure within the tank's vapor space and thus allow ambient air (at a slightly higher pressure) to enter the tank through the peripheral nozzles and into a slight partial vacuum created by the fan.  This is a more complex situation than the natural circulation you want, need, or desire.  But if you really want (or need) to circulate ambient air into the tank's vapor space that, in my opinion is what you have to do.






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