3 replies to this topic

[Chemical_Eng_7]

All,

Please excuse me if this question is quite simple, I have minimal experience and I am trying to learn all that I can. I have a question about a tank vent that does not seem very typical. The tank is not very large and does not hold a very volatile liquid (aqueos mixture about 70 percent water used for fertilizer industry). The tank is about 20,000 gallons with a 16 ft diameter and 12 ft length S/S. The incoming flow is about 155 gpm while the flow out is 120 gpm. I have calculated the inbreathing and outbreathing of the tank as follows: Inbreathing - 40 SCFM, Outbreathing - 36 SCFM. It is basically an atmospheric tank. The tank vent is essentially a 2 ft overhead flange which is separated from the tank opening by 1 inch nuts to act as a vent to atmosphere. So, the area of the opening is about 0.52 sq. ft. My question is how would go about calculating if the flange opening is in fact large enough to withstand the pressure and vacuum of the filling and pumping out of the tank. The path I am trying to take seems basic but I am having problems. I will try to calculate the velocity of the air flow in and out of this opening using the pressure drops from the know pressures. 12.0 psia atmospheric, 26.9 psia in the tank, and 8.5 psia vacuum pressure. These pressure are the max design pressure and I assure this is being treated as an atmospheric tank. I would really appreciate any kind of guidance with this situation. Thanks so much in advance.

CHE7

[proinwv]

Please clarify your statement, "my question is how would go about calculating if the flange opening is in fact large enough to withstand the pressure and vacuum of the filling and pumping out of the tank"

Do you mean what would the pressure drop be across this vent based on the in-breathing and out-breathing flows? Or alternately, what would the flow be at your tank pressure? Also, how is your atmospheric pressure butt 12 psia? Are you on a mountain top, or higher?

Atmospheric tank at 26.9 psia and -8.5 psia???

Fluid mechanics handbooks may give examples of your "nozzle" and how to calculate flow vs delta P, but I must say your pressure statements are difficult to believe.

[Art Montemayor]

CHE7:

I would love to help you on this application – as I am sure other members in this Forum would also – mainly because I sense something potentially wrong in the description you have given. I would not like for this application to turn into a bad scene. Allow me to explain my concern(s):

• This tank is either a storage tank, or it is a process tank. You state: “The incoming flow is about 155 gpm while the flow out is 120 gpm.” If this is a process tank and both flows are occurring at the same time under steady state conditions, then what is going in MUST come out. Otherwise, you will have accumulation – and overpressure.
• If what you meant to state was that the tank is a storage vessel that receives a batch quantity of liquid at a rate of 155 gpm while it is filling and after filling, it discharges the contents at a rate of 120 gpm, then you would have outbreathing requirements of 155 gpm (without even allowing for heating effects) which is equal to 20.7 cfm. The inbreathing would be the equivalent air to displace 120 gpm of exiting liquid which is 16 cfm. Note that the outbreathing is more than the inbreathing. However you state you calculated that the inbreathing is more than the outbreathing (40 cfm versus 36 cfm). Without your calculations to check, all I can deduce is that something doesn’t make sense.
• Another problem I have is that your total tank volume equals 18,050 gallons, not 20,000 gallons. So your calculations or information is erroneous.
• I do not know what you mean by “a 2 ft overhead flange which is separated from the tank opening by 1 inch nuts”. Do you mean to state that you have a vent with a flange that is 2ft in diameter? The simplest and most concise and fool-proof method of giving us the basic data would be a detailed drawing of the tank system. A written description cannot compete with a simple sketch in tell us all that we need to know.
• You state: “I will try to calculate the velocity of the air flow in and out of this opening using the pressure drops from the know pressures. 12.0 psia atmospheric, 26.9 psia in the tank, and 8.5 psia vacuum pressure. These pressure are the max design pressure and I assure this is being treated as an atmospheric tank.” I really am having a hard time in understanding this. Are you saying that you have design information on the tank that states the maximum design pressure is 26.9 psia? This would not describe a normal atmospheric tank. Where did this information come from? Better still, furnish a fabrication drawing of the tank and the documents (or nameplate data) that describe the Maximum Allowable Working Pressure (MAWP) and the Maximum Allowable Working Vacuum (MAWV) of the tank. With this information and the tank drawing, we can start to furnish you with some positive help.

[Chemical_Eng_7]

Art,

I very much appreciate the reply and thank you for pointing out the many holes in my description. I will try my hardest to provide the details that you requested in order to obtain positive help.

I attempted a simple sketch of the situation and it is attached as filename Tank Vent Sketch. My main objective is to determine if the 1 inch air space gap between the tank and the flange is sufficient to provide the necessary air flow for tank inbreathing and outbreathing capacity requirements. In lamens terms, is the tank vent large enough to allow for the pressure and vacuum of the tank during fill up and emtpying. If not, we will have to make a design correction in order to compensate.

In my original post, I stated that this was an atmospheric tank. Although it is open to the atmosphere through this small vent, the tank is rated as an ASME pressurized tank as you will see on the attachment. Also, it is safe to assume a batch style storage tank and NOT steady state operation process tank. We will assume that this is a tank that will be filled and after filled be emptied (just as you mentioned in your reply). I understand the major discrepancy between the flow in and out of the tank. But for now, it is okay for us to assume the 155 gpm into the tank, and also the 120 exiting the storage tank.

About the pressures, I calculated the pressure as 14.9 psig plus atm pressure 12.0 psia to obtain 26.9 psia pressure (Design/ MAWP) and the vacuum as -3.5 psig + 12.0 psia to obtain the 8.5 psig vacuum (Design/MAWV) pressure. Is this accurate?

Lastly, I calculated the same inbreathing and outbreathing values as you have in your reply (without thermal consideration). Converting flow rates into cfm we obtained the same values, 20.7 cfm outbreathing and 16 cfm inbreathing.

My original post showed values of inbreathing (40 cfm) and outbreathing (36 cfm). These values included the thermal compensation. I used a method of generic thermal venting as follows:

Thermal Venting Vacuum - 1.80763268*18,047^(.68303278) = 24.3 cfm; added to pump out = 40 cfm

Thermal Venting Pressure - 1.19546087*18,047^(.67714311) = 15.2 cfm; added to pump in = 36 cfm

I was giving these thermal equations and I am not sure where they orginated from.

And yes, you were correct in calculating the tank volume as 18,050 gallons.

I hope this clarifies my situation. If you could please provide some guidance as to how I can continue on to finding a solution. And also, if you need any more details please let me know. Thank you so much for your time and effort.

CHE7

 Tank Vent Sketch.xlsx   16.73KB   37 downloads