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# Vent size calculation for closd tank.

## Vent size calculation for closd tank.

(OP)
Hello every one,
I hope you are doing well. Currently i am a student and working as an intern in an industry. While working i encountered with an issue.
Issue - A liquid (e.g. water) cooling tank with capacity 30,000 liters is collapsed due to vacuum created by condensation when the washing process was carried out in tank. Hot water cycle with temperature 65° C causes the vapor in the tank and after the 30 second time gap, cold water cycle at 10° C was performed. This causes the condensation in tank. Pressure/ vacuum free Vent with Dia 104 were used initially. As it wasn’t sufficient to provide enough inbreathing, tank collapse.
So to solve this issue I did some calculations to find the diameter, can any one tell me that it is correct or not? I also referred API 2000 but i am not sure about it.

Calculation:
Volume of the tank (V1) : 30 m3
T1 – 65° C – 338 K
T2 – 10° C – 283 K
Time gap between hot and cold water cycle (t) – 30 seconds

:- V1/V2 = T2/T1 so V2 = 25.31 m3
∆V = V1 –V2 = 4.93 m3
Qair = ∆V / t = 600 m3/h

Now ,
𝑄 = (𝑉 𝐷²)/363.5 (V – velocity of air – 10 m/s )(assumed)
∴𝐷= √ (𝑄 ×363.5)/𝑣 = 147.42 mm

New D = 147.42 mm
Old D = 104 mm

thankyou.

### RE: Vent size calculation for closd tank.

Instead of using the gas law, use the thermodynamics either the Mollier diagram or Steam table to determine the initial and final specific volumes (cu.ft/lbm) at T1 and T2; assume the initial and final amount (lbm) of saturated vapor has not changed during the 30sec, otherwise, a time step analysis will need to be done; also assume no heat loss thru the shell. You'll need to figure out the quality of the saturated vapor in order to determine the specific volumes which will be probably on the low side such as 10% or less.

### RE: Vent size calculation for closd tank.

I agree with chicopee. Use the Mollier diagram. Ideal gas law is reasonable for superheated steam, but it looks like you don't have that.

I would do the stepwise function in excel and use an if statement to open a vacuum breaker at a certain pressure (unless you're planning on an atmospheric vent). I've done similar calculations in excel.

My gut tells me that you won't need a significant vent (relative to vessel diameter) based on the temperature differences and the apparent 30 seconds you have to vent. I don't know all the parameters, so I could be wrong. Good luck!

Jim Breunig P.E.
XCEED Engineering
Mechanical Engineering
FEA Consultants

### RE: Vent size calculation for closd tank.

You should compare the volume of saturated vapor at 65C vs volume of condensed liquid (not the saturated vapor) at 10C to get the volume reduction resulting in the vacuum imposed.

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