Pressurised Gas Tank - how long to empty??

[hydrogen_joe]

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Hi all, thanks for reading.

I am trying to model the emptying of a pressurised gas body (e.g. tank/line) when a leak is present. I know the initial choked mass flow rate of the leak due to the internal pressure (350 bar), however, I am not sure how best to model the decreasing flow rate due to the reducing pressure within the tank.

Ideally, I would be able to model the pressure decrease from 350bar to atmospheric conditions against leak time, and thus the time taken for the tank to 'empty'. I am struggling to find any reading on this, could anyone help me out or point me in the correct direction.

Thanks in advance!

 

[LittleInch]

As transient event you ether need to use a transient process analysis tool or create a spreadsheet and do it in time steps small enough so that the mass flow rate is more or less the same over the time step.

So you start with a certain mass of gas at a certain pressure in a fixed volume. You can calculate the mass flow.

Let that run for X seconds. Subtract the mass which has left from the start mass.

Then convert that mass to volume lost and re calcualte the internal pressure.

Rinse and repeat until you get down to non choked flow.

Note tha tthe tail will be very long. Ususally people set a certain cut off in terms of pressure ( say 1 bar or 5 bar ) and calculate up that point.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydrogen_joe]

Thanks for your reply.

The internal density would also decrease with decreasing pressure, I assume this would need to be found with each iteration to find the actual density at the new, lower pressure?

 

[georgeverghese]

Best to get this done on a commercial process simulator which has a utility for depressurisation simulations - Hysis or Pro II, especially since this starts at 350barg.

 

[goutam_freelance]

A derivation from First Law of Thermodynamics is enclosed.

Assumptions:
1. Fluid=air,perfect gas
2. Leak dia=2 mm
As can be seen from the curve the pressure decays to about 2 bar in 420 s=7 mins.

Please inform actual time for depressurization if available.

https://drive.google.com/file/d/1FJ8KsrXFvco_5ST3fkPK6BBmgKs-osK3/view?usp=sharing

 

[LittleInch]

The calculation needs to know volume of tank and flowrate. Neither of which the OP has sent us.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

The calculation assumptions are given below:
Assumed data:
P’=350 bar
V=0.5 m3
T’=30 0C=303.15 K
D=2 mm(hole dia)
A=3.142e-5 m2