Time to discharge a gas from a tank 4

[JASR76]

Consider the case in which a gas (natural gas in my case)flows from one line (where it's trapped, this line could be consider a pressurized 'tank') to atmosphere through a vent pipe . This is my case in a compression station and my objetive is to get my discharge line empty in 5 min (or less) during a vent operation.

How can I calculate this time and how accurate could be this number?

My initial pressure of the gas trapped into the line is 80 bar, so at the beginning and during a certain time we'll have sonic flow and after this time, we'll have subsonic flow.

Could you tell me a way to calculate:
-Flow into the line in every second
- Pressure into the 'tank' during every second
- Temperature into the 'tank' during every second

Thank you in advance and sorry for my English


Juan Antonio

 

[MortenA]

This is faily standard problem. Crane has a "Compressible flow manual" that will take you through the various steps - allthough you seem to know them yourself.

Set uo a spread sheet where you keep tabs of inventory, adiabatic expansion in the vessel and flow through the pipe. Evaluate stepvise and solve untill the vessel is empty.

Best regards

Morten

 

[JASR76]

Thank you very much, Morten

But I dont have this "Compressible flow manual" in my posession,I have only the "Flow of fluids Tech Paper 410" and it isn't very clear with this task.

Does anybody know where could I find a clear example similar to my problem?

Best regards & thanks

Juan Antonio

 

[quark]

You can start from here.

http://www.air-dispersion.com/usource.html

Regards,

 

[Montemayor]

quark:

Great contribution to the problem's solution, quark. You beat me to the punch. Milton Beychok's webpage is a process engineer's best-kept secret and really should be revealed more to all engineers doing compressible fluid work. However, Milton has a lot more than just compressible fluid answers to offer. His book is full of hands-on practical engineering solutions on process and environmental concerns.

But I'm sure you're more than aware of this. Juan Antonio should enjoy reading the specific answer to his problem there.

Art Montemayor
Spring, TX

 

[JASR76]

Thank you very much for all your answers

The only 'problem' of this web page

http://www.air-dispersion.com/usource.html

is that you dont have an example to contrast and you can't read the theory that goes under these equations.

I suposse that there is not a web or an article or a book as good as this one...but if you know one or you have an excel spreadsheet to contrast, i would be very thankful.

And last, but not least, I have been calculating and I see that the temperature at the end of the discharge is very very low, so it doesnt seem very realistic. Do you have any correction to the temperature in the vessel??

thank you again and again

 

[MortenA]

JASR76 the formula for gas flow is common and can be found in e.g. API 520 (PSV sizing). It seems from your question that you were looking for an engineering solution... (not that theory is utterly important in this case).

If one of us just supplies you with a spread sheet that wouldnt shed light on te theory in any case.

Normally wou would expect a temperature drop around 1/2 deg C pr bar og pressure drop (unless you get condensation) for the gas exiting.

The temperature drop in the vessel (assuming adiabatic expansion) will however be much larger.

Best regards

Morten

 

[quark]

Mr. Montemayor!

Actually I was in a dilemma to post this reply, for I picked up this link from one of your previous posts long time back, as far as my memory is concerned. I had a chance to view the book afterwards and it is a treasure indeed.

phew, now I am much relieved.

Thanks for your kind comments.

Regards,

 

[Latexman]

JASR76,

If the author gave everything away on the website, how many books do you think he'd sell?

Good luck,
Latexman

 

[JASR76]

LAtexman,

I'm sure he wouldn't sell many books, that's why I asked for a reference and not for a rule of thumb.

So I suppose that Milton Beychok's book is the best reference for this task and I am very grateful to Montemayor, MortenA and Quark for their information


Thank you all

 

[Montemayor]

Juan Antonio:

In order to be specific and direct in ensuring that you were given a direct and positive response to your query let me state the following regarding the recommendation you were given by quark:

You are definitely wrong in stating that the recommended website “doesn’t have an example to contrast (I suppose you mean ‘compare’) and you can't read the theory that goes under these equations”. Not only does Milton Beychok give you a specific example problem (methane gas leaking out of vessel at 60 oF and 3,430 psia through a 0.5 inch diameter hole) but he goes to the extreme in giving you the specific sources of all his equations and derivations[/], such as:

(1) Beychok, M.R., Fundamentals of Stack Gas Dispersion, published by the author, Irvine, California, USA, 1995
(2) Ermak, D.L., User's Manual for Slab - An Atmospheric Dispersion Model for Denser-Than- Air Releases, Lawrence Livermore Nat'l. Laboratory, Livermore, Calif., USA, 1990
(3) Spicer, T. and Havens, J., User's Guide for the Degadis 2.1 Dense Gas Dispersion Model, EPA-450/4-89-019, Research Triangle Park, North Carolina, USA, 1989
(4) Rasouli, F. and Williams, T.A., J. Air & Waste Management Association, March 1995
(5) Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, John Wiley & Sons, New York, New York, USA, 1960

He further continues to give you a table of Cp/Cv values for some common gases, a comparison between the results of TWO equations used (Rasouli and Williams Model vs. Bird, Stewart and Lightfoot Model) and then tops off everything else he gives you by further developing curves showing the rate of pressure decrease in the source vessel and the gas release rate.

I believe that Milton Beychok has given you everything you requested – and more! What you are being given is not "a rule of thumb", but hard-core engineering derivations and calculations based on classic fluid dynamics and Thermodynamics. Are we reading the same website?

As everyone on this forum knows, I'm a rigid stickler for referencing sources of engineering data, equations, and calculations. In my opinion Beychok has gone to the extreme limit in giving generous and ample references and bases for his calculations. I hope that you have read all through his example.


Art Montemayor
Spring, TX

 

[Guidoo]

It seems like Juan Antonio missed the link on the referred webpage. You can go directly to the example and detailed theory via:

http://www.air-dispersion.com/feature2.html

 

[Montemayor]

Guidoo:

Thanks for the sharp detective work; that is probably what has happened. Your keen observance and intuitiveness will probably take Juan Antonio directly to his solution.

Art Montemayor
Spring, TX

 

[quark]

Btw, Milton Beychok has been a member of eng-tips.com since 1999. Check this thread135-97506.

Regards,