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Choked Tank Depressurisation Calculation

fanno flow depressurisation choked flow

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#1 lambonumber5


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Posted 08 April 2021 - 05:35 AM

Hi Guys, 

I have been trying to write a model to calculate the depressurisation of hydrogen out of a tank through a vent line. I've used the fanno flow calculations as the tanks are at about 350bar so the flow will be choked. The results are in the expected ballpark but no one in my company can validate the results. I'd really appreciate it if someone with more knowledge and experience in this field would check what I've done.

Much appreciated 


Attached File  Depresurization calculator.xlsm   498.49KB   7 downloads

#2 lambonumber5


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Posted 08 April 2021 - 06:20 AM

Hi guys some information to help understand what I've done. First I used the Colebrook equation to find the frictions factor using an assumed Reynolds number, I used the goal seek function in excel to solve this. using that and the goal seek again i solved the Fanno flow equation and found the Mach number at the inlet of the vent pipe. These functions are coded in a macro and linked to the run button. from the Mach number the flow rate through the vent pip can be found and using a user defined time step the the new tank pressure and temperature can be calculated (adiabatic assumption). then assuming the Reynolds number hasn't changed significantly the friction factor and Mach number can be assumed to stay reasonably constant for the majority of the depressurisation process. the new pressure and temperature can be solved for each time step using the trapezium rule to approximate the correct depressurisation profile. I know that the Fanno flow equations breakdown at lower pressures but I think that for the majority of the depressurisation process it will be under the Fanno flow regime. 

Thanks again 


#3 latexman


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Posted 14 April 2021 - 08:08 AM



Milton Beychok had an outstanding website on Fundamentals of Stack Gas Dispersion and related topics. I used his website many, many times over the years. When he passed away, the website went dark. It was a great loss, but not a total loss, because one can buy his book of the same title, or, do like I do, and use the Internet Archive Wayback Machine.


It's not an exact match to your problem, but the section on "Calculating Accidental Release Flow Rates From Pressurized Gas Systems" is close enough to be worthy of mention.  If nothing else, you can solve his example with your spreadsheet and compare.

#4 lambonumber5


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Posted 14 April 2021 - 09:32 AM

Thanks I'll Take a look. I have spent a lot more time on the calculator and have improved the functionality. Along the way I found that the flow profile along the pipe matched fairly accurately the profile found on a peer reviewed paper. 

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