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Propane Tank Depressurization - Low Temperature Realistic?

propane tank depressurization

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#1 rs20170808@gmail.com

rs20170808@gmail.com

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Posted 25 January 2018 - 10:34 PM

Hi,
 
I am trying to simulate maintenance depressurization of a propane tank.  
Ive actually modelled a propane tank with 10% liquid and rest vapor. I've to depressurize it from 138 psig to 2 psig with manual globe valve
I have CV and opening of the valve
I get low temp at valve outlet when depressurization occurs in HYSYS. I use HYSYS depress. utility and provide 2 psig as backpressure. 
Now the line after globe valve is connected to flare
I want to make sure these low temp are realistic
Should we checking backpressure in flarenet. I think depress. utility gives low temp. as it depressurizes with 2 psig backpressure. What would be a good approach to simulate this.
 
Thanks


#2 serra

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Posted 26 January 2018 - 02:14 AM

a simple alternative which can be useful to verify the outputs from the software is the direct integration procedure,

for a pure fluid you need only a mollier chart or (for mixtures) some procedure to solve at each step phase equilibria (from vapor pressure for a pure fluid) and heat balance (enthalpy - latent heat),

at each step you konow pin (vessel), pout (flare, atm) and can calculate the flow of propane with ISA or similar correlations,

there are several threads discussing these procedures... I have used Excel+Prode and compared results with built-in blowdown utility, you can do the same with Aspen.



#3 Pilesar

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Posted 26 January 2018 - 11:12 AM

If you have propane liquid in the tank, the temperature will be the bubble point temperature at the tank pressure. This will get quite cold as you reduce the pressure. For maintenance depressurization, the more usual procedure is to drain the liquid out of the tank before venting the remaining vapor. 



#4 Technical Bard

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Posted 31 January 2018 - 11:05 PM

Depressuring liquid propane to atmosphere will result in temperatures approaching the TBP of propane at the downstream pressure, which at sea level is close to -43ÂșC.   The steel pipe may not get that cold - you would need to do a transient heat transfer calculation to see how much heat it loses to the propane and gains from the environment.






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