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#1 LeoLeo

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Posted 03 September 2013 - 02:02 AM

Dear all,

 

I am trying to do the depressurizing calculation for a flowline. There are two scenarios available for depressurizing of this flowline, Summer case and Winter Case.

 

The summer case I used to size the vent line and orifice (depressurizing will be done manually).

 

My first question is regarding the winter case. If I assume that the natural gas at temperature of 92C is trapped in the flowline at pressure of 350 barg and then it is cooled down to ambient temperature which is -4C. Then what will be the pressure in the flowline. The natural gas contains some water as well.

 

My second question is; I will have ice/hydrate formation in the winter case. What is common to do in this situation? Should I consider Methanol injection prior to shut in the flow line?

 

Thanks,

 

ARM



#2 PaoloPemi

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Posted 03 September 2013 - 06:38 AM

first case

if the initial amount of fluid trapped in flowline doesn't change,

you know initial V1 and final V2 volume (which is the same, that of flowline) T1,P1 and (final) T2,

you can solve a V-T flash operation to find (final) P2,

with PRODE and Excel I use the VTF() macro but the exact procedure will depend from your software.

 

second question,

yes, alcohols as methanol or salts or other chemicals can help to prevent ice / hydrate formation as they alter the phase equilibria.



#3 LeoLeo

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Posted 03 September 2013 - 08:08 AM

Paolopemi,

 

Thanks for the reply. The problem is that when the temperature changes from 92C to -4C, we will have condensation of water. Therefore, we cannot assume that the volume is the same.



#4 PaoloPemi

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Posted 03 September 2013 - 08:17 AM

when you solve a V-T or V-P flash operation

the procedure calculates the total volume (vapor+liquid+...)

so it takes in account any change of phase as condensation / evaporation etc.

at least so it does PRODE PROPERTIES,

with a different software you may use a different procedure.



#5 LeoLeo

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Posted 17 September 2014 - 03:40 AM

I thought, it will be nice to leave an update regarding this subject.

 

Injection of Methanol prior to depressuring is not common. To prevent temperature drop of the system (below hydrate formation temp. or low design temperature of system) IPF will be considered to protect the system.






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