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Back Pressure In Aspen Flare System Analyzer


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#1 jmit

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Posted 07 March 2017 - 08:59 PM

Hello All,

 

My question may sounds like basic but I really appreciate if someone can reply.

 

This is my initial exposure to Flarenet.

 

We have a Main flare system with total allowable pressure drop of 65psi across the tip, seal and stack (based on governing scenario and flowrate). This means the pressure at the bottom of stack should be equal to 65psi so as to reach atmospheric conditions at the time of flaring.

 

Now to simulate this situation in flarenet, do I need to input the fitting loss coefficient in "Flare Tip" till the pressure in flare upstream pipe shows 65psig or I can input the curve with static pressure drop of 65psi (with pressure correction). This will be for governing scenario though.

 

Also, how to simulate other scenarios and cases for checking back pressure and mach number ? what parameters we should specify in flare tip editor ?

 

We are yet to receive the flare vendor data. The requirement of simulation is to calculate back pressures for control valves (preliminary).

 

Kindly let me know if my question is not clear.

 

Thank you.



#2 jmit

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Posted 09 March 2017 - 10:05 PM

anyone ?



#3 cea

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Posted 10 March 2017 - 12:40 AM

It is long time that I worked last on flarenet. So, I try to answer with whatever I could recall.

 

Firstly, maximum allowable back pressure is by entire flare system (including header) & due to maximum flow for identified contingency.

65 psi allowable back pressure across tip, seal & stack appears to be erroneous. Please recheck & reconfirm.

 

With above perception, you are expected to build model starting from flare tip, wherein you need to indicate only constant back pressure due to pressure drop at tip, in stack & seal. (Generally, it ranges from 5 - 15 psig). Subsequently, you start connecting PRDs with all required process data & pipe fittings to the header that joins finally to stack. Needless to say, sequence of PRDs in model shall follow layout sequence.

 

The model thus developed, indicates maximum back pressure at each PRD that is participating during said contingency. As I understand, maximum allowable back pressure as mentioned in query, refers to thus calculated back pressure, which is not expected to cross 65 psig value.

 

Correct me, if I am wrong.

 

Further, you can built multiple scenarios for each PRD in same model. You need to select respective scenario while running the software (execute), so that  only relevant PRD data gets computed. 

 

Hope, this clarifies both queries.


Edited by cea, 10 March 2017 - 12:44 AM.


#4 jmit

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Posted 12 March 2017 - 09:39 PM

Thank-you for the reply.

Yes the allowable pressure drop across the stack is 65psi, this is for a flow of 2000mmscfd (client specified).

I understood the basics but unable to understand the flare tip editor specification. I can enter the single curve based on 2000mmscd and corresponding MW with specified allowable pressure drop of 65psi. How about the other cases (other relief valves and control valves) where the flow is quite less than the governing flow (2000mmscfd) ? Will it be correct to specify constant back pressure only in curves section (in flare tip editor); if yes then what flowrate shall I specify?

How about generation of curves for various random flowrates (till we receive the curves from vendor) ? like we knowfor 2000mmscfd corresponding drop is 65psi then for 1500,1000,500mmscfd we can calculate the pressure drop {dp=(flowrate)^2}. Is this the right approach ?

Suggestions will be highly appreciated.

Thank you!

#5 cea

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Posted 14 March 2017 - 12:13 AM

Thank-you for the reply.

Yes the allowable pressure drop across the stack is 65psi, this is for a flow of 2000mmscfd (client specified).

I understood the basics but unable to understand the flare tip editor specification. I can enter the single curve based on 2000mmscd and corresponding MW with specified allowable pressure drop of 65psi. How about the other cases (other relief valves and control valves) where the flow is quite less than the governing flow (2000mmscfd) ? Will it be correct to specify constant back pressure only in curves section (in flare tip editor); if yes then what flowrate shall I specify?

How about generation of curves for various random flowrates (till we receive the curves from vendor) ? like we knowfor 2000mmscfd corresponding drop is 65psi then for 1500,1000,500mmscfd we can calculate the pressure drop {dp=(flowrate)^2}. Is this the right approach ?

Suggestions will be highly appreciated.

Thank you!

You are expected to give only flare tip pressure, which is barometric pressure. Other details, that you need to provide is stack details, such as diameter & height. You may add some piping component to compensate for pressure drop in seal & tip.

 

Flarenet calculates system pressure drop for each identified case.

 

Please note that client have specified maximum allowable back pressure & not actual back pressure. Hence, you do not have to mention any back pressure value, but just have to ensure the actual back pressure is remaining within the range.



#6 jmit

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Posted 19 March 2017 - 07:00 PM

Thank you Cea.




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