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Elimination Of Final Stage Separator

separator design compressor design simulation basis rvp operating pressure

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#1 Danish John Paul

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Posted 13 November 2020 - 02:01 AM

Hi

In one of my project, I came across 2 stages of separators namely 3phase separator and a final stage separator. Where the liquid outlet of the 3 phase separator is fed again into the final stage separator and gas outlet is fed directly into LP compressor. While, The liquid from the final stage separator is pumped out to export and gas outlet is connected to stock tank compressor. The stock tank compressor discharge passes through a aftercooler joining to the LP compressor inlet. 
The operating pressure of the 3-phase separator is 3.75 barg and design pressure is 13.8 barg while for final stage separator has an operating pressure of 0.6 barg and design pressure is same 13.8 barg. The RVP of the export crude oil should be <10psia. The suction pressure of the LP compressor is 2.75 barg and that of stock tank compressor is 0.6 barg.
My question here why is that we can't take the gas outlet of the Final stage separator directly to the LP compressor therefore bypassing the stock tank compressor leading to the elimination of the stock tank compressor (this is by changing the operating pressure of the final stage separator to 2.75 barg). My understanding is that by increasing the operating pressure of a separator the requirement of horse power compressor can be reduced. 

please guide experts



#2 Bobby Strain

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Posted 13 November 2020 - 09:54 AM

Seems like we have seen this before.

 

Bobby



#3 Danish John Paul

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Posted 17 November 2020 - 11:06 PM

I have put this post a long time back. As there was no response, I have deleted the post and reposted Sir. Since I need an explanation for this i reposted this :-)



#4 Pilesar

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Posted 18 November 2020 - 07:27 AM

Separation pressure affects the material balance of the phases. Low pressure separating can help meet product specs. If you eliminate the stock tank compressor, what will happen to the vapor generated at the stock tank?



#5 Danish John Paul

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Posted Yesterday, 05:42 AM

Separation pressure affects the material balance of the phases. Low pressure separating can help meet product specs. If you eliminate the stock tank compressor, what will happen to the vapor generated at the stock tank?

 If the Stock Tank Compressor is eliminated, the gas from the final stage separator will be routed directly to LP Compressor suction. The liquid from the final stage separator can be routed to a stock tank where the operating pressure of the tank can be maintained @ 0.6~0.7 barg to meet the product specs ( Crude RVP<10 psig) .



#6 Pilesar

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Posted Yesterday, 08:41 AM

From your posts:
1) The "final stage separator has an operating pressure of 0.6 barg." 
2) "The suction pressure of the LP compressor is 2.75 barg." 
3) "If the Stock Tank Compressor is eliminated, the gas from the final stage separator will be routed directly to LP Compressor suction."
 
All three of the above statements cannot be true at the same time. What does your steady state simulation tell you about the feasibility of your plan? Are you allowing for the pressure drop in the vapor line and pressure control valve at the final stage separator? If you are not generating vapor at the final stage separator at low pressure operation, then your stock tank compressor horsepower is already very low. If you stock tank compressor horsepower is high enough that you think it worthwhile to reduce it, that shows you need it to compress the vapor generated at the lower pressure of the final stage separator. From what you describe about the operation, I suspect you need the low pressure final separator and the stock tank compressor.

Edited by Pilesar, Yesterday, 08:54 AM.





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