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Backpressure Checking Using Flarenet


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#1 Jeen


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Posted 08 April 2019 - 09:19 AM

Dear Sirs,

Apologies if this query is already mentioned elsewhere however I have searched and could not find out the right answer.

I am working on Flarenet to calculate actual backpressure of a PSV.
As per the experts in this Forum as quoted below, for actual backpressure calculation, it is obvious that the required capacity is used for the flare header while the rated capacity is used for tail pipes.
1. API Std 521 (5.4.2) : Typically, the required relieving rate is used for the flare header, flare tip, and knockout drum design with spring loaded PRVs.
(5.5.3) : In general, laterals and tail pipes from individual PRVs are sized based on the rated capacity.
2. When evaluating a flare header, use the required flowrate. For tailpipes, one uses the rated capacity of the valve, and that`s true regardless of whether the tailpipe discharge to atm or a flare header. If you`re using a software program (e.g. Flarenet) for modeling the flare system, the program will automatically switch from rated flowrate to required flowrate once the stream exits the tailpipe and enters the flare header.

Meanwhile, colleague said that rated capacity is used for tail pipe and header as well in case of single PRV case as a conservative approach. But this results high back pressure due to long existing flare header.

Therefore, I would like to have your advices what is engineering practice applied in Flarenet in the field to check actual backpressure in existing system. For evaluation (rating), It seems like that required capacity is used for the flare header regardless of single or multiple PRVs.

For your understanding, prepared the sketch of existing system, which case (among case A,B,C) shall be considered to check actual backpressure using Flarenet rating.

Thank you for advices in advance.


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#2 PRVBoss


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Posted 10 April 2019 - 01:01 PM

Hi Joey,


Good question and it is almost never clear where a lateral stops and a subheader or header or main flare spine starts. Often a rule of thumb based on pipe size increase is prescribed; for example, the lateral stops after the discharge pipe increases greater than two NPS as the pressure loss becomes nominal. 


Applying this to your schematic the only PRV where one would stop using rated capacity is PSV-6. Include the discharge line 10", and the next two pipe sizes (12" and 14", which aren't present) and stop when the line size increases to 16". For those PRV's with a 16" outlet one would be carrying the pressure loss calculations even downstream of the 20" connection because 16", 18", and 20" would be included as significant contributors and the calculation would stop at the next expansion. 


Good luck.

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