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Flare Load Upon Instrument Air Failure


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#1 SawsanAli311

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Posted 22 November 2019 - 12:21 PM

Dear All,

 

Based on API 521 6th edition section 5.3.3, a common mode failure such as instrument air can have coincident opening of the emergency depressuring normally energized valves along with the pressure control valves installed across the plant which fail in open position upon loss of instrument air. I know that we can avoid the opening of the BDVs by providing sufficiently sized secured instrument air bottles. However, do we really have to consider or check the above case while designing the flare system? Will the basis of the fact that the starting pressure under the instrument air failure scenario would be relatively lower than the usual conservative basis of relief or emergency fire blowdown design calculations and hence the combined PCV + depressurization load would be hence covered? Do we also have to check the case of all PCVs opening simultaneously to flare across the entire plant? 

 

Thanks 



#2 Pilesar

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Posted 22 November 2019 - 09:38 PM

You must identify all possible causes of overpressure, determine if they are credible, and document them. Each credible scenario must be accounted for when sizing the relief system. If it is a credible case that all PCVs would open simultaneously to flare across the entire plant, then the relief system must be sized for that.



#3 fallah

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Posted 28 November 2019 - 09:43 AM

 

Based on API 521 6th edition section 5.3.3, a common mode failure such as instrument air can have coincident opening of the emergency depressuring normally energized valves along with the pressure control valves installed across the plant which fail in open position upon loss of instrument air. I know that we can avoid the opening of the BDVs by providing sufficiently sized secured instrument air bottles. However, do we really have to consider or check the above case while designing the flare system? Will the basis of the fact that the starting pressure under the instrument air failure scenario would be relatively lower than the usual conservative basis of relief or emergency fire blowdown design calculations and hence the combined PCV + depressurization load would be hence covered? Do we also have to check the case of all PCVs opening simultaneously to flare across the entire plant? 

 

 

Hi,

 

IA failure is one of common failure among other ones, hence the total relief load upon IA failure should be brought into flare load summary to evaluate the governing case for flare header and stack sizing. Then no matter the relieving fluids following to IA failure are due to BDV or PCV opening individually or simultaneously...






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