Dear All,
When evaluating the depressuring time for a section of plant composed of some equipment and their connected piping, it is a good practice that we consider the effect of a component with the lowest thickness which in most of the cases I have seen is connected piping.(becaue at the onset of an emergency situation we should depressurise as soon as possible). If the concerned piping section thickness be less than 25 mm the depressuring should be faster(according to API 521). Suppose that following a logical trend we set the depressuring time(for example decrease the depressuring time from 15 minutes by 3 minutes for every 5 mm for thickness values lower than 25 mm). The problem arise after that, is a huge depressuring load for example depressuring a section of plant whithin 6 minutes which necessitates a flare network with huge capacity!
How to cope with this major concern?
Your valuable comments are appreciated.
Full Version: Effect Of Thin Equipment And Piping On Depressuring Time
QUOTE (jprocess @ Apr 9 2008, 05:41 PM) 
Dear All,
When evaluating the depressuring time for a section of plant composed of some equipment and their connected piping, it is a good practice that we consider the effect of a component with the lowest thickness which in most of the cases I have seen is connected piping.(becaue at the onset of an emergency situation we should depressurise as soon as possible).
When evaluating the depressuring time for a section of plant composed of some equipment and their connected piping, it is a good practice that we consider the effect of a component with the lowest thickness which in most of the cases I have seen is connected piping.(becaue at the onset of an emergency situation we should depressurise as soon as possible).
I am sorry to say that this statement has two wrong messages...
First, "it is a good practice...". This shall be " It is required..."
Second, Allowable stress level is a function of wall thickness (wt) , internal diameter (ID), etc. Low wt with low ID may have higher allowable stress level...
QUOTE
If the concerned piping section thickness be less than 25 mm the depressuring should be faster(according to API 521). Suppose that following a logical trend we set the depressuring time(for example decrease the depressuring time from 15 minutes by 3 minutes for every 5 mm for thickness values lower than 25 mm). The problem arise after that, is a huge depressuring load for example depressuring a section of plant whithin 6 minutes which necessitates a flare network with huge capacity!
How to cope with this major concern?
Your valuable comments are appreciated.
How to cope with this major concern?
Your valuable comments are appreciated.
Cut it into smaller section and conduct staged blowdown...
Dear JoeWong,
Thanks a lot for your valuable comments.
I fully agree with you.
You mean the last statement in API 521-2007 edition which indicate that 15 minute requirement is no longer valid and instead allowable stress level should be used as effective parameter?
This is true but all the work that a process engineer can do during evaluating the flare network is using softwares like HYSYS or Blowdown and considering a specific depressuring time. My problem is that how the process engineer should implement this new point into his calculations?
Is it a cost effective solution? I mean having staged blowdown system+a lower flare capacity instead of traditional blowdown system+a higher flare capacity?
Thanks again.
Thanks a lot for your valuable comments.
QUOTE
First, "it is a good practice...". This shall be " It is required..."
I fully agree with you.
QUOTE
Second, Allowable stress level is a function of wall thickness (wt) , internal diameter (ID), etc. Low wt with low ID may have higher allowable stress level...
You mean the last statement in API 521-2007 edition which indicate that 15 minute requirement is no longer valid and instead allowable stress level should be used as effective parameter?
This is true but all the work that a process engineer can do during evaluating the flare network is using softwares like HYSYS or Blowdown and considering a specific depressuring time. My problem is that how the process engineer should implement this new point into his calculations?
QUOTE
Cut it into smaller section and conduct staged blowdown...
Is it a cost effective solution? I mean having staged blowdown system+a lower flare capacity instead of traditional blowdown system+a higher flare capacity?
Thanks again.
QUOTE (jprocess @ Apr 10 2008, 04:23 AM)
You mean the last statement in API 521-2007 edition which indicate that 15 minute requirement is no longer valid and instead allowable stress level should be used as effective parameter?
This is true but all the work that a process engineer can do during evaluating the flare network is using softwares like HYSYS or Blowdown and considering a specific depressuring time. My problem is that how the process engineer should implement this new point into his calculations?
This is true but all the work that a process engineer can do during evaluating the flare network is using softwares like HYSYS or Blowdown and considering a specific depressuring time. My problem is that how the process engineer should implement this new point into his calculations?
My statement is "Allowable stress level is a function of wall thickness (wt) , internal diameter (ID), etc. Low wt with low ID may have higher allowable stress level...". Low wt does not mean it is the "weakest" component in the system...
Moreover, i don't see how my statement touch on negligence of the requirement of 15 minutes...
I would say 15 minutes is a good start but i don't see it is a mandatory requirement in API Std 521, 2007. You may read Depressuring within 15 minutes no longer applicable ?. Please correct my understanding if not true...
QUOTE
Is it a cost effective solution? I mean having staged blowdown system+a lower flare capacity instead of traditional blowdown system+a higher flare capacity?
The answer is no answer...it is subject to many parameters... but it is a way to minimize CAPEX.
QUOTE
My statement is "Allowable stress level is a function of wall thickness (wt) , internal diameter (ID), etc. Low wt with low ID may have higher allowable stress level...". Low wt does not mean it is the "weakest" component in the system...
As I mentioned in my first post my problem is that how process engineer can implement "allowable stress level" in his tools like HYSYS?!
QUOTE
Moreover, i don't see how my statement touch on negligence of the requirement of 15 minutes...
I would say 15 minutes is a good start but i don't see it is a mandatory requirement in API Std 521, 2007. You may read Depressuring within 15 minutes no longer applicable ?. Please correct my understanding if not true...
I would say 15 minutes is a good start but i don't see it is a mandatory requirement in API Std 521, 2007. You may read Depressuring within 15 minutes no longer applicable ?. Please correct my understanding if not true...
15 minutes or any other value for depressuring time is only a function of wt! (as per API)
QUOTE
The answer is no answer...it is subject to many parameters... but it is a way to minimize CAPEX.
What about using HIPPS?
QUOTE (jprocess @ Apr 11 2008, 07:45 AM)
As I mentioned in my first post my problem is that how process engineer can implement "allowable stress level" in his tools like HYSYS?!
You may program HYSYS to calculate stress in SPREADSHEET module...
QUOTE
15 minutes or any other value for depressuring time is only a function of wt! (as per API)
MAWP for 20 mm thickness vessel with 0.1 meter ID (other parameter same) would be higher than MAWP for 20 mm thickness vessel with 100 meter ID (other parameter same)...
QUOTE
What about using HIPPS?
HIPPS can be used to replace to PSV and reduce flare load...but use of HIPPS should be minimized as maintianing a HIPPS...huh...is difficult.
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