Rule of thumb for determining how much purge gas or sweet gas need to maintain flare header pressure 1

[Vivaldi M. Smith]

Good day ladies and gents.
I was wondering if there is rule of thumb to determine how much purge gas is needed or required to maintain the Flare System Header Pressure . Anyone knows what are the steps to make those calculations? is there ny API standard to refer to?
Muchas gracias

 

[shvet]

@georgeverghese
Thanks.

Flare / vent header purge is to prevent reverse migration of air into flare headers, which will happen if there is zero forward flow out of flare tips. Once air gets in, there is a risk of flashback or worse still, detonation in the FKOD / flare headers.

How can air migrate into flare header if there is a control valve located upstream of a burner? If flow is 0 then control valve is closed. If you are describing pipe segment downstream of control valve alone:
- this segment is not "header", it is a part of flare - what is the reason to purge header instead of final segment only?
- is it possible air backflows through small burner holes?
- is holes diameter high enough to flashback flame?
- why some burners are prone to a flashback (or may be a backflow) and need to be purged while neighboring burners inside of the same flare are not?
- is it cheaper to purge this final pipe segment once with air or inert gas to prevent flashback?
How it works? Sounds questionable.

Flare / vent header purge is maintain flame stability even in cases when forward flare stream flow is below the turndown of the flare burners.

It is a staggered flare, it is intended to prevent an accident/disaster:
- what is not ok when flame is unstable? A some small intermittent relief once a 25 years will be burned with efficiency say 98% because of capacity becomes below a turndown limit of a 1st stage burners while other large reliefs with efficiency 99.5% - and what? Is it a problem to be solved by so expensive means?
- a flare is staggered to provide stable range of capacity of burners - why during design one installs staggered burners taking in mind those have a capacity range so way that during operation those will work unstable outside of range? It is meaningless. May be cheaper to replace burners to better ones or provide more stages during design?
How it works? Sounds questionable.

 

[georgeverghese]

@shvet, Stage 1 burners are always open, while the others have shutdown valves upstream.
As you can see in my later post, I've corrected / revised the location of the min flow purge back to the upstream end of the plant flare headers - valid for both HP and LP flare headers.
If you have more questions, pls put up a new post.