flare header purge

[wenchop]

At the flare tip, it is clear what purge rates to use, depending on the seal if it is velocity seal or molecular seal or just a utility flare.

However, if there are a number of headers and subheaders in the relief system, each has to be purged, both at intermittent rates (e.g. during startup to displace air) and continuously (to prevent air ingress and flashback).

API 521 talks about purge rates at flare tip, but I cannot find guidance on purging flare subheaders and headers. If we assume API 521 continuous purge velocities, and we have many subheaders, the purge rate at the tip will be the sum of all the purge flow rates at each subheaders, and this is more velocity than what is required. Is this a waste of fuel gas/nitrogen or is this necessary?

Please advise, thanks.

 

[TD2K]

Long, big flare laterals are usually purged in my experience but I've not seen much in the way of guidance when does a flare lateral need to be purged? For example, individual PSV lines are not usually purged so when these start to combine, at what point should you consider purging them? I would normally not look at purging a line unless it was over say 150' length.

I had to install a new 16" flare lateral to pick up the discharge from 2 PSVs that used to go to atmosphere. The lateral was maybe 100' long, maybe somewhat more. The 16" lateral was tieing into the main flare that was 24" at that location so it was a good sized line in terms of comparison. We ended up installing a purge though I think it was questionable because if we had run the PSVs separately, I don't think we would have purged those lines. The existing flare lateral for that unit was around 12" I believe but substantially longer (several hundred feet) and I would definitely expect to see a lateral like that purged.

For just purging O2, I usually use between 0.05 to 0.1 ft/sec.