5 replies to this topic

[process En]

Dear all:
i am going to adjust&optimize fuel gas which is using as sweeping gas for flare tailpipe in our gas refinery.
the problem is that two different velocity has been founded in the contractor documents for fuel gas velocity.(0.2 & 0.02 ft/sec).

1) would you please advise me how can I find right velocity ?
introducing standard or other document are highly appreciated.

[fallah]

Dear all:
i am going to adjust&optimize fuel gas which is using as sweeping gas for flare tailpipe in our gas refinery.
the problem is that two different velocity has been founded in the contractor documents for fuel gas velocity.(0.2 & 0.02 ft/sec).

1) would you please advise me how can I find right velocity ?
introducing standard or other document are highly appreciated.

process En,

As per Fluor's process manual,continuous sweep gas velocity for elevated flare would be as follow:

- Normal operation: 0.10 ft/s (0.03 m/s)
in stack/tip
- Upset condition: 3.3 ft/s (1.0 m/s) in
stack/tip

Thus,you can use this recommended practice to calculate required gas velocity in tail pipe and to know which value is correct.

Fallah

Edited by fallah, 22 April 2011 - 12:22 AM.

[breizh]

Hi ,
Let you get a copy of API RP 521 , it should help you . Let you talk to the contractor who designed your equipment , he will advise you. Safety first!

Hope this helps
Breizh

[kkala]

Useful data can be found at http://www.cheresour...n-flare-header/. According to post by gvdlans there (8 Nov 06), purge gas velocity (depending on gas nature and header diameter) can be calculated (after H Husa & API 521) as per attached purge.xls (for example).
Nevertheless the thread continues by referring to BP standard RP44-3 (design guidelines for relief disposal system), that can be now downloaded from http://igs.nigc.ir/igs/BP/RP44-3.PDF. Recommended safety factors of 2 to 5 (the latter for densities close to air), to cover high wind conditions, indicate velocities of fuel gas purging close to 0.2 ft/s, not 0.02 ft/s.
Besides it is understood that purge velocities below 0.5 ft/s are not common (BP/RP-44-3, 7.1.6); also that Fluor practices may increase purge rate 33 fold in an upset condition (strong wind?), as indicated by Fallah.
So it is concluded that between a purge velocity of 0.2 ft/s and 0.02 ft/s, correct is the value of 0.2 ft/s. But I have the feeling that even this value is low. The last time I dealt with purging (1990), I was instructed to assume a velocity of fuel gas of 1 m/s = 3.3 ft/s. A lot may have changed since then; hopefully a member with experience will check this post (providing raw data at least) and give clearer information.

Attached Files

•  purge.xls   14KB   687 downloads

Edited by kkala, 25 April 2011 - 12:03 PM.

[process En]

dear All:
for more clarification:as all knows different stream from different units connected to flare header in the refinery,and two purge gas streams is used :first connected to end point of header in each unit which I named tail pipe flare purge gas and second which injected in the stack inlet.
I think all recommendation is related to second one (stack inlet purge gas).

my question:
1) is tailpipe flare purge gas calculated as per stack inlet purge gas?
2) what is the function of tailpipe flare purge gas?

[kkala]

for more clarification:as all knows different stream from different units connected to flare header in the refinery,and two purge gas streams is used :first connected to end point of header in each unit which I named tail pipe flare purge gas and second which injected in the stack inlet. I think all recommendation is related to second one (stack inlet purge gas).
my question:
1) is tailpipe flare purge gas calculated as per stack inlet purge gas?
2) what is the function of tailpipe flare purge gas?

1) As said, my knowledge on flaring is limited and experts' help would be welcomed. Meanwhile I have not heard of purge gas injected into flare stack.http://www.gasflare..../Flare_Type.pdf.
This could explain the low velocities reported for stack (0.02 - 0.2 m/s) versus higher purge velocities heard for tail pipe (previous post), if stack received injected purge gas only. Nevertheless purge gas in stack must be the sum of the two (tailpipe+injected) (plus some water vapors if seal drum is used).
In the few cases examined by me in the past, tailpipe flare purge gas continued into the stack, passing from the water seal drum. So stack inlet purge gas=0 (as additional flow).
2)Tailpipe flare purge gas creates a positive flow into the flare to prevent air back flow from tip under conditions that could occur (which could create combustible mixtures with the gas of the header).
After a peak of hot discharge, vacuum can be created in tail pipe when gas flow becomes (almost) 0. Tail pipe flare purge gas flow prevents air ingression. It is an additional protection to the water seal drum (or other seals).
Apart from this, air can intrude from valves after a hot discharge, since valves with guaranteed no external air ingression under internal vacuum can be hardly found (if at all). So tailpipe flare purge gas prevents it, even in case of no discharge to flare.

I have heard of controlled purge flow after a hot discharge, but not seen in design or at field. Also that said controlled purge flow can be local, which indicates a sophisticated purge system and instrumentation.

Edited by kkala, 07 May 2011 - 11:39 PM.