Cold Vent Re-ignition?

[cempass]

Dear Sirs,

I am working in the design of a cold vent system. We have estimated just two CO2 bottles (100# each) for the flame snuff at the end of the plant blow down event. The operation procedure is like this: Case Plant in the blow down scenario. 2.- Probable ignition of the flame of the stack. 3.- Leave the flame in the tip and Wait until the pressure of the plant reaches half of the initial pressure. 4.- Open the CO2 valves to snuff the flame. 5.- let relieve the reaimaining gas of the plant trough the stack just to avoid any flash get back into the stack.
My concern is about re ignition of the gas due to the high temperature of the tip. I know the ignition gas temperature is arround 1000 F.
I have never seen a cold vent re ignition of the flame since I ussually do not let the flame last more than 15 - 30 minutes. But in this event the flame can last up to two hours before I can open the CO2 snuff valves letting the tip reaches high temperature. It is quite dificult to estimate the TIP temperature when the remaining " purging " gas reaches the TIP. Is my concern correct?

I appreciate your help in this regard,

Sincerely,

CEMP

 

[Flareman]

Cempass

I made some comments in thread124-163305 which you may find useful relative to your snuffing scenario generally.

Regarding the possibility of reignition from hot metal, my sense of this is that it's a very low probablity. In most cases when there is a flame on a steel stack, only the pieces which are sticking into the flame normally become visibly hot, things like flame retention evices and pilot nozzles etc. You probably don't have any of those.

Visibly hot conditions usually indicate a metal temperature range of 800 - 1400 degF, so there is theoretically the possibility of reignition. The question is, how long can the metal hold that temperature after the flame is first extinguished. It can be estimated from heat transfer rates.
I doubt that it's very long and you can control the CO2 input period so that it precludes the reignition.

I don't have direct experience of your specific case but I have never managed to ignite a stream on a stack without having a concurrently active means of ignition like another flame or a concentrated energy source.

David