mflr
Sorry to be so late with this reply. I just got back onto the site after some time away.
What I think I see you say is that
- the flare normally has no flame
- when you get a relief valve lifting you get a visible flame
- when the valve closes, the flame dimishes, then disappears, then returns, - disappears , - returns etc.
If this is the case then what you see is the flame burning out the residual gas in the flare. The flame will burn down to the top of the tip and then begin to burn inside. It can go perhaps 1 metre inside before there is not enough air to burn the gas and the flame goes out. Some air then goes into the tip and, in a few minutes a mixture of air and gas comes up to the top and ignites from the pilots. That then burns back down, perhaps, 3 or 4 metres before it goes out. In another 5 minutes, the mixture comes up to the top and starts again. Next time it might burn down 10 or 15 metres .. and so on.
Sometimes, some of these events become flash-backs and there is an audible (explosive) noise.
The way to prevent this is to always have a base flow rate of flammable purge gas at a rate which does not allow internal burning.
I have a representative formula published at
navigate to purge | burnback.
You amy already have some purge on the system. if this is inert (Nitrogen ?) it will eventually prevent the mixture from flashing back because it suppresses the flammability of the mixture.
If you don't think that I have correctly understood your problem, submit another post and I will try to get back to you.
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David