Flare gas header below atmospheric pressure

[ASOTOR]

Hi,

Could be a flare gas header below atmospheric pressure? We have two flare systems (both with molecular seal on top of flare stack) under this condition: one has water seal pot and the other has not.

If pressure gauge measure is true, why this pressure?

Thanks for your reply.

 

[TD2K]

Flare headers where air is kept out by molecular seals will commonly run at slightly less than atmospheric pressure if purging is done with a lighter than air gas (such as natural gas).

Essentially, you have a column of natural gas in the flare stack. As the exit pressure is atmospheric, the pressure at the base of the flare stack is under a slight vacuum due to the lower density of the natural gas.

If you have a water seal, the pressure upstream of the water seal is unlikely to be less than atmospheric. You need to look at how much vacuum you have at the base of the flare stack and the depth of the water seal to estimate the pressure upstream of the wate seal. Once you are flaring, you will obviously be above atmospheric pressure.