Hi, Could be a flare gas header

[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.

 

[svanels]

Molecular seal ??

The flare header must not operate below atmospheric pressure. The purpose of the flare is to vent unwanted gases out of the system. 3 Things are necessary for a fire, heat, fuel and oxygen. The fuel and the temperature are already present, the oxygen will come when the flare header sucks air.

Best regards.
Steven van Els
SAvanEls@cq-link.sr

 

[joerd]

Yes, due to natural draught in the flare stack, you can get a low pressure in the flare stack. This is why you always have to purge the flare headers and the stack.

The water seal is there to make absolutely sure that no oxygen can flow backwards from the flare stack. With it, you can use much lower purge rates in your header system. From the seal to the stack, however, you will still need a full purge.

The molecular seal (see API RP521 fig 16) is a sort of labyrinth that traps heavy molecules and as such develops a seal for oxygen. It is appropriately named a purge-reduction seal, since there is always a chance of oxygen diffusion into the stack, so you still have to purge, at say 10% of the theoretical minimum without a seal. Regards,

Joerd