There was an issue that was identified in a HAZOP meeting where operations at a facility are seeing a small vacuum pressure in the flare system when no venting is taking place (i.e. purge only)
There was a disagreement with a more senior engineer regarding vacuum pressures in flare systems. It was said that it is acceptable to have a vacuum pressure in the flare system "as long as the velocity of the purge gas is maintained at or above the flare stack manufacturers minimum flow rate at the velocity seal". It was explained that the reason for the vacuum in the header has to do with buoyancy effects and the fact that flare gas is less dense than air.
This is the first time I have ever heard this and it makes no sense. The only flow I see at the flare tip when there is a vacuum in the flare header is the air ingress back into the header. I see this as a major hazard.
The answer to this question seems obvious to me but, I am would like to hear others thoughts on this.
Thanks,
Edited by KeJoSa2010, 10 March 2017 - 07:32 AM.