Reset this site with frames
Reset Main Menu / Purge / Elevated Flare / Purge

PURGE AND MINIMUM FLOW

ELEVATED FLARE
PURGE
Many elevated flares spend most of their active service in pseudo-dormant states. In these conditions, elevated flare stacks are generally purged continuously with only a small amount of gas in order to protect them against the possibility of air infiltration through the tip which could lead to flame flash-back and explosion or detonation.

Purge can also represent protection against corrosion or other undesirable conditions, but chiefly addresses the possible mixing of flammable gases with atmospheric air due to buoyancy forces in the stack, bearing in mind that an ignition source may exist at the flare pilots.

 

FLARE OPERATING STATES

Various levels of operation may exist which can require alternate purge conditions. These are outlined below.
 
(1) Normal "safe" operation of the flare
will result from maintaining a constant flow of gas in the system in order to exclude atmospheric air from the stack and also avoid "burn-back".  This is the case when
- all pilots are operative
and Flammable gas flows to the tip at or greater than the minimum "burn-back" rate ,
or flammable gas flows to the tip at minimum baseline rate with concurrent relief gas to make up to or exceed the minimum "burn-back"  rate.
or non-flammable gas flows to the tip at minimum baseline rate with concurrent relief gas to make up to or exceed the minimum "burn-back"  rate.
 
(2) A "stand-by" condition, exists when
- all pilots are operative
and non-flammable purge gas at baseline rate is used to seal the flare.
   
(3) A "dormant" condition results from interruption of purge flow at any time such that air can enter the riser from the top.  In this case,
- if pilots are burning,

any flammable gas flow should be immediately increased, at least to a safety rate,
- with no pilots or operating ignition source,
a pre-purge or post-purge sweep condition should be adopted as appropriate.

The individual terms describing the alternative purge conditions, baseline rate, burn-back rate, safety rate, pre-purge and post-purge are all separately explained.