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PURGE AND MINIMUM FLOW

ELEVATED FLARE
FLAMMABLE PURGE GASES AND "BURN-BACK"

For most flares, "Burn-back" flow and normal minimum baseline purge are NOT the same.

"Burn back" occurs at  extremely low rates  (a representative formula here) when the normal flare flame can regress into the tip under the effects of buoyant forces and localized wind turbulence.  Operational burn back can occur in all flares which are

-	operated or purged with flammable gas,
-	in service
	and	exhibiting a visible flame
	or	furnished with continuous pilots.

The basic phenomenon of "burn-back" is created by atmospheric air which enters the top of the flare tip and flows downward, counter to the gas flow, at the same time that the gas flow is flowing out of the flare.  For this to happen, the gas flow must be so small that it cannot generate turbulence

Burn-back can be a problem as it may overheat the tip material and can build solid carbon deposits internally. The hot flue gases produced, however, tend to be disruptive to air inflow patterns and it is not easy to predict burn-back using the normal theoretical models.

PRACTICAL GUIDE FOR ASSESSING BURN-BACK

A good practical guide for empirical determination of purging requirements against burn-back, is to use the visual flame regression condition for assessing stacks which are burning at purge only rates.

If there is a clearly visible flame above the top of the flare under purge only conditions, the flame should be "safe" with  no "burn-back" condition.
The visible flame may or may not make smoke depending on the concurrent smoke suppression treatment. If smoke is seen it will probably involves turbulent eddies with the smoke production from the outer edges of the flame.

If there is no visible flame or only the tip of a visible flame, then burn-back may be occurring.  In this  case, the hydrocarbon flame burning inside the tip will probably make smoke due to incomplete combustion.  Smoke produced in a tip due to burn-back will probably appear as a narrow column with either no supporting flame or from the extreme end of a very small flame.

The probable presence of smoke in each  of the above conditions can be confusing to operators who will probably react by  increasing the smoke-suppression steam for smokeless flaring.  In the "burn-back" case, this may actually exacerbate the burn-back problem rather than solve it.

Some flare tip designs permit augmentation of the minimum flow using steam in a center nozzle.

Smaller tips do not frequently display pronounced burn-back problems.