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PURGE AND MINIMUM FLOW

ELEVATED FLARE
"BASELINE" PURGE

The continuous BASELINE  purge rate for a flare stack is the minimum, base level flow of gas which is needed to just  resist air penetration from the top of the stack and hold an acceptable oxygen level within a "safe" distance of the top of the stack.  The minimum calculated baseline rate is NOT, usually, the same as the minimum flow condition which will ensure satisfactory day-to-day operation of a flare. (see operating states)

The basis of the continuous baseline rate is to ensure that the stack will remain "safe" during periods of inactivity and when the flame is not present on the stack.
It is used so that no dangerous flash-back will occur in the event of an unforeseen ignition condition, such as lightning or accidental operation of the flare ignition system.  It also provides protection against off-gassing of residual liquids in the flare system and associated vessels or inadvertent release of flammable gas into the relief system due to operator error.

When the continuous baseline rate is used, any additional relief flows which occur during operational periods will augment the base rate.  The combined flows should be checked against any other operating states and requirements such as burn-back.

The calculation to determine an appropriate baseline purge rate for any given flare stack includes the desired oxygen concentration in the flare, the distance into the stack at which that concentration should occur, the stack diameter and the buoyant diffusion characteristics of the gas in the stack.

In many cases, the base baseline rate can be reduced by the use of some assisting device such as a "gas seal".  In these cases, the effect of the "seal" is only valid in the system upstream of the seal. The downstream tip and any stack sections will still be subject to some infiltration and should be treated accordingly.

Using Nitrogen as the Baseline Purge

There is an argument that Nitrogen should be used as the baseline purge because

• it is often more cost effective to supply a small amount of N2 purge than a large amount of fuel gas. ( because the fuel gas rate needs to be sized to avoid burn-back, which is invariably greater than the baseline rate.)

 Additional benefits are

• the removal of the continuously visible flame   AND
• the smoke suppression flow (steam ?) can be reduced to a minimum ( whereas with flammable gas,  it is probably turned up so high that it hides the flame, and may actually put it out).

These arguments are, generally, true  BUT  they also needs the supporting argument that
the specific flare in question can safely be operated as a dormant flare with no flame, because

• there are "never" any relief gases going to flare ,          AND/OR
• if they do, the pilots are waiting to light the gas               AND
• it can't get there unless it has a total CV greater than 200 Btu/scf.  This is in acknowledgement of the US requirements in 40 CFR 60.18, and must include the N2 from the continuous input  AND  the stored volume of N2 which is discharged immediately preceding the relief.  This is probably the most difficult argument to make and is the one which "kills" most plans to use nitrogen purge.

Anyone looking to change the manner in which an existing  flare is purged needs to start with what is actually happening now.  A great many purge rates will have been calculated as header purge to sweep the sub headers and prevent pockets of potentially corrosive material.   It is not realistic to review the flare independently of these sub-headers with regard to purge.  It's a comprehensive solution.  Only after all the sub headers and unit purges are de-bugged, can the resulting accumulation be viewed as the flare purge.  Then, it's either not enough, or too much, for the minimum needs of the flare, with or without a gas seal.