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Flare Dispersion Analysis - Frequently Asked Questions (Faqs)

Flare Dispersion Analysis - Frequently Asked Questions (Faqs) Q1. What is Flare Dispersion Analysis?
A1. Flare dispersion analysis is a study of the dispersion of toxic and / or flammable pollutants (primarily gaseous) from a flare, either a ground or elevated flare.

Q2. Is it necessary to perform a flare dispersion analysis?
A2. Yes, it has now become mandatory in most parts of the world when a new industrial unit which requires a flare to burn waste gases requires to be set up and an environmental clearance is required from the local government for establishing the unit.

Q3. Is flare dispersion study required for existing old industrial units?
A3. This depends on the local laws and regulations related to environmental safety for the concerned industrial unit. From a moral and ethical viewpoint, it becomes obligatory for any organization to conduct not only a flare dispersion study for a new industrial unit it owns but also for old industrial units which may be more prone to emitting harmful pollutants to the atmosphere. Changing demographics make a compelling reason for periodic flare dispersion studies over the lifetime of the industrial unit. Most advanced nations have now made it compulsory for such kind of studies whether it be a new industrial unit, license renewal for an existing old unit or due to the changing demographics around a under construction / existing industrial unit.

Q4. Which are the reputed statutory/regulatory and guiding agencies which provide guidelines and assistance in conducting flare dispersion studies?
A4. Refer the list below:
a. United States Environmental Protection Agency (EPA)
b. European Environment Agency
c. Oil Industry Safety Directorate (OISD) - India
d. Ministry of the Environment – Government of Japan
e. Environment Agency – Gov. UK

Q5. What kind of dangerous pollutants are normally encountered from an industrial flare?
A5. Some of the most common pollutants are listed below:
a. SO2 (Sulfur Dioxide)
b. NO2 (Nitrogen oxide)
c. CO (Carbon monoxide)
d. CO2 (Carbon Dioxide) (Greenhouse gas)
e. Unburnt Hydrocarbons
f. Suspended Particulate Matter (SPM)
g. H2S (Hydrogen Sulfide)

Q6. What basic scenarios need to be considered when doing a flare dispersion analysis?
A6. Flares may operate under different conditions depending on the type of industrial unit where the flare is installed. However, the basic three scenarios that need to be considered for a flare dispersion study are as follows:
a. Normal flaring under normal plant / unit operating conditions
b. Emergency flaring under emergency or plant / unit upset conditions
c. Flameout when combustion is not taking place and cold venting from the flare occurs

Q7. Which is the most well known mathematical model used for flare dispersion studies?
A7. The most well known mathematical model widely used for air dispersion modeling is the “Gaussian Dispersion Model”. Refer the link below:

Q8. Which are the well known commercial software that can be utilized for flare dispersion studies?
A8. “PHAST” from DNV, “FlareSim” from Softbit Consultants, and “FRED” from Shell Global Solutions

Q9. Which other software can be used for flare modeling?
A.9 A freeware "SCREEN3" for point source (flare) modeling to estimate the ground level concentration can be found at the following link:
Refer the links below for some other commonly used software:

Look forward to comments and observations from the readers of my blog.


Sep 15 2014 04:54 AM

I had a question concerning zone selection in hazardous area classification:


in a plant, is it possible to have a zone 0 directly covered by zone 2?

A better formulation here would be:  is a jump among zones is allowed?   that means for example having a zone 2 adjacent to zone 0?,   or a non-hazardous area directly adjacent to zone 1? 


Thanks in advance

"Flameout when combustion is not taking place and cold venting from the flare occurs"


Sir, could you please help me understand this.

more precisely, how does a flare stack help during absence of combustion.

and what is cold venting of flare??


Great day to you, Thankyou.



Flares are supposed to operate continuously unless you have a "Flare Gas Recovery System". In other words, the flare is burning continuously using the flared gas, the purge gas (if it is a hydrocarbon or having some calorific value) and the ignition gas.


Flameout is a failure condition where the flare burning stops due to failure of the flare ignition system or due to flame not able to sustain because of environmental conditions such as high winds accompanied by heavy rain or hail. A flameout can prove to be an unsafe condition in terms of unburnt heavy gases (heavier than air) settling at ground level. In particular, toxic gases such as H2S can be a serious safety hazard. A flare dispersion analysis during flame out will provide an estimate that how much of toxic gases can accumulate ground level during a flameout condition and what safety measures need to be taken during a flameout occurrence.




thanks Again Sir.


Regards Mayuresh.

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