Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

IM PROVING FLA RE DESIGN 

A TRANSITION FROM ART-FORM TO EN GINEERIN G SC IEN C E

DAVID SHORE

FLAREGAS CORPORATION

NANUET NY. 10954 USA

Ph: 1-845-371-2519

davids@flaregas.com.

Prepared for presentation at

AFRC-JFRC

2007 Joint meeting

Waikoloa, Hawaii

October 22 - 24, 2007

Art_to_Science - AFRC.wpd 1 of 33
Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

ABSTRACT

Many of the day-to-day activities surrounding Flaring, at both the project design and 

operational stages, are vague and apparently outside the control of the Flare Engineer. 

Design specifications usually only cover a single case out of a multitude of possible 

compositions and flows. In service, flow rates, compositions, wind and w eather are largely 

decided by h ap p en stan ce and the control of utility addition is m anaged on the basis of 

visual perception. After the fact, Environmentalists try to estim ate the extent of the 

environmental dam age and combustion scientists develop theories and models which may

help to predict the extent of this impact.

The only point w here there exists an opportunity to affect the final outcom e is before the 

cycle even starts, when the main design concepts of the flare tip are developed. 

However, despite the best efforts of combustion scientists and the best intentions of som e 

m anufacturers, the majority of flare designers Worldwide still practice a design method 

which, in the view of the author, which ow es a s much to experience and "black art” a s it

does to scientific knowledge.

For the flare designer to achieve a fractional p ercentage gain in efficiency, or to reduce the 

ch an ces of generating hazardous pollutants, there is a need for a generalized developm ent, 

and wider dissem ination of param eters which are in control of those issues. This involves a 

serious effort by the Industry, together with the assistance of academ ia and the use of 

advanced technical skills. However, along with this effort com es the responsibility to 

exam ine the state of the current baseline. In short, the industry h as to m ove out of the arts

into the sciences.

This paper uses the three m ost common process related aspects of Flare engineering, 

(flame size/shape, minimum tolerable flammability and maximum discharge velocity) to 

highlight the com m on u se of "rules of thum b”, which may rep resen t flawed param etric input 

and su g g e sts alternative and m ore scientific ap p ro ach es which m ay be beneficial in 

improving an the global perform ance of flares, presenting e ach in a m anner which m ay be

recognizable and useful to most engineers.

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Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

IM PROVING FLA RE DESIGN 

A TRANSITION FROM ART-FORM TO EN GINEERIN G SC IEN C E

DAVID SHORE ©

FLAREGAS CORPORATION, NANUET N.Y. 10954

1-845-371-2519 : davids@flaregas.com

PR E FA C E

The combustion sciences encom pass a wide range of disciplines including automotive engineers,

engine designers, rocket engineers, heating engineers, furnace designers and many more. Within

this prestigious field, Flare engineering presents a rather lower profile.

As a practice, g a s flaring has apparently little associated complex technology and the two primary

techniques associated with Flaring involve

- being able to ignite an indeterminate stream of gas of largely unknown composition

and

- causing the resulting flame to burn without too much, obvious, environmental impact

Once the flare flame is burning, there are very few aspects of control and the applied principle is

usually to be well away from the flame and just watch it burn. Most of the better-known design

functions surrounding flaring are dedicated to allowing the flame to burn safely whilst unattended and

deciding just how far away personnel should be for safety.

Throughout the entire cycle of a flare, the overall ability to influence the eventual outcom e often

seem s vague.

At the beginning of any flare design process, most design specifications cover a single c a s e out of

a multitude of possible compositions and an almost infinite number of possible flows. Based on this,

it falls to an applications engineer to decide which piece of equipment will be the most likely to meet

not only the written specification but also the implied utilization, whilst remaining economically

attractive to the prospective customer.

Later, when the flare is in service, hardly anything in the process can be truly controlled. The flow

rate and the g a s composition, the wind and the weather are all decided by happenstance and the

control of utility addition (steam or air for example) is in the hands of an operator w h o se primary

motivation is avoidance of a visible emission and who, in consequence, will probably add sufficient

utility to completely hide the flame and significantly impact the concurrent combustion efficiency.

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Overall, a flare’s operational performance is generally evaluated in terms of visual perception of the

flame regardless of the concurrent combustion efficiency. Only after the fact, and when serious

pollution concerns may have b ecom e apparent, environmental scientists try to estimate the true

extent of the environmental damage, and in this spirit, combustion scientists then develop new

theories and models which may, someday, help to predict the extent of this environmental impact.

The only point within this entire cycle where there exists the opportunity to affect the final combustion

efficiency is before the cycle even starts, when the main design concepts of the flare tip are

developed but, even when potential design improvements may be introduced, the costs of

development and testing of potential new designs tend to impede widespread investigation of multiple

design candidates.

If the flare designer is to m a ssa g e new or existing designs to "squeeze” a fractional percentage gain

in combustion efficiency, or to reduce the ch a n ces of generating harmful pollutants, there is a need

for a generalized development and improved dissemination throughout the industry as a whole, of

the data and parameters which are in control of those issues. This involves a serious effort by the

Industry, together with the assistance of academ ia and the u se of advanced technical skills.

However, along with this effort c o m es the responsibility to exam ine the state of the current "baseline”

which, in the view of this author, still follows a practice which o w e s a s much to experience and "black

art” as it d o es to scientific knowledge. In short, the industry has to m ove out of the arts into the

sciences.

THE NEED FO R EN GINEERIN G “ QUALITY” PA R A M ET ER S

Any scientific input into flare design n eed s to be in a form which can be easily understood and

interpreted by those engineers responsible for creating or modifying those designs. Although

advanced computational analyses are available, these frequently require advanced operator skills

or a case-by-case investment, which both mitigate against common usage.

In the day by day practice of engineering, "real world” dimensions and properties are very significant

and, although an enormous amount of excellent scientific thought and combustion research is

already published, much g o e s largely unnoticed by the flare industry in deference to a widespread

u se of engineering "lore” and the "rule-of-thumb” together with a general acceptance of the "statusquo”.

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

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Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Possible contributors to this disconnect are

- the need within the scientific community to express complex scientific logic using

dimensionless terms that accurately describe the various phenomena and characteristics.

These, although accurate in nature, are som etim es indeterminate in value;

- the need for basic research to simplify research m odels to the least number of variables, often

making th ese models unrepresentative of "real world” equipment;

- the use of higher order equations and forms which require computational solutions;

- the engineer’s day-to-day involvement in project work which precludes the opportunity to

spend time in translation of the dim ensionless forms into "real world” numbers;

- the commercial requirements of engineering research which frequently condition the results

to prove or disprove a pre-defined commercial solution and, consequently, yield limited or

slanted results;

- the difficulty of obtaining reliable and meaningful large scale data.

Engineers frequently rely on pre-formulated design parameters and treat such as proven methods.

Consequently, useful engineering procedures need to be accurate and "solution-oriented”.

S o m e combustion parameters which apply to flaring are already widely recognized. Most engineers

will have a good working understanding of stoichiometry, calorific content and flammable limits and

th ese features are already used in so m e c a s e s of formulated design and can be easily incorporated

into others.

In this paper, the author d isc u s se s a few of the most common a sp e cts of flare engineering where

certain existing "rules-of-thumb” need to be re-evaluated in order to provide a meaningful and

scientifically sound baseline on which to build future data and standards.

Som e alternative and more scientific methods are proposed and other useful engineering

parameters, yet to be properly evaluated, are identified and recom m ended for further investigation.

THE FLA RE FLAME

The key to any meaningful design development, must be an understanding of the Flare flame itself.

This is, almost exclusively, a diffusion flame. Diffusion flare flame characteristics are listed below

in terms which might be widely understood within the general engineering community, cross

referenced to the enclosed Figure 1.

Art_to_Science - AFRC.wpd 5 of 33
1. Raw G a s leaves the end of the Flare pipe (Flare tip);

- Liquids, entrained in the flowing g a s stream are carried into the flame zone;

2. Unburned g a s mixes with air, by diffusion and/or local turbulence;

3. The flame can be ignited and establish when the local mixture c o m e s within a flammable

range;

4. The base (edge) of the flame establishes as a stable or unstable interface;

5. The down wind g a s column (flame / plume) develops a complex turbulent form

conditioned by the momentum forces of the gas discharge, the buoyancy forces

produced by the heat energy release and the eddies generated in the wind affected

"wake” of the flare pipe (Flare Stack);

6. The outer zo n e of the g a s column burns, rather like an envelope, wherever ga s can mix

with air in flammable concentrations;

7. The core of the gas column b ecom es hot due (mainly) to radiant heat from the burning

envelope;

- Entrained liquid droplets com m en ce to evaporate from the droplet surface;

- G a s e s and vapors in the core of the g a s column dissociate into radicals when the

local temperature creates molecular instability;

- As a function of the various transport properties of the liquid, molecular dissociation

may com m en ce in droplets prior to complete evaporation, fashioning new molecular

forms with carbon chains resembling polymers;

8. The distribution and concentrations of free, transient and unconverted radicals depend

on the local temperature, concentration of the radicals and the residence time since

dissociation;

9. With (residence) time, so m e free carbon radicals (atoms) combine to form solid

particulate (soot) [ this may also happen with other elemental solids such a S, Al, Si ];

10. Solid particulate may not burn (decay) completely during p a ssa g e through the zone of

oxygen availability (flame) resulting in solids em issions (commonly soot);

11. Particulate or unconsum ed liquid droplets, beyond a particular size and density, may lose

upward momentum and fall from the flame zo n e a s cold deposits or (som etim es) as

"burning rain”;

12. Fuel radicals b ecom e converted to flue g a s e s in the flame zone;

13. Unconsumed fuel pockets migrate to the flame ed g es and contribute to combustion

inefficiency;

14. At the e d g e s of the flame envelope, fuel radicals eventually reach a limiting concentration

and/or temperature which inhibits further conversion to flue gas;

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

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15. With increasing flame length, accumulating flue g a s e s increasingly impede diffusion of

ambient air into the core of the flame;

16. The overall dilution increases with downwind travel of the g as column, as a function of

atmospheric turbulence and wind speed;

17. Combustion c e a s e s at the end of the flame a s fuel radicals finally reach a limiting

concentration and/or temperature which inhibits any further conversion, adding to overall

inefficiency;

18. Thermal em issions from unsymmetrical molecules and black-body particulate produce

a zon e of high radiation which d ecrea ses with distance from the flame;

19. Complete and incomplete products of combustion combine with more air in a downwind

plume of hot flue gases.

Each of the foregoing characteristics is a candidate for individual examination by formula, but

conventional flare engineering a d d resses very few of them directly, and those which are addressed

may be covered by an unsophisticated or unscientific approach.

Computational Fluid Dynamics (CFD) d o es attempt to include many of th ese characteristics within

combustion models but, due to the degree of complexity n eed ed for the matrix, most tend to be

included as simplified parameters which require resolution by direct input of a corrective constant and

thus fall prey again to a level of subjective interpretation.

B ec a u se of the haphazard nature of flaring, flame models for general industrial u se do not need to

be critically accurate a s long a s they truly reflect specific selected conditions with a good probability.

However, within the industry as a whole, there are at least twelve different and independent,

published flame m odels( 1 )( 2 ). Each is intended to accurately represent a true flare flame for the

purposes of spacial and safety calculations but yet no two m odels s ee m to produce the sa m e results

for overall size and sh ap e of the flame and at least four different approaches are used for estimation

of radiant output. At the time of writing, there se e m to have been no attempts m ade to reproduce,

investigate or compare any of these flame models using CFD or by other independent large scale

study.

The API has implicitly endorsed two of the methods by including them in the commonly used

reference guide, API R P-521( 3 ). Neither model gives the s a m e results and the more complex of the

two (Brzustowski - BRZ)( 4 ) is fast becoming a de-facto industrial standard, despite having little

theoretical underpinning and generating flame shapes which do not reflect true conditions, insofar

as increasing wind speeds generate longer flames, contrary to the observed effects of wind

turbulence.

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 7 of 33
Although the flame may be represented pictorially, a s in Figure 1, and be satisfactory for general

discussions, an alternative scientific concept( 5 ) can be represented by the idealized presentation of

Figure 2 which shows, mathematically and theoretically, how g a s e s emerging from an orifice (flare

pipe) mix into the downwind atmosphere and create the flammable envelope around the central g a s

column. This provides a generalized flame sh ap e which is largely confirmed by full scale flame

temperature profiles generated by a DGMK( 6 } report, although fluid dynamicists frequently need to

break down the analysis to a much smaller scale when using more localized descriptors of turbulent

motion and mixing.

Only one of the various published models addresses the stream continuity of the emergent flue

g a ses to enable consistent downwind calculation of flue gas concentration and temperature. That

model is endorsed by this author (Buoyant Flame - BUO)( 7 } and derives largely from the standard

dispersion equations shown in Figure 2. The BUO model, which is also built around the familiar

Briggs( 8 } plume rise model, generates a variety of intermediate values which are used cumulatively

to model the flame and plume rise. T h ese include a value for total dwell time to the end of the flame,

which may be useful as a parameter in determining probable destruction efficiencies, and separate

com ponents for the three flame descriptors introduced by Gogolek( 9 \ Buoyancy, Momentum and

Wake influence.

The complete BUO procedure, which is reproduced in Figure 3, is simple to follow but lengthy due

to the u se of a number of parameters which are a function of the concurrent atmospheric stability and

which generate different flame profiles according to the appropriate stability condition.

FLAMMABILITY O F THE G A S STREAM

Flammability of the gas stream depends on the intrinsic characteristics of the gas composition and

is an essential aspect of g a s e s relieved to a flare. All g a s e s known a s fuels are intrinsically

flammable by definition. Many mixtures relieved to flares however, contain non-flammable

components and the mixtures themselves cannot always be guaranteed to be flammable.

The most commonly quoted "rules-of-thumb” in flare engineering relate to the flammability of a lean

stream and invoke heating value limits which derive directly from EPA Regulation 40C F R 60.18( 10 }

in which the minimum permissible heat content of g a s mixtures is set at 2 00 Btu/scf for raw g a s e s

(unassisted flares) and 300 Btu/scf for assisted flares.

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 8 of 33
The original Pohl( 11 )( 12 ) research leading to the regulation simulated lean g a s e s primarily by diluting

propane with nitrogen and the values selected for the subsequent regulation result directly from the

choice of th ese test g a ses. The published( 13 ), flammable properties of the tested mixture (propane

/ nitrogen) are shown in Figure 4. It is clear from this presentation that, although 200 Btu/scf allows

adequate flammability for a propane/nitrogen mixture, it is quite inadequate for a propane/carbon

dioxide mixture, which probably requires a value closer to 350 Btu/scf.

This inadequacy has since been recognized, but unresolved by Pohl et al( 14 ). Despite their view that

the presence of Inert com ponents has a weak effect on lower flammable limit, the effect is strong

when consideration is extended to the entire mixture, being generally in inverse proportion to the

concentration of flammable ga s in the mixture. B ec a u se the lean limit concentration, CL, is a flame

temperature based limit, the thermal properties of the diluent and other constituents play an

important part in determining the low limit for gas/inert mixtures. Shore( 15 ) proposed a unifying

property (Nitrogen Equivalent [NE]) providing relationships based on specific heat at representative

temperatures for the common diluents carbon dioxide (1.82), water vapor (1.35), sulfur dioxide (2.1)

helium (1.07) and argon (0.65). The characteristic is currently well recognized and the sa m e principle

is presently embodied in the International Standard - ISO 101 5 6 ( 16 ) a s a factor K. More recent works

by Molnarne et al ( 17 )( 18 ) have expanded th ese values for other diluents and mixtures.

This approach allows the easy construction of a flammable envelope for the generalized ca se of

flammable-inert mixtures, as shown in Figure 5. For a diffusion flame. the interpretations of minimum

mixture flammability must be made from the curves using the points where the gas/inert ratio lines

cross the theoretical (stoichiometric) line. This ensures that the mixture is able to combust

completely, with so m e e x c e s s air, at all lesser inert ratios.

When using published flammability limits and envelopes in this way it is w ise to also include a

minimum Factor of Safety (FOS). The implied FOS a s currently used by the EPA regulation (for

propane) is 1.5, based on the extreme concentration of flammable g a s in nitrogen, and this would

se e m to be just satisfactory, although a FOS of 1.75 would agree with that used by the USCG( 19 ) for

oxygen concentrations, in regulations governing flares for tanker operations.

The suggested procedure to determine the minimum practical heating value for a diluted mixture is

shown in Figure 6 and representative values for various simple mixtures are given in Figure 7. For

mixtures with a number of flammable g a s e s , the lean limit of the flammable mixture com ponents may

be determined using the common, Le Chatelier, approach.

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 9 of 33
Applying the specific heat of water to the flammable envelope of propane, in this way, gives a limiting

flammable condition (FOS = 1.5) of roughly 2 65 Btu/scf for propane in H2O. This gives so m e credence

to the use of the enhanced EPA minimum value of 300 Btu/scf for (steam) assisted flares as

developed from a propane based flame.

Generally, mixtures which are already approaching their limiting heating value generate relatively low

flame temperatures and are not likely to yield large quantities of solid particulate needing additional

treatment for sm oke suppression. Nonetheless, when th ese are flared in Flare tips which are fitted

with sm oke suppression capability, the potential additive flow should be considered a s an appropriate

inert diluent within the foregoing calculation. However, there is no recognition of the assist steam

rates by the current EPA regulation, and this allows a g a s with the regulatory minimum required heat

content for assisted flares, of 300 Btu/scf, to be easily overpowered by steam even with the

concurrent and mandatory pilot flame present at all times. Furthermore, the flare operator’s desire

to hide a flare flame by over-steaming may commonly lead to unrecognized and inadvertent flameouts, a condition which is indicated by a visible steam plume from the flare, signaling the probable

venting of unburned hydrocarbons. The Pohl reports clearly show an impact on combustion

efficiency as steam rates increase. Figure 9 shows this trend for the specific test condition and,

whilst the efficiency value here never falls below 99%, the inference of a further increase in steam

rate is apparent.

As more complex chemicals enter common usage, the need for flare engineers to understand the

interactions of flammable conditions b eco m es more significant and the simple procedure described

here can be applied generally and with equal confidence to all relieved materials.

FLAME LIFT, STABILITY AND INEFFICIENCY

The maintenance of a stable flame e d g e or b a se is essential a s a factor in reducing potential

em issions of unconverted materials. A lifted flame is a precursor of flame blow-off and total flame

loss from a flare can be a catastrophic environmental event.

There is general acceptan ce that flame stability is so m e function of discharge velocity and, within the

USA, flare engineering practice addresses flame stability largely by using the velocity-limiting

formulae which are to be applied as a characteristic of the EPA regulations noted previously and

discussed in Figure 8. The EPA formulae are only applied to g a s e s with a heating value less than

1000 Btu/scf, being therefore primarily hydrocarbon mixtures which are m ade lean by the inclusion

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 10 of 33
of inert content in the manner of the original Pohl research. An upper limit of 400 fps is arbitrarily

applied for all other c a s e s and the minimum value need not be less than 60 fps. Under the EPA

regulation, three separate and distinct rules apply as a function of the style of flare and hydrogen

content of the gases. As these formulae were derived empirically and independently, they have

minimal scientific commonality or basis.

Furthermore, the rules technically preclude a whole class of flares in common use, which are

specifically designed to operate beyond the normal constraints of the regulations.

When the EPA regulations are not a controlling factor, a completely different set of "rules of thumb”

are often applied across the industry. Following the recommendations of API, allowable exit

velocities are related to sonic sp e ed in the gas, on the basis of much earlier papers by Hottel( 20 } and

Kent( 21 ). T h ese generalizations implicitly a ssu m e a rich hydrocarbon g a s but have no codicils

w hatsoever regarding the energy content, flammability or dilution of the g a s in question.

This seemingly haphazard application of wildly different approaches to discharge velocity,

understandably c a u se s confusion amongst engineers who are not intimately associated with the

Flare Industry.

As described previously, a large diffusion flame burns as a quasi envelope of combustion around an

unburned core of gas. Much of the air used for this combustion is entrained around the root of the

flame, where the jet velocity is the greatest and the thermal turbulence is the least. This leads to a

disproportionate ratio of air to g a s in the outer envelope of the mixture in this zone. Since the stability

of the flame relies on its ability to "anchor” at the rim of the jet, the local mixture conditions are critical

to flame stability and, if this mixture is diluted beyond its lean flammable limit [CL], the b a se of the

flame will lift away from the jet and attempt to re-establish at s o m e distance above the jet. This is

a condition known as flame-lift. Once the flame is lifted, its stability is questionable and it can,

eventually, be sufficiently disrupted that the flame can extinguish in a condition termed blow-off.

Localized conditions of flame stability have been examined recently by many researchers, most

notably by Buckmaster( 22 \ but there have been several other significant studies( 23 )( 24 )( 25 } relating

to instability characteristics of the flame ed g e and co-flowing air and g a s streams. There s e e m s to

be agreement that the velocity of flame-lift and blow-off of such streams can be described using

formulae which typically incorporate discharge velocity, jet diameter and flame speed.

Karbasi and Wierzba( 26 } show ed a general pattern of stability for co-flowing g a s and air streams

which clearly indicates the influence of increasing air dilution on flame stability. The pattern of this

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 11 of 33
instability is reproduced in Figure 10. From this representation can be s e e n a relatively low

d ep en d en ce of the unstable flame conditions on co-flowing air stream velocity, which su g g e sts that

the governing conditions are related very locally to the flame e d g e and are dominated by the g a s flow

itself. A stronger d ep en d en ce of the blow-off limit on co-flowing air velocity may be an indicator of

a critical total air volume and could even possibly be related to the dimensional constraints of the test

equipment. A similar pattern is discernible in papers by other researchers( 27 ).

Although the published formulae for blow-out have a good correlation with test values in the small

scale, the u se of flame s p e ed a s a parameter, which is not commonly available as a single value in

engineering tables, and an un-measurable flame diameter factor both combine to make them

unusable for flare design engineering. However, the published results for lift-off of various mixtures

by several authors provide a useful cross reference to facilitate further investigation.

Pohl et al. also considered flame lift in the empirical EPA studies of flame efficiency. That report

concluded that the velocity n eed ed for satisfactory efficiency, together with no lift-off, varied only with

g a s energy content. However, the Pohl results appear to contain inconsistencies in that they yield

different patterns for the two primary flammable g a s e s in the study, propane and hydrogen sulfide.

S o m e of th ese results are presented in Figure 11, together with other results and the relevant EPA

parameters. There seem s to be no clearly apparent relationship between the various results

them selves or the EPA regulations which are intended to reflect them. As with the matter of

minimum heating value noted previously, these discrepancies are recognized but unresolved.

A careful re-examination of the Pohl data, available in published EPA reports, show s that the

variations in flame stability with g a s composition do not fall into the pattern described only by the

heating value (as embodied in 40 CFR 60.18) but can be more easily explained by derivation from

the minimum energy transfer or temperature function for the flame. This analysis suggests that a

re-statement of stability using the characteristics of the lower flammable limit for the total mixture

[ CLM ], and including any diluents in the manner previously described, may provide a suitably

universal parameter to permit prediction of unstable discharge velocity for any g a s or mixture in a

flare tip.

The analysis in this paper u se s the m a ss ratio of air to g a s [RAM ] at the lower flammable limit of the

mixture a s one parameter. A separate analysis of this parameter against data( 28 ) from

hydrocarbon/CO2 mixtures finds a good correlation between this and flame s p e e d providing a

confidence level for consequent suppression of the flame sp e ed a s a required input in the manner

Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 12 of 33
Improving Flare design - From Art to Science

David Shore / AFRC-JFRC 2007

of the Wierzba formulae. A very similar parameter w a s used by Johnson and Kostiuk( 29 } in unifying

a number of other results for wind-blown inefficiency of various lean mixtures.

This analysis also introduces the parameter UO/p/DO (velocity/jet periphery) a s a rearrangement of

the Wierzba formula. This develops a relationship with dimensions of Hertz and resem bles the

acoustic Ring Frequency (Mach1/p/DO) which defines a critical vertical shedding frequency for stream

eddies at the point of discharge and may, therefore, have a bearing on the vertical stability of the

flame at the sa m e point.

When the interpolated Wierzba results for lift-off are plotted together with the modified Pohl results

on the sam e basis, they show a remarkably close correlation, despite being performed on different

g a s e s and at vastly differing scales. Additional work on propane dilution of hydrogen flames by Wu

et al( 30 ) displays a similar correlation. All th ese results are plotted a s Figure 12.

The resulting formula of the common line becom es

Uo/p/Do = [Ram / 6.85 ] - Hz

where Uc

Do

RAM

p OM

Pa

CLM

stable discharge peak velocity

discharge diameter

air/gas m a ss ratio at lean limit

((100 - C l m ) / C LM ) ( p OM / p A )

density of discharge mixture

density of air

lean flammable limit of mixture

feet per second

feet

wt/wt

lb/ft3

lb/ft3

%

5

The empirical constant [ 6.85 ] may contain elem ents of viscosity and other transport properties of

the g a s and air, however, it is clear from the close correlation of the curve, that variations in th ese

values may be relatively insignificant in the larger scale of the problem.

As the basis of this relationship is the Lower Limit of Flammability [ CL ] a s used previously in

determination of the minimum recom m ended heating value, u se of the s a m e safety factor [ FOS ]

s e e m s to be appropriate. For FOS = 1.5, this would result in a diameter limitation, for an unstabilized

flame, based on the formula

Do = [ 805 * qo / (Ram)5 ](1'31

where qO = volumetric flow - cfs

Art_to_Science - AFRC.wpd 13 of 33
The involvement of volumetric flow in this manner raises a question of volumetric normalization. As

the conditions of flammability which control this feature will be dominated by ambient conditions, use

of published values of CL at the conventional temperature of 7 7 O F [25O C] is suggested a s a basis

for calculation of RAM. However, the initially cautious approach to sizing, in the a b se n c e of contrary

indications, would u se the actual design g a s temperature when determining g a s density [ pO ] and

subsequently impacting the diameter in proportion to (TO R)(4/3).

The included figure also displays a marker showing a representative refinery fuel gas and a typical

line indicating the condition of Mach 0.5 in a 30 inch flare tip, for the various mixtures represented

by the points on the graph. It is interesting to note that this marker and the intersection of the two

lines appear in the s a m e general stability region, providing a justification for those prior results which

indicate a satisfactory relationship between Mach number and stability for common undiluted

hydrocarbons.

Clearly, a s this result is derived from interpolated results, additional independent verification by other

research is desirable and is strongly encouraged.

There are several associated conditions surrounding this specific result which should be reproduced

in further work

- the condition represented is flame lift/instability, not blow off;

- the flame is not stabilized by active or passive devices;

- no pilot or continuous re-ignition is present;

- the downward propagation limit should be used;

- mixtures with inert g a s content should be corrected using the Nitrogen Equivalent (NE) to calculate

the true value of CLM_DOWN for the entire mixture.

The potential predictive ramifications of this relationship are significant:

- large flares may be operated at higher velocity than small flares on the sa m e gas;

- many large flares already in the field can be cross referenced for suitability, overcoming the need

to re-permit under restrictive regulations;

- a pattern of air entrainment at the root of the flame may em erge to permit formulation of the

performance of sonic or other high pressure flares;

- g a s e s rich in hydrogen may be flared at higher velocities without requiring oversized discharge;

- by relating eddy size or shedding frequency to stability, a pattern and additional formulae may

em erge for other types of flame stabilization such a s sudden expansion and bluff body;

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David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 14 of 33
- there may be a separate relationship (additional formula) based on the differences between

flammable limits for upward and downward propagation which describes piloted flames (upward

propagation) versus un-piloted flames (downward propagation)

- by extension, the latter may define a further formula to determine the minimum number of

external flame sources or pilots to properly stabilize an intrinsically unstable flame.

Many of the above points would also seem to be intuitively correct and figure 13 presents many of

th ese characteristics pictorially.

If the approach can be extended to piloted and stabilized flames, then calculation of a working value

of Ram, for stability purposes, may involve Flam m able Limits for D ow nw ard v s Upward

Propagation. These may not always be published, requiring an estimate to be made by the

engineer. In th ese c a s e s the practical approach would be to consider the given value to be CL UP.

The relativity of CL UP and CL DOWN is a function of a g a s e s propensity to release radiant transient

solids into the flame and is, therefore, intimately connected to the chemistry of the compound.

B ased on a very modest regression analysis of relatively limited published data for directional limits

of common lean g a ses, it would se e m that a rough approximation of the ratio may be obtained from

the formula -

CL_UP / CL_DOWN = ZUD = 1.2 - C/5 + H/30 +S/5 + N/10 + O/4 >= 1

where C, H, S, N and O are the number of atoms of carbon, hydrogen, sulfur, nitrogen and

oxygen respectively in a representative molecule of the original g a s mixture of flammable

components.

Empirical results for flammable limits s e e m to be apparatus dependent and can vary by a s much as

+/- 20% or 30%. A more carefully studied review of the true stability properties of g a s e s is

recommended for definitive or critical results.

Complex discharges may require special analytical treatment when using the RAM approach.

For example, most A ir A s s is te d F la re s may be generally described as an exterior duct carrying a

flow of fan-blown air, with an interior g a s distribution nozzle which releases the ga s to be flared into

the air stream in the vicinity of the b a se of the flame. As there is no standardization of g a s nozzle

design or flare exit conditions across the industry, the flame properties and stability will undoubtedly

vary across the available commercial models and relative duct size.

To coincide with the arguments of this model, for a practical design, the blown air n eed s to be

considered as a variable feature of the design procedure a s it is often in the direct and independent

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David Shore / AFRC-JFRC 2007

Art_to_Science - AFRC.wpd 15 of 33
control of the plant operator. Two different operating states may be relevant for individual

consideration;

- the blown air is greater than the required ratio to produce a CL (lower limit) concentration of ga s

in air, resulting in flame collapse and stabilization on the inner nozzle;

- the blown air is less than the required ratio to produce a CU (upper limit) concentration of g a s in

air, such that it acts a s a diluent in the g a s stream, with stabilization on the outer diameter.

As the mixture transitions between th ese two states there will also be two points of discontinuity, one

of which may possibly relate to the blow-off condition of Figure 10.

CA RB O N AND S O O T FORM ATION, AND RADIANT LOAD

A great deal of the design effort associated with flares involves attempting to suppress particulate

carbon formation in the flame. Initially, this is for the obvious visual effect and associated community

relations, but is also to facilitate compliance with Environmental standards which correctly u se sm oke

production a s a marker for other inefficiencies. Previous papers presented to AFRC have indicated

the occurrence of heavy aromatic pollutants from smoking flames.

Simplistically, carbon forms initially in a hydrocarbon flame b e c a u s e of the thermal breakdown of the

hydrocarbon molecule (pyrolysis) into component radicals. When this occurs in the core of the flame,

and in the a b se n c e of oxygen, so m e of the carbon atoms agglomerate into larger crystalline forms

such a s chains or spheres. T h ese forms are solid in nature and must decay physically, from the

outer surface toward the center, when the solid form m eets an oxygen-rich interface which permits

the eventual conversion to CO2. During this time, the particulate acts rather like a thermal black body

at constant temperature, and provides a highly radiant contribution of thermal energy within and from

the flame zone. A much smaller radiant contribution, will com e from the unsymmetrical molecules

in the flue gas, primarily CO2 and H2O, and probably less than 10% of the thermal load of the flame

will be radiated in this manner.

At this time there is no common agreement about the methods n eed ed to calculate solid carbon

fraction within the flame, nor its radiant effect. Schwartz( 31 } acknowledges this in a 1996 review

through a recommendation against using similar radiative fractions for alternative flame models.

Adding to the accumulation of "flare lore”, radiant heat (emissive) fraction is treated more like a

marketing tool than a scientific condition, leading to a significant amount of inconsistency and

confusion across the Industry.

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David Shore / AFRC-JFRC 2007

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The conditions which generate solid carbon fraction within the flame zo n e are not d iscussed in detail

here but must finally involve

- a time b a se covering the oxygen-deficient particulate growth;

- a time b a se covering the d ecay in the zo n e of oxygen availability;

- the potential carbon loading attributed to a single representative g a s molecule;

- the intrinsic thermal stability of the original molecular compound;

- the free energy potential of the molecule when compared with that of molecular carbon.

T h ese ideas remain to be developed in a future paper.

CO N C LU SIO N

This paper primarily d is c u s se s just three major asp ects of flare and flame modeling and presents

each in a manner which may be recognizable and useful to most engineers, rather than as

dimensionless analyses.

The flammability and stability m odels which have been proposed in this paper are more

comprehensive and involved than the basic m odels currently published by the API and EPA (as part

of 40CFR60.18). It is believed that th ese new m odels more accurately describe the minimum

requirements for a satisfactory and stable flame which meet the current em issions standard for flares

set by the EPA.

T hese new models do not address whether a standard of 98% conversion is appropriate for the

maximum design case, which is an ongoing debate to be d iscussed further in a subsequent paper.

On the one hand, there s e e m s to be evidence within the public domain which could support a

standard of 99% conversion, for a variety of operating conditions, particularly in concert with

increased confidence in a stability model. On the other hand, so m e research( 32 } appears to show

reduced efficiency for wind blown flames when accompanied by a wake influenced condition

(downwash). Further examination of th ese issu es will need to build on the s a m e flammability

concerns which form the basis of this paper.

The author acknowledges that the development of s o m e asp ects of th ese current m odels are based

on limited research data and, in many ways, represent hypotheses rather than results. In proposing

the models here, the primary intentions are

- to highlight confusing discrepancies in the various published documentation and regulations and,

hopefully, correct those inaccuracies (perceived by this author) which give rise to Global

misconceptions of the true requirements for satisfactory flaring of gas mixtures;

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David Shore / AFRC-JFRC 2007

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David Shore / AFRC-JFRC 2007

- to encourage the Flare Industry as a whole to take a more scientific position on the technical

issues surrounding the flare business;

- to provide ideas which may stimulate additional research into flame size, stability, lift off and

associated combustion inefficiency by other researchers. Such research is strongly encouraged

by the author, in the interests of improving a general level of technical knowledge and building

a stronger technical foundation for future developments.

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T h e A u th o r

David Shore has been designing and working with Flares and Combustion since

1965 and is presently Chief Engineer of Flaregas Corporation in N.Y. USA.

Mr. Shore served a s a member of the original steering committee responsible for the

research behind the current EPA regulation 40 CFR 60.18 and has also worked in

Europe a s a Chartered Engineer (Eur-Ing) where he w as also associated with Flare

research by the DGMK.

Mr. Shore’s previous papers include presentations to A.I.Ch.E and Noise.Con.

A c k n o w le d g m e n ts

The author acknowledges, with thanks and appreciation,

the contribution and assistance of family

and the Management of Flaregas Corporation

in facilitating the preparation and presentation of this paper.

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How a Flame Envelope establishes

b y a t m o s p h e r i c d i f f u s i o n / d i s p e r s i o n

R = V [ - 2 * ( c r 2 ) * l n { x * 2 * 7 i * ( a 2 ) } ]

0 2 0 4 0 6 0 8 0 1 0 0

L E L U E L

Figure 2 - A theoretical Flam e Envelope

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BUO - BUOYANT FLAME PROCEDURE

At the end-of-flame location XF; for conventional vertical flares; units to be consistent Btu; Ibm; ft; OR sec

a) determine the flow characteristics for the discharged gas

qF = heat flux

CV = calorific value

Po = density

Cl = lean flammability limit

e = flame emissivity

b) determine the physical characteristics of the flare

Hs = stack height

Ds = stack top outer diameter

rO = inner radius

c) determine the ambient conditions

Ua = wind speed

Ta = temperature

P a = density

cpa = specific heat

d) determine the atmospheric conditions

Stability category varies A / B / C / D default = D

S = varies 0.039 / 0.0459 / 0.0318 / 0.0142

N = varies 2.0016/1.8580 /1.7973 /1.7727

e) determine the terrain conditions

Ho = wind speed reference height

Y = wind height exponent (varies; 1/5 to 1/11.5 ref relevant wind code) default =1/ 9.5

d) estimate or calculate corrected values of

Xc = down wind travel to mid flame

Hc = mid flame height CD

Uas = ZfY * Ua for Zf = Hs / Ho = wind speed at stack height 3

hfi

Uac = ZcY * Ua for zc = Hc / Ho = wind speed at mid flame i-jSx = S * X f - 2 = stability constant at mid flame

g = 32.174 = gravitational constant

Kb = 1.6 = buoyancy constant

Km = 2.3 = momentum constant

Ks = [ 1 / (2 * p * Sx) ] = stability constant

Kf = CV * Po * Cl = flame constant

tF = [ ( qF * Ks ) / ( Kf * Uac3 ) ]TM = flame dwell time

Xf = [ Uac * tF ] = downwind flame length

Uo = [ ( qF / ( CV * p o * P * ro2 ) ] = true discharge velocity

Ud = [ ( 2 * p a / p o W * Uas ) ] = downwash velocity modifier

U01 = [ Uo - Ud ] = modified discharge velocity

Fb = [ 1- ( P o / Pa ) ] * Uo1 * ro2 = density buoyancy factor

Fh = [ g / ( P * Cpo * Ta * Pa ) ] = thermal buoyancy factor

Ff =

*

[

1

-

1

.5

*

e

]

=flame buoyancy factor

Fm = [ ( P O / P A ) * U012 * ro2 ] = momentum factor

DHd = [ 5 * Ds * Uq1 / Ud) ] <= 0 = downwash estimate

DHF1 = { 0.5 * Kb * [ ( Ff(1/3) * qJ1/3)) / Uaf ] * Xj2/3)} = buoyant rise of flame

DHm = [ Km * [ F jI/3) / Ua/ 2/3) ] * Xj1/3) ] = momentum rise

DHf = h + dHm + dHd = total rise

Down wind plume rise in any condition is given by

dhp = { DHP1 or AHP2 or DHP3} + DHm + DHd

AHP1 = { Kb * [ (Fj1/3> / Uas ] *X/2/3)} = buoyant rise; raw gas ; density base

DHp2 = { Kb * [ ( Fh(1/3) * q(1/3)) / Uaf ] * Xj2/3)} = buoyant rise; hot gas; thermal base

DHf3 = [ DHp2 - H X f} ] = buoyant rise; flue gas beyond flame

DHeXf} = DHp2{Xf} - DHf{Xf} = height correction at flame end

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F la m m a b ility E n v e lo p e s f o r P ro p a n e / I n e r t M ix t u r e s in A\r

r e f : Fig 3 2 : B O M B ulletin 5 0 3

0 10 20 30 40 50 60

% I n e r t in O rig in al M ix tu r e

Figure 4 - Propane F lam m ab ility

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Figure 5 - Generalized Flammability

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FLAM M ABILITY AND NITROGEN EQUIVALENCE

Cl = Lean Flammable Limit for Flammable component in Air %

R s = Air / Gas Stoichiometric Ratio

R l l = Air / Gas ratio at Lean Limit = (100 - C l) / C l

R il = Limit Ratio o f Inert/Flammable in N2 (or w ’ xs Air) = R l l - R s

N e = Combined Nitrogen Equivalent o f the Inert content = S { NEi * I%I}

% =fractional quantity o f ith Inert component

NeI = nitrogen equivalent o f ith Inert component

F OS = Factor o f Safety

R ix = Limit Ratio o f Inert/Flammable in true mixture = R i l / N e / F OS

C V VF = Combined Calorific value (vol) o f Flammable components

CVvm = min flam m able mixture heating value (vol) = CVvf / ( Rix + 1 )

Figure 6 - N itro gen Equivalence

REPRESENTATIVE LIMITING HEATING VALUE FOR SOME GAS/ INERT MIXTURES

CALCULATED USING

STANDARD FLAMMABLE LIMITS AND NITROGEN EQUIVALENCE OF INERT

propane / nitrogen 200 Btu/scf methane / nitrogen 140 Btu/scf

propane / water 265 Btu/scf ethylene / water 160 Btu/scf

propane / carbondioxide 340 Btu/scf ammonia / CO2 250 Btu/scf

propane / sulfur dioxide 385 Btu/scf hydrogen sulfide / SO2 105 Btu/scf

propane / helium 135 Btu/scf hydrogen / helium 25 Btu/scf

propane/argon 215 Btu/scf carbonmonoxide/ argon 115 Btu/scf

Uses FOS =1.5; LEL for upward propagation only from BOM 503; ambient temperature properties

Figure 7 - Minimum Flammability o f Mixtures

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C O M M E N T A R Y O N TH E EPA R E G U L A T IO N - 40 CFR 60.18

The well-known and widely discussed EPA Regulation, 40 CFR 60.18, (and 63.11) is one example o f a generalized

parametric input intended to assist in improving the global performance o f Flares by setting limits on volumetric

heating value o f gases which may be flared and on gas discharge velocity, thereby controlling flare size (in the

U.S.A.).

Insofar as the EPA’s purpose is to regulate and monitor emissions, the associated control o f flare tip diameter

suitably focuses the flare user’s attention on flare design. However, a major concern with the EPA regulation is

that the formulation o f the rules for heating value and velocity give an impression on a Worldwide scale that,

because the formula is implicitly endorsed by the U.S. Government, it properly describes the flame functions. In

consequence, the various limits which are set by the regulation on heating value and velocity tend to be universally

applied as a “rule-of-thumb” and with a high level o f confidence w hich m ay be unwarranted.

The underlying concept o f this regulation considers that the combustion efficiency o f any stable, active flame will

be more than 98% and that the intrinsic heating value o f gases and mixtures can describe both a lowest value to

support any flame and the upper velocity able to avoid flame lift-off. Compliance with the regulation is deemed to

imply a conversion efficiency o f 98% or better which then allows flare operators to evaluate their probable output

o f emissions with some reasonable accuracy.

Although subsequent investigations lead to a conclusion that the base value o f 98% may need review, particularly

for wind blown flames, the regulation stands, and some o f its basic characteristics are included in the recently issued

API Recommended Practice 537(333 and are likely to be included in the proposed new standard ISO 25457(34)

The EPA formulae resolve to

for non-assisted and steam assisted flares,

UO1 = 26.6 * 10(8507CV) (min = 60 fps; max = 400fps)

except for gases with a Hydrogen content greater than 8%, which may be calculated from

UO2 = 12.8 * (H2 vol% - 6) (max = greater o f 122 fps or UO1)

for air assisted flares

UO3 = 28.6 + 0.0867 * CV (min = 60 fps; max = 400fps)

where UO# = free discharge velocity - feet per second

CV = volumetric heating value - Btu/scf

(non asisted flares min = 200 Btu/scf) (assisted flares min = 300 Btu/scf)

Figure 8 - 40 C F R 60.18

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Efficicncy

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David Shore / AFRC-JFRC 2007

INFLUENCE OF 5TEAM ON GLOBAL EFFICIENCY

Mass ratio of Steam / Gas

Figure 9 - Steam and Combustion Efficiency

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JET D ISC H A R G E V E L O C IT Y

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S T A B I L I T Y P A T T E R N F O R A D I F F U S I O N F U M E

(A FT E R K A R B A S I A N D W IE R Z B A )

C O - F L O W I N G A I R S T R E A M V E L O C IT Y

AIR AXIS INTERVALS MAY BE APPROX 0.01 * JET VELOCITY INTERVALS

Figure 10 - Flame Stability

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C a lo rific V a lu e - B t u / s c f

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Figure 11 - EPA Flare Stability Curves

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Uo / ( Pi * Do ) - Hz

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C o n d i t i o n s f o r S t a b l e D i f f u s i o n F l a m e

E P A -600 / 2-84-095:2-3 & 2-85-123:2-4

A ir/G a s R atio a t Lean Limit - w t/w t 

eb C 3/N 2 a H 2S /N 2 * W ierzba C 1/H 2

— Typ Mach 0.5 Typ mid range H/C ■ Kalghatgi C 3/H 2

Figure 12 - A ir M ass ratio relationship

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Figure 13 - Lean L im it Flam e Stability

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