FROM: Carrie Richardson Fry and Jeff Coburn, RTI International 

TO: Andrew Bouchard and Brenda Shine, EPA/OAQPS 

DATE: July 3, 2012 

SUBJECT: Peer Review of “Parameters for Properly Designed and Operated Flares” 

This memorandum details the process for the peer review of the report entitled 

“Parameters for Properly Designed and Operated Flares” prepared by the U.S. Environmental 

Protection Agency’s (EPA’s) Office of Air Quality Planning and Standards (OAQPS) dated 

April 2012. To ensure a comprehensive and balanced peer review, RTI sought to establish a peer 

review panel consisting of individuals representing a variety of backgrounds and perspectives 

who could be considered “technical combustion experts.” To assist in this process, RTI utilized 

search engines and reviewed technical literature pertaining to flares and combustion. Experts 

were identified within four categories: the refinery industry, industrial flare consultants, 

academia and the environmental arena. 

RTI contacted multiple reviewers within each category to assess their availability and 

interest in participating in the peer review panel, and selected from among the interested parties 

to form an eight-person panel comprised of two individuals from each category. The eight peer 

reviewers on the panel are as follows: 

 Technical Lead on Flare Combustion Efficiency from a major oil company 

 Dr. Murty Kanury, Professor at Oregon State University 

 Gary Mueller, Principal Consultant Air Quality, Shell Global Solutions (US) Inc. 

 Lucy Randel, Research Director, Industry Professionals for Clean Air, Houston, 

Texas 

 Dr. Ranajit Sahu, Independent Consultant 

 Christopher Schaeffer, President and General Manager, Control Instruments 

Corporation 

 Dr. Jim Seebold, Independent Consultant 

 Dr. Joseph Smith, Laufer Endowed Chair of Energy, Missouri University of 

Science and Technology 
2 

Each peer review panel member was provided a charge statement with ten charge 

questions, a copy of the draft report, the corresponding data base, and, if requested, a copy of any 

references listed in the draft report. The ten charge questions follow: 

Section 2: Available Flare Test Data 

1. Please comment on the agency’s criteria for excluding available flare test run data 

from final analyses, and whether application of these criteria may have lead to 

inappropriate exclusions of relevant data points. 

Section 3: Steam and Flare Performance

2. Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as 

an operating parameter for indicating over steaming situations on steam-assisted 

flares. Comment on the agency’s use of the ratio of the net heating value of the 

combustion zone gas (NHVCZ) to the net heating value of the flare vent gas if diluted 

to the lower flammability limit (NHVLFL) as an alternative to LFLCZ. Does the flare 

data adequately support the EPA’s observations? 

3. Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is 

there other data available (that is not discussed in this report) that may help clarify 

our discussion about specific chemical interactions related to lower flammability 

limits of gas mixtures? 

4. Did the agency adequately examine other operating parameters (different from 

LFLCZ; or the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming 

situations? Are there specific other parameters that should be given more or less 

emphasis? 

Section 4: Air and Flare Performance 

5. Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. Additionally, also 

comment on whether the lower flammability limit of the flare vent gas (LFLVG) is an 

appropriate operating parameter for determining whether the flare vent gas being sent 

to an air-assisted flare is capable of burning? Does the flare data adequately support 

the EPA’s observations? 

Section 5: Wind and Flare Performance 

6. Please comment on the momentum flux ratio (MFR) as an operating parameter in 

crosswind velocities greater than 22 mph at the flare tip to indicate wake-dominated 

flame situations. Additionally, also comment on the agency’s observation that in the 

absence of crosswind greater than 22 mph, a low MFR does not necessarily indicate 

poor flare performance. Comment on the effectiveness of observations identifying 
3 

wake-dominated flames. Does the flare data adequately support the EPA’s 

observations? 

7. Did the agency adequately examine other operating parameters (different from MFR) 

for identifying wake-dominated flames? Are there specific other parameters that 

should be given more or less emphasis? 

Section 6: Flare Flame Lift Off 

8. Please comment on the maximum allowable flare tip velocity equation which 

considers combustion zone gas composition, the flare tip diameter, density of the flare 

vent gas, and density of air. Does the flare data adequately support the EPA’s 

observations? Are there specific other parameters or methods/equations that should be 

given more or less emphasis? 

Section 7: Other Flare Type Designs to Consider 

9. Please comment on the applicability of the LFLCZ parameter, maximum allowable 

flare tip velocity equation, and the observations regarding crosswind velocity to 

non-assisted flares, pressure-assisted flares, and other flare designs. 

Section 8: Monitoring Considerations 

10. Please comment on the appropriate monitoring equipment needed to ensure good 

flare performance and on any other known monitoring methods (not discussed in this 

report) for monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of 

NHVCZ to NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating 

scenarios and conditions where less robust monitoring equipment could be used to 

determine the operating parameters of interest. 

The panel members performed a thorough review of the draft report and provided 

answers to each of the questions to the extent possible, based on their technical expertise. Each 

panel member’s response is provided as an attachment to this memo (see Attachments A 

through H). Each response is provided exactly as submitted, with the exception of modifications 

to remove identifying information from the submission of the technical expert who wished to 

remain anonymous. 
Attachment A 

Technical Lead on Flare Combustion Efficiency from a major 

oil company 
We appreciate the opportunity to participate in the technical peer review of Parameters 

for Properly Designed and Operated Flares. The Panel of technical reviewers was asked 

to focus on a set of ten charge questions, with invitation to further comment on any 

aspects of the information in the report or other flare topics. To that end, please find 

specific responses to the charge questions below followed by a few general comments. 

Finally, the type of rigorous peer review that would be conducted for the purposes of 

journal publication seemed beyond the scope of this task. As such, comments for the 

purpose of technical editing of the paper were not included in this review. 

Charge Questions

Section 2: Available Flare Test Data 

1. Please comment on the agency’s criteria for excluding available flare test run 

data from final analyses, and whether application of these criteria may have 

lead to inappropriate exclusions of relevant data points. 

Response: Exclusion of data sets B and I seems wholly appropriate. As stated, there was 

insufficient information regarding conditions (flare vent gas flow rate) in data set B and 

data set I was on a unique flare design, which resulted in data inconsistent with other 

types of flares. Table 2-6 identified reasons for removal of data prior to “any final 

analysis.” In general, it is difficult to find fault with the reasoning that lead to data 

exclusion on these individual accounts; however, one concern with that approach 

remains. The report reads as though the maximum LFLCZ of 15.3% conclusion has driven 

the process of data exclusion insofar as the data was more highly scrutinized if 

inconsistent with that conclusion. It is difficult to infer which other data points would 

have been excluded under similar criteria had all of the data been similarly scrutinized. 

Section 3: Steam and Flare Performance
2. Please comment on the lower flammability limit of combustion zone gas 

(LFLCZ) as an operating parameter for indicating over steaming situations on 

steam-assisted flares. Comment on the agency’s use of the ratio of the net 

heating value of the combustion zone gas (NHVCZ) to the net heating value of 

the flare vent gas if diluted to the lower flammability limit (NHVLFL) as an 

alternative to LFLCZ. Does the flare data adequately support the EPA’s 

observations? 

Response: The primary observations detailed in this report state that LFLCZ is the most 

appropriate operating parameter toward the identification of oversteaming. The question 

of appropriateness for any regulatory operating parameter should take into account 1) 

practical considerations of operating the individual unit with respect to the parameter, 2) 

ability of the operating parameter to achieve environmental objectives and 3) cost 

effectiveness. This analysis does not consider the practicality of operating a flare with a 

new operating parameter nor does it consider the cost effectiveness of implementation. 

The report addresses part of the second consideration in exploring operating parameters 

to ensure good combustion efficiency in flares. There is no attempt made to quantify 

emissions reductions through implementation of such a parameter. 

It is difficult to follow how the authors arrived at the conclusion that an LFLCZ of 15.3% 

is the appropriate limiting value. The authors did an exceptional job of carefully 

reviewing the data with respect to LFLCZ approach. Monitoring the LFLCZ would require 

the maximum instrumentation (flow monitoring on waste and assist gas and full 

speciation of flared gas). Furthermore, a limit of 15.3% would require a large proportion 

of flares with high combustion efficiency to supplement flare gas with auxiliary gas to no 

benefit and an unnecessary cost. 

The LFLCZ concept relies on the lower flammability limit of a gas or mixture of gases to 

determine the minimum concentration below which a gas or mixture of gases will not be 

flammable. Theoretically, below the lower flammability limit (LFL), a gas will simply 

not burn. One would expect to see essentially a step-function change (high combustion 

efficiency independent of the dilution until a threshold were reached where combustion 

efficiency fell to zero) if the combustion efficiency of a particular gas mixture were 

plotted against the dilution of that gas (i.e. a plot of steam to vent gas ratio vs. 

combustion efficiency). Practically speaking, the combustion efficiency around the LFL 

will be greater than zero because the mixture will not be perfectly mixed and there will be 

pockets of rich material that will combust, resulting in a monotonically decreasing 

function that rapidly declines to zero near some threshold value of dilution. (It is also 

important to recall that the accuracy and precision of combustion efficiency 

measurements themselves decline with degrading combustion efficiency for a variety of 
reasons.) Individual data sets plotted against steam to vent gas ratio exhibit this type of 

behavior. 

A gas or mixture of gases should be capable of burning irrespective of how high or low 

the LFL is, provided the gas is present at a concentration between the upper and lower 

flammability limits in the combustion zone. By concluding that there is an upper bound 

to the LFLCZ where one expects to see combustion, the authors essentially conclude that 

gases with a higher LFLCZ are either less likely to be released in adequate volumes to be 

present in sufficient concentration in the combustion zone relative to their own LFL to 

combust, or there is inadequate mixing – flare operations specific assumptions. A 

conclusion that there is a maximum LFLCZ at which gases are less likely to combust is 

more of a conclusion about what concentrations the authors believe the average flare 

releases gas to the combustion zone and less of an inherent property of the gas 

composition. The question remains, in this case, of whether this data set has an adequate 

representation of flares to arrive at conclusions of average flare operations. The LFLCZ is 

essentially a semi-empirical parameter that may not be appropriately extended to 

additional flares outside of this data set, especially future flares or those with differing 

flare designs. 

There was not sufficient information provided to separately comment on the net heating 

value ratio other than to say that the report concludes it is nearly mathematically identical 

to the inverse of the LFLCZ, so the comments above likely apply. The report states that 

the ratio may be “easier for flare owners and operators to understand and implement 

because it uses a familiar parameter (net heating value).” While the term net heating 

value, particularly in the context of flare operations, may be more familiar, it is not 

obvious why the ratio described would be easier to implement. It appears to have the 

same instrumentation requirements as the LFLCZ methodology given that a net heating 

value at the lower flammability limit would need to be computed. 

3. Is there sufficient evidence that chemical interactions are occurring that make 

the calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold 

discussed? Is there other data available (that is not discussed in this report) 

that may help clarify our discussion about specific chemical interactions 

related to lower flammability limits of gas mixtures? 

Response: It is not clear to which chemical reactions the question refers. The response to 

question #2 addresses the physical basis for a maximum LFLCZ threshold and the semiempirical nature of that parameter. 
4. Did the agency adequately examine other operating parameters (different from 

LFLCZ; or the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming 

situations? Are there specific other parameters that should be given more or 

less emphasis? 

Response: Previous work focused on the latent heat value of the flare gas. That approach 

should not be discarded. Additional studies in the recent past are reasonably consistent 

with previous observations, including the work in the 1980s that set the basis for current 

flare requirements. That work recognized a change in flare combustion efficiency 

resulting from the addition of assist gas and attempted to address that with different 

minimum net heating value requirements dependent upon the presence of assist gas (300 

Btu/scf for assisted flares as compared to 200 Btu/scf for unassisted flares.) One would 

expect that the treatment of assisted flares should converge with unassisted flares in the 

limiting case where there is zero assist gas used. The data suggests that the minimum 200 

Btu/scf is likely the appropriate limiting net heating value, but assist gas should be 

handled by considering its volumetric dilution of the waste gas. That is the approach used 

in the combustion zone net heating value (CZNHV) concept. 

The CZNHV approach, however, appears to be dismissed, out of hand. The report 

correctly remarks that “using the NHVVG as an indicator of good combustion efficiency 

ignores any effect of steaming,” and notes that the NHVCZ factors in steam usage. Then 

the report states that if the data set included here were used “to determine appropriate 

operating conditions for flares, operators would need to maintain a NHVCZ between 

approximately 300 and 350 Btu/scf to ensure good combustion.” That conclusion was not 

further documented or justified. In the identification of an LFLCZ limit, there was a 

tolerance for some data inconsistency with the limit. A similar philosophy could be 

applied here. 

The data is plotted as CZNHV vs. combustion efficiency below and fit to the inset 

function. The function essentially interpolates between two lines – one representing good 

combustion efficiency and one rapidly approaching zero combustion efficiency. The 

particular fit adjusted the variable β to 114 (to fit the low combustion efficiency data 

visually) and then used the Microsoft® Excel solver to derive the best fit for α, in this 

case, 2.85. (One could use a regression technique for both α and β.) Interestingly, the fit 

crosses 96.5% combustion efficiency right around 200 Btu/scf. 

It is true, as noted in the report for a CZNHV parameter (as well as the others examined) 

that there exist data points of both high and low combustion efficiency on either side of a 

presumed limiting value, however, one could reasonably anticipate good combustion 

efficiency were an NHVCZ of 200 Btu/scf maintained. This type of analysis requires only 

a consideration of dilution at the combustion zone with assist gas above current 
regulatory requirements. In principle, a revision that simply relocated the point of 

calculation from the waste gas to the combustion zone would require the addition of 

engineering calculations or flow monitors (incurring an investment and operating cost) 

for both the waste and assist gas. CZNHV can be calculated by volumetric dilution of the 

NHV. 

The report states that “NHV may not be the best operating parameter for determining 

flare performance because the net heating value of a mixture can vary significantly at 

certain constant lower flammability limits (and vice versa),” though it is not clear from 

the report why NHV is assumed to be the inferior parameter of the two. Neither the 

CZNHV nor the LFLCZ approach described here recognizes the unique combustion 

chemistry of hydrogen. The NHV, on a volumetric basis, of hydrogen (290 Btu/scf) 

would lower the calculated CZNHV compared to propane (983 Btu/scf) and the LFL of 

hydrogen (4%) would raise the calculated LFLCZ compared to propane (2.1%), though 

data suggests that hydrogen is supportive of combustion. 

Perhaps most compelling, however, is that flare combustion practices should be 

established on a flare-by-flare basis. The challenge in identifying a single parameter and 

limit that best represents good combustion efficiency for all flares is a testament to the 

appropriateness of addressing flare combustion on a case-by-case basis. The correlation 

between a flare operating at the incipient smoke point and good combustion efficiency 

has been enumerated throughout the literature. For example, a major finding of the 

referenced 2010 TCEQ/UT study states that “the most efficient flare operation, as 

measured by DRE and CE, for the flare operating conditions tested, was achieved at or 

near the incipient smoke point.” One could envision a low cost and perhaps quite 
effective paradigm where combustion efficiency was more or less monitored visually 

much in the same way visible emissions are monitored today. 

Section 4: Air and Flare Performance

5. Please comment on the stoichiometric air ratio (SR) as an operating parameter 

for indicating excess aeration situations on air-assisted flares. Additionally, 

also comment on whether the lower flammability limit of the flare vent gas 

(LFLVG) is an appropriate operating parameter for determining whether the 

flare vent gas being sent to an air-assisted flare is capable of burning? Does 

the flare data adequately support the EPA’s observations? 

Response: The stoichiometric air ratio was a concept developed as a result of the recent 

TCEQ-commissioned study by UT at the John Zink facility. The conclusions of that 

report appear reasonable only for the limited set of data. Regarding air assisted flares, 

data were presented on only two flares. The entire data set derived from one of the flares 

was correctly excluded from this analysis due to fact that the flare had a diameter of 1.5” 

and it is documented that flares with diameters less than 3” do not scale to industrial 

flares in the determination of flare combustion efficiency. It is important to note that the 

complexity of parameterizing steam assisted flares became more apparent as the data set 

grew. The TCEQ data set is limited to one composition and one flare design. 

Furthermore, the data near the proposed limiting stoichiometic air ratio is limited to just a 

few points. Figure 4-4 shows zero data points between a stoichiometic air ratio of 7 and 

9, where the single point around 7 shows high combustion efficiency. Given the limited 

data available on air assisted flares, it is difficult to discern a limiting parameter, 

particularly with confidence for extrapolation to other flare compositions, designs and 

sizes. 

As described in the response to question 2, the LFLCZ limiting value was established as a 

semi-empirical parameter describing combustion efficiency observations specific to the 

steam assisted flares in the present analysis. The authors believe that the LFLVG operating 

parameter is a natural extension of the conclusions from the steam-assisted flare data; 

however, a similar conclusion on air-assisted flares is not derived from the present data. 
Section 5: Wind and Flare Performance 

6. Please comment on the momentum flux ratio (MFR) as an operating 

parameter in crosswind velocities greater than 22 mph at the flare tip to 

indicate wake-dominated flame situations. Additionally, also comment on the 

agency’s observation that in the absence of crosswind greater than 22 mph, a 

low MFR does not necessarily indicate poor flare performance. Comment on 

the effectiveness of observations identifying wake-dominated flames. Does 

the flare data adequately support the EPA’s observations? 

Response: There is inadequate data available to understand the effect of wind on an 

industrial flare. All of the studies targeting the study of wind effects were conducted on 

small pipe flares, almost all of which were below 3” in diameter. The report notes that 

these flares are generally not scalable to industrial sized flares. Regarding the analysis of 

the available test data on industrial size flares, “no test runs were performed in winds any 

higher than 22 mph.” The assumption that observations from small pipe flares should 

apply in the absence of data on larger flares does not seem appropriate. 

The data, as shown in section 5, do support the observation that a low MFR does not 

necessarily indicate poor flare performance for this data set. There are not data available 

to comment on the effectiveness of visual inspection of wake-dominated flames. 

7. Did the agency adequately examine other operating parameters (different from 

MFR) for identifying wake-dominated flames? Are there specific other 

parameters that should be given more or less emphasis? 

Response: There are inadequate data to investigate wind effects and correlate operating 

parameters to those observations. 

Section 6: Flare Flame Lift Off 

8. Please comment on the maximum allowable flare tip velocity equation which 

considers combustion zone gas composition, the flare tip diameter, density of 

the flare vent gas, and density of air. Does the flare data adequately support 

the EPA’s observations? Are there specific other parameters or 

methods/equations that should be given more or less emphasis? 

Response: There are no data presented to adequately assess equation 6-1. It is unclear 

that any conclusions can be drawn from Figure 6-1. 
Section 7: Other Flare Type Designs to Consider 

9. Please comment on the applicability of the LFLCZ parameter, maximum 

allowable flare tip velocity equation, and the observations regarding 

crosswind velocity to non-assisted flares, pressure-assisted flares, and other 

flare designs. 

Response: There are no data to support the application of new parameters to non-assisted 

flares or other flare types. One can envision that a non-assisted flare is the limiting case 

of an assisted flare with zero contribution of assist gas. However, it does not seem 

appropriate to apply the LFLCZ parameter to non-assisted flares given that limiting value 

was derived from a specific set of steam-assisted flares. 

Section 8: Monitoring Considerations 

10. Please comment on the appropriate monitoring equipment needed to ensure 

good flare performance and on any other known monitoring methods (not 

discussed in this report) for monitoring the following parameters: LFLCZ, 

LFLVG, LFLVG,C, the ratio of NHVCZ to NHVVG-LFL, CCZ, SR, MFR, and Vmax. 

Also, please comment on operating scenarios and conditions where less robust 

monitoring equipment could be used to determine the operating parameters of 

interest. 

Response: Section 8 of the report details the equipment needed in order to continuously 

demonstrate, with online analyzers and associated investment and operating cost, 

compliance with the parameters listed above. Another monitoring scenario worth 

consideration toward compliance demonstration includes grab samples. A discussion on 

operating scenarios and conditions where less robust monitoring equipment is indicated 

belongs in the context of cost effectiveness and the ability for demonstration of a 

parameter to meet environmental objectives. In that vein, one can readily envision 

scenarios or conditions in which significantly less robust monitoring equipment / 

methods or alternate operating parameters would serve to fulfill a flare monitoring 

function in a vastly more cost effective, yet environmentally protective fashion than the 

methods and equipment discussed above. 
General Comments: 

Effort and expense has been invested across the industry between the time of the 1980s 

promulgation of flare rules and the present time to achieve flare minimization. Some 

present day flares include equipment such as flare gas recovery units or are in staged flare 

configurations. As a result, there exist cases with little opportunity to reduce VOC/HAPs 

emissions as a result of combustion efficiency monitoring. An API study titled 

API/NPRA/ACC Steam-Assisted Flare Operations and Control Survey, investigated the 

cost-effectiveness of continuous monitoring of a combustion parameter. Conclusions 

from that study are below: 

• Flares that sometimes operate with high steam assist ratios do not necessarily 

have high VOC emissions. 

• Several variables influence the potential benefit from monitoring and controlling 

steam-assist ratio or CZNHV. 

o VOC content of the flare gas 

o Annual VOC load 

o VOC load during periods of high steam-to-flare gas ratio 

o Duration of high steam-to-flare gas periods 

o Combustion efficiency achieved through current operating practices 

• The cost-effectiveness for adding extensive monitoring and control is very poor 

for flares with a low potential for significant VOC emissions reduction. 

• For some flares, opportunities exist to cost-effectively reduce VOC emissions. 

However, most studied flares have no cost effective potential for significant VOC 

emission reductions through additional monitoring and controls. 

• The degree to which VOC emissions might be reduced depends upon a flare’s 

specific design, service and operation. 

• Potential cost-effective VOC reductions are achieved primarily through control 

of steam. 

• There is little opportunity to reduce VOC emissions from flares with no or 

minimal flow or VOC load during normal operations (i.e. flares that only handle 

startup, shutdown (equipment clearing), and malfunction; flares with flare gas 

recovery; and flares that combust non-VOC gases). 

• To assure cost-effective VOC emissions reductions, monitoring and control 

requirements should be based upon individual flare operations. 
Attachment B 

Dr. Murty Kanury 

Professor at Oregon State University 
RTI-EPA FLARE PEER REVIEW PANEL MEMBER REPORT BY

MURTY KANURY

June 4, 2012

 A Memorandum, dated May 10, 2012, from RTI for U.S. EPA to the Flare Peer Review Panel 

members, poses ten “charge questions’’ and asks each panel member to present a review of the 

Report in the form of answers to, and comments on, each question. 

 This document is a collection of responses of one member of the peer review panel to these ten 

charge questions. These responses are based primarily on the report titled "Parameters for Properly 

Designed and Operated Flares," with Appendices, prepared by the U.S. EPA (OAQPS) for Flare 

Review Panel, dated April 2012. Throughout this review, this report is referred to as the "subject 

report." Two earlier reports (respectively dated January 2012 and April 2010 by Gogolek, et al., 

under the sponsorship of the International Flaring Consortium) have also served this reviewer by 

providing some of the principles underlying the current design and operating parameters of flares.

 Prologue: Presented in Appendix A of the subject report are brief descriptions of ten flare test 

reports and results, published between 1983 and 2011. Their authors come from five (or six) 

different public and private establishments. The test reports contain flare performance (in terms of 

carbon combustion efficiency and vent gas destruction efficiency). Among the many factors 

expected to influence the flare performance are: the flare tip design, configuration and size; 

composition and feed rate of the flare gas; air or steam addition rate in assisted flares; flare tip exit 

velocity; cross-wind speed; and the use of pilot flame as a flare flame stabilizer. The vast array of 

possible combinations of these and other variables (which may now remain unidentified) makes it 

quite difficult to design, build and conduct a versatile experiment in which repeatable measurements 

can be made. It also makes much difficult any attempts to compare, contrast, correlate and interprest 

different sets of experiments and their results obtained by different investigators. 

 It is hoped that the flare tip designs and implementations are based upon a thorough 

consideration of combustion fundamentals from principles of flow, heat/mass transfer, chemical 

kinetics, reaction mechanisms, turbulence, flare gas mixing with air, flow separation, wake 

formation, flame lift-off and blow-off. A number of references dealing with the fluid mechanical 

aspects of flow around the flare tip are refernced in Sec. 3 of the 2010 report. It is not clear whether 

and how these aspects had been incorporated into the designs and builds of the tips engaged in the 

tests considered in the subject report under review.

Section 2: Available Flare Test Data

QUESTION #1: 

 Comment on the criteria for excluding available flare test run data from final analyses, and 

whether application of these criteria may have lead to inappropriate exclusions of relevant data 

points.
2

 Eleven “criteria for excluding data points” are presented and explained in Table 2-6 of the 

subject report. These are identified below as criteria (i)-(xi). As can be seen in the following 

comments, all the eleven criteria appear to be convincing and reasonable to this reviewer. While 

some of the criteria are obvious, others need a closer scrutiny and thinking. 

 (i) The first criterion goes thus; “Test report did not record combustion efficiency (CE) for 

a specific test run.” And its explanation reads, “It was determined that there was not enough 

information to be able to use the data point.” No comment is needed.

 (ii) The second criterion is that the “Test report recorded CE as 0% ...” The explanation was 

that “the flame was completely snuffed out...” Paraphrasing the continued explanation, further 

review indicated that LFLcz was greater than 15.3%, thus indicating that good combustion was not 

obtained. This appears quite reasonable and agreeable to this reviewer. The matter of the threshold 

LFLcz being 15.3% shall be discussed later under Question #3. 

 (iii) The third criterion in Table 2.6 reads, “Test report recorded that the extraction probe ... 

was located in the flame.” The essence of explanation is that the probe gives a good product sample 

only if it is in the flare plume, not if it is in the flame.

 

 (iv) The fourth criterion is stated as “Test report recorded that ... probe positioning ... was 

uncertain.” The run was considered invalid because “the ...technique may not have obtained a good 

sample of the flare plume.” The clause “... may not have obtained ...” present a troublesome and 

ambiguous justification. 

 

 (v) The fifth criterion is “Test report recorded a specific test run time as less than 5 minutes.” 

This test run was discarded because of too much uncertainty and variability in the reported values. 

The explanation is continued saying that also discarded were four other tests in Data Set D which 

had run times longer than 5 minutes. The reason for discarding was stated as “... several of the 

minutes in the average of the test run ... showed zero entries for the PFTIR data. These runs had less 

than 5 minutes of data that were not zero or not affected by wind.” 

 

 This reviewer is convinced that a reliable way of gas sampling with an extraction probe at a 

precisely known location in the plume is at best very difficult and impossible at worst. There appears 

to be enough unresolved variation (or fluctuation) even with a well-placed probe, leading one to 

question the reliability of these composition data. 

 

 (vi) The sixth criterion is “Test report recorded single test runs and an average of the specific 

single test runs, the single test runs were removed, but the average was kept.” The explanation was 

that “The single test runs were considered duplicative because each run was performed at the exact 

same conditions.” 

 

 In the absence of error-bars or scatter-bars, (i.e., departure of the result of one run from that of 

another), this explanation is not quite convincing.

 
3

 (vii) The seventh criterion reads, “Test report recorded a specific test run as smoking.” The 

attendant explanation is that “The specific run was considered out of compliance because visible 

emissions are a violation with the current regulation and such runs should not be used to establish 

operating parameters for good combustion.” 

 

 (viii) “...did not record enough information to determine flare vent gas flow rate for a specific 

run.” It is obvious that there is not enough information to use the data point.

 (ix) The ninth criterion relates to the flare vent gas flow rate. “... flow rate was less than 10 

pounds per hour.” The explanation was that in these extractive tests with low flow, the combustion 

efficiency values were very different from those in other similar runs but with higher flow rates. 

“The extractive test method may not have correctly detected the waste gas compositions because the 

flow rate was too low.” The reviewer cannot complain.

 

 (x) The tenth criterion is specific only to 10 runs in Data Set H. In these runs, vent gas 

involved nitrogen content in excess of 30v%. For these runs, the flare vent gas flow rates are 

reported to be inaccurate.

 

 (xi) The last criterion is specific only to 2 test runs in Data Set C. The combustion efficiency 

was reported to exceed 99%, yet fraction of combustible in the stream was less than 2%. It is 

explained that these two test runs are based on extractive test method which may not have correctly 

detected the waste gas compositions. The reviewer agrees with the statement that “... It is not 

possible for these two test runs to have achieved greater than 99% combustion efficiency (the 

combustibility of the stream is too low).” 

 In closing of this answer to QUESTION #1, it is the reviewer’s opinion that application of the 

foregoing eleven criteria has led only to appropriate exclusions of data points.

Section 3: Steam and Flare Performance

QUESTION #2: 

 (a) Please comment on the lower flammability limit of combustion zone gas (LFLCZ)as an 

operating parameter for indicating over steaming situations on steam-assisted flares. 

 (b) Comment on the agency’s use of the ratio of the net heating value of the combustion zone 

gas (NHVCZ) to the net heating value of the flare vent gas if diluted to the lower flammability limit 

(NHVLFL) as an alternative to LFLCZ. 

 (c) Do the flare data adequately support the EPA’s observations?

 Background: Like the flame at the mouth of a bunsen burner, (with its port of primary air shut 

off), a flame at the tip of the flare is expected to be a diffusion flame. A diffusion flame is one in 

which combustion reactions occur infinitely fast compared to the physical convective/diffusive 

transport (mixing) processes. Due to the larger size of the tip, slower feed of the combustible waste 

gas and complications resulting from cross wind at the tip of the flare, the flare flame will be much 

different from the orderly ideal laminar diffusion flame over a bunsen burner in a lab in which the 

ambient air atmosphere is quiescent.
4

 The flame will be broken up with serious undulations and folds, it will be lifted-off the flare 

tip, it might even be blown off and reattached in the wake to the flare tube which acts as a flame 

holder. The flame under these conditions if not entirely a diffusion flame. Combustion chemistry will 

be of finite rate rather thanof infinite rate. Dilution with steam or someother gas which is presumed 

to be 'inert' will further reduces the rate of chemical reaction kinetics in the flame. The definition and 

control of such a complicated beast as this flame is nearly impossible. Here lies the horrendous 

difficulty, this reviewer believes, in designing and operating flares to effectively and predictably 

destroy waste gas streams.

 With the slowed-down chemistry, the flame ceases to be one controlled solely by physics as it 

becomes a flame in premixed reactants. Flame propagation in a premixed reactant mixture stream is 

governed by both physics abd chemistry. The propagation becomes impossible if the mixture is too 

lean or too rich in 'fuel,' the combustible portion (% 'fuel' by volume or moles) of the waste mixture 

stream. Thus arise the lower (or lean) flammability limit (LFL) and the upper (or rich) flammability 

limit (UFL). Below the LFL and above the UFL flame propagation is impossible. 

 The details belabored in the preceding three paragraphs are important because the LFL is 

determined definitely by the combustion chemistry (in conjunction with whatever physical 

processes). This limit flame chemistry, however, may or may not be the same as in the chemistry in a 

premixed flame away from both the flammability limits. It is definitely not the same as the infinite 

rate chemistry in an ideal diffusion flame. To produce the highest carbon conversion efficieny and 

also destruction efficiency, combustion in the hottest possible flame seems to be desirable. The lean 

limit flame is far from this desirability. 

 (a) With this understood, this reviewer is comfortable to characterize the goodness of flaring 

with the chemistry of flare flame that is available only through the 'black-box' parameter LFL. 

Calculation of LFL at the combustion zone by using the mixture rule of LeChatelier and its 

extensions is reasonable. The description in Secs. 3.1 and 3.1.1 along with Figure 3.3 are sufficiently 

convincing to arrive at the (LFLcz less than equal to 15.3v% and CI equal to or greater than 96.5%) 

rule for practical use. The reviewer cannot move to the next item of the question without 

(presumptuously or, worse, naively) wondering about the significance of the decimal place digits 3 

and 5 in these thumb rule figures. It appears debatable if these digits could not be dropped, without 

harming the hypothesis that supports the correlations, so as to state the thumb rule limits in whole 

numbers, i.e.,15v% and 96%. 

 (b) As the independent parameter that correlates the measured combustion efficiencies, LFLCZ 

has been replaced by the ratio of NHVCZ to NHVVG-LFLwhere the former denotes the net heating 

value of the combustion zone gas while the later stands for the net heating value of the flare vent gas 

if diluted to the lower flammability limit. (Contrast Fig 3.10 with Figs. 3.3, et seq.) Analysis showed 

that this ratio being greater than 6.54 is a signal that the combustion efficiency will be greater than 

96.5%. EPA's first observation is fairly well supported by the flare data correlations using this ratio. 

These correlations appear to be useful in estimating the goodness of flare performance. But, yet an 

improved understanding of the meaning of the ratio and its relation to the LFLcz criterion will be 

valuable. 
5

 In order to discuss the meaning of this ratio, let us recall that the flare vent gas (VG) mixture 

includes all waste gas, sweep gas, purge gas, and supplemental gas, but include neither the pilot gas 

nor the assist media. For a steam-assisted flare, the combustion zone gas (CZ) mixture is formed by 

mixing the flare vent gas (VG) with various quantities of the steam supplied to the flare. If it is an 

air-assisted flare, air replaces steam in forming the CZ mixture. It is clear that the CZ mixture 

contains all the gases injected into the combustion zone of the flare except the pilot gas. It is not 

stated anywhere in the report but it appears that the CZ mixture does not contain any ambient air 

induced into the combustion zone. Noting this may be of later use in discussing air-assist flares. 

 

 Several comments can now be made. First, in all cases, steam, nitrogen, excess oxygen and 

such products of combustion as carbon dioxide and carbon monoxide are considered to be inert. 

This is probably an acceptable assumption because the temperatures involved are sufficiently low 

enough to neglect dissociation and chemical equilibria. Second, it has been said (somewhere in the 

flare combustion literature received by this reviewer through RTI) that one of the reasons for 

injection of steam is to form H and H2 in the CZ mixture in order to increase the flame reaction rates 

as well as carbon/soot combustion rates. At a flame temperature near 1,200K, it is doubtful if H2O 

decomposition will be measurable, leave alone the dissociation of H2 to H atoms. 

 Third, the reference to composition at which the denominator is evaluated at LFL indicates an 

implicit account for the (especially the chemical) factors that determine LFL in the behavior of flare 

flame. Is an implicit account sufficient in the important practical application this work is intended to 

serve? Fourth, even after a careful study of the report as well as the paper of Evans and Roesler, this 

reviewer did not understand why the denominator of this ratio is NHVVG-LFL rather than NHVCZ-LFL. 

Use of the later appears to be more logical so as to reckon NHVCZ in units of the same at the LFL, 

i.e., NHVCZ-LFL.

 Fifth, the work of Evans and Roesler has prompted the authors of this report to pursue the 

potential of the net heating value of the vent gas (VG) mixture (or of the CZ mixture which is the 

VG mixture diluted with steam) as an indicator of the combustion efficiency of the flare. In the 

second sentence of the first paragraph of Sec. 3.3, it has been remarked that the heating value is "... 

nearly identical mathematically to the reciprocal of the inverse of the LFLCZ ...". With apologies for 

nitpicking, this reviewer points out that it is not "... identical to the inverse," but rather "... inversely 

proportional to ...". At the 9th line in the second paragraph of the same section, it has been repeated 

that "... this ratio is mathematically equal to the inverse of the LFLcz ..." Again, it is not "... equal to 

the inverse ..." but "inversely proportional ..." 

 Sixth, an inverse relation should not be surprising. Nor its important meaning and potential be 

under-estimated. When one multiplies the combustible content in the VG mixture (or the CZ mixture 

which is the quantity on the x-axis of Figure 3.9) by the average heating value of this combustible 

mixture, one obtains the total chemical energy released in thermal energy form (i.e., heat) upon 

complete combustion. [Notice that the quantity on the x-axis of Figure 3.12 is the "net" heating value 

of the VG.] All constituents of the mixture other than the combustible content (with inclusion of 

nitrogen associated with the burned oxygen plus both N2 and O2 of the excess air, if any) have 

nothing to do with thermal energy release. They are there only to absorb a significant portion of the 
6

heat release in order to raise their temperature from the precombustion state to the combustion 

product stream state. As a result, the "flame temperature" (i.e., the temperature of product mixture 

resulting from combustion of the CZ mixture) is dramatically reduced; and with it, the flame reaction 

rates are diminished to a level at which it is susceptible to extinction without or with blow-off by the 

cross wind.

 It is not very difficult to combine the x-axes of Figs. 3.9 and 3.12 (after the basis of one or the 

other is converted to CZ or VG, to be consistent) and to plot the combustion efficiency as dependent 

on the product of combustible volume fraction by the average heating value. One need not be 

surprised if such a correlation turns out to be at least as meaningful as shown in Figs. 3.9 and 3.12, if 

not better. We will refer to this and the preceding paragraph in answer to Question #4 part (b). 

 And finally, the seventh comment at this junction relates to the matter of dilution due to 

adding steam, air or nitrogen to the VG flow at the flare tip. The intended purpose of injecting the 

assist-steam is three fold: first, to control smoke, hydrocarbon pyrolysis and soot formation; second, 

to enhance the homogeneous reaction rates by possibly increasing the supply of H2 from 

decomposition of H2O and H atoms from dissociation of H2; atoms; and third, to reduce the flame 

temperature due to raising the sensible enthalpy of H2O in the products of combustion. It may be that 

the third intended pupose is in conflict with the first two. It is clear why serious head aches arise due 

to "over-steaming" the stream to be burnt. 

 Here are a few questions this reviewer would like to pose at this point. Are there any tests 

done to inject superheated steam to keep the flare "flame temperature" from getting lowered too 

much due to the heat sink effect of the assist-fluid? Is there ever a buoyant flow up the pipe from the 

VG supply to the flare tip? Would such a buoyant "chimney flow" help obtain a healthy jet flame 

flame based at the flare tip? What is the pipe height, approximately? What is the temperature of VG 

at its inlet into the pipe? The answers are certain to be available in the published literature to make 

estimation possible of buoyancy and its relative importance in controlling the shape and stability of 

the flare flame at the rim. 

 (c) Do the flare data adequately support the EPA's observations? The data do indicate that 

the lower flammability limit of combustion zone gas (LFLCZ) is the most appropriate operating 

parameter. Analysis of existing data lead the Agency to conclude that, in order to maintain good 

combustion efficiency, the LFLCZ must be 15.3 percent by volume or less for a steam-assisted flare. 

 The data and analysis also suggest, and show, as an alternative to LFLCZ, that the ratio of the 

net heating value of the combustion zone gas to the net heating value of the flare vent gas if diluted 

to the lower flammability limit, (NHVCZ)/(NHVVG-LFL), must be greater than 6.54 to give 

combustion with efficiency greater than 96.5%. 

QUESTION #3: 

 (a)Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? 

 (b) Is there other data available (that is not discussed in this report) that may help clarify our 

discussion about specific chemical interactions related to lower flammability limits of gas mixtures?
7

 (a) A brief tutorial may be appropriate here. Most of the LFL and UFL data are in textbooks 

and Handbooks. They were originally measured (in flame-tubes, bunsen burners, flat flame burners, 

soap bubbles, etc.) in the last 75 years, by combustion scientists at research laboratories, especially 

at the U.S. Bureau of Mines in Pittsburgh, Pennsylavania. Much theoretical work is available in the 

combustion literature which includes the Combustion Institute's Proceedings of the the International 

Symposia, Journals such as Combustion and Flame, Combustion Science and Technology, and 

others. Most of the theoretical models have the goal of calculating the "fundamental" speed of flame 

propagation in premixed gaseous reactants (i.e., [pure-fuel-gas + air] mixtures) under the balance of 

heat conduction and energy release in the reaction. When conduction and convection are excessive, 

the reaction zone temperature decreases to such an extent that the flame gets "quenched" or 

"extinguished." 

 Even if the reaction temperature is high, the reaction rate may be diminished due to a 

diminishment of the fuel content or oxygen content in the mixture due to their consumption in the 

progress of the reaction. When either the fuel content or the oxygen content are sufficiently small, 

the reaction rate will become so small that the flame propagation ceases to occur. This is called 

"extinction," different from "extinguishment" or "quenching" due to excessive heat losses. The 

concepts of fuel-lean composition limit (LFL) on one side and (oxygen-lean) or fuel-rich 

composition limit (UFL) on the other side have thus evolved as measures of limits of composition 

between which flame can propagate. 

 The flammability limits are usually tabulated for premixed [pure-fuel-gas + air] mixtures; not 

for "diffusion flames" in whichthe chemistry is infinitely fast compared to the physical processes 

such as flow, diffusion, conduction, turbulence and other mixing process rates. Flare flames, like a 

bunsen flame with the primary air supply port is shut off, are diffusion flames. Yes, near the burnerrim even the a diffusion flame would suffer heat loss by conduction and the flame is quenched near 

the rim. It is not inconceivable that all flames near extinction are premixed limit flames. 

 Based on this tutorial, the first half of the question can now be answered. Even in the limit 

flames, chemical reactions still occur, albeit at a substantially rates smaller than in nearstoichiometric mixtures, by rather complicated reaction mechanisms and interactions. Concerns that 

these slow chemical interactions may make the LFLCZ < 15.3 threshold criterion for efficient 

combustion inaccurate are most probably unwarranted. A dying flame shall surely die at the 

flammability limits. 

 Critique of the LFLCZ: The CZ stream is composed of the VG stream plus the assist-fluid. 

What is to be burned is the mixture formed by the mixing of the CZ strem and entrained air. It is this 

mixture which has to be maintained at sufficiently high fuel content in the spirit of the lower 

flammability limit. This notion seems to be missing in the literature of flare design and operation. If 

the reviewer is unaware of any published open literature to the contrary, he offers a sincere apology 

for this critique of the state of the art of flare technology. 

 (b) Assuming that the "data" referred are "flare flame data," this reviewer's answer to the 

question is "most likely, No." Section 2 and Table 2.6 convince this reviewer that the agency had 
8

diligently collected all the available test data and screened them thoroughly for inclusion in the 

subject report. This reviewer, being a combustion scientist rather than an expert in flare technology, 

is unable to point out to the agency any data that may have been missed to consider in their work. 

We will refer to this paragraph below in the answer to Question #4 part (a). 

QUESTION #4: 

 (a) Did the agency adequately examine other operating parameters (different from LFLCZ; or 

the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming situations? 

 (b) Are there specific other parameters that should be given more or less emphasis? 

 (a) Answer is "No." Only the work reported in the subject report and its appendices has been 

reviewed here. In it, other possible flare operating parameters appear not to be considered. The 

answer to Question #3 part (b) above applies to this question also. 

 (b) As answer to this question, the reader is referred to the reviewer's response to Question #2 

part (b), the two paragraphsconstituting the sixth comment. 

 

Section 4: Air and Flare Performance

QUESTION #5: 

 (a) Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. 

 (b) Additionally, also comment on whether the lower flammability limit of the flare vent gas 

LFLVG is an appropriate operating parameter for determining whether the flare vent gas being sent 

to an air-assisted flare is capable of burning? 

 (c) Do the flare data adequately support the EPA’s Observations?

 (a) (1/SR) is related to what is known in combustion literature and practice as the 

"equivalence ratio." If SR = 1, fuel and air are supplied in stoichiometric proportions. SR < 1 

represents a "fuel-rich" (or deficient-air) combustion which is of no interest in flare design and 

operation. SR > 1 stands for "excess-air" (or fuel-lean)) combustion. (SR minus 1) is a measure of 

the excess air. Whereas SR in most engine combustion processes is set to be only 10 to 30 % greater 

than unity, a camp fire involves buoyancy-induced air to yield an SR lying in the range 5 to 10. (A 

peripheral note: SR is numerically the same in both mass units and volume (or molar) units.) 

 Based on the correlations shown in Figs. 4.1-4 of the subject report, one of the conclusions 

reached in Sec. 4 was that SR be 7 or less to obtain good combustion in the flare. This is also 

consistent with the combustion literature in that a higher the SR value leads to a lower flame 

temperature because the excess air acts as a thermal diluent of the product mixture. The threshold 

number 7 established in the subject appears to be quite reasonable in magnitude. The quantity (SR 

minus 1), the excess air fraction, is thus desirable to be 6 or less to produce good combustion in the 

flare. 

 A description of how the value of SR is obtained is either absent in the subject report or so 

meager that it failed to catch the eye of this reviewer. Is all of the air involved in SR calculation 
9

introduced at the flare tip to mix with the vent gas to form the CZ mixture, just as steam is 

introduced in the steam-assist tests? 

 If the answer to this question is in the affirmative, an interesting dilemma arises. Assuming 

that the efficacy of the assist-fluid, in doing what it is supposed to do, is roughly independent of the 

nqature of the assist-fluid, it appears reasonable to expect the same amount of ambient air entrained 

("educted," as called by Evans and Roesler), into the flame of both the air and steam assist cases, 

into the CZ mixing zone to burn the combustible portion of the same vent gas. In the case of airassisted flare, air has been first provided in a controlled manner as the assist-fluid and then again as 

the ambient air which is entrained (in an autonomously controlled manner) into the CZ mixture. In 

the case of steam-assist, however, air is entrained into the CZ mixture only once, i.e., via the second 

step. Has the resulting drastic difference between these two assist situations been noted, observed or 

described in any of the test reports? Has this mechanism been discussed and effects accounted for, in 

any analysis? 

 [Taking a digression, 

 Note that in Sec. 3, steam-assisted flare data of combustion efficiency are attempted (Fig. 

3.15) to be correlated with steam to vent-gas ratio S/VG by mass as the independent parameter. This 

ratio is somewhat analogous to SR. The analogy and similarity or dissimilarity between the two 

parameters appears to deserve some attention. 

 The conclusion about S/VG analysis was that the combustion efficiency degrades steeply to a 

value below 90% when S/VG exceeds 0.5. The data contained in Fig. 3.15 are replotted in Fig. 3.16 

with units of S/VG ratio changed from "by mass" basis to "by volume" basis to claim, with little 

explanation, that the trends are "consolidated" by the use of volume basis. Figure 3.17 is similar to 

Fig 3.16, with S/VG, again by volume, but the vent gas fuel is a well-defined hydrocarbon. The 

conclusion is that "The figure does seem to show further improvement ..." The reviewer is able to 

justify neither the "consolidation" nor the "improvement." 

 Steam is a diluent; it has simultaneously two undesirable effects on the performance of the 

flare. As a concentration diluent steam reduces the combustible concentration in the mixture entering 

into the combustion region; this would tend to decrease the reactivity of the mixture with the 

entrained air. As a thermal diluent with a large heat capacity, steam takes up a significant fraction of 

the heat released in the combustion process, thus reducing the "flame temperature." The reduction in 

temperature tends, in its own turn, to lower the reaction rate. These two effects alone are expected to 

lead to a significantly decreased flare performance. Use of "superheated steam" may curtail some of 

the the damage. 

 Not wanting to be a nay-sayer, the reviewer remains reluctant to raise the issues of soot and 

smoke emissions from a flare the performance of which is negatively influenced by steam's role as a 

diluent. 

 End of the digression.]

 

 (b) Because SR deals only with thermodynamics but not with chemistry, the authors invoke 

LFLCZ, (which is equal to LFLVG in the air-assisted case), as an indicator of how well a vent gas+air
10

mixture would burn. The conclusion was that along with the criterion of SR equal to 7 or less, an 

LFL less than 15.3v% will likely result in a combustion efficiency in exceess of 96.5%. "There is 

simply not enough air-assisted test data" to confirm this conclusion. More air-assist test runs appear 

to be required to arrive at an upper limit value of LFLVG more reliable than the 15.3v% for efficient 

combustion of the VG. 

 

 (c) Do the flare data adequately support the EPA's observations? The answer is this: "only 

partially." An explanation is immediately warranted. That SR is one of the appropriate operating 

parameters is quite correct. It is true that data suggest an SR of 7 or less would give a good 

combustion efficiency for an air-assisted flare. To declare SR is the "most appropriate" parameter is 

misleading and incorrect. By itself SR < 7 does not guarantee good combustion in the flare. The vent 

gas lower flammability limit LFLVG has to be lower than about 15.3v% to make the CZ mixture to 

result in a vigorous burn with the abundant air supply from the the SR < 7 rule. 

 For air-assisted flares, the exact value of the maximum allowable LFLVG per centage number 

is not as firmly established as is the 15.3v% or lower rule for steam-assisted flares. One might argue 

that the EPA's statement "The LFLCZ should be 15.3% or less to ensure ... adequate burning ..." is a 

safely conservative rule. But still there is no assurance of the rule for air-assisted flares in the 

absence of more test data. It has been candidly stated in the subject report, in the last sentence of the 

penultimate paragraph of Sec. 4 that "... there is simply not enough test data to determine whether a 

new LFLCZ threshold would be warranted for air-assisted flares (i.e., a LFLCZ threshold different 

than 15.3 percent)."

Section 5: Wind and Flare Performance

QUESTION #6:

 (a) Please comment on the momentum flux ratio (MFR) as an operating parameter in 

crosswind velocities greater than 22 mph at the flare tip to indicate wake-dominated flame 

situations. 

 (b) Additionally, also comment on the agency’s observation that in the absence of crosswind 

greater than 22 mph, a low MFR does not necessarily indicate poor flare performance. 

 (c) Comment on the effectiveness of observations identifying wake-dominated flames. 

 (d) Do the flare data adequately support the EPA’s observations?

 

 (a) MFR is the ratio of the momentum flux of the jet at the flare tip to the momentum flux of 

the cross wind. A large value of MFR leads to a stable vertical flare flame jet on which the cross 

wind effects are ignorable and the combustion efficiency is high. A low value of MFR, on the other 

hand, indicates large cross wind effects which bend the flame over, drive it into the wake of the wind 

flow (in the lee of the flare tip or stack) and even blow the flame off. The combustion efficiency is 

rapidly decreased as MFR is decreased to and below 0.1. Unburned fuel is observed to be ejected 

into the underside of the flame in the wake. The following paragraph of summary notes leaves the 

reviewer disappointed that there are not enough quantitative test data to fully explore the power of 

MFR to characterize the degradation of flame performance by cross wind wakes. 

 Most of the existing large scale test runs are not only incomplete in measurements but also 

uncoordinated with what measurements are needed in MFR analysis. Not many of larger scale flare 
11

tests included measurement of the CG mixture flow velocities at the flare tips. Based on the best 

possible rough estimates from the limited available tests and their data correlations of combustion 

efficiency as dependent on MFR are shown in Figures 5.6 and 5.7. 

 It is found that most of the tests were done at low MFR, mainly due to low flare tip velocities. 

Most of the tests are with MFR values less than about 0.2. All runs were tested at a MFR less than 

7.0. No tests were done at cross wind speeds above 22 mph. Most of the tests were done with LFLCZ

> 15.3v%. Among those not in this populous group of unacceptable tests, about 25 runs are in the 

useful group with MFR between 1 and 7, LFLCZ < 15.3v%, and CE is above 96.5%. The correlation 

indicates that for MFR less than 3, wake effects dominate. 

 

 (b) Comment on the agency’s observation that in the absence of crosswind greater than 22 

mph, a low MFR does not necessarily indicate poor flare performance. 

 When the agency's statement is parsed into the following five separate and smaller remarks, 

the reviewer finds them logical and meaningful in describing the state of the art of cross wind effects 

on the flame and the combustion efficiency as mapped on the basis of roughly estimated MFR and 

its range. 

 (1) "The data suggest that flare performance is not significantly affected by crosswind 

velocities upto 22 miles per hour (mph)." 

 (2) However, a wake-dominated flame in winds greater than 22 mph may affect flare 

performance. 

 (3) The data available indicate that the wake-dominated region begins at a momentum flux 

ratio (MFR) of 3 or greater. 

 (4) The MFR considers whether there is enough flare vent gas and center steam (if applicable) 

exit velocity (momentum) to offset crosswind velocity. 

 (5) Because wake-dominated flames can be identified visually, observations could be 

conducted to identify wake-dominated flames during crosswind velocities greater than 22 mph at the 

flare tip. 

 (c) The reviewer agrees with the agency's statement (remark 5 above) about the ease with 

which useful visual observations can be made on the flames in the wake. Video is also a visual 

technique.

 (d) Yes. 

QUESTION #7:

 (a) Did the Agency adequately examine other operating parameters (different from MFR) for 

identifying wake-dominated flames? 

 No, not in the subject report. We learn (from the 2010 and 2012 Consortium reports by 

Gogolek, et al.), that the flare literature contains, in addition to MFR, the definitions and bases of 

two other nondimensional parameters for characterizing the wake regime, its inception, its capture of 

the flame and degradation of combustion efficiency. These parameters are the "Buoyant Plume, BP" 

group and the "Power Factor, PF" group. The BP group is the wind speed Vwind divided by the 

product (gDpVtip)

1/3 where g is gravitational acceleration, Dp is the effective diameter of the flare pipe 
12

and Vtip is the actual flare tip velocity, (i.e., VG plus center steam velocity). The combination of Vg = 

(gDp)

1/2 denotes a characteristic g-induced velocity. But it is not a velocity representative of 

buoyancy. Vtip represents jet inertia. While increases in either or both of Vg and Vtip compete against 

the Vwind to preserve the stability of the flame column, Vg does not really represent the buoyancy. 

The group (gDpVtip)

1/3 indeed has units of a velocity and it contain gravity g and tip velocity but it 

has no rational meaning. The thoughts underlying the invention of the BP group and its use these 

days are unavailable to me. So I cannot critique it. I can, however, rewrite the BP group into the 

product of [Vwind/Vtip].[Vtip/Vg]

2/3 or into Vwind/[Vtip

1/3.Vg2/3] if they make any useful sense. The 

velocity induced by buoyancy will be discussed in answer (b) below. 

 The PF group makes no more sense to this reviewer than the BP group. Both of their 

definitions account for different (and wrong) mechanisms which ignore the meaning given in the 

field of fluid mechanics to the term "buoyancy-dominated regime" in jet flames, fire and dust 

plumes or submerged jets. Even if taken for their face value, the merits of BP and PF groups cannot 

be judged until analyses using them are developed. Their success is likely stunted by the same 

difficulties as encountered in the application of the notion of Momentum Flux Ratio due to the test 

engineers having not been given a list of variables and properties that have to be measured or 

estimated in a series of tests.

 

 (b) Are there specific other parameters that should be given more or less emphasis? 

 An increased value of the ratio [(P of the flare tip flow)/(P of the cross wind flow)] is 

appropriate in assessing the stability of a vertical flare plume. [Here P is a property which can be: 

inertia indicated by the flow velocity; mass flux which is a product of density and velocity; 

momentum flux which is a product of mass flux and velocity; or energy which is velocity squared]. 

 The stability can be made even better if we can generate buoyancy in the stack. This can be 

done by placing a small auxiliary methane+air flame in the flare pipe a foot or two beneath the flare 

tip. The tip velocity will be increased due to the buoyancy in the hot auxiliary combustion products. 

Density differences which are caused by temperature differences produce the buoyancy. Due to 

buoyancy, he velocity at the flare tip is increased in an order magnitude of [Xg(Tf-Ta)/Tf]

1/2 where g 

is gravity constant, Tf and Ta are the absolute temperatures of the auxiliary flame and ambient air 

respectively and X is the height diffrence beween the auxiliary burner and the tip. A mere 1 ft of this 

head results in 4 or 5 ft/s of velocity increase at the tip. This is the beauty of buoyancy-driven flows. 

Is there a new idea here for designing flare tips and flare stacks? Surely, such an invention must have 

been done by somebody out there before! 

Section 6: Flare Flame Lift Off

QUESTION #8:

 (a) Please comment on the maximum allowable flare tip velocity equation which considers 

combustion zone gas composition, the flare tip diameter, density of the flare vent gas, and density of 

air. 

 This is known as the Shore equation (D. Shore, "Improving flare design: a transition from artform to engineering science," presented at AFRC-JFRC October 2007 Joint Meeting, Waikoloa, HI) 

for the maximum allowable flare tip stream velocity Vmax including (if any) center steam in order to 
13

marginally avoid the flame from lifting off the flare rim. Its units, of course, are ft/s. The Shore 

equation gives Vmax as dependent on: the unobstructed area of flow at the vent rim Au ft2 obtained 

from the flare tip unobstructed diameter; the LFLVGCS (with the subscript VGCS standing for vent gas 

plus center steam, if any; and the ratio of the densities of VGCS and ambient air. The equation contains 

an "empirical" constant 6.85 sec0.2. Having no access to a copy of Shore's presentation, this 

reviewer cannot explain what the empirical constant accounts for where the value comes from. The 

equation also appeared in Appendix D of the subject report with Eqs. D.42-45 dealing with the 

calculation. 

 Let us take a moment here to see an analysis of the test data presented in Sec.6.2 of the 

subject report. Of the total of 356 steam- or air-assisted test runs, only 108 satisfied the requirements 

of good flare combustion (i.e., good steam information, LFLCZ < 15.3v%, air-assist runs with SR < 

7, and flare diameter not too small). Figure 6.1 is a 2x4 cycle log-log plot to portray the power law 

of the equation. On the x-axis is the right hand side of the Shore equation and on the y-axis is the left 

side which contains the maximum allowable flare tip velocity Vmax. The Shore equation is computed 

and shown as a straight line with a positive slope. 

 Also shown on the graph are the flare tip velocities, VVG-S, of the 108 test runs. All but three 

of the 108 points nicely fell under the predicted Vmax line implying that their VVG-S is less than the 

Vmax allowed by the equation and therefore did not experience a lift off. The three points lying above 

the prediction are expected to have exprienced a flame lift-off. One of them is known to have not 

lifted off, probably because the LFL of its vent gas is substantially smaller than the 15.3v%. The 

reports of the other two tests did not contain any remarks on lift-off. The behavior of each set of tests 

on this plot are examined and commented on. One conclusion, in Sec. 6 of the subject report caught 

this reviewer's eye, says that flame lift-off does not necessarily mean poor flare performance. Each 

of the three tests in which flame was expected to have lifted had combustion efficiency in excess of 

96.5%. 

 (b) Does the flare data adequately support the EPA’s observations?

 Yes, quite definitely. 

 (c) Are there specific other parameters or methods/equations that should be given more or 

less emphasis? 

 This reviewer has no more to say than that the Agency's comments in the Sec. 6.3 are 

noteworthy. 

 

Section 7: Other Flare Type Designs to Consider

QUESTION #9:

 Please comment on the applicability of the LFLCZ parameter, maximum allowable flare tip 

velocity equation, and the observations regarding crosswind velocity to non- assisted flares, 

pressure-assisted flares, and other flare designs.

 EPA's observations quite accurately summarize the answer to this question. The combustion 

zone (CZ) mixture is same as the vent gas (VG) mixture for non-assisted flares. Therefore the LFLCZ 

is simply the same as LFLVG. Because the vent gas is well defined, the LFLCZ is better defined and 
14

controlled so as to make application of the "15.3v% or less" rule can be implemented more 

confidently. The cross wind analysis by the measure of MFR and flame lift off analysis from the 

Shore Vmax equation are also directly applicable to non-assist flares; but not to pressure-assisted 

flares and other flare designs because of the lack of any test data on them to confirm. 

 

Section 8: Monitoring Considerations

QUESTION #10:

 (a) Please comment on the appropriate monitoring equipment needed to ensure good flare 

performance and on any other known monitoring methods (not discussed in this report) for 

monitoring the following parameters: LFLCZ, LFLVG, the ratio of NHVCZ to NHVVG-LFL, CCZ, SR, 

MFR, and Vmax. 

 (b) Also, please comment on operating scenarios and conditions where less robust monitoring 

equipment could be used to determine the operating parameters of interest.

 This reviewer does not feel qualified to comment on these matters of monitoring and 

measuring. He hopes the rest of the review will make up for the absence of his contribution on this 

question. Sorry! 
Attachment C 

Gary Mueller 

Principal Consultant Air Quality 

Shell Global Solutions (US) Inc. 
Review of the Parameters for Properly Designed and Operated Flares 

Gary R. Mueller, Shell Global Solutions (US) Inc. 

May 21, 2012 

Summary

This paper correlates a number of combustion zone composition parameters (LFLCZ, NHVCZ, NHVVG, CCZ, 

etc.)with observed flare combustion efficiency data. While fundamentally these parameters can be 

used to estimate when a flame will be extinguished, their ability to correlate when the flare flame 

transitions from a visible, stable flame to flame extinction is dependent upon far more parameters than 

these simple composition measures. While these data may be useful in describing this transition for a

given set of conditions (flare designs and atmospheric conditions), the ability of the parameters 

proposed to insure when a flare flame transitions from stable to unstable, is only as representative as 

the data used to develop the empirical correlation. It is unrealistic to expect these simple parameters to 

describe all combinations of the factors that can impact flare performance. This observation does not

diminish the utility of these parameters to describe the expected trend in combustion efficiency under

similar circumstances. This reviewer believes that in developing the empirical expression for these 

composition parameters, that the authors have put more emphasis on minimizing false positives (poor 

observed combustion efficiency when the correlation would predict good combustion) at the expense of 

producing a large number of false negatives (good combustion when the correlation would predict poor 

combustion). Since it is unrealistic to monitor all the variables necessary to predict every case, this 

reviewer believes that the in selecting the value of the parameter used to insure good combustion, an 

attempt should be made to minimize the standard error of all false positives and false negatives. While 

this reviewer has not done this analysis quantitatively. This would appear to be done at a LFLCZ closer to 

20% and a safety factor of 4.8 on NHVVG-LFL versus the values of 15.3% and 6.54 suggested in this paper. 

While available data do seem to support that MFR can be used to determine flow regime transitions in 

flares, the data do not support that MFR has any broadly applicable ability to correlate combustion 

efficiency impacts of crosswinds. This parameter is not scalable to large industrial flares, and no 

substantive data has been shown that combustion efficiency of industrial flares is impacted by high 

crosswinds. The data presented for flame lift-off has not been shown to add any value over that 

provided by the much simpler current Vmax equation that is a function of vent gas NHV. The case 

presented is not compelling that this parameter needs to be changed. As the data presented for 

combustion efficiency are empirical in nature, one should not expect that data developed on different 

types of flares would necessarily extend to other flares (steam-assisted, air-assisted, unassisted, 

pressure-assisted). While it is the engineer’s hope that there exists a unifying parameter that will 

describe all situations, this is not likely for the universe of industrial flares. As far as monitoring, the 

presence of a visible and stable flame is in fact the clearest indication of good combustion. While the 

composition parameters suggested are indeed useful in correlating operating envelopes that insure a 

visible and stable flame, they are not the only parameters that can be used. A myriad of parameters can 

be used to insure that assist gas stays in an acceptable range to produce a visible and stable flame. In an 
effort to make this monitoring cost effective, approaches that define the operating envelope that 

produces a visible and stable flame using existing flare instrumentation should be explored. 

Responses to Charge Questions

Section 2: Available Flare Test Data 

1. Please comment on the agency’s criteria for excluding available flare test run data from final 

analyses, and whether application of these criteria may have led to inappropriate exclusions of 

relevant data points. 

While the criteria for excluding data from the analysis appear justified, what is striking is the high

percentage of data excluded (270 of 582 (46%) for steam-assisted data, and 67 of 111 (60%)of the airassisted data were excluded). This reviewer would find it useful to break the excluded data into a 

smaller subset of reasons for excluding, such as: insufficient data recorded to calculate necessary 

parameters (no CE recorded, no flows recorded, etc.), data compromised (extraction probe in flame or 

uncertain position, flow data compromised by high N2 content, flow rate too low for extractive probe to 

quantify, etc.), duplicative data (e.g. averages kept, individual runs removed). Of interest to this 

reviewer is the percentage of data collected that was excluded because data protocols were 

compromised and tests where insufficient data was recorded to calculate the necessary parameters, as 

this would seem to indicate inappropriate data protocols or potentially flawed execution of protocols. 

Further, this reviewer sees no reason to exclude 0% CE data points or data points where smoking occurs. 

Indeed both of these represent a violation of current regulatory code, but they do serve a purpose in the 

analysis by defining the current limits of acceptable practice of operation. 

Section 3: Steam and Flare Performance 

2. Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as an operating 

parameter for indicating over steaming situations on steam-assisted flares. Comment on the 

agency’s use of the ratio of the net heating value of the combustion zone gas (NHVCZ) to the net 

heating value of the flare vent gas if diluted to the lower flammability limit (NHVLFL) as an 

alternative to LFLCZ. Does the flare data adequately support the EPA’s observations? 

This reviewer believes the LFLCZ parameter is a useful parameter in estimating the impacts of 

combustion zone gas composition on the inherent flammability of a mixture. As calculated it illustrates 

the impact that various combustibles and non-combustibles will have on the ability to maintain a flame, 

in particular when a flame cannot be propagated. However, the paper spends virtually no time 

discussing the factors that contribute to not being able to produce a stable flame when the LFLCZ is 

greater than 15.3%. In this reviewer’s opinion, as long as a combustible mixture can be obtained when

mixing air with the combustion zone gas, the ability to produce a stable flame has little to do with what 

the LFLCZ value is, but in fact is a function of the turbulent mixing conditions present during the test. The 

LFLCZ parameter is useful in predicting when the flame will be extinguished, but not very useful in 

predicting the transition from a stable flame to an extinguished flame. In essence, the data attempt to 

correlate combustion efficiency data over a range of unspecified turbulent mixing conditions against a
parameter that measures only the composition of the combustion zone gas. While a relationship is 

illustrated in these data, it is this reviewer’s contention that the relationship depicted between LFLCZ and 

combustion efficiency is an artifact of the unspecified mixing conditions present during the test runs. 

The paper spend a significant amount of time explaining the 10 data points which did not achieve a 

combustion efficiency of 96.5% when the LFLCZ was less than 15.3%, but virtually no time discussing the 

greater than 50 data points that achieved a combustion efficiency of greater than 96.5% while having a

LFLCZ of greater than 15.3%. This would seem to be an indication that the relationship depicted is not a 

cause and effect relationship, but rather a convenient way to display the data. While it acceptable to 

attempt such a correlation, it should be acknowledged that such a correlation’s ability to predict other 

combustion conditions is limited to situations that fall within the range of the data presented, and that 

in fact there is no reason to expect this correlation to be generally applicable. 

While the correlation of combustion efficiency versus NHVCZ also suffers from the same pitfall as the 

LFLCZ, parameter (e.g. correlating parameter is unrelated to factors impacting turbulent mixing), Figures 

1 and 2 below would seem to indicate that it suffers from far fewer false negatives than the LFLCZ

parameter (e.g. fewer instances when poor combustion is predicted but good combustion is observed) 

and has a similar number of false positives (e.g. good combustion is predicted but poor combustion is 

observed), depending upon what Safety Factor is applied to the NHVVG-LFL value. Safety Factors of 4.8 

and 7.2 were selected because they produced average limiting values of NHVCZ of the data set of 

approximately 200 BTU/SCF and 300 BTU/SCF, respectively. A safety factor of 4.8 increases the number 

of false positives, but drastically reduces the number of false negatives, as opposed to a safety factor of 

7.2. While not technically rigorous, the average limiting NHVCZ for the steam assisted data using a safety 

factor of 4.8, also reduces to the limit previously determined to insure a stable flame for unassisted

flares (e.g. 200 BTU/SCF). One would expect as the steam assist approaches zero, the limiting NHVCZ

should approach that of an unassisted flare. Selection of a safety factor should consider minimizing not 

only the false positives, but also the false negatives . Since the NHVCZ is a function of the LFLCZ, one 

would expect that this safety factor approach would also apply to the LFLCZ parameter. While the safety 

factor chosen is obvious within the calculation of the NHVlimiting approach, it is less obvious in the 

somewhat arbitrary selection of the limiting LFLCZ of 15.3%. While there are some false positives with a 

safety factor of 4.8, the bulk of these observations have combustion efficiencies above 90% and/or are

relatively close to the NHVlimiting value (e.g. while in error, not grossly in error). No experimental 

uncertainty was placed upon either the combustion efficiency value or the NHVCZ value, but the bulk of 

the false positive values at a safety factor of 4.8 are probably close to being within the uncertainty of the 

experimental data. 
Figure 1. NHVlimiting calculated as 4.8 times the NHVVG-LFL

Figure 2. NHVlimiting calculated as 7.2 times the NHVVG-LFL

0.00%

10.00%

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-200.00 -100.00 0.00 100.00 200.00 300.00 400.00 500.00 600.00 Combusiton Efficiency,%

NHVcz - NHVlimit, [BTU/SCF]]

Series1

0.965

Safety Factor =4.8

0.00%

10.00%

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-400.00 -300.00 -200.00 -100.00 0.00 100.00 200.00 300.00 400.00 500.00 Combusiton Efficiency,%

NHVcz - NHVlimit, [BTU/SCF]]

Series1

0.965

Safety Factor =7.2
3. Is there sufficient evidence that chemical interactions are occurring that make the calculated LFLCZ

inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is there other data available (that 

is not discussed in this report) that may help clarify our discussion about specific chemical 

interactions related to lower flammability limits of gas mixtures? 

As stated in charge question 2 above, while there may be some slight inaccuracies involved with the 

calculation of the LFLCZ, this reviewer believes this has little bearing on the 15.3% threshold proposed. 

The LFLCZ can be used to determine when a flame will not be present, it has little value in determining 

the transition from a stable flame to the extinction of that flame. This would be governed by the 

turbulent mixing created at the flare tip. To the degree the data set used is representative of all flare 

tips and all atmospheric conditions encountered at these tips, the 15.3%threshold presented is 

representative. As stated in charge question 2 above, the selection of the 15.3% threshold would 

appear to be based upon minimizing the false positives (bad combustion when good combustion is 

predicted), at the expense of creating many false negatives (good combustion when poor combustion is 

predicted). This reviewer believes that both false negatives and false positive should be minimized, with 

the intent of minimizing the magnitude of the error in both false negatives and false positives. While it 

may be convenient to use these combustion zone composition data to correlate the performance data, 

there is no fundamental reason these parameters should correlate all the data. The ability to fit the

data is only as good as the data used to develop the correlation, and is only representative of all flares to 

the extent the data set used is representative of all flares and all atmospheric conditions encountered. 

As such, there will always be exceptions to these empirical correlations, and the regulatory structure

should acknowledge that in the absence of a fundamental dependence with the correlating parameter, 

a procedure to determine alternate compliance metrics should be allowed. 

4. Did the agency adequately examine other operating parameters (different from LFLCZ; or the ratio 

of NHVCZ to LFLVG-LFL) that could indicate over steaming situations? Are there specific other 

parameters that should be given more or less emphasis? 

The report did not specifically mention the Reduced Steam Volume Fraction (RSVF) introduced by 

Gogolek (Gogolek, 2010a), but in essence this parameter is but a rearrangement of the LFLCZ. Just like 

LFLCZ, the RSVF only predicts where a flame will be extinguished, and it too uses an empirical plot of 

available data in an attempt to predict where a high combustion efficiency will be insured (see Mueller, 

et al., 2011). What is lacking is a discussion of the impacts that turbulent mixing parameters can have 

on observed combustion efficiency (Smith, 2009). While use of Large Eddy Simulation (LES) is a 

developing science in regards to assessing combustion efficiencies for flares, which relies upon 

computational fluid dynamics that may not be warranted in many situations, some discussion of the 

variables that impacts the results of these LES analyses is warranted. Some discussion of the parameters 

that are important in the LES analysis of flare combustion efficiency, and whether the data set used for 

the correlations presented are felt to be representative of these turbulent mixing variables would go a 

long way toward addressing if the correlations being proposed are truly representative or merely an 

artifact of the data sets available. 
Section 4: Air and Flare Performance 

5. Please comment on the stoichiometric air ratio (SR) as an operating parameter for indicating 

excess aeration situations on air-assisted flares. Additionally, also comment on whether the 

lower flammability limit of the flare vent gas (LFLVG) is an appropriate operating parameter for 

determining whether the flare vent gas being sent to an air-assisted flare is capable of burning. 

Does the flare data adequately support the EPA’s observations? 

In theory, if the LFLCZ parameter were a fundamental correlating parameter for combustion efficiency, it 

should work equally as well with air as the inert (being as air is 80% nitrogen) as with steam as the inert. 

While this analysis was not conducted for the air-assisted data, the reviewer has attempted to use the

TCEQ air assist data and plot it versus a modified RSVF as defined by Mueller (2011). This preliminary 

(unpublished analysis) indicated that combustion was seen to occur well past and RSVF of 1.0, which 

should be the theoretical flame extinction point. What this indicates to this reviewer is that the 

turbulent mixing of the air-assisted flare tested is considerably different than that of the steam assisted 

flares, with potentially much less penetration of the combustion zone gases by the assist air than is 

observed with steam in the steam-assisted flare tips. While the combustion efficiency data can be 

correlated to SR for the data set presented in this paper, there is little data or reason to believe that the 

small data set presented is necessarily representative of the entire universe of air-assisted flares. In fact, 

the two data sets presented in the paper are dramatically different (TCEQ and EPA-600/2-85-106). 

Clearly, the calculated LFLVG is appropriate for assessing whether the vent gas going to an air-assisted 

flare is capable of burning. What is not immediately obvious to this reviewer is what that value needs 

to be in order to insure a high combustion efficiency. This would be a function of the turbulent mixing 

achieved at the air-assist tip. While the paper correctly states that . . .”, there is simply not enough airassisted test data to determine whether a new LFLCZ threshold would be warranted for air-assisted 

flares.”, there also is not enough data to assess whether there is an appropriate value of LFLVG that 

needs to be achieved to insure good combustion, as the turbulent mixing behaviors of the steamassisted tips and the air-assisted tips are clearly different. While LFLCZ or LFLVG can be used to determine 

a composition that can burn, it is the turbulent mixing of the tip that will determine when the flame 

becomes unstable. The analysis presented in Section 4 is not technically rigorous, and the conclusions 

presented are not warranted by the data. 

Section 5: Wind and Flare Performance 

6. Please comment on the momentum flux ratio (MFR) as an operating parameter in crosswind 

velocities greater than 22 mph at the flare tip to indicate wake-dominated flame situations. 

Additionally, also comment on the agency’s observation that in the absence of crosswind greater 

than 22 mph, a low MFR does not necessarily indicate poor flare performance. Comment on the 

effectiveness of the observations identifying wake-dominated flames. Does the flare data 

adequately support the EPA’s observations. 

While MFR has been shown to be a useful parameter is assessing the transition from one flare flow 

regime to another (inertia-dominated, buoyancy dominated, and wake-dominated), the ability of this 
parameter to be predictive of combustion efficiency has been mixed at best. While impacts of 

crosswinds have been demonstrated on small flares (1”-3”), it has also be demonstrated that the impact

on combustion efficiency is less as the flare size gets larger (Gogolek, 2010a & 2010b). Further the data 

published in Figure 5-5 by Seebold et al. showing a strong correlation with MFR have been further 

researched by Evans et al. (2011), and virtually all the data points showing diminished combustion 

efficiency have been traced back to 1” flare studies, which do not necessarily scale to larger flares. In 

addition, MFR is a hydrodynamic parameter that fails to incorporate the buoyant energy released at the

flare tip due to combustion of the gas. Gogolek’s Power Factor makes an attempt to incorporate the 

impact of this force on the flow regime. Note that for the Power Factor to be valid, combustion has to 

be occurring at the tip. Gogolek uses an RSVF of less than 0.8 to insure good combustion is occurring. 

While RSVF was not calculated in the data set presented, most of the full-scale test data points with 

combustion efficiency below 90% are likely at an RSVF of greater than 0.8 and thus would be excluded 

from Gogolek’s PF analysis. In summary, MFR has been successfully correlated with flare flow regime, 

but MFR’s ability to correlate combustion efficiency is size dependent, and has to this reviewer’s 

knowledge never been shown to correlate to a noticeable decrease in combustion efficiency in any flare

larger than 6” in diameter. While full scale flare data have illustrated some instances of diminished

combustion efficiency, it remains to be demonstrated that this was due to high crosswinds. As in 

virtually all the cases were a significant decrease in combustion efficiency was observed, significant

combustion zone composition deficiencies were present. The technical case presented for a limiting 

MFR is not technically robust nor complete, and in this reviewer does not see that sufficient evidence

has been presented to warrant establishing a minimum MFR at this time. 

7. Did the agency adequately examine the other operating parameters (different from MFR) for 

identifying wake-dominated flames? Are there specific other parameters that should be given 

more or less emphasis? 

While the Power Factor was examined, the analysis failed to recognize that test points conducted at an

RSVF of greater than 0.8 are representative of points where inadequate combustion was occurring for 

the Power Factor to apply. Since RSVF was not calculated for the data set, it is impossible to say with 

certainty from the data presented. However, by analogy it is reasonable to assume that the bulk of the 

data points with combustion efficiencies below 90% were conducted at RSVFs above 0.8. Thus, for the 

full scale data, the only conclusion that can be drawn is that for the PFs calculated from the test data, no 

discernible negative impact on combustion efficiency was observed. While the IFC data (Gogolek, 2010b) 

illustrate the potential for some small combustion efficiency impacts at PFs in excess of 0.05-0.07 (note 

data in this paper appear to be using the uncorrected PF text, not the January 2012 corrected version of 

Gogolek, 2010b), the corrected PFs are a factor of 100 lower (e.g. scale in Figure 5-9 should be 0.0005 to 

0.0055). None of the full scale tests were conducted at a wind speed where a significant impact on 

combustion efficiency would be expected. Also, Figure 14 from Gogolek, 2010b (corrected) is produced 

below. Please not the apparent dependence of flare tip size in these data. While no significant impact 

of wind was noted until wind speeds exceeded 22 mph, it is uncertain that any impact would be seen on 

commercial scale flares. Evans and Seebold (2012) have postulated that the negative pressure force 

that causes wake-stabilized flames is proportional to, v2/r (velocity squared divided by flare radius), as 
such, the negative force leading to wake-stabilized conditions diminishes significantly as the size of the 

flare increases. While it remains to be shown, it plausible that the PF correlation shown in Figure 14 

below continues to increase with increasing flare diameter, such that it is quite possible that no 

noticeable impact of ambient wind will be observed on large commercially available flares. Which is 

certainly was the current full-scale data have shown (Evans, 2011). The data presented do not 

demonstrate a compelling case that high ambient winds have any impact on the degradation of 

observed combustion efficiencies of commercially available steam- or air-assisted flare tips. 

 

Section 6: Flare Flame Lift Off

8. Please comment on the maximum allowable flare tip velocity equation which considers 

combustion zone gas composition, the flare tip diameter, density of the flare vent gas, and density 

of air. Does the flare data adequately support the EPA’s observations? Are there specific other 

parameters or methods/equations that should be given more or less emphasis? 

While the data presented in Section 6.0 are intellectually interesting, these data are really of little 

probative value in assessing whether Eq. 6-1 offers any advantages over the current Vmax equation (e.g 

Vmax as a function of vent gas NHV). It would be worthwhile to plot the 108 data points against the 

current Vmax equation. I suspect a similar type of graph would be drawn, with most, if not all, data points 
below the controlling Vmax equation. If such proves to be the case, there would seem to be little case for 

action for changing the existing Vmax equation. 

Section 7: Other Flare Type Designs to Consider 

9. Please comment on the applicability of the LFLCZ parameter, maximum allowable flare tip velocity 

equation, and the observations regarding crosswind velocity on non-assisted flares, pressureassisted flares, and other flare designs. 

The LFLCZ parameter clearly is not directly applicable to non-assisted flares, as the expression for LFLCZ

approaches LFLVG as the assist gas goes to zero. While not technically rigorous, it would be intellectually 

pleasing if the limiting LFLCZ or NHVCZ approached that previously determined for non-assisted flares, as 

the amount of assist gas approached zero. To this reviewer’s knowledge no new data on non-assisted 

flares has been generated, so the minimum NHVCZ determined for zero assist gas should approach 200

BTU/SCF. Note that in charge question 2, this reviewer demonstrated that such is the case when the SF 

chosen for NHVlimiting is selected as ~4.8 (e.g . NHVlimiting = 4.8 * NHVVG-LFL). Note that this approach also 

tends to minimize not only false positives, but also false negatives, while minimizing the mean error. 

Since the current fit of LFLCZ with combustion efficiency is but an empirically fit expression, and the 

turbulent mixing parameters of pressure-assisted flares are believed to be considerably different from

that of steam-assisted and air-assisted flares, this reviewer would agree that it is unlikely that the

correlation developed for steam- assisted flares nor air-assisted flares will apply to pressure-assisted 

flares. 

This reviewer sees little value in implementing Eq. 6-1 for steam assisted flares, and therefore it is likely 

that this equation will not be applicable to non-assisted or pressure-assisted flares. 

This reviewer does not see that a credible case has been presented for a minimum MFR to insure the 

combustion efficiency of commercial steam-assisted industrial flares, and lacking a clear case for flares 

were data exists, it seems unlikely that MFR is a meaningful parameter to assess the combustion 

efficiency of non-assisted or pressure-assisted flares either. 

Section 8: Monitoring Considerations

10. Please comment on the appropriate monitoring equipment needed to ensure good flare 

performance and on any other known monitoring methods (not discussed in this report) for 

monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of NHVCZ to NHVVG-LFL, CCZ, SR, 

MFR, and Vmax. Also, please comment on operating scenarios and conditions where less robust 

monitoring equipment could be used to determine the operating parameters of interest. 

Clearly, the simplest and most straight forward monitoring to ensure good combustion is the presence 

of a stable and visible flame. While data on the composition of the vent gas and the combustion zone 

gas have been correlated to good flare performance in this data set, they speak to only the impact of 

composition on combustion and not the turbulent mixing that occurs at the flare tip. These impacts are 

only accounted for to the degree the existing data set is representative of the flares being monitored. 
There is the potential to correlate many variables with observed combustion efficiency results. However, 

this does not mean that combustion efficiency is solely a function of the correlated variable. In that 

regard, the presence of a stable and visible flame integrates all of these variant factors, and many 

measured parameters can probably be correlated to the presence of a visible and stable flame, in 

addition to the parameters mentioned in Section 8.0. Part of assessing what monitoring is appropriate

is matching the frequency of changes in key parameters to the frequency of monitoring these 

parameters. For cases where the vent gas composition does not change (e.g. a loading flare), sampling

vent gas composition periodically is probably sufficient. When flow is associated with specific events, 

then tracking those events may be appropriate to assess flow and composition. In large complex flare 

systems with the potential for the addition of many streams, flow measurement and more frequent 

composition measurement may be indicated. The point is that flare performance can be correlated to 

multiple operational parameters that may be measured and recorded at existing facilities. For a given

flare it is impossible to monitor and record all parameters that impact flare performance, but a viable 

operating envelope that insures a stable and visible flame can be developed using any number of 

measurable operating parameters. The parameters listed in Section 8.0 are clearly potential parameters, 

but are by no means inclusive of all parameters that can be used to correlate good flare performance. 

References

Evans, S. (2011). Insights from Passive FTIR Flare Performance Testing. NPRA Environmental Conference.

New Orleans: National Petrochemical and Refiners Association. 

Evans, S., & Seebold, J. (2012). Real Industrial Flares in the Field: No Evidence of Wind Induced 

Combustion Efficiency Degradation. IFRF 17th International Members Conference Proceedings.

International Flame Research Foundation. 

Gogolek, P., A. Caverly, R. Schwartz, J. Seebold, and J. Pohl. (2010a). Emissions from elevated flares – a 

survey of the literature. Prepared for the International Flaring Consortium, CanmetENERGY (April). 

Gogolek, P., A. Caverly, R. Schwartz, J. Seebold, and J. Pohl. (2010b). Flae Test Facility – Results.

Prepared for the International Flaring Consortium, CanmetENERGY (October), Amended January 2012. 

Mueller, G. (2011). Combustion of mixtures: a modified IFC approach. American Flame Research 

Committee, Combustion Symposium, Sept. 18-21, 2011, Houston, TX. 

Smith, P & Thornock, J. “LES Simulations of Sour Gas Flares in Western Canada” presented at June 1, 

2009 American Flame Research Committee Meeting. 
Attachment D 

Lucy Randel 

Research Director 

Industry Professionals for Clean Air, Houston, TX 
1 

MEMORANDUM 

TO: Research Triangle Institute for U.S. Environmental Protection Agency 

FROM: Lucy Randel 

DATE: May 21, 2012 

SUBJECT: Review of the Parameters for Properly Designed and Operated Flares

Following are my comments in response to the charge questions for review of Parameters for 

Properly Designed and Operated Flares. Overall, the paper is well documented and uses the 

most current flare test data. For the most part, the conclusions appear well supported. My 

detailed comments are provided below.

Section 2: Available Flare Test Data 

1. Please comment on the agency’s criteria for excluding available flare test run data 

from final analyses, and whether application of these criteria may have lead to 

inappropriate exclusions of relevant data points. 

Response: 

For several studies or runs within studies, very limited data were available, making it 

impossible to determine parameters of interest at all or with any reasonable level of 

confidence. These were appropriately excluded. 

Also, exclusion of data where errors or significant uncertainty were noted in the original 

report is justified. 

An entire run from SDP EPF was removed because of problems with GE panametric flow 

readings. Flows were adjusted using alternate data and then used in original SDP report. 

Data excluded appear consistent with conclusions of this report. Their inclusion could 

provide additional support of the observed data trend, but also subject the conclusions to 

criticism of data quality. Excluding these data points does not appear to change

conclusions reached, however. 

Use of average values with statistical range is an appropriate representation of the data. 

The confidence intervals of the average give an indication of variability within test 

conditions. 
2 

Section 3: Steam and Flare Performance 

Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as an operating 

parameter for indicating over steaming situations on steam-assisted flares. Comment on the 

agency’s use of the ratio of the net heating value of the combustion zone gas (NHVCZ) to the net 

heating value of the flare vent gas if diluted to the lower flammability limit (NHVLFL) as an 

alternative to LFLCZ. Does the flare data adequately support the EPA’s observations? 

Response: 

EPA conducted a thorough analysis of multiple data sets and the relevant literature in 

identifying LFLCZ as an operating parameter for indicating over steaming situations. 

While the empirical flare data are used to establish the basic correlations, the parameter 

can be applied to mixtures of different composition by using published data for the LFL 

of individual components. This factor can be calculated from vent gas composition and 

flow rates of vent gas and steam. An advantage of this parameter is that the literature 

referenced provides equivalencies that can be incorporated for different diluents of 

nitrogen, steam and carbon dioxide. 

In the data analysis, Figures 3-3 and 3-4 both show a much greater level of certainty of 

good combustion at an LFLCZ of 10.0%. EPA had plausible explanations for several of 

the outlying points between 10.0% and 15.3% LFLCZ. Nonetheless, as the complexity of 

the flare gas mixture increases, the uncertainty in the value of the calculated LFLCZ can 

be expected to increase. In addition, in actual flaring circumstances, vent gas 

compositions will not be constant and lag times in steam adjustment can occur resulting 

in variable LFLCZ . For these reasons, EPA would need to consider various options for 

time-averaging of parameters and may want to consider adding a factor of safety to 

parameters used to define acceptable combustion conditions. 

The flare test data should be correlated to actual operating conditions to see if the more 

stringent value can be met while still meeting flare manufacturer’s operating guidelines. 

Further the ability to meet this level could be related to the flare design, with more recent 

designs better able to achieve good combustion with less assist gas. Further, flare 

operation at low flows and high turn-down ratios, such as for routine venting, is more 

likely to be operating in the wake-stabilized region and thus more susceptible to wind 

effects. Under such circumstances a more conservative value for LFLCZ may be 

warranted. 

Sine the results are fairly similar, the ratio of the net heating value of the combustion 

zone gas (NHVCZ) to the net heating value of the flare vent gas if diluted to the lower 

flammability limit (NHVLFL) appears viable as an alternative to LFLCZ if the agency 

found it to be a much more practical monitoring parameter that would reduce monitoring 

costs. However, the speciation of the mixture is likely to be one of the larger costs and 

that would be required in both cases. 
3 

2. Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is 

there other data available (that is not discussed in this report) that may help clarify 

our discussion about specific chemical interactions related to lower flammability 

limits of gas mixtures? 

Response: 

The report gives examples from Azatayan et al. on potential reactions that could inhibit 

combustion and describes the differences with the outlier results in the flare data (page 

3-19). Based on this discussion, it does not appear the magnitude of any competing 

reactions is sufficient to invalidate the 15.3% LFLCZ threshold discussed. One would 

expect that reactions are occurring which enhance as well as inhibit combustion and that 

these are both incorporated into the empirical results.

Zhao (2008) conducted upper and lower flammability limit measurements for binary

hydrocarbon mixtures in addition to estimating the flammability limits through CFT-V 

(calculated flame temperature at constant volume) modeling prediction. Measurements 

were conducted on mixtures of methane and n-butane, methane and ethylene, ethylene 

and propylene, and ethylene and acetylene. The model has potential application for flare 

gas mixtures with components whose LFL values are not available. 

3. Did the agency adequately examine other operating parameters (different from 

LFLCZ; or the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming 

situations? Are there specific other parameters that should be given more or less 

emphasis? 

Response:

The discussion of other parameters explains their limitations and how their use could 

require stricter requirements than necessary. On pages 3-33 to 3-34 comparing the 

similar heats of combustion with the widely different flammability limits of ethylene and 

ethanol area illustrates well why NHVCZ would not be a good operating parameter. The 

LFLCZ appears more robust in being applicable to variations in flare design and other 

parameters such as steam to flare gas ratios between the different studies. 
4 

Section 4: Air and Flare Performance

4. Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. Additionally, also 

comment on whether the lower flammability limit of the flare vent gas (LFLVG) is an 

appropriate operating parameter for determining whether the flare vent gas being sent 

to an air-assisted flare is capable of burning? Does the flare data adequately support 

the EPA’s observations? 

Response: 

The amount of data available to evaluate air-assisted flares was much less than for 

steam-assisted flares. However, the 2010 TCEQ data was conducted with multiple 

analysis techniques and repetition of runs with excellent agreement. The TCEQ 2010 

data clearly illustrate that excess air impedes combustion; only nine of 44 runs achieved 

greater than 96.5% combustion. The TCEQ data also show, that for the very high turndown ratios studied, a very narrow range exists where an adequate DRE is achieved 

without exceeding the smoking limitation. (TCEQ, 2011, pg. 92, 94). Also, the data for 

propylene and propane differed somewhat in that the smoking observed with propylene

requiring a higher SR to prevent smoking and thereby limiting the SR that would achieve 

acceptable combustion to only the runs with SR near 7. 

Since all the TCEQ air runs were conducted with the same design and their

correspondence to earlier EPA studies was limited, it is not clear if the same value of SR 

would be appropriate to other flare designs and other gas mixtures. 

Because the change from smoking conditions to poor combustion is very narrow,

accurate monitoring of low flow air flares becomes critical in minimizing emissions. 

An alternate monitoring parameter for air flares could be optical monitoring of the flame 

combined with recording data on air flow rates so that the stoichiometric ratio could be 

determined and compared to the flame monitoring data. 

The detailed discussions of LFL related to steam-assisted flares can be considered in the 

context of air-assisted flares. Calculation of LFL for the mixture would follow the same 

principles. The main difference, as discussed by EPA, is that air in the combustion zone is 

augmented by ambient air making it difficult to get a useful value for LFL in the 

combustion zone. That said, if the lower flammability limit of the flare vent gas (LFLVG) 

is such that unassisted vent gas will not burn, addition of air will not make it combustible. 

Nonetheless, if significant air-assist is required to prevent smoking, as with the TCEQ 

propylene runs, the LFLVG may not provide much information related to actual flare 

performance. 

Section 5: Wind and Flare Performance 

5. Please comment on the momentum flux ratio (MFR) as an operating parameter in 

crosswind velocities greater than 22 mph at the flare tip to indicate wake-dominated 
5 

flame situations. Additionally, also comment on the agency’s observation that in the 

absence of crosswind greater than 22 mph, a low MFR does not necessarily indicate 

poor flare performance. Comment on the effectiveness of observations identifying 

wake-dominated flames. Does the flare data adequately support the EPA’s 

observations? 

Response: 

In EPA’s analysis of flare test data in this report in section 5.4, the MFR does not appear 

to be a controlling variable. Efficiencies appear determined by the LFLCZ. On page 5-15, 

the report states ”it is reasonable to conclude that the momentum flux ratio (MFR) 

should be 3 or greater in crosswinds that are greater than 22 miles per hour (mph) at the 

flare tip.” This seems to be a reasonable hypothesis, but without data above 22 mph and 

MFR of 3, it is not a well-supported conclusion. 

While the cumulative data provide evidence that high winds can affect flare performance, 

the data do not appear sufficient to use MFR as an operating parameter at this time. 

However, collection of data to calculate MFR coupled with visual observations and other 

monitoring parameters could provide useful information. 

The studies on momentum flux ratios that show a strong correlation between wind speed 

and combustion efficiency of a flare are referenced from the University of Alberta and the 

models of Cassimere and Edgar (2006 and 2008)(Johnson and Kostiuk, 2000. They are 

based on data from 1 inch and 0.5 inch simple pipe flares. Gogolek et al investigated 

larger diameter pipe flares as well as flares with flare retention rings and found 

combustion efficiency was not directly correlated with MFR, but more closely related to a 

power factor. 

The amount of data available do not provide any definitive result with respect to 

crosswind effects because most of the runs were conducted in what was most probably 

the wake dominated region. 

6. Did the agency adequately examine other operating parameters (different from MFR) 

for identifying wake-dominated flames? Are there specific other parameters that 

should be given more or less emphasis? 

Response: 

It may be useful to request monitoring data from facilities that will include calculation of 

MFR and LFL, combined with visual or optical monitoring of flares that can be used to 

identify whether it is wake or jet dominated. 

EPA notes “there are two main concerns with relying on observations: (1) that some 

flames may simply be too difficult to see, and (2) it may be difficult to recognize when a 

flame is wake-dominated.”(pg 5-15). Various optical monitors can be used to provide a 

thermal as well as visual picture of the flame. Examples are discussed under question 10. 
6 

 Data could be reviewed on a schedule of every few hours, to be increased in winds over 

20 mph or in upset conditions when more frequent monitoring could inform operators

trying to maintain stable, smokeless flame. 

Infrared cameras were used in the 2010 TCEQ study to define the flame and thereby 

accurately aim the PFTIR detectors and position the sampling probe. 

Section 6: Flare Flame Lift Off 

7. Please comment on the maximum allowable flare tip velocity equation which 

considers combustion zone gas composition, the flare tip diameter, density of the flare 

vent gas, and density of air. Does the flare data adequately support the EPA’s 

observations? Are there specific other parameters or methods/equations that should be 

given more or less emphasis? 

Response: 

The maximum flare tip velocity equation developed by Shore uses the lower 

flammability limit parameter, which is reasonably accessible if the mixture 

composition is known. In Shore’s development of this equation, he found a good fit

for a variety of vent gas compositions including the presence of hydrogen sulfide, 

hydrogen and nitrogen. The flare test data presented by EPA in Figure 6-1 also fit 

this correlation fairly well. 

 As noted by EPA, “all of the recent flare test data were collected during high 

turndown rations” and do not validate the equation at higher flare tip velocities. 

As discussed in section 6.3, other proposed equations for determining exit velocities 

would be difficult to apply to complex mixtures in flare gases because the parameters 

are not easily obtained. 

Section 7: Other Flare Type Designs to Consider 

8. Please comment on the applicability of the LFLCZ parameter, maximum allowable 

flare tip velocity equation, and the observations regarding crosswind velocity to 

non-assisted flares, pressure-assisted flares, and other flare designs. 

Response: 

It appears that EPA’s analysis of non-assisted flares adequately addresses the applicable 

parameters that affect combustion. The one exception would be for non-assisted, 

hydrogen-fueled flares with greater than 8% hydrogen content, which currently are 

subject to different requirements under 40 CFR 60.18 and were not specifically studied in 

this white paper. 
7 

EPA’s conclusion that the observations made in this report for non-assisted, steamassisted and air-assisted flares cannot be directly applied to pressure-assisted or other 

flare designs, based on currently available data, appears justified. The LFLCZ addresses 

the combustibility of the flame, however, which is important in any flare. Since the value 

of the LFLCZ parameter that achieves good combustion may very well be different, 

additional data specific to those designs would be needed before using this parameter to 

evaluate ideal operating conditions. Pressure-assisted flares with high exit velocities are 

more likely to operate in the jetting regime, suggesting wind effects could be expected to 

have less impact on performance. 

Pressure assisted flares are designed to operate at much higher exit velocities than the 

flares studied in this report; hence the maximum allowable flare tip velocity equation is 

unlikely to be applicable. 

Enclosed ground flares have very different designs that operate with lower vent gas 

heating values and isolation from wind effects. The parameters reviewed in this study are 

not likely to apply. 

Section 8: Monitoring Considerations 

9. Please comment on the appropriate monitoring equipment needed to ensure good 

flare performance and on any other known monitoring methods (not discussed in this 

report) for monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of 

NHVCZ to NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating 

scenarios and conditions where less robust monitoring equipment could be used to 

determine the operating parameters of interest. 

Response: 

Several methods exist for monitoring flare performance with several described in the 

2003 presentation, “Flare System Emission Controls” by Zeeco Company presented at 

the Texas Technology Conference and referenced in the white paper: 

“Steam Control Methods 

 Flare Gas Measurement and Ratio Control 

 Optical Analysis of Flare Flame with Output Steam Controller 

 Manual Adjustment of Steam Flow for Smokeless Flame 

 Radiant Temperature Measurement for Steam Control 

Flare Gas Flow Measurement Options 

 Ultrasonic Insertion Type 

 Thermal Mass Flow Insertion Type 

 Orifice Plate 

 V-cone orifice plate 

 Annubar Device 
8 

 Vortex Meter 

 Turbine Meter” 

In California, the BAAQMD Regulation 12, Rule 11 requires continuous flow monitoring 

of vent gas and composition monitoring either continuously or by sampling. The 

manufacturer SICK is just one company that offers ultrasonic flow meters that meet the 

BAAQMD requirements: 

SICK Volume Flow Monitors FLOWSIC100 Flare Ultrasonic flow meter 

https://www.mysick.com/eCat.aspx?go=FinderSearch&Cat=Row&At=Fa&Cult=English&FamilyID

=339&Category=Produktfinder&Selections=35223

A product brochure is attached. (accessed May 21, 2012) 

 Optical flame monitors that monitor flare gas and link into control systems to maintain 

optimum steam composition are also readily available. Using such monitors eliminates 

the manual steam adjustment, which so often results in oversteaming in an attempt to 

eliminate smoking. Effective use of these monitors could potentially greatly reduce 

emissions from large upset and maintenance flare events. At large flow rates, just 

reducing efficiency from 99 % to 98% would double the already large emissions. 

Operators manually adjusting steam are much more likely to err on the side of more 

steam than less. Two examples that use infrared technology are listed below. (Websites 

were accessed May 21, 2012): 

Powertrol SLX-302 Flare Monitor analyzes the flame with infrared technology and 

controls steam flow based on a set point. 

http://www.powertrol.com/flaremon.htm

Williamson Smokeless Flare Stack also uses infrared technology. 

http://www.williamsonir.com/content10646

Hamworthy Combustion Flarscan Automatic Steam - Control System for Smokeless 

Flaring uses radiant temperature measurement. 

http://www.hamworthy-combustion.com/products/flares-thermal-oxidation-gas-cleaningsystems-6/flarscan-automatic-steam-control-system-for-smokeless-flaring-18.aspx

Episodic Emissions 

Emergency releases with high LHV content and flow rates were not typical of the flare 

data referenced in this report, but that does not mean the findings are not relevant to 

these conditions. These episodic emissions release large quantities of VOCs and 
9 

HRVOCs, even under ideal combustion conditions. They also have been correlated with 

high ozone events in Texas. (Murphy and Allen, 2004) However, current operating 

practice often includes manual adjustment of steam flow rate to eliminate smoking. 

Excess steam was clearly shown in this study to reduce DRE. At high flow rates even 

small reductions in efficiency mean large increases in emissions. Education of operators 

with respect to best operating practices is important, but improved monitoring and 

automatic controls such as those described above would be much more reliable and 

should be required in most circumstances. 

Flare Gas Recovery 

Scenarios where flare gas recovery systems are used to eliminate routine flows to flares 

and minimize upset releases may not require continuous flow monitoring. However, 

provisions for controlling steam other than manually based on visual observations should 

be employed. Optical flame monitoring would still be important to identify flame 

properties that cannot be seen visually, especially when steam is used. 

Appendix D: Calculations 

Errata on page D-11, equation D-14 

The equation listed is calculating a mass flow rate, not a molar flow rate. The correct 

equation, which was used in the spreadsheet calculations, substitutes Qj1 and Qj2 

respectively for mj1 and mj2 

Where 

Qj1 =volumetric flow rate of component 1 (i.e., propylene flow or nitrogen flow)), scf/hr 

and 

Qj2 =volumetric flow rate of component 2 (i.e., propylene flow or nitrogen flow)), scf/hr

References 

Murphy, Cynthia Folsom and David T. Allen. “Event Emissions in the Houston Galveston Area” 

(HGA), January 14, 2004, p. A-31, 

http://files.harc.edu/Projects/AirQuality/Projects/H013.2003/H13AppendixA.pdf, accessed June 

19 2011. 

Zhao, Fuman ”Experimental Measurements and Modeling Prediction of Flammability Limits of 

Binary Hydrocarbon Mixtures” May 2008 

http://repository.tamu.edu/bitstream/handle/1969.1/ETD-TAMU-2688/ZHAOTHESIS.pdf, accessed May 16, 2012. 
Attachment E 

Dr. Ranajit Sahu 

Independent Consultant 
1 

Comments by Reviewer Dr. Ranajit (Ron) Sahu on EPA’s Draft Report 

entitled “Parameters for Properly Designed and Operated Flares” 

May 20, 2012 

Please see EPA’s charge questions in bold followed by my numbered comments in italics. 

Please note that comments sometime do and sometimes do not explicitly address the charge 

questions but are considered to be relevant nonetheless, given the context of the discussion. 

Also, although not specifically requested, comments are provided for Section 1 also. 

Comments on Section 1 Introduction 

1. The purpose of the report in so far how EPA intends to use it is not discussed clearly. For 

example, is it EPA’s intent that the report will be used as a basis for the ultimate revision of 40 

CRF 60.18/40 CFR 63.11(b)? Is it intended to identify data gaps in testing that will help refine 

the technical analysis further? Is it both? EPA should provide a clear discussion of purpose. 

Specifically, the report, later in Section 3.4.1, clearly states the shortcomings of using net 

heating value of the vent gas as a criterion in assuring that destruction efficiencies of flares 

remain high – directly undercutting the basis of 40 CFR 60.18 etc. While this is technically 

valid, it begs the question of what EPA intends to do with 40 CFR 60.18 as a result. 

2. This section notes that flares are used as “control” devices in many applications. While that 

may be true in practice, a broader contextual discussion of why that may be inappropriate, 

without first satisfying the requirements of what a proper VOC or SO2 control device should be, 

is in order. The fact that elevated stack flares, which are the subject of discussion in the report, 

cannot guarantee a specified residence time at temperatures necessary for specific hydrocarbon 

destruction should be noted. How this shortcoming can/may be overcome in design and 

operation, coupled with monitoring, can then be logically explored. 

3. This section notes, without data support, that “industry has significantly reduced the amount of 

waste gas being routed to flares....” EPA should provide data support for this assertion. Does this cover 
2 

both routine and non-routine flaring? Is it true of all industrial flares or just those at refineries? Is it 

true in all parts of the country? Also, as a follow-up the report should discuss the implications of this 

statement – in terms of the shift in the operating point (i.e., low turn-down ratio) of flares in routine 

flaring mode, as compared to design, and how that may affect destruction efficiencies. 

4. EPA should avoid (ideally) or carefully caveat all assumptions made in the report. Specific examples 

will be discussed later. Words or phrases like “assume”, “could”, “seems likely” etc. should be 

carefully considered. Ideally, the report should not stray into assertions that have no data support. 

5. In every section, as appropriate, EPA should identify data gaps such as need for additional tests, etc. 

that can be used to test the various hypotheses that are put forth in this report. 

6. As a general matter, before using each of the data sets, such as in Sections 3 and 4, EPA should first 

conduct a proper data validation/data usability analysis. As it stands, EPA seems to assume that all of 

the data (except those excluded via Section 2 discussions) are valid. Only after portions of the data set 

don’t seem to confirm to a specific hypothesis, EPA embarks on a discussion of the “anomalous” data 

and their quality. This is backwards. All validation/usability should be conducted first and decisions on 

acceptability/flagging/rejection should be made up front. Thereafter, all of the valid and useable data 

should be used in the analysis. That way, any “anomalies” can be discussed in the context of the 

hypothesis being tested (i.e., whether or not a particular parameter or metric best predicts combustion 

efficiency etc.).

Section 2: Available Flare Test Data 

Please comment on the agency’s criteria for excluding available flare test run data from 

final analyses, and whether application of these criteria may have lead to inappropriate 

exclusions of relevant data points. 

7. First, a fundamental comment on this section and Table 2-1. EPA notes that this section 

identifies the data and reports “...that were used to support our investigation...” However, it is 

clear from Table 2-1 that there are no reports or data sets listed between September 1985 and 

May 2010 – a period of around 15 years. Is it EPA’s position that there are no relevant data 

sets available, from researchers or from operating flares, in this entire time period? If so, EPA 
3 

should explicitly state so and justify. If not, EPA should expand this table and include additional 

available data sets or provide a discussion of why they could not use such data. Regardless, 

EPA should broadly discuss its data gathering approach (was data limited to just US or did EPA 

look globally? for example, etc. ) and help the reader understand how it arrived at the list of 

reports in Table 2-1. As it stands, the selection of the starting data set and reports, without this 

contextual and supporting discussion, seems arbitrary. 

8. Exclusion of data sets B and I appears to be proper. 

9. However, as noted in the general comments above, this exclusion or rejection should be done 

as part of a broader data validation/data usability exercise for each data point that would 

consider the data point in detail. As this stage, EPA seems to be excluding data sets/points that 

have obvious flaws (such as missing parameters, non-representative flare type etc.). The 

implication is that data not excluded at this stage are all valid and useable. But, in later 

discussions, EPA states that certain non-excluded data points could still be problematic. For 

example, in Section 3.1.3.1, EPA notes that certain data runs may be invalid because “...wind 

came from a certain direction steam may have interfered with the pilot due to a “shaping steam ring 

failure..” during these runs. In Section 3.1.4.1, EPA speculatively discusses the extent to which steam 

may have partially mixed or not in certain runs. In Section 3.1.4.2, EPA states that “...there could be 

issues with the testing of the combustion efficiencies for these runs. Information was not available to 

judge the accuracy for the FHR test runs. The SDP EPF test report cited the possibility of inaccurately 

high combustion efficiency measurements during times of unstable combustion conditions. However, the 

test runs were not identified in the report where this may have occurred.” In Section 4.4, EPA rejected 

all data from EPA-600/2-85-106 and retained only the TCEQ data. All of these examples raise issues 

with non-excluded data from Section 2. While the deficiencies EPA points out may or may not be true, 

such “opportunistic” exclusions/aspersions of data, raised later, simply because certain data do 

not fit certain hypotheses, is properly avoided, by completing a more thorough 

validation/usability analysis up front in this section. Otherwise, the integrity of the conclusions 

is questionable. 

10. EPA should consider including one or two additional technical appendices describing AFTIR 

and PFTIR, and additional techniques (DIAL?) that have been used to collect data remote flare 
4 

data. Specifically, the actual form of implementation of these techniques that was used, and 

limitations, should be discussed. 

11. Combustion Efficiency, Destruction Efficiency, and “Good” Performance Level. 

Section 2.8 touches on these extremely critical foundational issues, which affect the entire report. 

(i) First, the report should clearly define combustion efficiency (CE) and destruction 

efficiency (DE). In particular, this reviewer does not believe that the definition of destruction 

efficiency implicitly assumed in the report is correct. This is a fundamental problem that affects 

ALL subsequent analyses because most of the report assumes a fixed relationship between DE 

and CE, namely that a CE of 96.5% is enough to assure a DE of 98%. 

(ii) The report should note that DE depends on each compound – in other words, DE = 

1- (mass out/mass in), and that this is calculated on a per species basis in the flare. Other DE 

definitions for hydrocarbons are not relevant. For sulfur compounds, the term conversion 

efficiency, denoting the oxidation of sulfur compounds in the vent gas, to SO2 may be more 

appropriate. 

(iii) The report should discuss the role of products of incomplete combustion (PIC) – i.e., 

hydrocarbons that are invariable created, even in well controlled combustion conditions, and 

certainly so in flares, in the definition of DE. Consider an example, of a simple natural gas 

diffusion flame. It is well known that, in addition to the expected CO2, water vapor, CO, etc., we 

also measure benzene, formaldehyde, and numerous other PICs in the post-flame region. Even if 

the CE in such a flame is very high, denoting a high conversion of the carbon molecules to CO2, 

how does one define destruction efficiency for benzene in this instance? Since no benzene was 

input into the flame, DE in this case for benzene, is actually negative, denoting benzene creation. 

Thus, as this simple example illustrates, there cannot be any basis for simply assuming that CE is 

always lower than DE and that DE is at least 1.5% more than CE. 

(iv) This reviewer does not believe that the report does an adequate or even passable job 

of assuming that “...Baukal’s estimation of 1.5% difference is a reasonable assumption...,” referring to 

the difference between combustion and destruction efficiencies. A careful review of Baukal shows that 

Baukal does not justify the underlying statement that “...a flare operating with a combustion efficiency of 

98 percent can achieve a destruction efficiency in excess of 99.5 percent.” Baukal does not provide any 

citations or studies to support this. And, Baukal’s definition of DE, neglecting formation of PICs, is 
5 

simply inadequate in this instance. Given the criticality of this assumption for all of the conclusions of 

the report, there needs to be significantly more discussion on this point. 

(v) Finally, on an important policy note, the report should refrain from equating 98% DE 

with “good” performance of a flare, simply because current regulations, that are known to be 

flawed, assume this. 

Section 3: Steam and Flare Performance

Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as an 

operating parameter for indicating over steaming situations on steam-assisted flares. 

Comment on the agency’s use of the ratio of the net heating value of the combustion zone 

gas (NHVCZ) to the net heating value of the flare vent gas if diluted to the lower 

flammability limit (NHVLFL) as an alternative to LFLCZ. Does the flare data adequately 

support the EPA’s observations? 

Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is there 

other data available (that is not discussed in this report) that may help clarify our 

discussion about specific chemical interactions related to lower flammability limits of gas 

mixtures? 

Did the agency adequately examine other operating parameters (different from LFLCZ; or 

the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming situations? Are there 

specific other parameters that should be given more or less emphasis? 

12. Please see comments provided under Sections 1 and 2 that address fundamental issues 

affecting the analysis and conclusions of this section. The analysis of the data presented and the 

conclusions reached cannot be properly judged without a resolution of the basic issues raised in 

comments previously provided for Sections 1 and 2. 

13. With regards to the used of LFL, there is a fundamental issue that the report does not 

address. Without stating so, the report seems to assume that LFL (whether in air, or oxygen, and 
6 

even more so in the presence of inerts), even for a pure compound, is a fundamental property of 

the substance. It is not. It is well known that the LFL for a pure compound, say, even methane, 

in air, is: (i) a function of the underlying test, typically and ASTM (or similar) test involving a 

specific apparatus such as a specified tube of specified diameter and made of a specified 

material, etc.; (ii) a function of temperature; and (iii), less importantly, a function of pressure. 

In fact, it is well known, in safety design, for example, that one cannot simply design for safety by 

assuming that mixtures below their LFL are safe. Usually significant safety factors, such as 

assuming that mixtures have to have concentrations one quarter of the LFL or lower etc. are 

used, precisely because it is understood that LFL values tabulated in standard references cannot 

be assumed to hold in specific combustion situations that bear little or no resemblance to the 

standard ASTM (or similar) test conditions under which LFL values are measured. 

Based on the above, the report should first discuss why even a pure component LFL, tabulated 

elsewhere, should be relevant or how it should be adjusted for typical flare tip conditions. Then, 

having established that, the further use of the LFLCZ in the manner used in the report would 

make more technical sense. 

14. With regard to the question posed, namely whether the data supports the use of LFLCZ as a 

proper parameter for denoting good flare combustion conditions, answering this question is 

premature given data quality and other issues raised previously. Certainly the conclusion that 

15.3% LFLCZ is a proper value for demarcating good versus not good operation is premature. 

15. Throughput this section, the report discusses steam injection but does not discuss steam tests. 

This reviewer expects that the incremental enthalpy added via steam made be a fundamental 

factor affecting combustion conditions in steam-assisted flares. EPA should at least request that 

stream quality or conditions be properly noted in any future testing. 
7 

Section 4: Air and Flare Performance

Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. Additionally, also comment on 

whether the lower flammability limit of the flare vent gas (LFLVG) is an appropriate 

operating parameter for determining whether the flare vent gas being sent to an airassisted flare is capable of burning? Does the flare data adequately support the EPA’s 

observations? 

16. Please see comments provided under Sections 1 and 2 that address fundamental issues 

affecting the analysis and conclusions of this section. The analysis of the data presented and the 

conclusions reached cannot be properly judged without a resolution of the basic issues raised in 

comments previously provided for Sections 1 and 2. 

17. Please also see comments discussion the fundamental issues with regards to use of LFL. 

18. While SR and LFLVG may be proper parameters for use in describing air-assisted flares, the 

specific technical discussion and the identification of specific demarcation values of SR 

discussed in this section are premature until issues discussed above are first addressed. 

Section 5: Wind and Flare Performance 

Please comment on the momentum flux ratio (MFR) as an operating parameter in 

crosswind velocities greater than 22 mph at the flare tip to indicate wake-dominated flame 

situations. Additionally, also comment on the agency’s observation that in the absence of 

crosswind greater than 22 mph, a low MFR does not necessarily indicate poor flare 

performance. Comment on the effectiveness of observations identifying wake-dominated 

flames. Does the flare data adequately support the EPA’s observations? 

Did the agency adequately examine other operating parameters (different from MFR) for 

identifying wake-dominated flames? Are there specific other parameters that should be 

given more or less emphasis? 
8 

19. EPA should explicitly stress in this Section (as it does note in later Section 8.5) that all wind 

speeds have to be measured or estimated at the flare tip. 

20. It is unclear how EPA has weighed the findings from Johnston et al (2001). As EPA notes, 

“...Johnston et al. (2001) provide a sketch of the unburned hydrocarbons detected downwind in a wakedominated flare with a crosswind velocity of 8 meters/second (m/s) or about 18 mph (Figure 5-3).” EPA 

should explain how this 18 mph value was factored into its conclusions regarding 22 mph being an 

appropriate threshold for wind effects. 

21. A major uncertainty in this analysis is the issue of scaling (i.e., at what size of test flares – 2”, 3”, 6”, 

etc. can the results be scaled to actual operating flares that are considerably larger? EPA notes 

significant uncertainty in this regard. In Section 5.3, EPA notes that “...Gogolek et al. (2010b) states 

that results of pipes smaller than 3 inch do not scale-up to larger pipes; and it has not been determined 

whether results for 3 inch to 6 inch pipes can successfully be applied to full-scale industrial flares.” This 

reviewer believes that resolution of this scaling issue is critical before further conclusions can be 

properly drawn regarding wind-effects. EPA should note this as a critical data gap. 

22. Finally, as noted earlier, all conclusions regarding the proper cut-off for MFR for wind effects are 

premature until fundamental issues relating to combustion and destruction efficiency discussed earlier 

are resolved.

Section 6: Flare Flame Lift Off 

Please comment on the maximum allowable flare tip velocity equation which considers 

combustion zone gas composition, the flare tip diameter, density of the flare vent gas, and 

density of air. Does the flare data adequately support the EPA’s observations? Are there 

specific other parameters or methods/equations that should be given more or less 

emphasis? 

23. While the analysis is generally sound, EPA’s caveat that “...this analysis does not test Equation 

6-1 over a very large range of possible flare tip velocities....” is important. It is also noted that the 

effects of other variables such as cross-wind, etc. on Eq. 6-1 is not clear.
9 

Section 7: Other Flare Type Designs to Consider 

Please comment on the applicability of the LFLCZ parameter, maximum allowable flare tip 

velocity equation, and the observations regarding crosswind velocity to non-assisted flares, 

pressure-assisted flares, and other flare designs. 

24. EPA should not venture into speculation. Having stated that “[W]e are unable to verify 

whether any of the analyses presented in this technical report could apply to non-assisted flares because 

there are minimal test data available for non-assisted flares....” EPA has no basis to then suggest that 

“[I]t seems reasonable to assume that the LFLCZ analysis in Section 3.0 of this report could apply to a 

non-assisted flare.” Rather EPA should properly identify the underlying lack of data as a data gap. 

Section 8: Monitoring Considerations 

Please comment on the appropriate monitoring equipment needed to ensure good flare 

performance and on any other known monitoring methods (not discussed in this report) for 

monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of NHVCZ to 

NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating scenarios and 

conditions where less robust monitoring equipment could be used to determine the 

operating parameters of interest. 

25. Please see prior comments on the use of LFL and therefore how that might affect the 

calculations of LFLCZ, LFLVG, LFLVG,C, etc. 
Attachment F 

Christopher Schaeffer 

President and General Manager 

Control Instruments Corporation 
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	1	

Review of The Parameters for Properly Designed and Operated Flares

Answers to Charge Questions 2‐5 and 10

By Christopher Schaeffer

Section 3: Steam and Flare Performance

Question 2

“Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as an 

operating parameter for indicating over steaming situations on steam-assisted flares. Comment 

on the agency’s use of the ratio of the net heating value of the combustion zone gas (NHVCZ) to 

the net heating value of the flare vent gas if diluted to the lower flammability limit (NHVLFL) as 

an alternative to LFLCZ. Does the flare data adequately support the EPA’s observations?” 

“To identify over steaming situations that may occur on steam-assisted flares, the data suggest 

that the lower flammability limit of combustion zone gas (LFLCZ) is the most appropriate 

operating parameter. Specifically, the data suggest that, in order to maintain good combustion 

efficiency, the LFLCZ must be 15.3 percent by volume or less for a steam-assisted flare. As an 

alternative to LFLCZ, the data suggest that the ratio of the net heating value of the combustion 

zone gas (NHVCZ) to the net heating value of the flare vent gas if diluted to the lower 

flammability limit (NHVVG-LFL) must be greater than 6.54.“ 

Answer 2

The operating parameter is a threshold value separating good combustion efficiency (CE) from 

bad. The ideal operating parameter would ensure the desired CE without the unnecessary 

addition of fuel gas, but the data does not point to an ideal parameter. Each of the operating 

parameters has some false indications. 

The most appropriate operating parameter must minimize false indications, especially “false 

positives” in which the operating parameter indicates good combustion efficiency (CE) when the 

actual CE is insufficient to ensure good destruction efficiency (DE). 

The most appropriate operating parameter should be readily measureable to an accuracy that 

allows its effective use. 

As a means to evaluate operating parameters, an analysis of the data (Appendix D) can be made 

that classifies the outcomes as “positive,” “negative,” “false positive” or “false negative.” 

Positive The operating parameter exceeds the threshold and the CE is above the required 

level. The operating parameter successfully indicates good CE. 

Negative The operating parameter does not exceed the threshold and the CE is below the 

required level. The operating parameter successfully indicates insufficient CE. 
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False positive The operating parameter exceeds the threshold and the CE is below the required 

level. The operating parameter fails to indicate insufficient CE. This is the 

outcome to be most avoided. 

False negative The operating parameter does not exceed the threshold and yet the CE is above 

the required level. The operating parameter fails to indicate good CE. This 

outcome causes unnecessary use of added fuel. 

Comments for some of the operating parameters that were considered: 

NHVvg 

The net heating value of the vent gases does not account for the effect of added steam on 

CE. The data shows that for steam-assisted flares it is a poor predictor of CE and should 

not be used. Even at high thresholds this method produces an unacceptable percentage of 

false positive outcomes. 

Ccz > 0.32 

This parameter can be a good predictor of CE, can be measureable, but can require a 

significant amount of added fuel compared to other parameters. There are very few false 

positive results, even when the threshold is lowered to Ccz > 0.28 (only two false positive 

“near misses”), or even Ccz > 0.26 (nine false positives). 

NHVcz 

The net heating value in the combustion zone accounts for much, but not all, of the effect 

of added steam on CE. It has the advantage of being readily measured with sufficient 

accuracy. The quantity of false positive and results false negative results are comparable 

to the LFLcz parameter. 

LFLcz = 15.3 percent by volume and 

The ratio NHVcz/NHVvg-lfl = 6.54 

These equivalent parameters are good indicators of CE. They most fully account for the 

effects of different combustible gases and added inert gases, especially steam. But they 

can be difficult to measure, and depend on flammability values that can have significant 

variations in published values. 

Examination of the false results for NHVcz and LFLcz show they have many in common, and 

among them are the suspect data, for example the MPC Detroit 4-1, 4-2, 5-1, 5-2, etc. 

Given the wide variety of flare designs and operating conditions, it may be useful to determine 

more than one appropriate operating parameter. There are cases where the otherwise most useful 

operating parameter cannot be measured effectively or quickly enough to allow proper control. 

   
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Question 3

“Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is 

there other data available (that is not discussed in this report) that may help clarify our 

discussion about specific chemical interactions related to lower flammability limits of gas 

mixtures?” 

Answer 3

One concern for any parameter, including LFLcz, is how well operating parameter will apply to 

other combustible gases and conditions not found in the data. 

For example, the combustion properties of ammonia might require special consideration. The 

narrow flammable range, high LFL in air, and low heating value might produce false results for a 

threshold value developed for hydrocarbons. 

A second concern for the accuracy of the calculated LFLcz is error arising from differences in 

published values of the Lower Flammable Limit in air. The LFL in air is an experimentally 

determined value. The LFLcz threshold is directly proportional to the LFL value in air, so an 

error in the LFL value will carry directly through to an error in the threshold. For the most part, 

the differences in published LFL values are attributable to test conditions: static or flowing 

gases, open or closed systems, and “go” versus “no go” criteria. The values themselves are most 

often determined only to the nearest 0.1 percent by volume. Differences of 0.2 percent by 

volume or more are not uncommon. If the LFL value is in error by 10%, which is not unheard of, 

the LFLcz threshold will be in error the same amount. To prevent ambiguity and error, the LFL 

values used to generate LFLcz should be traceable to flare test data. At a minimum, it should be 

determined by the same test methods that were used to generate the LFL values in the flare data. 

To a lesser extent, it would be worthwhile to investigate the data involving hydrogen, acetylene, 

and other outliers to see if their outcomes have a different percentage of false results compared to 

those without. For example, hydrogen has unique combustion properties compared to 

hydrocarbon: a wide flammable range, low LFL compared to its stoichiometric concentration, 

and at the same time, a relatively low heat of combustion. It can easily be seen that a threshold 

value of 300 BTU/SCF NHVvg for hydrogen would not be useful in predicting its combustion 

efficiency since it would produce a false result even for 95% by volume hydrogen (285 

BTU/SCF). 

   
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Question 4

“Did the agency adequately examine other operating parameters (different from LFLCZ; 

or the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming situations? Are 

there specific other parameters that should be given more or less emphasis?” 

Answer 4

The agency examined the relevant parameters. 

The parameter NHVcz should be given more emphasis because in some cases it could perform as 

well as LFLcz with the virtue of being more readily measurable. 

Comments on other parameters: 

Burning velocity 

The burning velocity of a combustible gas mixture might be expected to indicate 

conditions for good combustion efficiency, but this parameter is not characterized well 

enough to be useful. Furthermore, it often true that the maximum burning velocity for 

many combustible gases is related to flammability, a better characterized parameter. 

Oxygen concentration 

Oxygen concentration does not appear to provide additional insight for the flare, which 

burns in open air. Much of the literature for the effect of oxygen concentration on 

flammability relates to its effect within a homogeneous mixture, would be important to 

consider in a closed system, but not in a flare. 

Temperature and Pressure 

Pressure and temperature variations in the open air are not large enough to produce a 

significant effect. 

Stoichiometric concentration (Cst) 

For many combustible gas mixtures, the highest burning velocity and lowest ignition 

energy are found at concentrations just above Cst. Cst also correlates will with 

flammability. (Appendix C) 

Flammable Range 

Zabetakis includes the Upper (U25) and Lower (L25) Flammable Limits in air in tables 

of gas properties. One might predict that the wide flammable range of hydrogen helps 

promote good combustion efficiency, while the narrow range of flammable of ammonia 

presents a problem. (Appendix C) 

   
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Section 4: Air and Flare Performance

Question 5

“Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. Additionally, also comment on 

whether the lower flammability limit of the flare vent gas (LFLVG) is an appropriate 

operating parameter for determining whether the flare vent gas being sent to an airassisted flare is capable of burning? Does the flare data adequately support the EPA’s 

observations?” 

Answer 5

In the flare study, there is little data, concerning only methane and propylene combustible gases. 

This may not be representative enough to draw a conclusion from the flare study data alone and 

apply it to a wide variety of combustible gases. 

LFLvg as an operating parameter can be seen as a special case of LFLcz without steam assist. In 

this limited case, correction for nitrogen equivalence is unnecessary because there is no steam 

and no added inert beyond what is already present in the vent gases. So the vent gases are 

effectively the same as the combustion zone gases, and the 15.3% LFL figure should be 

applicable. 

For many combustible gases, the LFLvg threshold of 15.3% by volume at SR = 7 would lead to a 

concentration near LFL if there was complete mixing of the assist air and the vent gases. In 

practice, the method of injection of assist air should not produce complete mixing prior to 

combustion. Since the stoichiometric concentration in air is approximately two times the LFL in 

air, the mixture of vent gas and air assist gas should pass through the flammable range and 

present the opportunity of a stable flame envelope burning at the stoichiometric concentration. 

In a similar manner LHVvg as an operating parameter should be given consideration because the 

main difficulty with the use of LHVvg arose from its failure to correct for the added inert water 

in steam-assisted flares. In air assisted flares this limitation on the use of LHVvg will not be so 

important, and so the benefit of using LHVcg for air assist flares, namely the ability to monitor 

effectively with a less robust monitoring system such as a calorimeter. 

 
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Section 8: Monitoring Considerations

Question 10

“Please comment on the appropriate monitoring equipment needed to ensure good flare 

performance and on any other known monitoring methods (not discussed in this report) 

for monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of NHVCZ to 

NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating scenarios and 

conditions where less robust monitoring equipment could be used to determine the 

operating parameters of interest.” 

Answer 10

The monitoring equipment includes an extractive analyzer to measure the desired combustible 

gas characteristic (volume concentration, net heating value, flammability) of the vent gas, plus a 

means to measure flow rates. 

This answer will look at extractive analyzers that can directly or indirectly measure LFLCZ, 

LFLVG, LFLVG,C, the ratio of NHVCZ to NHVVG-LFL, CCZ, and SR. 

The focus on an extractive analyzer is justified by the well-founded observations of the difficulty 

in sampling from the combustion zone itself, and on the limits to the use of PFTIR and AFTIR 

methods as described in the report. 

The principal requirements for an extractive combustible gas analyzer are speed and accuracy. In 

some cases, a gas chromatograph will not be fast enough. So although the GC is most accurate 

for monitoring LFLcz, a less robust analyzer such as a calorimeter will sometimes be more 

suitable, even if it measures a less than perfect parameter such as NHVcz. 

Accuracy 

Response Factors (see Appendix A) 

This is not an issue for analyzers such as FTIR and GC that can measure the 

concentration of each individual species of combustible gas in the sample. For 

other analyzers that make a total (or combined) measurement of all combustible 

gases, it is important to know the response factors. These analyzers should have a 

uniform response factor for the parameter being measured for each species of 

combustible gas that it must measure. It is common for the response factors of 

analyzers to be published by the analyzer manufacturer, and in some cases these 

response factors are verified by a third party. The response factors are typically 

normalized to a standard reference gas whose factor is 1.0. So when the analyzer 

is calibrated using the reference gas, the response to another gas can be 

understood. The indicated reading can be interpreted. A response factor of 1.1 

will mean that the indicated gas reading made by the analyzer is 1.1 times the 

actual concentration. 
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Other influences on accuracy are usually less significant and can be adequately 

addressed by installation, operation and maintenance of the equipment according 

to industry norms. 

Sample Handling and Conditioning 

The sample of vent gases should be conveyed to the analyzer without 

adulteration. It is preferable to make a measurement on a wet basis, with all parts 

of the sampling system that come in contact with the sample heated to at least 

(and preferably 10 degree C above) the temperature of the vent gases being 

measured. Otherwise, condensation of water vapor will cause false readings 

unless the measurement is corrected for loss of the water volume. 

Filtration of particulate matter is routine and necessary to protect the analyzer 

from clogging. 

If the sample is passed through a pump before it reaches the analyzer the pump 

should not raise the pressure of the sample to a point where it could condense out 

water vapor. Notably, vent gases saturated with water vapor at the measuring 

point temperature and pressure will not readily tolerate an increase in pressure or 

a reduction in temperature without condensation. This would require correction of 

the measurement to account for the lost water’s volume. 

Speed 

Response Time 

If the analyzer is to be used to control the flare it must respond quickly enough to 

allow stable control, preferably more quickly than the vent gases can change. For 

example, consider waste gases from different chemical batches being vented to a 

flare. Each batch may require (only) 30 to 60 minutes to be vented, which would 

present a challenge to a slow analyzer, such as a gas chromatograph having a ten 

minute response (cycle) time. 

If the analyzer is to be used solely to monitor and not for control, a longer delay 

can be tolerated, and in some cases the interval between measurements can be 

extended, provided the interval does not produce a significant error. 

The total response of the analyzer’s measurements includes the “sample transport 

time” to convey the sample from the measuring point to the analyzer’s inlet plus 

the “T90” time for the analyzer to reach a final stable reading. The analyzer 

should be located as close as possible to the point of measurement. Particulate 

filters should be small, tubing diameters small, and flow rates high enough to 

transport the sample quickly from the measuring point to the analyzer. 

The delay in sample transport for a filter having a dead volume V and a sample 

flow rate Q is approximately V/Q. The delay in sample transport for standard1⁄4 

inch OD tubing at a flow rate of 1 liters per minute is approximately one second 

per meter. 
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Comments on analyzer types: 

Gas Chromatograph 

The sample is separated and each constituent is individually determined to a high 

degree of accuracy. The response for each combustible gas is optimized. The 

separation and analysis steps require a significant amount of time, for example, 

ten minutes. It may require sample conditioning to remove gases that interfere 

with its normal operation. 

Flame Ionization Analyzer 

A small continuous sample flow is burned in a small hydrogen flame (40 cc/min) 

in the presence of an electric field, which results in an ion current that is 

proportional to the “total hydrocarbon” in the sample. It response correlates best 

to the density of C-H (carbon hydrogen bonds) and can be characterized as PPM 

Cv (parts-per-million carbon by volume). It has a weakened response to oxygenbearing hydrocarbon, and no response at all to hydrogen or carbon monoxide. It is 

most commonly used for very low concentration (up to 300 PPM), and also for 

measurements in the flammable range (1 to 3 percent by volume) but can saturate 

at high concentration. It has a fast response time – a few seconds. 

Flame Temperature 

This analyzer uses a small sensing flame to measure total flammability in the 

range from 0 to 100% LFL (Lower Flammable Limit in Air). A sample of the gas 

to be measured enters the sensing flame by diffusion and produces a temperature 

rise proportional to the “total flammability.” It has a fast response time – a few 

seconds. It is relatively unaffected by corrosive gases and can make 

measurements on a wet basis. To measure vent gases, the limitation on range 

would have to be overcome by an accurate dilution of the sample gas of 10x or 

more in a sample conditioning system. 

Combustion-type Calorimeter 

There are many types designed to measure the heating value of gases across a 

wide range. One type presently used for flares burns a small continuous sample 

flow premixed with hydrogen to produce a temperature rise proportional to the net 

heating value of the sample. It has a uniform response to many different 

combustible gases in terms of the net heating value, including hydrogen and 

carbon monoxide. .” It has a fast response time – less than ten seconds. It is 

relatively unaffected by corrosive gases and can make measurements on a wet 

basis. 

Non-dispersive Infrared Sensor 

Relatively simple and inexpensive, it quickly measures the “total absorbance” of 

infrared energy. It has a wide range of response factors to different combustible 

gases, which usually limits or even prohibits its use for mixtures. 

Sensors: Electrochemical / Catalytic bead / Semiconductor (tin-oxide) 

These detectors are intended for leak detection in air and so in the great majority 

of cases do not have the range and specificity for use with flares. 
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Conditions where “less robust” monitoring equipment (combustible gas analyzer) could be used: 

Note: For this answer “less robust” is not meant to not imply reliability (such as the 

availability of the measurement, or MTBF). This answer addresses the analyzer used to 

measure the combustible gas characteristics. In general here, the less robust system is one 

which is not able to identify the concentration of each individual combustible gas in the 

vent gases. 

There is an acceptable margin of error 

The operating parameter is defined as a threshold beyond which the CE and thus DE is 

sufficient. This is an important feature affecting the required accuracy. An accurate 

monitoring system can be controlled at the threshold. A less accurate monitoring system 

can operate equally successfully if controlled at a safe margin beyond the threshold, in 

such a way that the inaccuracy of the less robust monitoring system does not defeat 

(exceed) the safe margin. 

This may allow successful operation in cases where the measurement accuracy is difficult 

to achieve with existing monitoring equipment. For example, monitoring equipment that 

is accurate to within +/-5% could be employed to control LFLcz below 15.3% by volume 

by operating with an indicated measurement not greater than 14.5%. This provides a 

balance between cost and complexity of the monitoring system and the cost of added fuel 

to reach the threshold (if needed) without a compromise in CE. 

A single combustible gas is present 

If just one species of combustible gas is present, then any analyzer that measures a 

proportionally related characteristic of the gas over the range of interest may be used in 

combination with a conversion factor between the characteristic being measured and the 

operating parameter that is needed. For example, is propylene alone is being flared, then 

a BTU/SCF measurement of propylene could be made with a calorimeter and it could be 

converted to volume fraction or flammability as needed. 

One combustible gas dominates a mixture 

If a single species of combustible gas dominates a mixture such that the other 

combustible gases in the mixture can be considered trace gases, then a measurement of 

the dominant gas alone can be used as long as the resulting error is within an acceptable 

margin of error (see above). For example, if a trace amount of hydrogen is present, and it 

could be ignored (not measured) without compromising CE (the actual CE would be 

better than the indicated CE made by excluding hydrogen from measurement), then an 

analyzer able to measure the dominant combustible gas would be acceptable. 

The analyzer’s response factors are uniform 

If all response factors for all combustible gases that can be present in the vent gases are 

within an acceptable margin of error, then the less robust system can be used. The 

standard mixing rule for response factors is to use the weighted average on a volume 

basis. For a worst case analysis, this simplifies to a consideration of only the lowest and 

highest response factors. 
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The analyzer is calibrated for the combustible gas having the weakest response 

If the analyzer is calibrated to read “true” for the combustible gas to which it is least 

sensitive, then all indicated readings will be at or above the actual gas concentration such 

that all operation will meet or exceed the threshold. 

   
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Appendix A – Response Factors 

Response factors show how a sensor that is calibrated for one particular gas, usually a 

"reference" gas, will read when exposed to other gases. If two gases both have the same response 

factor, they respond equally, unless the sensor is non-linear (infrared), in which case linearization 

of the signal can introduce more error, or less, depending on the individual characteristics of the 

sensor for each gas. 

The degree of error that results from attempting to measure two different gases that have 

different response factors can be understood by taking the ratio of the two factors. Thus an 

attempt to measure two gases, one with a response factor to 0.5, and another with a response 

factor of 1.5, could yield a reading one third (0.5/1.5) or three times (1.5/0.5) the actual 

concentration. 

The industry standard accuracy requirement for a flammable gas sensor is +/-10%. Response 

factors are therefore one of the most significant influences on accuracy. They easily introduce 

large errors. 

Response factors are obtained by testing. The response factors should be obtained from the 

manufacturer of the sensor. In addition, the manufacturer's response factors can have 

independent verification as part of the third-party approval process(FM, CSA, ATEX, 

CENELEC). 

The response factors given here are from various sources, including several manufacturers. The 

factors were all put into the same format (calibration readings in some cases were converted into 

response factors). This information is for illustrative purposes only. The calibration of actual 

sensors should be performed only according to the manufacturer's instructions. 

For many process monitoring applications, the sensor must be calibrated so that all gases to be 

detected read the actual concentration or higher, but do not under-report the actual concentration. 

Therefore, the sensor is calibrated for the gas with the lowest response factor. 

The flame temperature type sensor is said to have a "universal calibration" for common solvent 

vapors, because the response factors for common solvent vapors are in the range from 0.9 to 1.1. 

By contrast, response factors for FID and Catalytic sensors are usually in the range from 0.5 to 

1.5 (a three-to-one ratio), and factors for infrared sensors can easily reach 0.25 to 2.0 (an eightto-one ratio). 

The response factors for catalytic sensors are subject to drift as the sensor ages. The sensor loses 

its response to hydrocarbons, which are harder for the sensor to detect, while maintaining its 

response to more easily catalyzed gases like hydrogen. The sensor eventually reads only 

hydrogen and not hydrocarbon. 

   
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Table of response factors

Response factors for several sensor and analyzer types measuring %LFL in air

Relative to standard test gas = 100, equivalent to factor of 1.0

Gas FTA Infrared

1

Infrared

2

Infrared

3

Catalytic

1

Catalytic

2

FID1 FID2

Methyl formate 81    194 100

MIBK 90    113 82

Methoxypropanol 90 102   

Ethoxypropanol 90 100   

Amyl alcohol 90    91

Toluene 94 38 39 103 93 111 124

Tetrahydrofuran 94    133 82

Benzene 95 91 108 100 98

Propyl acetate 96    69 96 81 94

Propanol 99 113 101 67 111 66 75 67

Cyclohexane 100 52 180 95 109 91 103 101

Heptane 100 97 180 97 89 100    116

Butanol 100 106 126 74 105 67

Butyl acetate 100 91 73 143 200

Triethylamine 100    105 81

Acetone 101 67 29 108 77 85 73

Hexane 102 100 180 95 89 92 90 73

MEK 102 66 47 111 92 86

Ethyl benzene 104    87 127

Ethyl acrylate 104    111 97

Propylene 104    65 80

Xylene 105 88 47 106 99 110 119

Methanol 106 230 180 130 58 57 70

Ethanol 107 139 148 97 133 66 63 81

Ethyl acetate 110 78 74 91 82 136 93

Ether 110 145 140 133

Butane 110    113 108 103

Butadiene 110    32 106

Naptha 111    113 121

Ethylene 117    27 72

Methane 120    51 88

Propane 128    121 138 86 88

Ethane 128    117

The table above illustrates how analyzers and sensors of different types and manufacture have differing

response to combustible gases. Because these types of “less robust” analyzers make readings of the

total gas mixture, not individual components, variations in the relative proportions of each combustible

gas in the mixture will cause error in readings.

In the accompanying chart (below) the distribution of response factors can be seen. A perfect response

to all gases would be represented as a flat line with all values equal to 100 (i.e. 1.0).
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Response Factors – Conversion of Factors

The chart below gives response factors for a FTA (Flame temperature) analyzer for measurement of flammability and

also for measurement of heating value. The analyzer’s principle of operation yields a uniform response to a variety of

combustible gases in terms of flammability. For the combustible gases listed (“solvents”), the standard deviation of

response factors is 5%.  

To illustrate the conversion of response factors, the chart includes a heating value (BTU) response factor for the same

combustible gases. The response factors converted from flammability to heating value have a much greater standard

deviation, 22%. This is expected, since the relationship between flammability and heating value is not perfectly

correlated.

Even so, this shows how response factors can be converted.  

Solvent LFL MW

Liquid

Density

Pounds

per

Gallon

Heat of

Combustion

BTU per

pound

Heat of

Combustion

BTU per

gallon

LFL Definition:

Cubic Feet of

Air per Gallon

of Solvent at

100% LFL

Pounds

per cubic

foot at

LFL

BTU per Cubic

Foot  of

Solvent @LFL

BTU "Factor"

normalized

to Toluene =

1

LFL for flame

temperature

response factor

Flame

temperature

LFL response

factor

Flame

temperature

BTU relative

response

factor

Acetone 6.7 12,250 82,075 1,780 46 0.97 2.50 1.01 0.95

Butyl acryla te 1.4% 128.2 13,860 0.003923 54 1.15 1.70 1.00 1.11

Cell osolve 7.8 13,000 101,400 1,210 84 1.77 1.70 1.02 1.75

Cyclohexane 6.5 18,684 121,446 2,320 52 1.11 1.33 1.00 1.07

Cycl ohexanone 7.9 15,430 121,897 2,640 46 0.98 1.10 0.84 0.80

Dime thyl Formami de 7.8 11,280 87,984 1,780 49 1.04 2.20 0.95 0.96

Ethyl Ace ta te 7.5 10,110 75,825 1,630 47 0.98 2.00 1.00 0.95

Ethyl Alcohol 6.6 11,570 76,362 1,630 47 0.99 3.30 1.08 1.04

Heptane 5.6 19,170 107,352 2,210 49 1.03 1.00 1.02 1.02

Hexane 1.2% 86.2 19,246 0.002261 44 0.92 1.10 1.00 0.89

Hexane 5.5 19,246 105,853 2,330 45 0.96 1.10 1.00 0.93

i‐Propyl Alcohol 6.6 12,960 85,536 2,070 41 0.87 2.20 0.99 0.84

Me thanol 6.5 8,419 54,724 1,264 43 0.91 6.00 1.06 0.94

Methyl Ethyl Ketone 6.7 13,480 90,316 2,540 36 0.75 1.80 1.01 0.74

n‐Butyl Ace ta te 7.3 13,130 95,849 1,450 66 1.40 1.70 1.00 1.36

n‐Butyl Alcohol 6.7 14,230 95,341 2,410 40 0.84 1.70 1.00 0.81

n‐Propyl Alcohol 6.7 13,130 87,971 1,920 46 0.97 0.99 0.93

o‐Xylene 7.3 17,558 128,173 2,670 48 1.01 1.10 1.04 1.02

sec‐Butyl Alcohol 6.7 15,500 103,850 1,980 52 1.11 1.00 1.08

Toluene 7.2 17,430 125,496 2,650 47 1.00 1.20 1.03 1.00

Turpenti ne 0.8% 136.2 19,000 0.002383 45 0.96 0.80 1.00 0.93

ace tic aci d 4.0% 60.1 5,645 0.005251 30 0.63 0.80 0.80 0.49

formi c aci d 18.0% 46.0 2,045 0.018114 37 0.78 0.90 0.90 0.68

mean 1.00 1.01

1.  Liqui d densi ty from NFPA‐86 (1990) s tdev 0.05 0.22

2.  Heat of combus tion from Noyes Solvent Sa fe ty Handbook

3.  LFL De fi ni ti on from NFPA‐86 (1990)

4.  BTU per cubi c feet from Solvent = Heat of combus tion / LFL de fi ni ti on

Propane 1.7% 44.09 19,782 0.00164 32 0.67 1.70 0.97 0.65

Propane 2.0% 44.09 19,782 0.00193 38 0.79 1.14 0.97 0.76

Methane 4.4% 16.04 21,517 0.00154 33 0.68 4.40 1.20 0.82

Ethylene 2.3% 28.05 20,290 0.00141 29 0.59 2.70 1.17 0.69

Methane 5.0% 16.04 21,517 0.00175 38 0.78 5.00 1.40 1.09

Ethylene 2.7% 28.05 20,290 0.00166 34 0.69 2.70 1.17 0.81

Propane 2.1% 44.09 19,782 0.00202 40 0.82 1.14 0.97 0.80

Gases that are used during calibration ‐ for reference only
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	15	

Appendix B‐ LFL Values from Various Authorities

Substance 

IEC 79-

20 1996 

NFPA 

325M 

1991 

NFPA 

325M 

1969 

Bureau 

of Mines 

Bulletin 

627 

CCOHC 

(Canada) 

Redeker / 

Schoen 

1990 

(Germany) 

Nabert / 

Schoen 

1991 

(Germany) 

acetaldehyde 4.0 4.0 4.0 4.0 4.0 

acetone 2.5 2.5 2.6 2.6 2.6 

acetone 2.5 2.5 

benzene 1.3 1.2 1.3 1.3 1.2 

butadiene 1,3- 1.4 2.0 2.0 2.0 1.4 1.1 

butyl acetate 1.4 1.7 1.4 1.7 1.2 1.2 

cyclohexane 1.2 1.3 1.3 1.3 1.2 

cyclohexanone 1.0 1.1 1.1 1.0 1.3 

dichloromethane 13.0 13.0 12.0 13.0 

dimethyl ether 2.7 3.4 3.4 2.7 2.0 

dimethyl sulfide 2.2 2.2 2.2 

dioxane 1,4- 1.9 2.0 2.0 1.9 

ethane 2.5 3.0 3.0 3.0 2.7 3.0 

ethanol 3.3 3.3 3.3 3.3 3.5 

ethyl acetate 2.2 2.0 2.2 2.2 2.0 

ethylene 2.5 2.7 2.7 3.1 2.3 2.7 

ethylene glycol 3.2 3.4 3.2 

heptane 1.1 1.1 1.1 1.1 1.1 

hexane 1.0 1.1 1.2 1.1 1.0 1.2 

isobutane 1.8 1.8 

isobutyl alcohol 1.7 1.7 

isobutylene 1.8 1.8 1.6 

isopropyl alcohol 2.0 2.2 2.5 2.0 

methane 4.4 5.0 5.0 5.0 5.0 

methanol 5.5 6.0 6.7 7.3 

methanol 6.7 5.5 5.5 

methyl acrylate 2.4 2.8 2.8 2.4 2.8 

methyl chloride, chloromethane 8.1 7.0 7.1 

methyl ethyl ketone 1.4 1.8 1.9 2.0 1.8 

methylene chloride 13.0 12.0 13.0 

N,N-dimethylformamide 1.8 2.2 1.8 2.2 2.2 

n-butane 1.4 1.9 1.8 1.8 1.4 1.5 

n-butyl alcohol 1.4 1.4 

o- xylene 1.0 0.9 1.0 1.1 1.0 1.0 

p-, m- xylene 1.1 1.1 1.1 

propane 2.0 2.1 2.2 2.1 2.2 1.7 2.1 

propanol 2.2 2.2 2.1 2.2 2.2 2.1 2.1 

styrene 0.9 1.1 

t-butyl methyl ether 1.5 1.6 

tert-butyl alcohol 1.4 1.9 2.4 2.3 

toluene 1.2 1.1 1.2 1.2 1.3 1.2 1.2 
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	16	

 

Appendix C – Cst, with U25 and L25 ratios

In tables of gas properties, Zabetakis includes the stoichiometric concentration air (Cst), and also  the ratio

“L25/Cst,” which is the ratio of the Lower Flammable Limit of the combustible gas in air to the stoichiometric

concentration of the combustible gas in air. For many combustible gases, the Lower Flammable Limit in air is

within 10% of the value 0.54*Cst. The exceptions to this general rule L25 = 0.54*Cst are themselves interesting,

for example hydrogen and ammonia.  

Cst may be of interest as a parameter interesting because it can be calculated, and so it is less ambiguous than

the Lower Flammable Limit in air.  

Substance L25/Cst U25/L25 U25/Cst Cst

1,3‐Butadiene 0.52 6.00 3.13 3.8%

1‐Butene 0.50 5.29 2.67 3.4%

Acetylene 0.32 40.00 12.93 7.7%

Benzene 0.44 6.50 2.87 2.7%

Carbon Monoxide 0.42 5.92 2.51 29.5%

Cis‐2‐Butene 0.50 5.29 2.67 3.4%

Ethylene 0.41 13.33 5.51 6.5%

Hydrogen Sulfide 0.33 11.00 3.59 12.3%

Hydrogen 0.14 18.75 2.54 29.5%

Iso‐Butane 0.58 4.67 2.69 3.1%

Iso‐Butylene 0.53 5.33 2.85 3.4%

Methane 0.53 3.00 1.58 9.5%

Methyl Acetylene 0.34 6.88 2.35 5.0%

n‐Butane 0.45 6.07 2.72 3.1%

Pentane   0.59 5.20 3.06 2.6%

Propane 0.52 4.52 2.36 4.0%

Propylene 0.54 4.63 2.50 4.4%

Toluene 0.53 5.92 3.12 2.3%

Trans‐2‐Butene 0.53 5.39 2.87 3.4%

Ammonia 0.69 1.87 1.28 21.8%

Butadiene 1,2‐  0.50 6.00 2.98 4.0%

Cyclopropane 0.54 4.33 2.34 4.4%

Ethane 0.53 4.17 2.21 5.6%

Ethyl Benzene 0.41 8.38 3.42 2.0%

Heptane 0.54 6.70 3.58 1.9%

Hexane 0.51 6.82 3.48 2.2%

Propadiene 0.50 6.80 3.42 5.0%

Xylene 0.55 6.36 3.50 2.0%

   
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	17	

Appendix D – Analysis of Operating Parameters  

For each operating parameter, the “steam data used all analysis” is categorized as “positive,” “negative,” “false positive”

or “false negative.” This allows for a comparison of operating parameters and an understanding of the type of outcome

the use of the parameter will have. The effect of changes to the threshold or to the CE limit can be readily seen.

The data is more readily viewed in the Excel spreadsheet, but the first page of each of several representative views is

included in the following pages.

   
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	18	

The net heating value in the vent gases for various threshold values 300, 500 and 700 BTU/SCF shows a large percentage

of false outcomes.

   

NHVvg Threshold 300 NHVvg Threshold 500 NHVvg Threshold 700

CE Threshold 96.5% CE Threshold 96.5% CE Threshold 96.5%

Sums of outcomes Sums of outcomes Sums of outcomes

160 1 147 4 139 51 97 25 118 71 77 46

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

1 2183 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 2183 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 2183 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 2183 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

5 309 98.7% 1 0 0 0 0 0 0 1 0 0 0 1

6 2183 99.7% 1 0 0 0 1 0 0 0 1 0 0 0

7 294 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

8 298 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

9 2183 82.2% 0 0 1 0 0 0 1 0 0 0 1 0

10 305 99.5% 1 0 0 0 0 0 0 1 0 0 0 1

11 342 99.9% 1 0 0 0 0 0 0 1 0 0 0 1

12 364 99.8% 1 0 0 0 0 0 0 1 0 0 0 1

13 339 99.7% 1 0 0 0 0 0 0 1 0 0 0 1

14 405 99.8% 1 0 0 0 0 0 0 1 0 0 0 1

15 192 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 232 99.3% 0 0 0 1 0 0 0 1 0 0 0 1

17 275 99.4% 0 0 0 1 0 0 0 1 0 0 0 1

18 329 97.9% 1 0 0 0 0 0 0 1 0 0 0 1

19 425 99.7% 1 0 0 0 0 0 0 1 0 0 0 1

20 425 99.6% 1 0 0 0 0 0 0 1 0 0 0 1

21 732 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

22 732 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

23 732 97.5% 1 0 0 0 1 0 0 0 1 0 0 0

24 748 96.3% 0 0 1 0 0 0 1 0 0 0 1 0

25 732 94.8% 0 0 1 0 0 0 1 0 0 0 1 0

26 753 89.7% 0 0 1 0 0 0 1 0 0 0 1 0

27 705 92.6% 0 0 1 0 0 0 1 0 0 0 1 0

28 732 85.9% 0 0 1 0 0 0 1 0 0 0 1 0

29 826 82.8% 0 0 1 0 0 0 1 0 0 0 1 0

30 901 98.4% 1 0 0 0 1 0 0 0 1 0 0 0

31 932 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

32 908 97.7% 1 0 0 0 1 0 0 0 1 0 0 0

33 940 97.4% 1 0 0 0 1 0 0 0 1 0 0 0

34 932 92.7% 0 0 1 0 0 0 1 0 0 0 1 0

35 926 93.9% 0 0 1 0 0 0 1 0 0 0 1 0

36 873 97.1% 1 0 0 0 1 0 0 0 1 0 0 0

37 876 89.2% 0 0 1 0 0 0 1 0 0 0 1 0

38 777 99.1% 1 0 0 0 1 0 0 0 1 0 0 0

Flare Vent

Gas Net

Heating

Value

(NHVVG)

[Btu/scf]

Reported

Combustion

Efficiency

(CE)

Nbr [%]
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	19	

The net heating value in the combustion zone gas accounts for much of the effect of added steam and so produces

relatively low percentages of false outcomes, similar to LFLcz. Many of the false positive results are suspect data points.

250 275 300

96.5% 96.5% 96.5%

110 135 13 54 101 138 10 63 84 141 7 80

True

Positive

True

Negativ

False

positive

False

negativ

True

Positive

True

Negativ

False

positive

False

negativ

True

Positive

True

Negativ

False

positive

False

negativ

1 793 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 471 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 693 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 267 99.5% 1 0 0 0 0 0 0 1 0 0 0 1

5 242 98.7% 0 0 0 1 0 0 0 1 0 0 0 1

6 241 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

7 236 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

8 298 99.9% 1 0 0 0 1 0 0 0 0 0 0 1

9 153 82.2% 0 1 0 0 0 1 0 0 0 1 0 0

10 305 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

11 342 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

12 364 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

13 339 99.7% 1 0 0 0 1 0 0 0 1 0 0 0

14 405 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

15 192 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 232 99.3% 0 0 0 1 0 0 0 1 0 0 0 1

17 220 99.4% 0 0 0 1 0 0 0 1 0 0 0 1

18 295 97.9% 1 0 0 0 1 0 0 0 0 0 0 1

19 333 99.7% 1 0 0 0 1 0 0 0 1 0 0 0

20 373 99.6% 1 0 0 0 1 0 0 0 1 0 0 0

21 235 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

22 224 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

23 172 97.5% 0 0 0 1 0 0 0 1 0 0 0 1

24 204 96.3% 0 1 0 0 0 1 0 0 0 1 0 0

25 129 94.8% 0 1 0 0 0 1 0 0 0 1 0 0

26 166 89.7% 0 1 0 0 0 1 0 0 0 1 0 0

27 133 92.6% 0 1 0 0 0 1 0 0 0 1 0 0

28 106 85.9% 0 1 0 0 0 1 0 0 0 1 0 0

29 120 82.8% 0 1 0 0 0 1 0 0 0 1 0 0

30 461 98.4% 1 0 0 0 1 0 0 0 1 0 0 0

31 456 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

32 313 97.7% 1 0 0 0 1 0 0 0 1 0 0 0

33 305 97.4% 1 0 0 0 1 0 0 0 1 0 0 0

34 259 92.7% 0 0 1 0 0 1 0 0 0 1 0 0

35 266 93.9% 0 0 1 0 0 1 0 0 0 1 0 0

36 261 97.1% 1 0 0 0 0 0 0 1 0 0 0 1

37 215 89.2% 0 1 0 0 0 1 0 0 0 1 0 0

38 411 99.1% 1 0 0 0 1 0 0 0 1 0 0 0

39 435 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

Net Heating

Value of

Combustion

Zone Gas

(NHVCZ)

[BTU/scf]

Reported

Combustion

Efficiency

(CE)

Nbr [%]

NHVcz Threshold

CE Threshold

Sums of outcomes

NHVcz Threshold

CE Threshold

Sums of outcomes

NHVcz Threshold

CE Threshold

Sums of outcomes
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	20	

The LFLcz‐adj accounts best for the effect of added steam and so produces relatively low percentages of false outcomes.

Many of the false positive results are suspect data points.

   

13.8% 15.3% 16.8%

96.5% 96.5% 96.5%

82 142 6 82 98 138 10 66 106 135 13 58

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

1 6.7% 100.0% 10001000100 0

2 11.5% 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 7.7% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 20.9% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

5 22.0% 98.7% 0 0 0 1 0 0 0 1 0 0 0 1

6 23.4% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

7 22.5% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

8 25.1% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

9 38.5% 82.2% 0 1 0 0 0 1 0 0 0 1 0 0

10 19.8% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

11 15.3% 99.9% 0 0 0 1 0 0 0 1 1 0 0 0

12 14.4% 99.8% 0 0 0 1 1 0 0 0 1 0 0 0

13 15.5% 99.7% 0 0 0 1 0 0 0 1 1 0 0 0

14 14.1% 99.8% 0 0 0 1 1 0 0 0 1 0 0 0

15 27.3% 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 22.6% 99.3% 0 0 0 1 0 0 0 1 0 0 0 1

17 22.9% 99.4% 0 0 0 1 0 0 0 1 0 0 0 1

18 16.9% 97.9% 0 0 0 1 0 0 0 1 0 0 0 1

19 15.0% 99.7% 0 0 0 1 1 0 0 0 1 0 0 0

20 13.3% 99.6% 1 0 0 0 1 0 0 0 1 0 0 0

21 14.3% 99.5% 0 0 0 1 1 0 0 0 1 0 0 0

22 15.1% 99.5% 0 0 0 1 1 0 0 0 1 0 0 0

23 20.3% 97.5% 0 0 0 1 0 0 0 1 0 0 0 1

24 17.6% 96.3% 0 1 0 0 0 1 0 0 0 1 0 0

25 28.0% 94.8% 0 1 0 0 0 1 0 0 0 1 0 0

26 22.1% 89.7% 0 1 0 0 0 1 0 0 0 1 0 0

27 25.3% 92.6% 0 1 0 0 0 1 0 0 0 1 0 0

28 35.6% 85.9% 0 1 0 0 0 1 0 0 0 1 0 0

29 32.6% 82.8% 0 1 0 0 0 1 0 0 0 1 0 0

30 9.2% 98.4% 1 0 0 0 1 0 0 0 1 0 0 0

31 9.3% 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

32 14.0% 97.7% 0 0 0 1 1 0 0 0 1 0 0 0

33 14.4% 97.4% 0 0 0 1 1 0 0 0 1 0 0 0

34 17.3% 92.7% 0 1 0 0 0 1 0 0 0 1 0 0

35 16.9% 93.9% 0 1 0 0 0 1 0 0 0 1 0 0

36 17.0% 97.1% 0 0 0 1 0 0 0 1 0 0 0 1

37 21.4% 89.2% 0 1 0 0 0 1 0 0 0 1 0 0

Nbr

Lower

Flammability

Limit of

Combustion

Zone Gas

Adjusted for

Nitrogen

Equivalency

(LFLCZ)

[vol frac]

Reported

Combustion

Efficiency

(CE)

[%]

Sums of outcomes Sums of outcomes Sums of outcomes

LFLcz‐adj

Threshold

LFLcz‐adj

Threshold

LFLcz‐adj

Threshold

CE Threshold CE Threshold CE Threshold
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	21	

The LFLcz also produces relatively low percentages of false outcomes. Many of the false positive results are suspect data

points.

   

14.5% 15.3% 16.0%

96.5% 96.5% 95.5%

96 137 11 68 103 136 12 61 116 118 10 68

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

1 6.6% 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 11.1% 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 7.6% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 19.6% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

5 21.6% 98.7% 0 0 0 1 0 0 0 1 0 0 0 1

6 21.7% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

7 22.2% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

8 25.1% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

9 34.1% 82.2% 0 1 0 0 0 1 0 0 0 1 0 0

10 19.8% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

11 15.3% 99.9% 0 0 0 1 0 0 0 1 1 0 0 0

12 14.4% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

13 15.5% 99.7% 0 0 0 1 0 0 0 1 1 0 0 0

14 14.1% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

15 27.3% 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 22.6% 99.3% 0 0 0 1 0 0 0 1 0 0 0 1

17 22.4% 99.4% 0 0 0 1 0 0 0 1 0 0 0 1

18 16.7% 97.9% 0 0 0 1 0 0 0 1 0 0 0 1

19 14.8% 99.7% 0 0 0 1 1 0 0 0 1 0 0 0

20 13.2% 99.6% 1 0 0 0 1 0 0 0 1 0 0 0

21 13.5% 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

22 14.2% 99.5% 1 0 0 0 1 0 0 0 1 0 0 0

23 18.6% 97.5% 0 0 0 1 0 0 0 1 0 0 0 1

24 16.3% 96.3% 0 1 0 0 0 1 0 0 0 0 0 1

25 24.6% 94.8% 0 1 0 0 0 1 0 0 0 1 0 0

26 19.9% 89.7% 0 1 0 0 0 1 0 0 0 1 0 0

27 22.8% 92.6% 0 1 0 0 0 1 0 0 0 1 0 0

28 30.1% 85.9% 0 1 0 0 0 1 0 0 0 1 0 0

29 28.0% 82.8% 0 1 0 0 0 1 0 0 0 1 0 0

30 8.9% 98.4% 1 0 0 0 1 0 0 0 1 0 0 0

31 9.0% 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

32 13.1% 97.7% 1 0 0 0 1 0 0 0 1 0 0 0

33 13.4% 97.4% 1 0 0 0 1 0 0 0 1 0 0 0

34 15.8% 92.7% 0 1 0 0 0 1 0 0 0 0 1 0

35 15.4% 93.9% 0 1 0 0 0 1 0 0 0 0 1 0

36 15.5% 97.1% 0 0 0 1 0 0 0 1 1 0 0 0

Nbr

Unadjusted 

Lower 

Flammability 

Limit of 

Combustion 

Zone Gas 

(LFLCZ)

[vol frac]

Reported

Combustion 

Efficiency

(CE)

[%]

Sums of outcomes Sums of outcomes Sums of outcomes

LFLcz Threshold LFLcz Threshold LFLcz Threshold

CE Threshold CE Threshold CE Threshold
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	22	

The Ccz parameter would be very useful in cases where the flare normally handles a high concentration of combustible

gas.

   

26% 28% 32%

96.5% 96.5% 96.5%

88 139 9 76 75 146 2 89 55 147 1 109

Nbr

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

True 

Positive

True 

Negative

False 

positive

False 

negative

1 36.3% 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 21.6% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

3 31.7% 99.8% 1 0 0 0 1 0 0 0 0 0 0 1

4 12.2% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

5 11.1% 98.7% 0 0 0 1 0 0 0 1 0 0 0 1

6 11.0% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

7 10.8% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

8 9.6% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

9 7.0% 82.2% 0 1 0 0 0 1 0 0 0 1 0 0

10 12.1% 99.5% 0 0 0 1 0 0 0 1 0 0 0 1

11 15.7% 99.9% 0 0 0 1 0 0 0 1 0 0 0 1

12 16.7% 99.8% 0 0 0 1 0 0 0 1 0 0 0 1

13 15.5% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

14 17.1% 99.8% 0 0 0 1 0 0 0 1 0 0 0 1

15 8.8% 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 10.6% 99.3% 0 0 0 1 0 0 0 1 0 0 0 1

17 9.4% 99.4% 0 0 0 1 0 0 0 1 0 0 0 1

18 12.5% 97.9% 0 0 0 1 0 0 0 1 0 0 0 1

19 14.2% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

20 15.9% 99.6% 0 0 0 1 0 0 0 1 0 0 0 1

21 27.3% 99.5% 1 0 0 0 0 0 0 1 0 0 0 1

22 26.1% 99.5% 1 0 0 0 0 0 0 1 0 0 0 1

23 19.9% 97.5% 0 0 0 1 0 0 0 1 0 0 0 1

24 23.2% 96.3% 0 1 0 0 0 1 0 0 0 1 0 0

25 15.0% 94.8% 0 1 0 0 0 1 0 0 0 1 0 0

26 19.2% 89.7% 0 1 0 0 0 1 0 0 0 1 0 0

27 15.5% 92.6% 0 1 0 0 0 1 0 0 0 1 0 0

28 12.3% 85.9% 0 1 0 0 0 1 0 0 0 1 0 0

29 12.3% 82.8% 0 1 0 0 0 1 0 0 0 1 0 0

30 48.1% 98.4% 1 0 0 0 1 0 0 0 1 0 0 0

31 45.9% 99.0% 1 0 0 0 1 0 0 0 1 0 0 0

32 32.4% 97.7% 1 0 0 0 1 0 0 0 1 0 0 0

33 30.5% 97.4% 1 0 0 0 1 0 0 0 0 0 0 1

34 26.1% 92.7% 0 0 1 0 0 1 0 0 0 1 0 0

35 26.9% 93.9% 0 0 1 0 0 1 0 0 0 1 0 0

36 28.2% 97.1% 1 0 0 0 1 0 0 0 0 0 0 1

37 23.0% 89.2% 0 1 0 0 0 1 0 0 0 1 0 0

38 49.3% 99.1% 1 0 0 0 1 0 0 0 1 0 0 0

Combustible

Components

in the

Combustion

Zone

(CCZ)

[vol frac]

Reported

Combustion

Efficiency

(CE)

[%]

Sums of outcomes Sums of outcomes Sums of outcomes

Ccz Threshold Ccz Threshold Ccz Threshold

CE Threshold CE Threshold CE Threshold
Review	by	Christopher	Schaeffer	of:	The Parameters for Properly Designed and Operated Flares.			 Page	23	

The “stoichiometric concentration in the combustion zone gas” is an invented parameter. It is the sum of the volume

fractions of each combustible gas in the combustion zone divided by that gas’s stoichiometric concentration in air (eg Cst

from Zabetakis). It is interesting to see if it compares favorably with LFLcz.

200.0% 237.5% 300.0%

96.5% 96.5% 96.6%

150 84 64 14 134 115 33 30 100 139 11 62

Nbr

Stoichometric

concentration

in combustion

zone gas

Reported

Combustion

Efficiency

(CE)

[%]

True

Positive

True

Negative

False

positive

False

negative

True

Positive

True

Negative

False

positive

False

negative

True

Positive

True

Negative

False

positive

False

negative

1 816.7% 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 484.6% 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 713.3% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 275.1% 99.5% 1 0 0 0 1 0 0 0 0 0 0 1

5 249.2% 98.7% 1 0 0 0 1 0 0 0 0 0 0 1

6 248.1% 99.7% 1 0 0 0 1 0 0 0 0 0 0 1

7 243.3% 99.9% 1 0 0 0 1 0 0 0 0 0 0 1

8 215.4% 99.9% 1 0 0 0 0 0 0 1 0 0 0 1

9 158.0% 82.2% 0 1 0 0 0 1 0 0 0 1 0 0

10 272.6% 99.5% 1 0 0 0 1 0 0 0 0 0 0 1

11 352.4% 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

12 375.3% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

13 348.9% 99.7% 1 0 0 0 1 0 0 0 1 0 0 0

14 383.7% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

15 197.8% 95.1% 0 1 0 0 0 1 0 0 0 1 0 0

16 239.0% 99.3% 1 0 0 0 1 0 0 0 0 0 0 1

17 232.9% 99.4% 1 0 0 0 0 0 0 1 0 0 0 1

18 311.9% 97.9% 1 0 0 0 1 0 0 0 1 0 0 0

19 353.0% 99.7% 1 0 0 0 1 0 0 0 1 0 0 0

20 395.1% 99.6% 1 0 0 0 1 0 0 0 1 0 0 0

21 274.2% 99.5% 1 0 0 0 1 0 0 0 0 0 0 1

22 261.4% 99.5% 1 0 0 0 1 0 0 0 0 0 0 1

23 199.9% 97.5% 0 0 0 1 0 0 0 1 0 0 0 1

24 234.5% 96.3% 0 0 1 0 0 1 0 0 0 1 0 0

25 150.7% 94.8% 0 1 0 0 0 1 0 0 0 1 0 0

Sums of outcomes Sums of outcomes Sums of outcomes

STOICHcz Threshold STOICHcz Threshold STOICHcz Threshold

CE Threshold CE Threshold CE Threshold

250.0% 300.0% 350.0%

96.5% 96.5% 96.5%

125 121 27 39 100 137 11 64 82 142 6 82

Nbr

Stoichometric

concentration

in combustion

zone gas

Reported

Combustion

Efficiency

(CE)

[%]

True

Positive

True

Negative

False

positive

False

negative

True

Positive

True

Negative

False

positive

False

negative

True

Positive

True

Negative

False

positive

False

negative

1 816.7% 100.0% 1 0 0 0 1 0 0 0 1 0 0 0

2 484.6% 99.9% 1 0 0 0 1 0 0 0 1 0 0 0

3 713.3% 99.8% 1 0 0 0 1 0 0 0 1 0 0 0

4 275.1% 99.5% 1 0 0 0 0 0 0 1 0 0 0 1

5 249.2% 98.7% 0 0 0 1 0 0 0 1 0 0 0 1

6 248.1% 99.7% 0 0 0 1 0 0 0 1 0 0 0 1

Sums of outcomes Sums of outcomes Sums of outcomes

STOICHcz Threshold STOICHcz Threshold STOICHcz Threshold

CE Threshold CE Threshold CE Threshold
Attachment G 

Dr. Jim Seebold 

Independent Consultant 
1

— Good Try —

Peer review of a report by

U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS)

“Parameters for Properly Designed and Operated Flares”

submitted by

James G. Seebold, Chevron (Retired)

Independent Consultant

May 21, 2012

In the United States, alleged wind-induced combustion efficiency (“CE”) degradation in the operation 

of real industrial flares in the field has become a big issue amongst environmental activists in 

consequence of which the United States Environmental Protection Agency is considering regulations 

related to wind effects. While it is true that wind effects have been reported in model-scale (typically 

≤3”D and often «3”D, soda-straw like) tests in wind tunnels, there is no evidence whatsoever in recent and 

extensive in situ full-scale (typically »18”D) remote-sensing field testing of any significant wind-induced 

CE-degradation.

Nothing has changed

The regrettable absence of 

systematic variation in combustion efficiency found in 

past and present flare 

studies leads inevitably to 

the idea of “stochasticness” 

and great difficulties for 

regulatory interpretation.

This fact is illustrated not 

only in recent USDOJ/USEPA 

consent-decree-imposed in 

situ PFTIR testing of real 

industrial flares in the field 

that is the subject of the 

USEPA OAQPS report now 

in peer review but over and 

over again in the decades 

since Siegel’s monumental work1in competently executed studies from the mid-1980s USEPAsponsored “Evaluation of the Efficiency of Industrial Flares” (illustrated above) on which the current failed 

federal law (40CFR60.18) was based to the most recent USDOJ/USEPA consent-decree-imposed field 

studies.

A summing up of the valiant PFTIR-based in situ field investigations carried out by eminently qualified

practitioners, the USEPA OAQPS report, “Parameters for Properly Designed and Operated Flares,”

 

1

Siegel, K. D., "Degree of Conversion of Flare Gas in Refinery Elevated Flares," PhD Thesis in Engineering Science, Feb, 1980,

 Chemical Engineering Department, University of Karlsruhe, Germany
2

appears merely to be an elaborately perseverated defense of the blatantly conjectural nature (illustrated 

in “Figure 3-3” below) of the so-called “conclusions” in a report that does not come any closer than 

previous efforts to producing the long sought but to date elusive flare researchers’ “Holy Grail;” viz.,

the omniscient, universal, all-encompassing, incontestable correlating parameter for flare combustion 

efficiency.

“... suggests ...” or “... most appropriate ...” hardly implies

data quality good enough for rule making or policy making

“To identify over steaming situations that may occur on steam-assisted flares, the data suggest that 

the lower flammability limit of combustion zone gas (LFLCZ) is the most appropriate operating 

parameter. Specifically, the data suggest that, in order to maintain good combustion efficiency, the 

LFLCZ must be 15.3 percent by volume or less for a steam-assisted flare.” 2 

Are you kidding me? No better than the OAQPS long ago jumped-to “conclusions” on which today’s

failed federal law (40CFR60.18) is based.

 

2 Quote highlighted in the Research Triangle Institute for U.S. Environmental Protection Agency instructions to 

 Peer Reviewers that are appended to this review.
3

Furthermore, “15.3” implies far more accuracy and reliability in LFLcz or any variant thereof than is 

actually there in the USEPA OAQPS adduced data. 

Similarly with the combustion efficiency demarcation arbitrarily set at “96.5.”

Those lines could be put pretty much any place else depending upon what one seeks to “prove.”

The fatal error lies in USEPA OAQPS’s compulsion to extract a bullet-proof activist-indemnifying

regulation-justifying argument from stochastically flawed data just as was done so many years ago in 

the formulation of today’s failed federal law (40CFR60.18).

MFR nonsense

There is no evidence whatsoever in the test report data to support the pseudo-scientific speculations

quoted below that are highlighted in the Research Triangle Institute for U.S. Environmental Protection

Agency instructions to Peer Reviewers that are appended to this review.

“The data suggest that flare performance is not significantly affected by crosswind velocities up to 22 

miles per hour (mph).” 

No, the USEPA OAQPS adduced data demonstrates conclusively that in the extensive USDOJ/USEPA 

consent-decree-imposed testing of real industrial flares in the field there is no evidence whatsoever of 

wind-induced combustion efficiency (CE) degradation! 

“There are limited data for flares in winds greater than 22 mph.” 

Actually, as far as I am aware, there are no data for real industrial flares in winds greater than 22 mph! 

Perhaps USEPA OAQPS might like to sponsor a study similar to Siegel’s in which the blower is crankedup above 6 m/s? 

Or perhaps not ...

“However, a wake-dominated flame in winds greater than 22 mph may affect flare performance.” 

And pigs might fly. Show me the data for real industrial flares in the field!

“The data available indicate that the wake-dominated region begins at a momentum flux ratio (MFR) 

of 3 or greater.” 

No, not the USEPA OAQPS adduced data in the report currently under peer review to which “peers” 

should restrict their attention and to which USEPA OAQPS should restrict their attention in attempting 

to formulate regulations; and certainly not the USEPA OAQPS adduced data on real industrial flares in 

the field.

This is pure pseudo-scientific speculation which, in my opinion, is ill-advisedly at best applied to 

reacting jets as explained below.

“The MFR considers whether there is enough flare vent gas and center steam (if applicable) exit

velocity (momentum) to offset crosswind velocity.”

No, it doesn’t; the USEPA OAQPS MFR concept is severely flawed in that it completely neglects the 

physical and chemical realities of reacting jets!
4

To say that this is poorly understood today would be a gross understatement.

But what is clear is that while it seemed appealing a few years ago, the manifest uselessness of the 

momentum flux ratio (MFR) in “correlating” combustion efficiency or anything else for that matter 

related to real industrial flare performance has by now become pretty well recognized. 

MFR may very well have some utility in describing the flow out industrial stacks or other unfired vents 

in windy conditions which is, in and of itself, a seriously complicated downwash, vortex street 

interactive fluid mechanics phenomenon.3

But MFR is not useful in the case of flares which are required by existing federal law to be lit. The 

reacting jet, even if it would not appropriately be described as a “jet” on its own compared with the 

crosswind velocity, gains a lot of upward and rapidly-building momentum the instant the reaction 

starts to heat it up. 

In short, the reactants get a sharp upward buoyancy-rise velocity jolt upon ignition. It may be that this 

marked upward momentum impulse partially accounts for the marked absence of downwash-induced 

wake-stabilized combustion efficiency degradation via partially reacted or unreacted eddy stripping in 

real full-scale industrial flares in the field.

Additionally, the USEPA OAQPS report overlooks 

the fact that there are a number of different 

wake regimes that establish themselves at 

various stages in the development of the vortex 

street, each of which has different pull-down and 

wake-formation propensities.

There exist Reynolds Number regimes in which 

the vortex street is more coherent than others 

and presumably, therefore, exerts a stronger 

pull-down to wake-stabilization and also, 

presumably, a greater propensity toward 

stripping partially-reacted eddies out of the 

trailing wake-stabilized combustion zone.

We don’t know. It hasn’t been studied. Not on 

real industrial flares in the field. Too bad, so sad, 

but it remains poorly understood. Perhaps we 

are in that critical Reynolds Number regime for 

toy flares in a wind tunnel but not for the 

conditions in which real industrial flares have 

thus far been tested in the field.

What we do know is that there is a great deal of 

data both recent and historical on real industrial

flares reliably tested in situ in the field on which 

no CE-degrading wind effects whatsoever have been seen.

 

3

See for example: http://library.usask.ca/theses/available/etd-04212008-124717/unrestricted/MAdaramola.pdf

Flow patterns for flow over a cylinder:

(A) Reynolds number 0.2; (B) 12;

(C) 120; (D) 30,000; (E) 500,000
5

And quite literally not “seen” – no trailing wake-stabilized plume and, therefore, no partially reacted 

eddy stripping combustion efficiency degradation.

Toy flares vs. real industrial flares in the field

Studies alleging a significant MFR influence on combustion efficiency are based on data from small 

diameter model-scale flares. How do these data scale up to real industrial flares? The key scaling 

concept can be found in any basic textbook on aerodynamics.

In brief, assuming a smooth vertical cylindrical stack, the relative vacuum in the low pressure region in 

the wake behind the flare stack is proportional to the square of the wind velocity and inversely 

proportional to the stack diameter; i.e.,  Vwind

2

/Dstack. This means that for a given vent gas flow rate, 

larger diameter stacks are inherently more resistant to wind effects than smaller diameter stacks.

In short, everything else being equal, the larger the stack diameter, the less “suck” there is in the wake 

of the stack. Therefore, at a given wind speed, larger diameter flares are more resistant to the 

formation of the stationary “suck-down” vortex and hence more resistant to crosswind-induced 

combustion efficiency degradation.

Thus, on large flare stacks, one would not expect to see wake dominated flow except perhaps 

under hurricane conditions nor any so-called “MFR”-related combustion efficiency degradation as, 

indeed, we do not.

Both historical and recent in situ tests of real industrial flares in the field show no evidence whatsoever 

of wind related combustion efficiency degradation nor any evidence of wake stabilized behavior.

What about real stacks terminated at the top?

Then there is the question of the fluid mechanics at the top of a finite circular cylinder and that local

influence on pull-down. It turns out that the strongest pressure deficit is not downwind 180-degrees 

from the upwind face of the cylinder but on the sides at 90-degrees and 270-degrees from the front, a 

perhaps inconvenient truth.

What does that mean with respect to alleged but increasingly discredited crosswind-induced 

combustion efficiency degradation on real industrial flares in the field?

We don’t know. Nobody has studied it. But it probably matters ...

“Because wake-dominated flames can be identified visually ...”

Maybe.

Maybe not.

What is certain is that in the USDOJ/USEPA consent-decree-imposed testing of real industrial flares in 

the field, NO (no) so-called “wake-dominated behavior” whatsoever was observed as far as I am 

aware!
6

Regulating the wind

There exists a veritable plethora of authoritative articles on the aerodynamics of flow across infinite

cylinders and finite cylinders terminated at the top.4

While I do not recommend it, in the event that USEPA OAQPS are determined to regulate or engineer 

the wind, perhaps the many and varied erudite dissertations that have appeared over the years in the 

Journal of Wind Engineering and Industrial Aerodynamics5 might be found useful.

But I doubt it ...

Flame liftoff hardly implies degraded combustion efficiency

“To avoid flame lift off, the data suggest that the actual flare tip velocity (i.e., actual flare vent gas 

velocity plus center steam velocity, if applicable) should be less than an established maximum 

allowable flare tip velocity calculated using an equation that is dependent on combustion zone gas 

composition, the flare tip diameter, density of the flare vent gas, and density of air.” 6 

Hardly! Sonic flares are characterized by flames that are lifted to a perfectly stable calculable position

some distance above the tip. Just because today’s failed federal law (40CFR6018) doesn’t recognize sonic 

flares doesn’t mean we can’t go there perfectly safely and with combustion efficiencies »98%!

MFR disconnect

Momentum Flux Ratio (MFR) is appropriately defined for unfired stacks as the momentum of the stack 

gas divided by the momentum of the wind. It is true that for MFR defined that way for relatively cold 

vents, an MFR < 1 generally indicates that the wind momentum predominates and the plume begins to 

deflect from the vertical as the plume trajectory develops.

However, for hot plume dispersion, depending upon just how hot, of course, buoyancy rise invariably 

greatly exceeds velocity rise. The USEPA OAQPS MFR calculation considers only the ambient process 

temperature of the vented flare gases.

Accordingly, by overlooking buoyancy rise for fired “stacks” such as flares, USEPA OAQPS are not even 

looking at the dominant plume rise mechanism!

 

4

See for example:

 Free end effects on the near wake flow structure behind a finite circular cylinder 

 (http://www.postech.ac.kr/lab/me/efml/html/publication/pdf/64.pdf);

 Flow Around a Circular Cylinder with a Free End 

 (http://library.usask.ca/theses/available/etd-07252011-090143/unrestricted/Heseltine_Johnathan_Lucas_sec_2003.pdf):

 Tubes: cross-flow over (http://www.thermopedia.com/content/1216/?tid=104&sn=1410)

5

http://www.sciencedirect.com/science/journal/01676105

6 Quote highlighted in the Research Triangle Institute for U.S. Environmental Protection Agency instructions to 

 Peer Reviewers that are appended to this review.
7

Upcoming papers/presentations

The foregoing issues and others will be amply illustrated and explained and in two upcoming papers, 

one to be presented at the International Flame Research Foundation’s triennial Members Conference, 

Chateau Maffliers, France, June 11-13, 2012 (Link:http://www.ngcom.it/17thMC/); and the other to be presented 

at the American Flame Research Committee’s Annual Meeting, University of Utah, Salt Lake City, Utah, 

September 5-7, 2012 (Link:http://www.afrc.net/assets/fordownload/pdfs/afrc_2012_call_for_papers.pdf).

I should like to take this opportunity once again to suggest that the latter meeting would be an 

excellent opportunity to promote agency/industry cooperation around flare innovation. I have 

extended to USEPA several cordial invitations to participate in any manner USEPA chooses, thus far 

without any affirmative commitment at the USEPA end, and I am more than pleased to extend the 

invitation again here.

AFRC would be happy to host the USEPA representatives. Supposing that USEPA are interested, at the 

AFRC end I can get it done in whatever form and format the agency would actually be interested in 

doing it.

At last year’s AFRC-sponsored Industrial Flare Colloquium in Houston in which USEPA and TCEQ 

participated, the industrial participation was great. I expect this year, especially after the recent USEPA 

announcements, the interest and attendance will be even stronger at the AFRC 2012 gathering. I am 

sure that, just as last year, there will be a lot of AFRC industrial delegates who are interested in “EPA’s 

Vision,” whatever that means, vis-à-vis industrial cooperation on flare issues going forward and how 

their companies might cooperate and fit in.

In the event that USEPA would be interested in taking advantage of what seems to me to be a 

marvelous opportunity to promote such agency/industry cooperation on flare innovation, I would be 

more than pleased to make all the arrangements.

Is there a better way?

Perhaps. One of the worst that mistakes “we” – regulators, activists and industrialists alike – ever 

made was to allow emergency flares to become “emission control devices,” thus compromising the 

emergency flare’s legitimate and overarching safety functionality.

Far better to have a fire in plain sight safely up there in the nighttime sky than a devastating explosion 

back there in the plant. Nobody would disagree with that.

Perhaps that historic mistake has been viewed, in some quarters as least, as a good thing, particularly 

today. Over the decades since the commission of that mistake it has, after all, provided gainful 

employment to generations of regulators, process engineers and activist lawyers seeking injunctive 

relief.

But rather than perseverating in trying to specify what have proven over the last three decades to be 

an increasing number of to date stochastically uncorrelatable independent governing parameters, a 

few of which doubtless have even yet to be discovered, why not consider returning to the use of flares 

strictly as emergency devices, not emissions control devices?

If a plant has “too many” emergencies, fine them. When the CEO takes notice, the plant operators and 

process engineers will figure out how better to run and design. They always have. And there are lots 

of other ways to deal with the frequent but typically piddly releases that require environmental 

control.

It just might be a better way to a better end. I commend it to your consideration.
8

Appendix

MEMORANDUM

TO: James G. Seebold, Independent Consultant

FROM: Research Triangle Institute for U.S. Environmental Protection Agency

DATE: April 5, 2012

SUBJECT: Review of the Parameters for Properly Designed and Operated Flares

This memorandum provides background information and specific charge questions to the Flare 

Review Panel in its review of a report on parameters for properly designed and operated flares prepared 

by U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS). The report provides an 

examination of several factors that are important for a properly designed and operated flare. Based on 

the analysis provided in the report, the data suggest that over steaming on steam-assisted flares and 

excess aeration on air-assisted flares degrade flare performance. In addition, the data suggest that high 

winds and flame lift off can influence flare performance on all types of flares. This document will be the 

focus of review by the Flare Review Panel that must be completed by the end of the day on May 21st, 

2012.

Background

In May 2005, the Ohio Environmental Protection Agency (OhioEPA) installed monitors at a 

school to investigate odor complaints. The monitoring showed high human health risk (i.e., hazard 

quotient 6.21 and cancer risk of 5 in 10,000) and the district closed the school. The school was located 

across the street from a chemical plant. In September 2005, the U.S. EPA Region 5 began investigating 

the chemical plant and determined that over steaming at the facility’s steam-assisted flare was the likely 

cause of the ambient air issues.

In February 2010, the EPA, OhioEPA, and facility agreed to a consent decree requiring a new 

paradigm in flare monitoring that focuses on steam usage at the flare tip (i.e., combustion zone heating 

value and steam-to-vent-gas ratio). The consent decree also required that Passive Fourier Transform 

Infrared Spectroscopy (PFTIR) remote testing be performed. PFTIR remote sensing involves using a 

spectrometer positioned on the ground to view hot gases from the flare plume, which radiate spectra that 

are unique to each compound. Around the same time this consent decree was being drafted, the EPA 
9

Office of Enforcement and Compliance Assurance (OECA) requested testing be conducted pursuant to 

section 114 of the Clean Air Act on several other flaring facilities using PFTIR remote sensing 

technology. OECA’s request included a requirement to test a range of operating conditions (including 

typical conditions) at each flaring facility. All of the PFTIR testing carried out through these actions and 

used as part of this report were performed and analyzed by a single company. 

In May 2009, the Texas Commission on Environmental Quality (TCEQ) contracted with The 

University of Texas at Austin to conduct a comprehensive flare study project on full-scale steam- and 

air-assist flares at the John Zink Company flare demonstration facility in Tulsa, Oklahoma. The purpose 

of the project was to conduct field tests to measure flare emissions and collect process and operational 

data in a semi-controlled environment to determine the relationship between flare designs, operation, 

flare vent gas lower heating value and flow rate, destruction efficiency, and combustion efficiency. The 

study also evaluated the performance of remote sensing technologies against extractive techniques.

EPA Review of Flares

EPA used the test data from the recent PFTIR testing and TCEQ flare studies (as well as other 

older experimental flare efficiency studies conducted by the EPA in the early 1980s) to investigate the 

effects of flare performance with varying amounts of steam (for steam-assisted flares) and air (for airassisted flares); and high wind and flame lift off situations (for both types of flares). EPA also reviewed 

available scientific information from peer-reviewed studies and other technical assessments about 

flammability, wind, and flame lift off to support our observations. Based on an analysis of the data, we 

have determined that there are numerous operating parameters that should be considered in order to be 

confident that a flare is operated consistently and properly to achieve good combustion efficiency.

We have developed a report that is organized into nine sections and nine technical appendices. 

Section 1.0 introduces the report and provides a summary of our primary observations. Section 2.0 

identifies the experimental flare efficiency studies and flare performance test reports used in this 

investigation. Sections 3.0 through 8.0 describe the development of our observations. Section 9.0 

provides a list of documents referenced in this report. 

The primary observations made in this report are as follows:

 To identify over steaming situations that may occur on steam-assisted flares, the data suggest 

that the lower flammability limit of combustion zone gas (LFLCZ) is the most appropriate 

operating parameter. Specifically, the data suggest that, in order to maintain good 
10

combustion efficiency, the LFLCZ must be 15.3 percent by volume or less for a steamassisted flare; i.e., ≤0.153. As an alternative to LFLCZ, the data suggest that the ratio of the 

net heating value of the combustion zone gas (NHVCZ) to the net heating value of the flare 

vent gas if diluted to the lower flammability limit (NHVVG-LFL) must be greater than 6.54. 

(Quoted from FINAL USEPA Flare Technical Report, p.1-2)

 To identify excess aeration situations that may occur on air-assisted flares, the data suggest 

that the stoichiometric air ratio (SR) (the actual mass flow of assist air to the theoretical 

stoichiometric mass flow of air needed to combust the flare vent gas) is the most appropriate 

operating parameter. Specifically, the data suggest that, in order to maintain good 

combustion efficiency, the SR must be 7 or less for an air-assisted flare. Furthermore, the 

data suggest that the lower flammability limit of the flare vent gas (LFLVG) should be 15.3 

percent by volume or less to ensure the flare vent gas being sent to the air-assisted flare is 

capable of adequately burning when introduced to enough air. (Quoted from FINAL USEPA 

Flare Technical Report, p.1-2)

 The data suggest that flare performance is not significantly affected by crosswind velocities 

up to 22 miles per hour (mph). There are limited data for flares in winds greater than 22 mph. 

However, a wake-dominated flame in winds greater than 22 mph may affect flare 

performance. The data available indicate that the wake-dominated region begins at a 

momentum flux ratio (MFR) of 3 or greater. The MFR considers whether there is enough 

flare vent gas and center steam (if applicable) exit velocity (momentum) to offset crosswind 

velocity. Because wake-dominated flames can be identified visually, observations could be 

conducted to identify wake-dominated flames during crosswind velocities greater than 22 

mph at the flare tip. (Quoted from FINAL USEPA Flare Technical Report, p.1-3)

 To avoid flame lift off, the data suggest that the actual flare tip velocity (i.e., actual flare vent 

gas velocity plus center steam velocity, if applicable) should be less than an established 

maximum allowable flare tip velocity calculated using an equation that is dependent on 

combustion zone gas composition, the flare tip diameter, density of the flare vent gas, and 

density of air. (Quoted from FINAL USEPA Flare Technical Report, p.1-3)

 LFLCZ could apply to non-assisted flares (i.e., the LFLCZ must be 15.3 percent by volume or 

less in order to maintain good combustion efficiency). Also, the same operating conditions 

that were observed to reduce poor flare performance associated with high crosswind velocity 

and flame lift off could apply to non-assisted flares. Finally, because of lack of performance 

test data on pressure-assisted flare designs and other flare design technologies, it seems likely 

that the parameters important for good flare performance for non-assisted, steam-assisted, 

and air-assisted flares cannot be applied to pressure-assisted or other flare designs without 

further information. (Quoted from FINAL USEPA Flare Technical Report, p.1-3)
11

Document Availability

The report is being made available to the Panel in the form of the attached electronic file, which 

we request be forwarded to all members of the Panel.

Specific Charge in Reviewing the Parameters for Properly Designed and Operated Flares

We ask the Panel to focus on the charge questions below in their review of the report, but we 

would appreciate comments on any aspects of the information in the report or other flare topics. In 

addition, all references used in this report are available upon request.

Section 2: Available Flare Test Data

1. Please comment on the agency’s criteria for excluding available flare test run data from final 

analyses, and whether application of these criteria may have led to inappropriate exclusions 

of relevant data points.

Section 3: Steam and Flare Performance

2. Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as an 

operating parameter for indicating over steaming situations on steam-assisted flares. 

Comment on the agency’s use of the ratio of the net heating value of the combustion zone 

gas (NHVCZ) to the net heating value of the flare vent gas if diluted to the lower flammability 

limit (NHVLFL) as an alternative to LFLCZ. Does the flare data adequately support the EPA’s 

observations?

3. Is there sufficient evidence that chemical interactions are occurring that make the calculated

LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is there other data 

available (that is not discussed in this report) that may help clarify our discussion about 

specific chemical interactions related to lower flammability limits of gas mixtures?

4. Did the agency adequately examine other operating parameters (different from LFLCZ; or the 

ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming situations? Are there specific 

other parameters that should be given more or less emphasis?

Section 4: Air and Flare Performance

5. Please comment on the stoichiometric air ratio (SR) as an operating parameter for indicating

excess aeration situations on air-assisted flares. Additionally, also comment on whether the 

lower flammability limit of the flare vent gas (LFLVG) is an appropriate operating parameter 

for determining whether the flare vent gas being sent to an air-assisted flare is capable of 

burning? Does the flare data adequately support the EPA’s observations?

Section 5: Wind and Flare Performance
12

6. Please comment on the momentum flux ratio (MFR) as an operating parameter in crosswind

velocities greater than 22 mph at the flare tip to indicate wake-dominated flame situations.

Additionally, also comment on the agency’s observation that in the absence of crosswind 

greater than 22 mph, a low MFR does not necessarily indicate poor flare performance.

Comment on the effectiveness of observations identifying wake-dominated flames. Does the 

flare data adequately support the EPA’s observations?

7. Did the agency adequately examine other operating parameters (different from MFR) for 

identifying wake-dominated flames? Are there specific other parameters that should be given 

more or less emphasis?

Section 6: Flare Flame Lift Off

8. Please comment on the maximum allowable flare tip velocity equation which considers 

combustion zone gas composition, the flare tip diameter, density of the flare vent gas, and 

density of air. Does the flare data adequately support the EPA’s observations? Are there 

specific other parameters or methods/equations that should be given more or less emphasis?

Section 7: Other Flare Type Designs to Consider

9. Please comment on the applicability of the LFLCZ parameter, maximum allowable flare tip 

velocity equation, and the observations regarding crosswind velocity to non-assisted flares, 

pressure-assisted flares, and other flare designs.

Section 8: Monitoring Considerations

10. Please comment on the appropriate monitoring equipment needed to ensure good flare 

performance and on any other known monitoring methods (not discussed in this report) for 

monitoring the following parameters: LFLCZ, LFLVG, LFLVG, C, the ratio of NHVCZ to 

NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating scenarios and 

conditions where less robust monitoring equipment could be used to determine the operating 

parameters of interest.
Attachment H 

Dr. Joseph Smith 

Laufer Endowed Chair of Energy 

Missouri University of Science and Technology 
A Technical Review of Parameters for Properly Designed and 

Operated Flares

Report for Flare Review Panel

April 2012

U.S. EPA Office of Air Quality Planning and Standards (OAQPS)

This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by EPA. It does not represent and 

should not be construed to represent any Agency determination or policy

Review Performed By:

Joseph D. Smith, Ph.D.

Professor and Laufer Endowed Energy Chair

Department of Chemical and Biological Engineering

Missouri University of Science and Technology

Rolla, Missouri

Submitted:

June 5, 2012
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Evaluation Rubric:

Scoring: 1 = POOR, 2 = FAIR, 3 = AVERGAE, 4 = GOOD, 5 = EXCELLENT

Technical Content

Poor (1):

 No reference to literature

 Inconsistent conclusions and/or recommendations

 Incorrect statements and/or calculations

Fair (2):

 Missing some references

 No conclusions/recommendations

 Incomplete calculations that partial support conclusions/recommendations

 Little support for technical statements

Average (3):

 Includes most references

 Limited conclusions/recommendations

 Limited calculations support conclusions/recommendations

 General support for technical statements

Good (4):

 Includes key references 

 High level conclusions and recommendations

 Key calculations included in discussion (no foundation for calculations or assumptions used) 

 Key technical statements based on supporting evident from literature and/or calculations

Excellent (5):

 Includes key references plus additional supporting references

 Strong conclusions and recommendations included

 Detailed calculations included with clear explanation and sample calculations to illustrate

 All technical statements supported with clear evidence from literature and/or calculations
Technical Review: JDS/05Jun2012

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I- Design and execution of the research activity 25/30

• Did the approach show creativity or new thought in attacking the design problem? Score: 7/10

Based on the analysis reported in this document, the following specific conclusions are made:

1. To identify over steaming situations that may occur on steam-assisted flares, the lower 

flammability limit of combustion zone gas (LFLCZ) is the most appropriate operating 

parameter,

2. To identify excess aeration situations that may occur on air-assisted flares, the 

stoichiometric air ratio (SR) (the actual mass flow of assist air to the theoretical 

stoichiometric mass flow of air needed to combust the flare vent gas) is the most 

appropriate operating parameter,

3. Flare performance is not significantly affected by crosswind velocities up to 22 mph,

4. Wake-dominated flare operation begins at a momentum flux ratio (MFR) of 3 or greater,

5. Actual flare tip velocity (i.e., actual flare vent gas velocity plus center steam velocity, if 

applicable) should be less than an established maximum allowable flare tip velocity 

calculated using an equation dependent on combustion zone gas composition, effective 

flare tip diameter, flare vent gas density, and air density,

6. For non-assisted flares, the same criterion LFLCZ ≤ 15.3% by volume is applicable to 

ensure good combustion efficiency.

This work introduces a new metric LFLcz (Lower Flamability Limit of gases in the combustion 

zone above a flare stack) with a criterion to quantify flare performance (i.e., LFLcz ≤ 15.3). The 

validity of using this metric to evaluate flare performance was quantified using several key factors

known to impact flare performance (i.e., tip diameter, tip velocity, waste fuel composition, inert 

composition, ambient wind speed, ambient air density, flare gas density, etc.). A method to 

calculate the LFL for a mixture including inert gases (i.e., nitrogen, carbon dioxide, and water 

vapor) was also introduced. Test results from several flare experiments from industry, academia, 

and government research organization were analyzed using this metric and criterion. It was 

concluded that this metric is valid and effective for quantifying flare performance for steam/air 

assisted flares, non-assisted flares, and pressure assisted flares. In addition, two additional 

performance metrics were briefly considered including “Net Heating Value” and “Steam Ratios”.

In general, two phenomena control flare performance: 1) reaction kinetics, and 2) gas mixing. 

The report compares available flare performance data in terms of levels of steam (for steam 

assisted flares); or air (for air assisted flares); high ambient wind conditions, and flame lift off. 

Unfortunately, the analysis did not evaluate the metrics proposed by Smoot, et.al. (2010)1 which 

was developed and reported to the EPA and used to evaluate steam flare performance for the 

Department of Justice. As a recommendation for future work, this additional analysis should be 

conducted.

 

1 Smoot, L.D., Jackson, R.E. and Smith, J.D., “Toward Combustion Efficiency Control Criteria in Open 

Industrial Flares,” Advances in Combustion Technology: Improving the Environment and Energy Efficiency, 

American Flame Research Committees - International Pacific Rim Combustion Symposium, Sheraton Maui, 

Hawaii, September 26 –29 (2010)
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The project plan was not documented in the report that was reviewed. However, the clear 

objective of this work was to assess flare performance to identify key operating parameters that 

quantify “efficient” flare operation. In this regard, the project plan appeared well designed and 

executed.

However, reading the analysis indicated a clear bias related to establishing the proposed metric 

LFLcz. In many instances data points that did not meet the proposed criterion were excused as 

“bad data resulting from inaccurate testing procedure or analysis”. The proposed metric is a single 

number that is being used to assess the very complex nonlinear interaction between reaction 

kinetics and turbulent gas mixing. Thus, it is expected by this author that this metric should not 

reasonable be expected to account for all flare performance data. No mention was made of 

instances where certain data points agreed with the metric but may also have been inaccurate due 

to poor testing procedure or data analysis. This data quality error is related to accepting bad data 

as good data to confirm a hypothesis. If certain data that do not agree with the proposed 

hypothesis are rejected to confirm the applicability of the metric, then the same standard must be 

applied for all data evaluated from a given test. Unfortunately, this did not appear to be the 

standard taken in this analysis.

• Was the project plan well designed? Score: 10/10

• Was the project executed efficiently? Score: 8/10

II- Evaluation of the research results: 32/40

• How reliable are the results? Score: 15/15

The flare test data as reported are reliable with discrepancies highlighted (i.e., plot of 

Combustion Efficiency vs. Momentum Flux Ratio by Seebold, et.al., 2004). The data are well 

referenced and are presented in a comprehensive fashion. It would be nice to have the original 

data sets for comparison against what is reported in this report. However, based on my review of 

the literature the data as presented appear reliable thus any analysis will be reliable.

• Are the results interpreted correctly? Score: 10/15

For the most part, the test results are evaluated in a consistent basis and the conclusions drawn 

are correct. However, there are examples throughout the report that illustrate a bias toward 

justifying all performance data based on this single parameter. Thus, the results are not interpreted 

correct but do reflect the most comprehensive analysis of existing flare test data available in the 

open literature.

• Are the conclusions based on sound data? Score: 7/10

The conclusions drawn as a result of this analysis appear to be based on a thorough evaluation 

of valid data. However, it appears there remains unaccounted for factors that affect flare 

performance. Clearly, kinetics is critical to the combustion process. Wang et.al., (1996) shows that 

small amounts of H2O increase the burning rate by modifying the OH radical. Brouwer et.al., 

(1994) points out that the production of products of incomplete combustion (PIC) is controlled by 
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Parameters for Properly Designed and Operated Flares

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complex interactions between turbulent mixing and chemical kinetics. This fact is evident when 

the authors attempt to explain why several data points exhibit combustion efficiency less than 

96.5% for a LFLcz < 15.3, which is proposed as the criteria for good flare performance.

III-Importance and/or relevance of the research: 10/30

• Did the report clearly identify relevance/importance of gas flaring and flare technology? Score: 10/10

Yes, the importance of defining the stable flame envelope of operating conditions for a flare 

was discussed in terms of 40 CFR60.18 and 40 CFR63.11(b) and the impact of reduced waste gas 

sent to flares on their operating efficiency (i.e., combustion efficiency).

• Did the report address the practical applications of gas flaring and flare technology Score: 0/10

No, this was not part of this report.

• Did the report discuss the economic and environmental implication flare technology? Score: 0/10

No, this was not part of this report.

Overall Score 67/100
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As requested of the Flare Review panel, I have focused my report review on the charge 

questions listed below. The comments listed below are a summary of my review 

comments and the entire report with my hand written review comments are available on 

request.

Section 2: Available Flare Test Data

1. Please comment on the agency’s criteria for excluding available flare test run data 

from final analyses, and whether application of these criteria may have led to 

inappropriate exclusions of relevant data points.

Data quality has been assessed using the following questions2

:

• What decision is being made that leads to action?

• What is the cost of being wrong?

• What is the cost of being right?

• What data are critical to the decision being made?

• How accurate and precise must the data be to ensure the correct decision is made?

• What are the time and resource constraints on data collection?

• How will the data be evaluated?

• How will clear and concise documentation for effective communication be achieved?

These questions were used to assess the flare test data evaluated in the present report 

considering the project objective (listed earlier):

• What decision is being made that leads to action? Identifying an accurate and defendable 

performance metric to assess flare efficiency will allow the EPA to regulate flare 

operation. The recent attention to flare emissions indicates that identifying the “correct” 

metric is an important decision that will allow the agency to act.

• What is the cost of being wrong? If the EPA selects a metric that is overly restrictive 

(inaccurate) in quantifying flare performance, it may result in unnecessary fines and 

additional legal costs of >$100MM (estimate assumes approximate legal costs/case of 

~$1MM, >100 U.S. chemical plants/refineries subject to restriction). If the inaccurate 

metric is too lax and allows excessive emissions (i.e., low combustion efficiency) the 

cost in human health could easily exceed this amount but may be unquantifiable.

• What is the cost of being right? The cost of performing additional testing to more fully 

quantify the reaction kinetic and turbulent mixing effects on combustion efficiency for 

the conditions described in the present report (i.e, high olefins/hydrogen content, varying 

inert composition and levels, ambient wind speed, tip velocity, tip design). Previous 

work reported in the literature and summarized in this report represent a cost of >$3MM 

(assumes ~$10,000/data point for >300 data points listed in report). Additional testing 

based on this analysis may cost on the same level.

 2 C. Christensen, Private Communication to J.D. Smith, Dow Chemical Company, 1993.
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• What data are critical to the decision being made? Reproducible combustion efficiency 

measurements taken from flares operating in a controlled (quantified) condition using 

validated test procedures with established monitoring equipment (see Section 8) is the 

critical data required to make this decision. A complete data set must include 

information from the different levels of the “Validation & Verification” pyramid (see

Figure 1).

Figure 1 - Validation and Verification Pyramid for Flare Testing

• How accurate and precise must the data be to ensure the correct decision is made?

Combustion efficiency > 96.5% ±1.5% at a 99% confidence interval was been 

established as representative of “good flare performance” (see Section 3.1). This implies 

the collected combustion efficiency data must be accurate to at least 3 significant figures. 

Given that the combustion efficiency is based on the measured composition of gases, this 

implies these measurements must also have at least 3 significant figures.

• What are the time and resource constraints on data collection? Since this data will be 

used to establish EPA guidelines, the time and resource constraint to collect this data will

be determined by the EPA.

• How will the data be evaluated? To avoid biasing in the data collection and analysis, this 

work should be performed by a dis-interested third party (not industrial consortium nor 

regulatory agency).

• How will clear and concise documentation for effective communication be achieved?

Results from this analysis should be published in the open literature with appropriate 

peer review to ensure the data and its analysis is thoroughly and critically reviewed. 

Results of this data and its analysis should be made available on the EPA website to 

ensure broad distribution to the technical community. 
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In addition to the data quality assessment, I’ve including the following conclusions based on my 

review of the data sets included in this analysis and the assessment of which data points should 

(should not) be included in the analyses.

Table 2.1 in section 2 lists ten different data sets that were analyzed including 582 steam-assisted 

flare test runs (270 test runs eliminated because no steam was used in the tests or there was not 

enough data to quantify flare performance) and 111 air-assisted flare test runs (67 test runs 

eliminated due to lack of test information). The report clearly states why each data set (point) 

was eliminated. However, later in the report in section 3 several data points appear to be 

dismissed because they don’t conform to the proposed metric. Eliminating data because of 

problems in the testing methodology (i.e., shaping steam ring failure)3 is a valid argument but no 

discussion was found throughout the report where a data point that agreed with the proposed 

metric was dismissed because of poor testing methodology or incorrect analysis. This gives the 

impression that the data analysis may be biased to support a proposed hypothesis. The authors 

do provide reasons for excluding 66 data points that do not fit the overall trend including: 1)

inaccurate calculation of LFLcz, 2) combustion efficiency measurements were inaccurate, 3) 

flammability is not an appropriate parameter to represent flare performance because it does not 

completely explain the performance of a flare. Of these three reasons for excluding the 66 data 

points it appears that the last reason is likely most accurate since it is hard to imagine that either 

the LFLcz calculation or the combustion efficiency measurements were incorrect for this many 

data points. This conclusion is not surprising since flare performance is governed by the 

nonlinear relation between reaction kinetics (see Figure 2) and turbulent mixing. Small reaction 

zones that sustain combustion have been shown to form inside small turbulent eddies generated 

by the shearing action occurring in the turbulent flow present inside the flare tip. Thus, the 

complex kinetics describing olefin/hydrogen/oxygen combustion reaction chemistry is closely 

linked to turbulent fluid mixing. Both of these phenomena are highly non-linear so the chance of 

fully quantifying this behavior with a single parameter is highly unlikely.

Section 3: Steam and Flare Performance

1. Please comment on the lower flammability limit of combustion zone gas (LFLCZ) as 

an operating parameter for indicating over steaming situations on steam-assisted 

flares. Comment on the agency’s use of the ratio of the net heating value of the 

combustion zone gas (NHVCZ) to the net heating value of the flare vent gas if diluted 

to the lower flammability limit (NHVLFL) as an alternative to LFLCZ. Does the flare 

data adequately support the EPA’s observations?

The application of LFLcz as a performance metric for flare operation is explained very well 

and clearly has some justification. As discussed earlier, flare performance is controlled by 

reaction kinetics and turbulent mixing. Finding a single parameter to quantify these highly 

 3 See pages 3-13 and 3-17 for a discussion of a potential problem with reported test data as a reason to exclude it from 

the combustion efficiency vs LFLcz plot shown in Figure 3-4.
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non-linear coupled phenomena is highly unlikely. However, as shown in Figure 3-4, a 

majority of the test data included in the analysis have a combustion efficiency above the 

96.5% level judged as representative of “good” flare performance. Thus, based on this and 

the subsequent analysis of the outliers not conforming to this metric, the LFLcz appears a 

good measure of flare performance. When the LFLcz metric is compared to the net heating 

value ratio (shown in Figure 3-11), little improvement difference is shown. Both metrics fail 

to capture 100% of all data accepted as quality data and the same argument holds for the 

second metric as the first – using a “single” number to quantify the complex behavior of 

reaction kinetics coupled to turbulent mixing is unreasonable.

Figure 2 - Reaction path analysis for combustion of 1,3 Butadiene at 980 K at a distance of 0.47 cm in 

front of burner4

2. Is there sufficient evidence that chemical interactions are occurring that make the 

calculated LFLCZ inaccurate with respect to the 15.3% LFLCZ threshold discussed? Is 

there other data available (that is not discussed in this report) that may help clarify 

our discussion about specific chemical interactions related to lower flammability 

limits of gas mixtures?

To assess this issue, the report discusses three conditions: 1) equal olefin/hydrogen cases 

(Table 3-3), 2) low olefin/high hydrogen (Table 3-4) and 3) high olefin/low hydrogen (Table 

3-5). Based on this analysis, it appears that the olefin concentration doesn’t seem to impact 

the measured combustion efficiency. Understanding that specific chemical compounds are 

more reactive than others (i.e., flame speed for acetylene is much higher than flame speed for 

 4 Gueniche, H.A., Glaude, P.A., Fournet, R., Battin-Leclerc, F., “Rich Premixed Laminar Methane Flames Doped by 

Light Unsaturated Hydrocarbons: II. 1,3-Butadiene,” Combustion and Flame, 151, 245-261 (2001).

67% of reaction goes through this channel
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methane) this conclusion does not make sense. Thus, it appears that this effect is somehow 

missed using the LFL metric. To properly capture these phenomena will require more detail 

related to complex chemical kinetics coupled with turbulent mixing. The best way to 

accomplish this is using detailed computational fluid dynamics models based on transient 

flow and reaction chemistry as discussed by Smith et.al., (2007)

5 and Smith et.al, 20096

.

3. Did the agency adequately examine other operating parameters (different from 

LFLCZ; or the ratio of NHVCZ to LFLVG-LFL) that could indicate over steaming 

situations? Are there specific other parameters that should be given more or less 

emphasis?

As discussed by the agency, “compounds with a high carbon to hydrogen ratio have a greater 

tendency to smoke and require better mixing for smokeless flaring. The required steam rate 

is dependent on the carbon to hydrogen ratio of the gas being flared.”7

 This clearly indicates 

other parameters that should be given more emphasis. Nowhere in the discussion of the 

LFLcz metric is the carbon-to-hydrogen ratio discussed other than indirectly when 

considering flare performance for various ratios of olefin and hydrogen (discussed earlier).

An entire separate analysis originally funded by the agency has not been included in this 

discussion nor has this work been referenced in this report. The work by Smoot et.al. 

(2009)8 (also discussed by Smoot et.al., [2010]1

) evaluates the correlation between flare gas 

adiabatic flame temperature (Tad) and flare gas Lower Heating Value (LHV) and finds that 

the flare gas LHV value may be a useful criterion for regulating flare operation. A similar 

graph for vent gas LHV vs. vent gas Tad indicates that vent gas LHV is not a reliable criterion 

for regulating flare performance. Computations also evaluated the Flammability Ratio (FR) 

in relation to the Lean Flammability Limit (LFL) to identify cases where flare gas 

combustion would not occur or would be highly inefficient. Adiabatic flame temperature 

correlated linearly with FR and declined with decreasing fuel molecular weight. A key 

recommendation from this work was to evaluate the FR criteria for flare performance using 

available flare performance data, as has been presented in the current work.

Section 4: Air and Flare Performance

1. Please comment on the stoichiometric air ratio (SR) as an operating parameter for 

indicating excess aeration situations on air-assisted flares. Additionally, also 

comment on whether the lower flammability limit of the flare vent gas (LFLVG) is an 

appropriate operating parameter for determining whether the flare vent gas being 

sent to an air-assisted flare is capable of burning? Does the flare data adequately 

support the EPA’s observations? 

 5 Smith, J.D., Suo-Ahttila, A., Smith, S.K., and Modi, J., “Evaluation of the Air-Demand, Flame Height, and Radiation 

Load on the Wind Fence of a Low-Profile Flare Using ISIS-3D,” AFRC-JFRC 2007 Joint International Combustion 

Symposium, Marriott Waikoloa Beach Resort, Hawaii, October 21-24, (2007). 6 Smith, J.D., Smoot, L.D., and Jackson, R.E., “Technical Foundations of New Performance Criteria for Efficient 

Operation of Industrial Steam-Assisted Gas Flares,” IFRF 16th International Members’ Conference: Combustion and 

Sustainability - New Technologies, New Fuels, New Challenges, Boston, Massachusetts June 8-10 (2009). 7 US EPA OAQPS, “Control Cost Manual: Chapter 7 – Flares,” 5th Edition, February (1996). 8 Smoot, L.D., Jackson, R.E., and Smith, J.D., “Combustion Efficiency Control in Single-Stage, Industrial, SteamAssisted Open-Flares,” Technical Report CR – SAS IF – 09A, May (2009). see http://sasieng.com/www/services/technical-seminars-presentations-papers-and-reports/
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Using the Stoichiometric Ratio (SR) to quantify combustibility of a gas mixture is well 

established (see Glassman, 19969 and Kuo, 200510). In the present application, the SR is 

being used to assess the combustibility of the vent gas coming up the flare stack. In this 

application, the SR is clearly applicable as is the LFLCZ (or LFLVG) for air flares. However, 

the fact remains that as with steam flares, it is difficult to completely characterize the 

complex coupled phenomenon of reaction kinetics and turbulent mixing which is clearly 

evident in the data plots of SR vs Combustion Efficiency (see Figure 4-1 through 4-4). The 

criteria of SR < 7 to quantify “good” air flare performance is questionable since most of the 

air responsible for smokeless flare operation comes from air entrained after the vent gas 

exists the flare stack11. 

Also, as the report correctly points out, relevant data for industrial air flare performance must 

be taken on flare tips with a diameter > 3” to be representative of larger flare tips (i.e., tips 

with diameters greater than 24”). Air flare data taken on small tips (i.e, < 3” Tip Diameter) 

is misleading because the mixing phenomena in these small tips is much different than 

occurs in large tips (i.e., mixing dominates the combustion process). 

Finally, the report reasons that since the assist media is mixed with the vent gas in the flare 

stack it is appropriate to use LFLVG instead of LFLCZ. For standard air flares this conclusion 

may be correct but for non-standard air flares (i.e., John Zink’s Annular Air Flare or Zeeco’s 

high pressure air flare) this will not be correct and the criteria should continue to use LFLCZ

as was used for steam flares.

Section 5: Wind and Flare Performance

1. Please comment on the momentum flux ratio (MFR) as an operating parameter in 

crosswind velocities >22 mph at the flare tip to indicate wake-dominated flame 

situations. Additionally, also comment on the agency’s observation that in the 

absence of crosswind >22 mph, a low MFR does not necessarily indicate poor flare 

performance. Comment on the effectiveness of observations identifying wakedominated flames. Does the flare data adequately support the EPA’s observations? 

As shown in Section 5, the MFR when based on wind speed and not flare gas tip velocity is 

an appropriate operating parameter. As pointed out, there are many examples where the 

MFR is less than 3 (or 0.1 for Figure 5-5 from Seebold)12 but the combustion efficiency 

remains above 96.5%. Also, it is pointed out that completely different operating conditions 

can produce the same MFR. Thus, the conclusion that MFR may only be applicable when 

the transition to wake dominated combustion is caused by increasing crosswind velocity and 

not a decrease in flare tip velocity.

2. Did the agency adequately examine other operating parameters (different from MFR) 

for identifying wake-dominated flames? Are there specific other parameters that 

should be given more or less emphasis?

 9 Glassman, Irvin, Combustion, Third Edition, Academic Press, New York (1996).

10 Kuo, Kenneth K. Principles of Combustion, Second Edition, John Wiley and Sons, NY (2005). 11 Baukal, C.E., The John Zink Combustion Handbook. Boca Raton, Florida, CRC Press (2001). 12 Seebold has indicated that the data shown in his 2004 paper represented a collection of all flare data he 

had gathered from the literature and plotted himself.
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Certainly, a wake-dominated flame represents a condition where the unburnt hydrocarbon 

could be stripped from the flare plume thus reducing the combustion efficiency of the flare. 

The report correctly assesses the Plume Buoyancy Factor (BP) and the Power-Factor (PF) for 

a flare flame. Neither of these metrics captures the complex coupling between reaction 

kinetics and turbulent mixing occurring at the flare tip exit. For crosswinds > 22 mph, the 

latter phenomena appears to dominate (i.e., there is enough momentum to strip flare gas from 

the plume before it reacts). Therefore, it appears reasonable to focus on a metric that 

explicitly includes the kinetic effects and the turbulent mixing effects. Lastly, the report 

correctly points out the significant effect wind has on steam flare performance due to the 

upper steam jets not participating in the flare flame (i.e., the steam jets pass overhead of the 

reaction zone thus reducing the steam jet impact on combustion).

Section 6: Flare Flame Lift Off

1. Please comment on the maximum allowable flare tip velocity equation which 

considers combustion zone gas composition, the flare tip diameter, density of the flare 

vent gas, and density of air. Does the flare data adequately support the EPA’s 

observations? Are there specific other parameters or methods/equations that should 

be given more or less emphasis?

The report focuses on the Shore equation because it considers the LFL. In this analysis, 213 

of the 330 flare tests were excluded from the analysis due to limited data required to use the 

equation or the tests were judged outside of normal operation (i.e., too much assist air 

because SR > 7). Thus, only 108 data points were analyzed and of these 10 which met the 

LFLcz criteria did not achieve “good” combustion efficiency (> 96.5%). In addition, the plot 

of flare metric vs lift off velocity using the Shore equation (see Figure 6-1) showed that all 

but three were judged as having no flame lift off. Several of the tests not having flame lift 

off had combustion efficiencies < 96.5%. This analysis shows that flame lift off is not itself 

an strong indicator of poor flare performance but flame liftoff does indicate unstable 

combustion which can lead to poor combustion efficiency. 

Flame liftoff is a balance between reaction kinetics (i.e., flame speed) and mixing (reactants 

mixed with combustion radicals to maintain combustion zone). When the tip velocity 

exceeds the flame speed the flame is known to “lift-off” from the tip. In this case, detailed 

transient LES based CFD analysis is required to capture this affect and a single parameter is 

not appropriate.

Section 7: Other Flare Type Designs to Consider

1. Please comment on the applicability of the LFLCZ parameter, maximum allowable 

flare tip velocity equation, and the observations regarding crosswind velocity to 

non-assisted flares, pressure-assisted flares, and other flare designs.

Pressure-assisted flares (i.e., multipoint ground flares) use momentum to induce (mix) 

surrounding air with the flare gas. For a Pressure-assisted flare, the LFL parameter may be 

appropriate as stated in the report. Also, the MFR and the Vmax methodologies are also likely 

applicable to non-assisted flares. 
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Section 8: Monitoring Considerations

1. Please comment on the appropriate monitoring equipment needed to ensure good 

flare performance and on any other known monitoring methods (not discussed in this 

report) for monitoring the following parameters: LFLCZ, LFLVG, LFLVG,C, the ratio of 

NHVCZ to NHVVG-LFL, CCZ, SR, MFR, and Vmax. Also, please comment on operating 

scenarios and conditions where less robust monitoring equipment could be used to 

determine the operating parameters of interest.

The Panametric gas monitor has been used to measure combustion effluent gas (see expert 

report for DOJ case on Questar vs DOJ).

The “presumptive” device for effectively and accurately monitoring flare gas effluent 

composition to determine flare combustion efficiency is the “Open Path” FTIR as developed 

and used by R. Spellicy and by Clean Air Engineering, Inc.

Certainly, a performance metric for flares relies on measureable data. The instruments listed 

in the report for the various metrics appear correct and no additional information can be 

added by this reviewer.