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MEMORANDUM 

TO: Flare Peer Review Panel Members 

FROM: Research Triangle Institute for U.S. Environmental Protection Agency 

DATE: May 1st, 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 [INSERT DATE]. 

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 steamassisted 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 Passive 
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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 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 air-assisted 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 
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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 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. 

 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. 

 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 wakedominated 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 wakedominated flames during crosswind velocities greater than 22 mph at the flare tip. 

 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. 

 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 
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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. 

Document Availability 

The report is being made available to the Panel in the form of an electronic file, which 

has been posted on and FTP site and shared with 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 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? 
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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 

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.