Technical Memorandum

TO: EPA Docket No. EPA-HQ-OAR-2014-0738

FROM: Andrew Bouchard, Environmental Engineer

Refining and Chemicals Group, SPPD (E143-01)

DATE: January 27, 2015

SUBJECT: Review of Available Test Data on Multipoint Ground Flares

I. Purpose

This memorandum documents the data review conducted regarding performance and operational 

requirements for pressure-assisted multipoint ground flares.

II. Background on Regulatory Flare Operating Requirements 

The U.S. Environmental Protection Agency (EPA) originally promulgated requirements for 

flares used as a control device on October 18, 1983 in the standards of performance for 

equipment leaks of Volatile Organic Compounds (VOC) in the Synthetic Organic Chemical 

Manufacturing Industry (SOCMI) at 40 CFR Part 60, Subpart VV. Shortly thereafter, on January 

21, 1986, the Agency modified the requirements for flare exit velocity limitations and moved the 

requirements from 40 CFR Part 60, Subpart VV to the General Provisions at 40 CFR Part 60, 

Subpart A “for easy reference by all subparts in Part 60 and Part 61” (51 FR 2700). Following 

the addition of the 1990 amendments to the Clean Air Act, the EPA then promulgated these same 

requirements for flares used as a control device on March 16, 1994 in the General Provisions at 

40 CFR Part 63, Subpart A for easy reference by rules being developed in Part 63. Finally, on 

May 4, 1998, the EPA added to the General Provisions of both Parts 60 and 63 requirements 

specifically for non-assisted flares that have a diameter of 3 inches or greater and have a 

hydrogen content of 8.0 percent (by volume) or greater. Today, flares used as a control device 

subject to national emissions standards for hazardous air pollutants (NESHAP) or new source 

performance standards (NSPS) are required to meet these certain design and operating 

requirements as specified in the General Provisions at 40 CFR 63.11(b) or 40 CFR 60.18(b) 

respectively.

The specific design and operating requirements for flares used as control devices laid out 

currently in the General Provisions specify that flares shall be: (1) steam-assisted, air-assisted, or 

non-assisted; (2) operated at all times when emissions may be vented to them; (3) designed for 

and operated with no visible emissions (except for periods not to exceed a total of 5 minutes 

during any 2 consecutive hours); and (4) operated with the presence of a pilot flame at all times.

Additionally, depending on the flare type, facilities using flares as control devices are also 

required to maintain both a minimum heat content of gas combusted in the flare and comply with 

a maximum exit velocity at the flare tip. While recent developments in measurement technology 

and emissions data gathering on flare performance suggest that some additional constraints may 

be needed to ensure a high level of control efficiency for flares used within certain source 
2

categories (see technical memorandum “Petroleum Refinery Sector Rule: Operating Limits for 

Flares” at EPA Docket ID No. EPA-HQ-OAR-2010-0682-0206) it is generally presumed that

“flares operating in accordance with these specifications destroy volatile organic compounds 

(VOC) or volatile HAP with a destruction efficiency of 98 percent or greater” (63 FR 24437). 

III. Summary of Pressure-Assisted Multipoint Ground Flare Technology

Pressure-assisted multipoint ground flares (MPGF) are conceptually similar, yet technically 

different in both design and operation compared to more traditional elevated flare tip designs 

(e.g., steam-assisted, air-assisted, and non-assisted flare tips). A review of the recently proposed 

petroleum refinery emissions risk modeling input file (see Risk Model Input File at EPA Docket 

ID No. EPA-HQ-OAR-2010-0682-0077) shows that, on average, more traditional elevated flares 

in this source category have an average stack height of approximately 170 feet and average 

effective stack diameter around 40 inches. Pressure-assisted burners on the other hand are 

uniquely different and can vary in burner shape, burner size, number and size of holes in the 

burner, and hole orientation. When comparing pressure-assisted MPGF to the more traditional 

flare tip designs, it is important to note two things. The first is that pressure-assisted flare burner 

arrangements are typically (but not always) setup in MPGF arrays that have stack heights near

ground level (e.g., Dow’s proposed MPGF design layout has individual burner heights of 8 feet).

The second is that individual pressure-assisted flare tips within the MPGF array have 

significantly smaller overall effective stack diameters (e.g., Dow’s proposed individual burner 

design has an effective area of 3.65 square inches, which equates to an effective diameter of a 

little over 2.1 inches per burner).

Due to their size and unique configuration, pressure-assisted flares operate by taking advantage 

of the pressure upstream of the burner to create a condition whereby air is drawn into contact and 

mixed with high exit velocity flared gas, resulting in smokeless flare operation if properly 

designed and operated. Pressure-assisted flares typically need a minimum flare header pressure 

of 15 psig to properly operate and typically operate at sonic velocity, also known as Mach = 1 

conditions (e.g., sonic velocity for typical hydrocarbon gases ranges from 700 to 1,400 feet per 

second). Pressure-assisted flares can be used as in a single burner type layout or in staged arrays 

of many identical burners. These staged arrays, when at ground level are commonly referred to 

as MPGF. MPGF, if constructed and operated properly, have the ability to handle a wide range 

of flare gas flow rates and compositions, operate with no smoke, and achieve high overall flare 

combustion efficiencies. The ability for MPGF to handle a wide range of gas flow rates is 

accomplished by operating a staging system. A staging system uses staging valves that open and 

close to certain arrays of burners in the MPGF according to set gas pressures such that as flow 

and pressure increase or decrease in the flare header, so to do the number of pressure-assisted 

flare tip burners being used. Each staging valve/burner array set-up also has a pilot flame, or in 

some instances multiple pilot flames, to ensure the pressure-assisted flare tips have an ignition 

source to combust gas when it is sent to the flare. 

One can best envision a MPGF system as looking like that of football field of burners. The 

MPGF system is surrounded by a tall fence (e.g., 51 feet tall for Dow’s proposed PDH-1 ground 

flare). The various layers of the vertical fencing are slightly offset to allow air flow in but also 

designed to protect workers from radiant heat from the flare. The ground surrounding the burners 
3

inside the fence is typically rock. An actual visual of what a MPGF system looks like can be seen 

in Dow’s February 2014 presentation to EPA titled “Flare Systems Multipoint Ground Flares

PDH-1 Flare Emissions Test Results TCEQ/EPA,” and has been added below for ease of 

reference.

Under the current spectrum of EPA flare regulations, pressure-assisted MPGF can comply with 

all applicable requirements in the General Provisions of 40 CFR Parts 60 and 63 except for the 

maximum velocity requirements. However, given the fact that these types of flares are designed 

to operate at high velocity, the remaining discussion in this technical memorandum will focus on

available test data on pressure-assisted MPGF and minimum operating requirements that need to 

be met to ensure these flare types achieve VOC and/or HAP destruction efficiencies of 98 

percent or greater when being utilized. 

IV. Available Multipoint Ground Flare Test Data

There have been a variety of flare performance tests done over the past 30 or so years on 

pressure-assisted flare tips, however the dataset currently available is somewhat small in both 

sheer number of test runs and waste gas composition profiles compared to, for example, steamassisted flares. This is less of a concern from a combustion efficiency standpoint because 
4

pressure-assisted flares don’t suffer from potential deteriorated flare performance issues due to 

over-assisting from steam or air assist. The more practical concern for these flare types is clearly 

defining operating parameters that will ensure high combustion efficiencies are achieved over the 

entire high velocity operating regime that pressure-assisted flares are designed to operate. 

There are seven tests currently known to be available for pressure-assisted flares. These tests 

consist of: 1)1982 John Zink testing immediately after EPA/CMA testing, 2) Pohl & Soelberg

1985 EPA study, 3) BP 2006 test, 4) Dow Chemical 2006 test, 5) John Zink 2010 test performed 

after TCEQ flare study, 6) Marathon Petroleum 2012 test (report by Clean Air Engineering), and 

7) Dow Chemical 2013 test (report by John Zink). Given the fact that one of the general results 

from the Pohl & Soelberg 1985 study was that flare head design can influence the flame stability 

curve, the focus of our remaining discussion will incorporate general results from this 1985 study 

as well as be limited to the test data gathered using Passive Fourier Transform Infrared 

spectroscopy (PFTIR) testing on the John Zink Linear Relief Gas Oxidizer (LRGO) flare burners 

(Tests 6 & 7). Specific focus is being given to these tests in particular because the same style 

flare burner was tested in both cases and because assessment of flare flameout is more 

challenging when assessing extractive test urn data compared to minute by minute PFTIR test 

data. 

The test report for Test 6, the Marathon Petroleum 2012 study, presented PFTIR testing for three

different types of flare head designs. One flare head design was a traditional steam-assisted 

elevated flare tip (Model No. EEF-QSC-36), one was a steam-assisted ground flare burner 

(Model No. SKEC) and the last one was a pressure-assisted ground flare burner (Model No. 

LRGO-D). There were 3 different test series (PA1, PA2, and LTS) that specifically looked at 

flare performance of the LRGO pressure-assisted ground flare burners when combusting 

different mixtures of natural gas and nitrogen at differing velocity operating regimes. The 

objective of test series PA1 was to determine the minimum combustion zone gas net heating 

value of a natural gas/nitrogen mixture that supports good combustion at sonic velocity (burner 

pressure around 13 psig); the objective of test series PA2 was to determine the minimum 

combustion zone gas net heating value of a natural gas/nitrogen mixture that supports good 

combustion at the minimum velocity needed to properly operate the burner (around 4 psig). Each 

PA1 and PA2 test series consisted of two sets of triplicate runs, one at a higher average of net 

heating value around 900 BTU/scf (Run series 1) and one at a lower average net heating value of

around 600 BTU/scf (Run series 4). There was also another test, PA1 Run 4(2) that was aborted 

after the burner went out after only 2 minutes and this test is discussed further in Section V 

below. The PA1 and PA2 test series were conducted on three LRGO burners in series (and not an 

individual LRGO burner like Dow’s 2013 test). The third test series, LTS, was excluded because 

it was done to demonstrate repeatability of PFTIR measurements over time. While test series

LTS could be useful in potentially providing information on effects of uncontrolled variables 

such as wind on PFTIR measurements and CE test results, all test runs in this series were 

performed using only natural gas and were shorter in duration (in the 11-17 minute range) than 

tests analyzed for purposes of assessing flame stability (20 minutes or more). Given that test 

PA1, Run 4(4) of the Marathon test was stopped after the burner went out at minute 17 

(discussed further in Section V. below), assessment of the LTS series dataset for purposes of 

flare flame stability was determined to be inappropriate due to the short run lengths. However,
5

one would expect the conditions tested in LTS series to provide stable flare operation given the 

flammability of natural gas.

The test report for Test 7, the Dow Chemical 2013 study, presented PFTIR testing for two 

different types of flare head designs. One flare head design was a steam-assisted ground flare 

burner (Model No. SKEC) and the other was a pressure-assisted ground flare burner (Model No. 

LRGO-HC). Specifically for the LRGO ground flare burner, Dow conducted flare performance 

testing for seven different operating scenarios on a single LRGO burner. One of those test runs, 

test series P4, did not collect any emissions data as this test was strictly a stability test conducted 

to ensure a high hydrogen content vent gas stream (90 vol % hydrogen/10 vol% natural gas) 

could produce a stable flame on a pressure-assisted flare tip burner. The other six test series 

consisted of triplicate stable flare flame tests that did take emissions measurements as well as 

assess flare flame stability (three additional runs, referred to as test runs P2H Run 1, P2L Run 1, 

and P3L Run 1 in supplement 1 to the Dow report, were unstable and are discussed further in 

Section V below). There were 3 different gas mixtures tested on the Dow LRGO burner. Test 

series P1 used only propylene fuel, test series P2 used a propylene/ nitrogen gas mixture, and test 

series P3 used a propylene/natural gas/nitrogen gas mixture. Each of these mixtures were tested 

at both sonic exit velocity conditions (i.e., around 13 psig for test series P1H, P2H, and P3H) and

at the minimum burner fuel pressure conditions (i.e., around 4 psig for test series P1L, P2L, and 

P3L). 

V. Performance Data for Pressure-Assisted MPGF Operations

Review of Combustion Indicators for Fuels

The technical memorandum “Petroleum Refinery Sector Rule: Operating Limits for Flares” 

(EPA Docket ID No. EPA-HQ-OAR-2010-0682-0206) provides details of combustion theory 

and the net heating value (NHV) and lower flammability limit (LFL) parameters, the difference 

between flare vent gas and combustion zone gas, and how each of these parameters are properly 

calculated. For ease of reference and in order to ensure consistency in terminology, the 

appropriate equations to determine said parameters in both the flare vent gas and combustion 

zone gas when there are multiple components are presented below.





n

i

vg iNHVi NHV x

1

(Eqn. 1)
6

where:

NHVvg = Net heating value of flare vent gas, British thermal units per standard cubic 

foot (Btu/scf).

i = Individual component in flare vent gas.

n = Number of components in flare vent gas.

xi = Concentration of component i in flare vent gas, volume fraction.

NHVi = Net heating value of component i determined as the heat of combustion where 

the net enthalpy per mole of offgas is based on combustion at 25 degrees 

Celsius (°C) and 1 atmosphere (or constant pressure) with water in the 

gaseous state from values published in the literature, and then the values 

converted to a volumetric basis using 20 °C for “standard temperature.” 

Table 1 summarizes component properties including net heating values.





















n

i i

vg

LFL

χ

LFL

i

1

1

(Eqn. 2)

where:

LFLvg = Lower flammability limit of flare vent gas, volume fraction.

n = Number of components in the vent gas.

i = Individual component in the vent gas.

χi = Concentration of component i in the vent gas, volume percent (vol %).

LFLi = Lower flammability limit of component i as determined using values 

published by the U.S. Bureau of Mines (Zabetakis, 1965), vol %. All inerts, 

including nitrogen, are assumed to have an infinite LFL (e.g., LFLN2 = ∞, so 

that χN2/ LFLN2 = 0). LFL values for common flare vent gas components are 

provided in Table 1.
7

Table 1— Individual Component Properties

Component

Molecular 

Formula

MWi

(lb/lb-mol)a

CMNi

(mole/mole)

NHVi

(Btu/scf)

LFLi

(vol%)a

Acetylene C2H2 26.04 2 1,404 2.5

Benzene C6H6 78.11 6 3,591 1.3

1,2-Butadiene C4H6 54.09 4 2,794 2.0

1,3-Butadiene C4H6 54.09 4 2,690 2.0

iso-Butane C4H10 58.12 4 2,957 1.8

n-Butane C4H10 58.12 4 2,968 1.8

cis-Butene C4H8 56.11 4 2,830 1.6

iso-Butene C4H8 56.11 4 2,928 1.8

trans-Butene C4H8 56.11 4 2,826 1.7

Carbon Dioxide CO2 44.01 1 0 ∞

Carbon Monoxide CO 28.01 1 316 12.5

Cyclopropane C3H6 42.08 3 2,185 2.4

Ethane C2H6 30.07 2 1,595 3.0

Ethylene C2H4 28.05 2 1,477 2.7

Hydrogen H2 2.02 0 274 4.0

Methane CH4 16.04 1 896 5.0

Methyl-Acetylene C3H4 40.06 3 2,088 1.7

Nitrogen N2 28.01 0 0 ∞

Oxygen O2 32.00 0 0 ∞

Pentane+ (C5+) C5H12 72.15 5 3,655 1.4

Propadiene C3H4 40.06 3 2,066 2.16

Propane C3H8 44.10 3 2,281 2.1

Propylene C3H6 42.08 3 2,150 2.4

Water H2O 18.02 0 0 ∞

a MWi = molecular weight of component I; lb/lb-mol = pounds per pound-mole; LFLi = Lower flammability limit of 

component i; vol% = volume percent.

  vg s a,premix

vg vg

cz Q Q Q

Q NHV

NHV

 





(Eqn. 3)

 

vg

vg vg s a,premix

cz Q

LFL Q Q Q

LFL

  



(Eqn. 4)

where:

NHVcz = Net heating value of combustion zone gas, Btu/scf.
8

NHVvg = Net heating value of flare vent gas, Btu/scf.

LFLcz = Lower flammability limit of combustion zone gas, volume fraction.

LFLvg = Lower flammability limit of flare vent gas, volume fraction.

Qvg = Cumulative volumetric flow of flare vent gas, standard cubic feet per minute 

(scfm).

Qs = Cumulative volumetric flow of total steam, scfm.

Qa,premix = Cumulative volumetric flow of premix assist air, scfm.

Assuming that the volumetric flows of both total steam (Qs) and premix assist air (Qa,premix) are 

equal to zero, Equation 3 simplifies into Equation 1 (NHVcz = NHVvg) and Equation 4 simplifies 

into Equation 2 (LFLcz = LFLvg). Thus, for the performance tests for pressure-assisted flares 

considered in this technical memorandum, the combustion zone gas and flare vent gas are 

interchangeable. The combustion zone gas terminology will be used in the remainder of the 

document for purposes of consistency in discussing compliance metrics for flare operating limits

while the flare vent gas terminology will be used in the discussion of flare tip velocity to be 

consistent with current General Provisions requirements. 

Unstable Pressure-Assisted Flare Performance Data

One of the primary objectives of both the Marathon 2012 test and Dow 2013 test was to gather 

data in support of establishing an appropriate flare operating envelope that ensures both a stable 

flare flame as well as high level of combustion efficiency when using the LRGO pressureassisted flare burners to combust differing flare vent gas mixtures. While the vast majority of test 

runs from these two tests achieved both a high level of combustion efficiency as well as a stable 

flare flame over a minimum duration of 20 minutes, five test runs were aborted as a result of the 

flame becoming unstable and extinguished before 20 minutes of data could be obtained. There 

was another test aborted, test P4 run 1 of the Dow test, but this run was aborted due to not 

achieving the target flow rates of hydrogen and natural gas and not due to flame stability issues. 

It should also be noted that no emissions data were collected during the test series P4 and 

therefore this test series will not be discussed further. 

Three of the five unstable test runs (P2H Run 1, P2L Run 1, and P3L Run 1) happened during 

the Dow 2013 test. When instability occurred, the flames went out, the test was stopped, the fuel 

composition was adjusted to a higher net heating value, and the test was started over and 

appended with a “b”. Subsequent testing for that test series was then performed two more times 

at those adjusted higher heating value levels to ensure triplicate runs were performed for a given 

test series. It is important to note however that two of Dow’s unstable test runs, P2H Run 1 and 

P2L Run 1, were aborted after approximately 4 minutes and 2 minutes, respectively, and were 

able to collect both emissions and process data. Dow’s other unstable test run, P3L Run 1, was 

aborted less than a minute into the test run and no emissions or process data due could be 
9

collected due to the short duration of this test run. Therefore, test P3L Run 1 is excluded from the 

data analysis discussion below. 

The other two unstable test runs (PA1 Run 4(2) and PA1 Run 4(4)) happened during the 

Marathon 2012 test. Each of these unstable test points are unique in their own respects. Test PA1 

Run 4(2) is unique in the sense that two of the three burners went out after approximately 2 

minutes of testing and the test was aborted. There are limited data for this test, however, there 

was one minute of PFTIR emissions data that showed a combustion efficiency of over 99% right 

before flameout. Due to the short duration of this test, it was not counted as being one of the 

triplicate runs for that test series; however, test PA1 Run 4(4) was counted at one of the triplicate 

runs of the test series. What is unique about test PA1 Run 4(4) is that while 17 minutes worth of 

emissions data were able to be gathered and analyzed, a full 20 minute test could not be 

completed because one of the three burners being tested went out. Thus, while this test run was 

included as one of the triplicate runs for that test series due to enough emissions data being able 

to be gathered and analyzed, we consider this test to be only partially stable due to a loss of 

flame before a full 20 minute test could be completed.

Being able to grasp and fully understand the stable flare flame operating envelope for high 

velocity pressure-assisted flares is of critical importance. One of the general results from the 

1985 Pohl & Soelberg report was that “stable flare flames and high (>98-99 percent) combustion 

and destruction efficiencies are attained when the flares are operated within envelopes specific to 

each flare head and gas mixture tested. Operation beyond the edge of the operating envelope 

results in rapid flame de-stabilization and a decrease in combustion/destruction efficiencies.” 

More recent evidence, particularly from the Marathon 2012 tests further suggests that the cliff

right beyond the edge of flame stability for pressure-assisted flares is so rapid and dramatic for

the specific operating conditions tested that as long as the LRGO burners are lit they maintain a 

high overall efficiency (on average greater than or equal to 99 percent combustion efficiency), 

right up to the point of flame extinguishment. Graphically, the unstable pressure-assisted flare 

data is depicted in this memo (i.e., Figures 1-4) by using the data gathered prior to flare 

flameout.

Flare Tip Velocity

The General Provisions requirements for maximum permitted flare tip velocity currently contain 

requirements for three different sets of flare types. These three different sets of flare types consist 

of (1) Steam-assisted and non-assisted flares, (2) Air-assisted flares, and (3) Non-assisted flares 

having a diameter of three inches or greater that have a hydrogen content of 8.0 percent (by 

volume) or greater and are already designed for and operated to meet certain exit velocities. If 

pressure-assisted flares were to be assigned into one of the aforementioned bins currently 

allowed under the General Provisions, they would be in bin (1) for non-assisted flares. 

The General Provisions requirements for maximum permitted flare tip velocity for non-assisted 

flares is dependent on the NHVvg being combusted. A minimum NHVvg of 200 BTU/scf is 

required at all times for non-assisted flare types, and this net heating value would allow flares to 

be operated with a maximum permitted flare tip velocity of 60 ft/s. On the other end of the 

spectrum, non-assisted flares operated with a NHVvg of 1,000 BTU/scf or greater are currently 
10

allowed to operate with a maximum permitted flare tip velocity up to 400 ft/s. In between these 

two values, and starting at a NHVvg of 300 BTU/scf, the maximum permitted flare tip velocity 

gradually increases with increasing net heating value, up to the 1,000 BTU/scf or greater and 400 

ft/s or less maximum velocity limit. The equations for non-assisted flare types burning flare vent 

gas with a net heating value between 300 BTU/scf and 1,000 BTU/scf are shown in Equation 5

(SI units) and Equation 6 (English units) below.

Log10 (Vmax) = (HT + 28.8) / 31.7 (Eqn. 5)

where:

Vmax = Maximum permitted flare tip velocity, meters per second (m/s).

HT = Net heating value of the gas being combusted, megajoules per standard cubic 

meter (MJ/scm).

Log10 (Vmax) = (HT + 1212) / 850 (Eqn. 6)

where:

Vmax = Maximum permitted flare tip velocity, feet per second (ft/s).

HT = Net heating value of the gas being combusted, British thermal units per standard 

cubic foot (BTU/scf).

Figure 1 below shows a plot of the current General Provisions maximum permitted flare tip 

velocity requirements for non-assisted flares versus the pressure-assisted flare testing done in 

both Marathon 2012 test and Dow 2013 test. While the current General Provisions requirements 

don’t rely on the lower flammability limit of the vent gas, Figure 2 presents a graphic depiction 

of velocity and this particular combustion indicator to better understand the overall flame 

stability trend. 

It should also be noted that there are some differences in the way flare tip velocity was reported 

and is graphically depicted in Figures 1 and 2 below for both the Marathon 2012 test and the 

Dow 2013 test. The General Provisions requires that the actual exit velocity of a flare be 

determined by the method specified in 40 CFR 60.18(f)(4) or 40 CFR 63.11(b)(7)(i) by dividing 

the volumetric flow rate of gas being combusted in the flare (in units of standard temperature and 

pressure), as determined by Method 2, 2A, 2C, or 2D as appropriate in appendix A to 40 CFR 

part 60 by the unobstructed (free) cross-sectional area of the flare tip. The Marathon 2012 test 

reported velocity values at standard conditions using the ideal gas law for all pertinent 

calculations. The Dow 2013 test, on the other hand, reported their velocities at actual conditions 

and used the ideal gas law for the low pressure test cases, but used the speed of sound calculation 

to estimate their exit velocities during the high pressure testing (see supplement 2 to Dow Report 

for more details). These reported velocity values are used in the Figures below.

Also, the NHV and LFL parameters in all the figures were calculated using the values in Table 1 

of this technical memorandum. For Marathon’s 2012 test, compositional data from Table A.7-2 

of the test report was used and normalized to 100 mole percent. The reported values lined up 

almost identically with the values calculated during this analysis. For Dow’s 2013 test, the 

process data reported in Appendix JZ-4 (John Zink Data Logs) was used. There are two issues
11

worth mentioning for this particular data analysis. First, variations exist in the values reported for 

NHV among the same test runs in the various appendices (e.g., JZ-3 vs. JZ-4 vs. JZ-5). We 

decided to use to use the data from appendix JZ-4 as a result of its higher calculated net heating 

value of 787 BTU/scf for unstable test P2H Run 1 (as opposed to a gas chromatograph analysis 

done by TRC indicating a net heating value of only 746 BTU/scf). Second, while the molecular 

weight and heat content of the natural gas were provided in supplement 1 to the Dow 2013 test, 

the LFL for the natural gas was not provided. Since there is not a LFL in Table 1 of this memo 

for natural gas, a default LFL for methane (5.0 volume percent) was assigned to natural gas for 

this test. A default NHV or LFL was not needed for natural gas in the Marathon test as it was 

already speciated in Table A.7-2. 

Figure 1. Flare Tip Velocity as a function of NHVvg

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000 2500

Velocity (ft/s)

NHVvg (BTU/scf)

Dow Stable

Dow Unstable

Marathon Stable

Marathon Unstable

Marathon Partial Unstable

EPA GP Vmax
12

Figure 2. Flare Tip Velocity as a function of LFLvg

The Agency’s current requirements would require that flares meet an increasing minimum net 

heating value with increasing velocity, all the way up to a minimum waste gas net heating value 

of 1,000 BTU/scf and maximum velocity of 400 feet per second. However, due to the unique 

nature of the technology and design of these pressure-assisted burners, flame stability can be 

achieved at velocities significantly higher than 400 feet per second (i.e., sonic velocity) with 

flare vent gas net heating values actually below 1,000 BTU/scf. Therefore, the data suggest that,

if pressure-assisted flares are operated properly and with sufficient heat content to produce a 

stable flame, the maximum permitted flare tip velocity requirements in the General Provisions 

should not apply to this particular type of control technology. 

Pressure-Assisted Flare Performance as a Function of Combustion Indicators

As discussed previously, stable flare operation is paramount to ensuring proper operation of 

pressure-assisted flares. When a flare flame is present, the data suggest that pressure-assisted 

flares will achieve a high level of combustion efficiency. In fact, when looking at all of the stable 

and unstable flare performance data for both the Marathon 2012 test and Dow 2013 test, when a 

0

200

400

600

800

1000

1200

1400

1600

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Velocity (ft/s)

LFLvg (vol %)

Dow Stable

Dow Unstable

Marathon Stable

Marathon Unstable

Marathon Partial Unstable
13

flame was present, the lowest achieved average combustion efficiency was measured was 98.5 

percent. As can be seen from Figures 3 and 4 below, stable pressure-assisted flare operation can 

occur for mixture of natural gas and nitrogen at a NHVcz of approximately 600 BTU/scf and 

greater and a LFLcz of 7.5 volume percent (vol %) or less. However, unstable test P2H Run 1, a 

gas mixture of propylene and nitrogen with a NHVcz of 787 BTU/scf and LFLcz of 6.6 volume 

percent (vol %), seems to be the limiting case for where flame instability would occur on the 

pressure-assisted burners tested. Given that pressure-assisted MPGF are designed to combust a 

wide range of gases with varying compositions and flows, as well as the fact that one of Pohl & 

Soelberg’s conclusions from their 1985 report was that the gas mixture tested can have a 

pronounced impact on the flare operating envelope, setting operational parameters based on this 

limiting case should ensure stable flare operation at all times. Thus, the data suggest that as long 

as pressure-assisted MPGF are operated with a NHVcz of 800 BTU/scf or greater and a LFLcz of 

6.5 volume percent (vol %) or less at all times that these designs will achieve both a high level of 

combustion efficiency and operate within the proper envelope of flame stability at all times.

Figures 3 and 4 also show the level of combustion efficiency for the unstable tests just before 

flame extinguishment.

 

Figure 3. Pressure-assisted flare combustion efficiency as a function of NHVcz

98.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

0 500 1000 1500 2000 2500

Combustion Efficiency(%)

NHVcz (BTU/scf)

Dow Stable

Dow Unstable

Marathon Stable

Marathon Unstable

Marathon Partial Unstable
14

Figure 4. Pressure-assisted flare combustion efficiency as a function of LFLcz

In summary, the above data suggest that, if pressure-assisted flares are operated properly and 

within a proper flare envelope capable of producing and maintaining a stable flame, pressureassisted flares can achieve equivalent reductions of VOC and HAP (i.e., 98 percent greater) to 

those achieved by steam-assisted, air-assisted, and non-assisted flares complying with the 

General Provisions at 40 CFR 63.11(b) or 40 CFR 60.18(b). Also, the data suggest that, if 

pressure-assisted flares are operated properly, the maximum exit velocity requirements for flares 

set forth in the General Provisions are unnecessary as this type of technology is designed to 

operate at velocities much higher than the 400 ft/s currently allowed. Lastly, in order to ensure 

pressure-assisted flares are operated in manner such that a stable flare flame is always 

maintained (and subsequently a high combustion efficiency is always achieved), the data suggest 

that maintaining a NHVcz greater than or equal to 800 BTU/scf or LFLcz less than or equal to 6.5 

volume percent (vol %) at all times will ensure proper pressure-assisted flare performance.

98.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Combustion Efficiency(%)

LFLcz (vol %)

Dow Stable

Dow Unstable

Marathon Stable

Marathon Unstable

Marathon Partial Unstable
15

VI. References

Coburn, J. 2013. Petroleum Refinery Sector Rule: Operating Limits for Flares. Memorandum to 

A. Bouchard and B. Shine, U.S. EPA Office of Air Quality Planning and Standards. 

December 12.

EPA (U.S. Environmental Protection Agency). 2012. Parameters for Properly Designed and 

Operated Flares. Prepared for U.S. Environmental Protection Agency, Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. April 2012.

John Zink Hamworthy Combustion. 2013. Report on Emissions Testing of Pressure Assisted 

LRGO-HC and Steam Assisted SKEC Burners. Prepared for The Dow Chemical 

Company. Test Conducted 19-22 November 2013.

John Zink Hamworthy Combustion. 2013. Report on Emissions Testing of Pressure Assisted 

LRGO-HC and Steam Assisted SKEC Burners – Supplement #1, Rev 0. Prepared for 

The Dow Chemical Company. Test Conducted 19-22 November 2013.

John Zink Hamworthy Combustion. 2013. Report on Emissions Testing of Pressure Assisted 

LRGO-HC and Steam Assisted SKEC Burners – Supplement #2, Rev 0. Prepared for 

The Dow Chemical Company. Test Conducted 19-22 November 2013.

Marathon Petroleum Company, LP. 2012. Performance Test of Steam-Assisted and PressureAssisted Ground Flare Burners with Passive FTIR – Garyville. Prepared by Clean Air 

Engineering, Inc. Final Report March 21, 2013.

Pohl, J. and N. Soelberg. 1985. Evaluation of the efficiency of industrial flares: Flare head 

design and gas composition. EPA-600/2-85-106. Prepared for U.S. EPA Office of Air 

Quality Planning and Standards. (September).

The Dow Chemical Company. 2014. Flare Systems Multipoint Ground Flares PDH-1 Flare 

Emissions Test Results TCEQ/EPA. February 2014.
Attachment 1

Spreadsheet Containing Dow’s 2013 Test Data and Associated Calculated Data 
17

Table A1-1: Average Test Run Mass Flow Rate Data

Test Run Propylene Flow Rate (lb/hr)a Nitrogen Flow Rate (lb/hr)a Natural Gas Flow Rate (lb/hr)a

P1H-1 8576 0 0

P1H-2 8201 0 0

P1H-3 8146 0 0

P1L-1 5241 0 0

P1L-2 5394 0 0

P1L-3 5633 0 0

P2H-1* 3691 4251 0

P2H-1b 3821 4144 0

P2H-2 3787 4130 0

P2H-3 3755 4104 0

P2L-1* 1982 2853 0

P2L-1b 2217 2738 0

P2L-2 2175 2746 0

P2L-3 2113 2706 0

P3H-1 2599 3882 1103

P3H-2 2524 3871 1108

P3H-3 2512 3824 1113

P3L-1* - - -

P3L-1b 1539 2394 666

P3L-2 1533 2387 657

P3L-3 1580 2355 667

*Denotes Unstable Test Run. Values reported in “Report on Emissions Testing of Pressure Assisted LRGO-HC and 

Steam Assisted SKEC Burners – Supplement #1, Rev 0.” 

a

Values as reported in Appendix JZ-4 (John Zink Data Logs) of Dow’s 2013 test report.
18

Table A1-2: Calculated Average Test Run Volumetric Flow Rate Data

Test Run

Ambient 

pressure 

(psia) a

Fuel 

Temperature at 

Burner (°F) a

Propylene Flow 

Rate (ACFH) b,c

Nitrogen Flow 

Rate (ACFH) b,c

Natural Gas 

Flow Rate 

(ACFH) b,d

Total Overall 

Gas Flow Rate 

(ACFH) e

P1H-1 14.38 107 86,188 0 0 86,188

P1H-2 14.29 92 80,743 0 0 80,743

P1H-3 14.35 122 84,209 0 0 84,209

P1L-1 14.29 107 53,003 0 0 53,003

P1L-2 14.35 102 53,843 0 0 53,843

P1L-3 14.40 111 56,931 0 0 56,931

P2H-1* 14.29 95 36,537 63,219 0 99,756

P2H-1b 14.29 89 37,415 60,961 0 98,376

P2H-2 14.36 98 37,506 61,450 0 98,957

P2H-3 14.39 98 37,112 60,936 0 98,048

P2L-1* 14.29 83 19,195 41,510 0 60,706

P2L-1b 14.29 82 21,432 39,764 0 61,195

P2L-2 14.36 92 21,309 40,418 0 61,728

P2L-3 14.36 93 20,740 39,902 0 60,641

P3H-1 14.30 82 25,107 56,338 26,144 107,590

P3H-2 14.35 84 24,387 56,190 26,268 106,845

P3H-3 14.38 89 24,443 55,901 26,573 106,918

P3L-1* - - - - - -

P3L-1b 14.36 66 14,368 33,576 15,256 63,200

P3L-2 14.36 82 14,747 34,497 15,508 64,752

P3L-3 14.37 87 15,329 34,325 15,878 65,532

*: Denotes Unstable Test Run. Values for ambient pressure and fuel temperature at burner reported 

in “Report on Emissions Testing of Pressure Assisted LRGO-HC and Steam Assisted SKEC Burners –

Supplement #1, Rev 0.” 

a

Values as reported in Appendix JZ-4 (John Zink Data Logs) of Dow’s 2013 test report.

b

Values calculated using Ideal Gas Law at reported pressure and temperature conditions. 

c

Table 1 of this memo provided component properties used in Ideal Gas Law calculations.

d

Natural Gas component properties needed for Ideal Gas Law calculation provided in “Report on 

Emissions Testing of Pressure Assisted LRGO-HC and Steam Assisted SKEC Burners – Supplement #1, 

Rev 0.”

e

Sum of propylene flow rate, nitrogen flow rate and natural gas flow rate.
Table A1-3: Average Test Run Flare Tip Velocity Data (at Actual Conditions)

Test Run

Calculated Ideal 

Gas Velocity (ft/s) a

Calculated 

MW of gas 

(lb/lb-mol) b

k

c

Calculated Speed 

of Sound (ft/s) d

Reported actual 

velocity (ft/s) e

P1H-1 945 42.08 1.142 875 875

P1H-2 885 42.08 1.145 864 865

P1H-3 923 42.08 1.138 885 885

P1L-1 581 42.08 1.142 875 581

P1L-2 590 42.08 1.143 871 590

P1L-3 624 42.08 1.141 877 624

P2H-1* 1093 33.16 1.241 1016 1016

P2H-1b 1078 33.36 1.241 1008 1008

P2H-2 1084 33.34 1.239 1015 1016

P2H-3 1075 33.34 1.239 1016 1016

P2L-1* 665 32.46 1.250 1020 665

P2L-1b 671 32.94 1.250 1011 671

P2L-2 676 32.87 1.249 1021 676

P2L-3 665 32.82 1.250 1023 665

P3H-1 1179 28.65 1.269 1093 1094

P3H-2 1171 28.55 1.270 1097 1098

P3H-3 1172 28.53 1.268 1102 1103

P3L-1* - - - - -

P3L-1b 693 28.59 1.275 1080 694

P3L-2 710 28.61 1.271 1094 711

P3L-3 718 28.67 1.267 1096 719

*: Denotes Unstable Test Run.

a

Values calculated using the “Total Overall Gas Flow Rate (ACFH)” column in Table A1-2, then converting 

into ACFS (by using 3600 seconds in 1 hour conversion factor) and converting into feet by using exit area of 

LRGO burner of 0.025 ft2(calculated by using reported burner exit area of 3.65 in2 and dividing by 144 ft2in

1 in2conversion factor).

b

Values calculated using volume percent of components in Table A1-4 below and molecular weight 

properties in Table 1 of this memo for propylene and nitrogen and natural gas molecular weight provided 

in “Report on Emissions Testing of Pressure Assisted LRGO-HC and Steam Assisted SKEC Burners –

Supplement #1, Rev 0.” 

c

Ratio of specific heats (Cp/Cv). Values provided in “Report on Emissions Testing of Pressure Assisted

LRGO-HC and Steam Assisted SKEC Burners – Supplement #2, Rev 0.” Assumed k values for unstable tests 

equivalent to that from run 1b of same series as these values were not provided. 

d

Calculated using methodology in outlined in “Report on Emissions Testing of Pressure Assisted LRGO-HC 

and Steam Assisted SKEC Burners – Supplement #2, Rev 0.” 

e

Reported velocity values from “Report on Emissions Testing of Pressure Assisted LRGO-HC and Steam 

Assisted SKEC Burners – Supplement #2, Rev 0.” Values for velocity for high pressure tests are reported as 

speed of sound whereas values for velocity for low pressure tests are reported using ideal gas law. In some 

instances, the calculated and reported values may vary by 1 ft/s, however this is most likely attributable to 

rounding from number of significant figures used for input values when performing calculations.
20

Table A1-4: Average Test Run Composition, NHV, LFL, and CE

Test Run

Propylene 

(vol%) a

Nitrogen 

(vol %) a

Natural Gas 

(vol %) a

Calculated NHV 

(BTU/scf) b

Calculated 

LFL (vol%) c

Reported 

Weighted 

CE (%)d

P1H-1 100 0 0 2150 2.4 100

P1H-2 100 0 0 2150 2.4 99.8

P1H-3 100 0 0 2150 2.4 99.8

P1L-1 100 0 0 2150 2.4 99.7

P1L-2 100 0 0 2150 2.4 99.9

P1L-3 100 0 0 2150 2.4 99.5

P2H-1* 36.6 63.4 0 787 6.6 99.98

P2H-1b 38.0 62.0 0 818 6.3 99.9

P2H-2 37.9 62.1 0 815 6.3 99.8

P2H-3 37.9 62.1 0 814 6.3 99.8

P2L-1* 31.6 68.4 0 680 7.6 99.74

P2L-1b 35.0 65.0 0 753 6.9 99.7

P2L-2 34.5 65.5 0 742 7.0 99.8

P2L-3 34.2 65.8 0 735 7.0 99.6

P3H-1 23.3 52.4 24.3 720 6.9 99.8

P3H-2 22.8 52.6 24.6 712 6.9 99.8

P3H-3 22.9 52.3 24.9 715 6.9 99.6

P3L-1* - - - - - -

P3L-1b 22.7 53.1 24.1 706 7.0 99.9

P3L-2 22.8 53.3 23.9 705 7.0 99.8

P3L-3 23.4 52.4 24.2 721 6.9 99.7

*: Denotes Unstable Test Run.

a

Values calculated using the volumetric flow rates for each component in Table A1-2. 

b

NHV presented here is both NHVvg and NHVcz as they are equal. NHV properties in Table 1 of this memo for 

propylene and nitrogen are used and for natural gas the NHV of 899 BTU/scf provided in “Report on 

Emissions Testing of Pressure Assisted LRGO-HC and Steam Assisted SKEC Burners – Supplement #1, Rev 0” is 

used. 

c

LFL presented here is both LFLvg and LFLcz as they are equal. LFL properties in Table 1 of this memo for 

propylene and nitrogen are used and for natural gas an assumed LFL equivalent to that of methane is used 

(LFL of 5.0 vol %) as speciated composition was not provided. 

d

CE values reported here are from Appendix JZ-5 (Clean Air Engineering Flare Performance Testing with 

PFTIR John Zink). The values for the unstable test runs reported here are from “Report on Emissions Testing 

of Pressure Assisted LRGO-HC and Steam Assisted SKEC Burners – Supplement #1, Rev 0.” 
21

Attachment 2

Spreadsheet Containing Marathon’s 2012 Test Data and Associated Calculated Data 
22

Table A2-1: Reported Vent Gas Composition Data

Condition Test Run

Nitrogen 

(vol%) a

Carbon 

dioxide 

(vol%) a

Methane 

(vol%) a

Ethane 

(vol%) a

Propane 

(vol%) a

N-Butane 

(vol%) a

Total 

vol %

a

PA1 1(1) 1.6 0.6 93.6 3.8 0.3 0.0 99.9

PA1 1(2) 1.2 1.1 93.6 4.2 0.4 0.0 100.5

PA1 1(3) 1.3 0.8 94.1 3.9 0.4 0.1 100.6

PA1 4(1) 36.8 0.3 61.0 2.5 0.2 0.0 100.8

PA1 4(2)* 35.5 0.6 62.0 2.1 0.2 0.0 100.5

PA1 4(3) 36.3 0.4 62.4 2.7 0.2 0.0 102.0

PA1 4(4)** 37.8 0.4 60.4 2.7 0.2 0.0 101.5

PA2 1(1) 1.6 0.6 94.4 3.9 0.4 0.1 101.0

PA2 1(2) 1.6 0.6 92.7 4.0 0.4 0.1 99.4

PA2 1(3) 1.6 0.7 92.9 3.7 0.4 0.1 99.4

PA2 4(1) 33.8 0.5 63.5 2.2 0.3 0.0 100.3

PA2 4(2) 39.1 0.4 61.6 2.7 0.2 0.0 104.0

PA2 4(3) 39.3 0.5 61.6 2.7 0.2 0.0 104.3

*: Denotes Unstable Test Run. Compositional data for this this specific test run was not provided in Marathon’s test 

report, however it was recorded and provided by Clean Air Engineering upon request. 

**: Denotes Partially Unstable Test Run 

a

Values reported same as in “Table A.7-2: Vent Gas Component Distribution for all LRGO Test Runs,” with minor 

exception of expressing units of volume percent rather than mole percent since they are the same. 
23

Table A2-2: "Normalized" Vent Gas Composition Data

Condition Test Run

Nitrogen 

(vol%) a

Carbon 

dioxide 

(vol%) a

Methane 

(vol%) a

Ethane 

(vol%) a

Propane 

(vol%) a

N-Butane 

(vol%) a

Total 

vol %

a

PA1 1(1) 1.6 0.6 93.7 3.8 0.3 0.0 100.0

PA1 1(2) 1.2 1.1 93.1 4.2 0.4 0.0 100.0

PA1 1(3) 1.3 0.8 93.5 3.9 0.4 0.1 100.0

PA1 4(1) 36.5 0.3 60.5 2.5 0.2 0.0 100.0

PA1 4(2)* 35.4 0.6 61.7 2.1 0.2 0.0 100.0

PA1 4(3) 35.6 0.4 61.2 2.6 0.2 0.0 100.0

PA1 4(4)** 37.2 0.4 59.5 2.7 0.2 0.0 100.0

PA2 1(1) 1.6 0.6 93.5 3.9 0.4 0.1 100.0

PA2 1(2) 1.6 0.6 93.3 4.0 0.4 0.1 100.0

PA2 1(3) 1.6 0.7 93.5 3.7 0.4 0.1 100.0

PA2 4(1) 33.7 0.5 63.3 2.2 0.3 0.0 100.0

PA2 4(2) 37.6 0.4 59.2 2.6 0.2 0.0 100.0

PA2 4(3) 37.7 0.5 59.1 2.6 0.2 0.0 100.0

*: Denotes Unstable Test Run. Compositional data for this this specific test run was not provided in Marathon’s test 

report, however it was recorded and provided by Clean Air Engineering upon request. 

**: Denotes Partially Unstable Test Run 

a

Values for each test run were “normalized” to equate the total volume percent to 100 vol % by dividing the individual 

component’s reported value in Table A2-1 by the value reported in the “Total vol %” column in the same table and then 

multiplied by 100. 
24

Table A2-3: Reported Average Test Run Volumetric Flow Rate and Flare Tip Velocity (at 

Standard Conditions)

Condition Test Run Standard Flare Gas Flow Rate (scf/hr) Reported Flare Tip Velocity 

(ft/s) a

PA1 1(1) 321,086 1,377

PA1 1(2) 314,583 1,349

PA1 1(3) 317,181 1,360

PA1 4(1) 275,855 1,183

PA1 4(2)* 284,089 1,218

PA1 4(3) 293,124 1,257

PA1 4(4)** 290,601 1,246

PA2 1(1) 181,306 777

PA2 1(2) 181,094 776

PA2 1(3) 181,265 777

PA2 4(1) 164,334 705

PA2 4(2) 161,703 693

PA2 4(3) 163,071 699

*: Denotes Unstable Test Run. Flow rate and flare tip velocity data for this this specific test run was not 

provided in Marathon’s test report, however mass flow rates of nitrogen and natural gas were recorded 

and provided by Clean Air Engineering upon request. These values were then averaged converted to a 

volumetric flow rate at standard conditions (1 atm and 68 °F). Assumed natural gas had a molecular 

weight of 17.15 lb/lb-mol (same as that reported by Dow in “Report on Emissions Testing of Pressure 

Assisted LRGO-HC and Steam Assisted SKEC Burners – Supplement #1, Rev 0.”) and nitrogen had 

component properties of those in Table 1 of this memo. 

**: Denotes Partially Unstable Test Run 

a

Reported flare tip velocity is at standard conditions (1 atm and 68 °F) and not actual conditions.

Conversion from volumetric flow rate to velocity was accomplished by converting into scf/sec (by using 

3600 seconds in 1 hour conversion factor) and converting into feet by using exit area of the LRGO burner 

of 0.022 ft2(calculated by using reported burner exit area of 3.11 in2 and dividing by 144 ft2in 1 in2

conversion factor) and then dividing by 3 since 3 LRGO burners in series were tested.
25

Table A2-4: Average Test Run NHV, LFL, and CE

Condition Test Run

Calculated "Normalized" 

NHV (BTU/scf) a

Calculated "Normalized"

LFL (vol%) a

Reported 

Weighted CE (%)b

PA1 1(1) 906 5.0 98.7

PA1 1(2) 915 4.9 100

PA1 1(3) 917 4.9 100

PA1 4(1) 591 7.7 98.5

PA1 4(2)* 595 7.6 99.6

PA1 4(3) 607 7.6 99.8

PA1 4(4)** 589 7.8 100

PA2 1(1) 920 4.9 98.9

PA2 1(2) 906 4.9 98.8

PA2 1(3) 903 5.0 100

PA2 4(1) 611 7.4 99.8

PA2 4(2) 600 7.8 99.4

PA2 4(3) 600 7.8 99.3

*: Denotes Unstable Test. The Reported Weighted CE (%) test run data was not provided in Marathon’s 

test report, however one minute of PFTIR CE emissions data was recorded and provided by Clean Air 

Engineering upon request. 

**: Denotes Partially Unstable Test Run 

a

Normalized vent gas composition data from Table A2-2 and component properties from Table 1 of this 

memo were used to calculate NHV and LFL parameters. NHV presented here is both NHVvg and NHVcz as 

they are equal. LFL presented here is both LFLvg and LFLcz as they are equal.

b

Except for test PA1 Run 4(2) (see first footnote above), Reported Weighted CE (%) values come from 

“Table A.1-2 Data Summary for All LRGO Test Runs” of the Marathon test report.