Download Flares-The John Zink Hamworthy Combustion Handbook-Robert Schwartz, Jeff White and Wes Bussman PDF

TitleFlares-The John Zink Hamworthy Combustion Handbook-Robert Schwartz, Jeff White and Wes Bussman
Tags Pressure Gases Natural Gas Smoke
File Size17.6 MB
Total Pages47
Table of Contents
                            11. Flares
	11.1 Flare Systems
		11.1.1 Purpose
		11.1.2 Objective of Flaring
		11.1.3 Applications
		11.1.4 Flare System Types
			11.1.4.1 Single-Point Flares
			11.1.4.2 Multipoint Flares
			11.1.4.3 Enclosed Flares
			11.1.4.4 Combination Systems
		11.1.5 Major System Components
			11.1.5.1 Single-Point Flares
			11.1.5.2 Multipoint Flares
			11.1.5.3 Enclosed Flares
	11.2 Factors Influencing Flare Design
		11.2.1 Flow Rate
		11.2.2 Gas Composition
		11.2.3 Gas Temperature
		11.2.4 Gas Pressure Available
		11.2.5 Utility Costs and Availability
		11.2.6 Safety Requirements
		11.2.7 Environmental Requirements
		11.2.8 Social Requirements
	11.3 Flare Design Considerations
		11.3.1 Reliable Burning
		11.3.2 Hydraulics
		11.3.3 Liquid Removal
		11.3.4 Air Infiltration
		11.3.5 Flame Radiation
		11.3.6 Smoke Suppression
		11.3.7 Noise/Visible Flame
		11.3.8 Air/Gas Mixtures
	11.4 Flare Equipment
		11.4.1 Flare Burners
			11.4.1.1 Nonassisted or Utility
			11.4.1.2 Simple Steam Assisted
			11.4.1.3 Improved Steam Assisted
			11.4.1.4 Advanced Steam Assisted
			11.4.1.5 Low-Pressure Air Assisted
			11.4.1.6 Energy Conversion
			11.4.1.7 Endothermic
			11.4.1.8 Special Types
		11.4.2 Knockout Drums
		11.4.3 Liquid Seals
			11.4.3.1 Prevent Upstream Contamination
			11.4.3.2 Pressurize Upstream Section
			11.4.3.3 Diverting Gas Flow
			11.4.3.4 Control Valve Bypass
			11.4.3.5 Liquid Seals as Arrestors
			11.4.3.6 Design Factors
		11.4.4 Purge Reduction Seals
		11.4.5 Enclosed Flares
		11.4.6 Flare Support Structures
		11.4.7 Flare Controls
			11.4.7.1 Typical Steam Control Valve
			11.4.7.2 Automatic Steam Control
			11.4.7.3 Typical Staging Control Valve
			11.4.7.4 Level Controls
			11.4.7.5 Purge Controls
		11.4.8 Arrestors
	11.5 Flare Combustion Products
		11.5.1 Reaction Efficiency
			11.5.1.1 Definition of Destruction and Combustion Efficiency
			11.5.1.2 Technical Review of Industrial Flare Combustion Efficiency
				11.5.1.2.1 Hydrogen Enrichment
		11.5.2 Emissions
		11.5.3 Dispersion
	References
                        
Document Text Contents
Page 1

251

11
Flares

Robert E. Schwartz, Jeff White, and Wes Bussman

CONTENTS

11.1 Flare Systems ................................................................................................................................................................. 252
11.1.1 Purpose .............................................................................................................................................................. 253
11.1.2 Objective of Flaring .......................................................................................................................................... 253
11.1.3 Applications ....................................................................................................................................................... 253
11.1.4 Flare System Types ........................................................................................................................................... 254

11.1.4.1 Single-Point Flares ............................................................................................................................. 254
11.1.4.2 Multipoint Flares ................................................................................................................................ 254
11.1.4.3 Enclosed Flares ................................................................................................................................... 254
11.1.4.4 Combination Systems ........................................................................................................................ 255

11.1.5 Major System Components .............................................................................................................................. 255
11.1.5.1 Single-Point Flares ............................................................................................................................. 256
11.1.5.2 Multipoint Flares ................................................................................................................................ 256
11.1.5.3 Enclosed Flares ................................................................................................................................... 256

11.2 Factors Influencing Flare Design ................................................................................................................................ 256
11.2.1 Flow Rate ............................................................................................................................................................ 256
11.2.2 Gas Composition ............................................................................................................................................... 257
11.2.3 Gas Temperature ............................................................................................................................................... 257
11.2.4 Gas Pressure Available ..................................................................................................................................... 258
11.2.5 Utility Costs and Availability ......................................................................................................................... 259
11.2.6 Safety Requirements ........................................................................................................................................ 259
11.2.7 Environmental Requirements ......................................................................................................................... 260
11.2.8 Social Requirements ......................................................................................................................................... 260

11.3 Flare Design Considerations ....................................................................................................................................... 260
11.3.1 Reliable Burning ............................................................................................................................................... 261
11.3.2 Hydraulics .......................................................................................................................................................... 262
11.3.3 Liquid Removal ................................................................................................................................................. 263
11.3.4 Air Infiltration ................................................................................................................................................... 264
11.3.5 Flame Radiation ................................................................................................................................................ 265
11.3.6 Smoke Suppression ........................................................................................................................................... 266
11.3.7 Noise/Visible Flame ......................................................................................................................................... 267
11.3.8 Air/Gas Mixtures .............................................................................................................................................. 270

11.4 Flare Equipment ............................................................................................................................................................ 270
11.4.1 Flare Burners ..................................................................................................................................................... 270

11.4.1.1 Nonassisted or Utility ....................................................................................................................... 270
11.4.1.2 Simple Steam Assisted ...................................................................................................................... 271
11.4.1.3 Improved Steam Assisted ................................................................................................................. 272
11.4.1.4 Advanced Steam Assisted ................................................................................................................ 272
11.4.1.5 Low-Pressure Air Assisted ............................................................................................................... 273
11.4.1.6 Energy Conversion ..............................................................................................................................274

© 2014 by Taylor & Francis Group, LLC

Page 2

252 The John Zink Hamworthy Combustion Handbook

11.1 Flare Systems

During the operation of many hydrocarbon indus-
try plants, there is the need to control process condi-
tions by venting gases and/or liquids. In emergency
circumstances, relief valves act automatically to limit
equipment overpressure. For many decades of the last
century, process vents and pressure relief flows were
directed, individually or collectively, to the atmosphere
unburned. Gases separated from produced oil were
also vented to the atmosphere unburned. The custom
of unburned venting began to change in the late 1940s
when increased environmental awareness and safety
concerns created the desire to convert vents to continu-
ously burning flares.

Burning brought about the need for pilots and pilot
ignitors and the need for awareness of the design factors
and considerations imposed on a system by a flame at
the exit. In many cases, the desirable flaring of the gases
was accompanied by objectionable dense black smoke
as shown in Figure 11.1. In addition to their develop-
ment of flare pilots and ignition systems, industry pio-
neers John Steele Zink and Robert Reed1 invented the
first successful smokeless flare burner (Figure 11.2) in

the early 1950s. This invention was an important point
in the transition from unburned vents to flaring and
from vent pipes to burners specifically designed for
flare applications.

While the combustion fundamentals discussed in
Volume 1 continue to apply, flare burners differ from
process and boiler burners in several respects, including

11.4.1.7 Endothermic ....................................................................................................................................... 276
11.4.1.8 Special Types ...................................................................................................................................... 280

11.4.2 Knockout Drums .............................................................................................................................................. 281
11.4.3 Liquid Seals ....................................................................................................................................................... 282

11.4.3.1 Prevent Upstream Contamination ................................................................................................... 283
11.4.3.2 Pressurize Upstream Section ........................................................................................................... 283
11.4.3.3 Diverting Gas Flow ............................................................................................................................ 283
11.4.3.4 Control Valve Bypass ......................................................................................................................... 284
11.4.3.5 Liquid Seals as Arrestors .................................................................................................................. 284
11.4.3.6 Design Factors .................................................................................................................................... 284

11.4.4 Purge Reduction Seals ...................................................................................................................................... 284
11.4.5 Enclosed Flares .................................................................................................................................................. 287
11.4.6 Flare Support Structures .................................................................................................................................. 288
11.4.7 Flare Controls .................................................................................................................................................... 289

11.4.7.1 Typical Steam Control Valve............................................................................................................. 290
11.4.7.2 Automatic Steam Control .................................................................................................................. 291
11.4.7.3 Typical Staging Control Valve .......................................................................................................... 291
11.4.7.4 Level Controls ..................................................................................................................................... 292
11.4.7.5 Purge Controls .................................................................................................................................... 293

11.4.8 Arrestors ............................................................................................................................................................. 293
11.5 Flare Combustion Products ......................................................................................................................................... 293

11.5.1 Reaction Efficiency ............................................................................................................................................ 293
11.5.1.1 Definition of Destruction and Combustion Efficiency ................................................................. 293
11.5.1.2 Technical Review of Industrial Flare Combustion Efficiency ..................................................... 294

11.5.2 Emissions ........................................................................................................................................................... 295
11.5.3 Dispersion .......................................................................................................................................................... 295

References ................................................................................................................................................................................ 297

Figure 11.1
Typical early 1950s flare performance.

© 2014 by Taylor & Francis Group, LLC

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Page 23

273Flares

The  smaller the discharge, the greater the perimeter to
area ratio and thus the quicker air can penetrate the inte-
rior of the gas bundle promoting smokeless combustion.
Each discharge has an associated external–internal tube,
similar to those present in the improved steam-assisted
design. The difference between the external–internal
tubes of the advanced design from those of the improved
design is that the advanced design tubes are straight.

Straight tubes are more efficient educting air than bent
tubes since there are no turning losses. Higher eduction
efficiency allows for more air to be drawn in with the
same amount of steam or the same amount of air with less
steam. Another feature of the advanced design is the inclu-
sion of a premix zone where the steam/air mixture from
the external–internal tube is mixed with the vent gas prior
to discharge and ignition. Premixing of air with a vent gas
prior to combustion greatly improves the smokeless per-
formance. The advanced steam-assisted flare design can
provide the same smokeless performance as an improved
design while only requiring 70% of the steam.

Operation of the advanced design is much simpler
than that of the improved design. While an improved
design typically has three different steam lines and care-
ful coordination is required between the three steam
flows, the advanced design uses a single steam line. The
advanced design brings together the best aspects of the
simple design (ease of operation) with the best aspects
of the improved design (improved smokeless perfor-
mance) while using less steam.

11.4.1.5 Low-Pressure Air Assisted

Not all plants have large amounts of steam available for
use by the flare. Some plants prefer not to use steam to
avoid freezing problems, others cannot commit water to
make steam for smoke control, and still others choose

Figure 11.28
State-of-the-art Steamizer® XP™.

0.70

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80
Tip diameter (in.)

P(
ai

r)
/A

(fu
el

)

First generation

�ird generation

Figure 11.26
A comparison of the perimeter/area ratio for simple and improved
steam-assisted flares.

Figure 11.27
Steamizer® flare burner and muffler.

© 2014 by Taylor & Francis Group, LLC

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Page 24

274 The John Zink Hamworthy Combustion Handbook

not to install a boiler. To meet this need, a series of air-
assisted flare designs was invented.12

Generally, the air-assisted flare burner consists of a
gas burner mounted in an air plenum at the top of the
flare stack (Figure 11.29). Relief gas is delivered to the
burner by a gas riser pipe running coaxially up the cen-
ter of the flare stack. Low-pressure air is delivered to
the burner from one or more blowers located near the
base of the flare stack. The air flows upward through the
annular space between the flare stack and the gas riser.

The first air-assisted flare applications were associ-
ated with operations some distance from the main plant
or totally remote from plant utilities support. Early air
flares were often designed to flare small to moderate
flow rates. The success of these flares led to the use of
air assist on flares of greater capacity. More recently, air-
assisted flares have come into use as the flare for large
process facilities. The flame similarity method and the
related near-field mixing region models are examples of
the design tools necessary for cost-effective application
of air-assisted flares. Today, air flare designs are available

with demonstrated tip life spans of 5–10 years. Smokeless
rates above 150 × 106 standard cubic feet per day (SCFD)
(4.2 × 106 standard cubic meters per day) are available for
saturated hydrocarbons such as production facility reliefs.

Figure 11.30 shows an example of a more recent air flare
design. Waste gas exits the burner in one or more narrow
annular jets, each surrounded by assist air. This design
makes good use of the perimeter/area ratio concept dis-
cussed earlier in the context of steam-assisted flares.

��������� ��
��������
�����

In the smokeless flaring discussions earlier, the focus cen-
tered on adding energy from an outside source to boost
the overall energy level high enough to achieve smokeless
burning. An advantage is gained if an outside source is
not required. This is the case with energy conversion flare
burners. Such burners are also referred to as high-pressure
flare burners or multipoint flare burners. Where they can
be employed, the use of energy conversion flare burners
can provide a significant reduction in flare operating cost.

There are two distinct groups of energy conversion
burners. The first group is distinguished by having a
single inlet and relatively close grouping of the waste
gas discharge points. The other group employs a means
beyond energy conversion to achieve smokeless flaring.
In both groups, an underlying principle is the conver-
sion of the static pressure of the waste gas, at the burner,
to jet velocity and ultimately into momentum.

Another concept employed by both groups is the
division of the incoming gas stream into multiple

Figure 11.30
Annular air flare.

Figure 11.29
Air-assisted smokeless flare with two blowers in a refinery.

© 2014 by Taylor & Francis Group, LLC

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Page 46

296 The John Zink Hamworthy Combustion Handbook

and vertical directions, takes the form of a Gaussian
distribution about the centerline of the plume and is
written as follows:


C

Q
U

H y

z y z y
=







−





−




σ σ π σ σ
exp exp

2

2

2

22 2
(11.16)

where
C is the predicted GLC concentration, g/m3
Q is the source emission rate, g/s
U is the horizontal wind speed at the plume centerline

height, m/s
H is the plume centerline height above ground, m
σy and σz are the standard deviations of the concen-

tration distributions in the crosswind and vertical
directions, respectively, m

y is the crosswind distance, m (see Figure 11.55)

This Gaussian dispersion model was derived assum-
ing a continuous buoyant plume, single-point source,
and flat terrain. Beychok22 discusses the shortcomings
of Gaussian dispersion models. Beychok suggests that
it is realistic to expect Gaussian dispersion models to

consistently predict real-world dispersion plume con-
centrations within a factor that may be as high as 10.
Gaussian dispersion models, however, are useful in
that they can give a rough and fairly quick estimation
and comparison of pollutant levels from elevated point
sources.

The accuracy of a Gaussian dispersion model depends
on how well one can determine the plume rise, H, at any
given downwind distance and dispersion coefficients, σy
and σz. A standard atmospheric stability classification
method, known as the Pasquill–Gifford–Turner classifi-
cation, is widely used in GLC models. This method cat-
egorizes the stability of the atmosphere into six classes
that vary from very unstable (class A) to very stable
(class F). An atmosphere that is stable has low levels of
turbulence and will disperse a pollutant more slowly
than an unstable atmosphere. The dispersion coeffi-
cients, σy and σz, are dependent on the amount of tur-
bulence in the atmosphere and are, therefore, related to
the atmospheric stability class. For more information on
the equations describing the dispersion coefficients, see
Turner.23

The plume height is defined as the vertical distance
from the plume centerline to grade, as illustrated in
Figure 11.55. There are several variables that can affect

x

z

y

Wind
direction

Plume
height

Stack
height

Dispersion of pollutants

Grade

Gaussian distribution
of pollutants

Figure 11.55
Geometry for plume dispersion calculations.

© 2014 by Taylor & Francis Group, LLC

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Page 47

297Flares

the plume height. These variables are divided into
two categories: emission factors and meteorological
factors. The emission factors include the (1) stack gas
exit velocity, (2) stack exit diameter, (3) stack height, and
(4) temperature of the emitted gas. The meteorological
factors include the (1) wind speed, (2) air temperature
with height, (3) shear of the wind with height, (4) atmo-
spheric stability, and (5) terrain. None of the equations
reported in the literature for estimating plume heights,
however, take into account all the emission and meteo-
rological factors. For a review of these equations, see
Moses et al.24

GLC analysis is very complex because the results can
depend on so many variables, as briefly discussed ear-
lier. In the past, engineers and scientists have described
GLC modeling as an art rather than a science. However,
this paradigm is shifting due to more sophisticated
computer models.25 Due to the complexity of these mod-
els, one should consult an expert when requiring GLC
analysis.

����������

1. J.S. Zink and R.D. Reed, Flare stack gas burner, U.S. Pat.
2,779,399, issued January 29, 1957.

2. R.E. Schwartz and S.G. Kang, Effective design of emer-
gency flaring systems, Hydrocarbon Engineering, 3(2),
57–62, 1998.

3. S.H. Kwon, D.I. Shin, D.D. Cobb, D.K. Kang, and
C.  Stacklin, Improve flare management, Hydrocarbon
Processing, 76(7), 105–111, 1997.

4. R.D. Reed, Furnace Operations, 3rd edn., Gulf Publishing,
Houston, TX, 1981.

5. API Standard 521, Pressure-relieving and Depressuring
Systems, 5th edn., American Petroleum Institute,
Washington DC, May 2008.

6. M.R. Keller and R.K. Noble, RACT for VOC—A Burning
Issue, Pollution Engineering, July 1983.

7. U.S. EPA AP-42, Compilation of Air Pollutant Emissions
Factors, 5th edn., Vol. 1, Section 13.5, Industrial Flares,
Washington DC.

8. C. Baukal, J. Hong, R. Poe, and R. Schwartz, Large-scale
flare testing, in Industrial Combustion Testing, C. Baukal
(ed.), Chapter 28, CRC Press, Boca Raton, FL, 2011.

9. J.S. Zink, R.D. Reed, and R.E. Schwartz, Temperature-
pressure activated purge gas flow system for flares, U.S.
Pat. 3,901,643, issued August 26, 1975.

10. R.E. Schwartz and J.W. White, Flare Radiation Prediction:
A Critical Review, John Zink Company, Tulsa, OK, 1996.

11. R.D. Reed, Flare stack burner, U.S. Pat. 3,429,645, issued
February 25, 1969.

12. R.D. Reed, R.K. Noble, and R.E. Schwartz, Air powered
smokeless flare, U.S. Pat. 3,954,385, issued May 4, 1976.

13. R.D. Reed, J.S. Zink, and H.E. Goodnight, Smokeless flare
pit burner and method, U.S. Pat. 3,749,546, issued July 31,
1973.

14. R.E. Schwartz and R.K. Noble, Method and apparatus
for flaring inert vitiated waste gases, U.S. Pat. 4,664,617,
issued May 12, 1987.

15. W.R. Bussman and D. Knott, Unique concept for noise
and radiation reduction in high-pressure flaring, Offshore
Technology Conference, Houston, TX, May 2000.

16. J.S. Zink, R.D. Reed, and R.E. Schwartz, Apparatus for
controlling the flow of gases, U.S. Pat. 3,802,455, issued
April 9, 1974.

17. H. Glomm, Anordnung und Betrieb von Notabblas-
esystemen (blow down systems), Rohrleitungstechnik in
der Chemishen Industrie, 199, 18–28, 1967.

18. McDaniel, Marc. Flare Efficiency Study. Rep. no. EPA-
600/2-83-052. Washington DC, U.S. EPA, 1983.

19. U.S. EPA, Code of Federal Regulations, Title 40, Part 60,
Standards of Performance for New Stationary Sources.

20. N.I. Sax and R.J. Lewis Sr., Dangerous Properties of
Industrial Materials, 7th edn., Van Nostrand Reinhold,
New York, 1989.

21. M. Miller and R. Liles, Air modeling, Environmental
Protection, 6(9), 34–37, 1995.

22. M. Beychok, Error propagation in stack gas dispersion
models, The National Environmental Journal, 6(1), 33–37,
1996.

23. B. Turner, U.S. EPA Office of Air Programs. Workbook
of Atmospheric Dispersion Estimates. Washington DC.
U.S. EPA, 1970.

24. H. Moses, G.H. Strom, and J.E. Carson, Effects of meteo-
rological and engineering factors on stack plume rise,
Nuclear Safety, 6(1), 1–19, 1964.

25. C. Seigneur, Understanding the basics of air quality
modeling, Chemical Engineering Progress, 88(3), 68–74, 1992.

© 2014 by Taylor & Francis Group, LLC

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