NTJS
United States Office of Air Quality EPA-450/4-88-009
Environmental Protection Planning and Standards September 1988
Agency Research Triangle Park NC 27711
Air
A WORKBOOK OF
SCREENING
TECHNIQUES FOR
ASSESSING IMPACTS
OF TOXIC AIR
POLLUTANTS
Of
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EPA-450/4-88-009
A Workbook Of Screening Techniques For
Assessing Impacts Of Toxic Air Pollutants
by.
TRC Environmental Consultants, Inc.
800 Connecticut Boulevard
East Hartford, CT06108
EPA Project Officer Jawad S Touma
Contract No. 68-02-3886
Prepared for
U S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 27711
September 1988
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This report has been reviewed by the Office of Air Quality Planning and Standards, US EPA, and has been
approved for publication. Mention of trade names or commercial products is not intended to constitute
endorsement or recommendation for use. Copies of this report are available, for a fee, from the National
Technical Services, 5285 Port Roval Road, Springfield VA 22161.
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ACKNOWLEDGEMENTS
This workbook was prepared by Daniel J. McNaughton and Paul M. Bodner of
TRC Environmental Consultants, Inc. under contract to the U.S. Environmental
Protection Agency, Office of Air Quality Planning and Standards, Source
Receptor Analysis Branch. The work was directed by Jawad S. Touma, Source
Receptor Analysis Branch.
-in-
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TABLE OF CONTENTS
Page
LIST OF FIGURES ix
LIST OF TABLES xi
1.0 INTRODUCTION 1-1
2.0 SELECTION OF SCREENING TECHNIQUES FOR TOXIC AIR
CONTAMINANTS 2-1
2.1 Release Categorization 2-1
2.2 Limitations and Assumptions 2-2
2.3 Technique Selection and Use 2-3
2.4 Determining Maximum Short-Term Ground Level
Concentration 2-8
3.0 SUPPORT DATA FOR SCREENING ESTIMATES 3-1
3.1 Meteorological Data 3-1
3.1.1 Wind Speed and Direction 3-1
3.1.2 Stability and Turbulence 3-2
3.1.3 Temperature 3-4
3.1.4 Atmospheric Pressure 3-4
3.2 Chemical and Physical Parameters 3-4
4.0 SCENARIOS AND TECHNIQUES FOR RELEASE AND EMISSIONS
ESTIMATES 4-1
4.1 Continuous Particulate and Gaseous Releases from
Stacks 4-3
4.1.1 Particulate Matter 4-3
4.1.2 Gases 4-4
4.2 Continuous Releases of Fugitive Dust 4-5
4.3 Ducting Failures With Dust Releases 4-6
4.4 Flare Emissions 4-7
4.5 Continuous Gaseous Leaks from Tanks/Pipes 4-8
4.6 Instantaneous Gaseous Releases from Stacks .... 4-11
4.7 Multiple Fugitive Continuous Gaseous Emission
Sources 4-12
4.8 Continuous Gaseous Emissions from Land Treatment . 4-13
4.9 Continuous Emissions from Municipal Solid Waste
Landfills 4-15
4.10 Continuous Emissions of Pesticides and Herbicides . 4-17
4.11 Instantaneous Emissions Due to Equipment Openings . 4-18
4.12 Evaporation from Quiescent or Aerated Surface
Impoundments 4-20
4.13 Continuous Relief Valve Discharges (Two-Phase Flow) 4-24
4.14 Instantaneous Relief Valve Discharges (Two-Phase) . 4-28
4.15 Low Volatility Liquid Leaks from Pipes 4-29
4.16 Low Volatility Liquid Leaks from Tanks 4-32
-v-
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Page
4.17 High Volatility Liquid Leaks from Pipes 4-34
4.18 High Volatility Liquid Leaks from Tanks 4-35
5.0 ATMOSPHERIC DISPERSION ESTIMATES 5-1
5.1 Cloud Densities 5-2
5.1.1 Calculations to Determine the Relative Density of
Instantaneous Releases 5-2
5.1.2 Calculations to Determine the Relative Density of
Continuous Releases 5-3
5.2 Plume Rise Calculations 5-6
5.2.1 Mean Molecular Weight for Mixtures of Gases . . . 5-6
5.2.2 Flare Plume Rise 5-6
5.2.3 Buoyancy Plume Rise 5-8
5.3 Dispersion Parameters 5-10
5.3.1 Horizontal and Vertical Dispersion Parameters
for Continuous Emission Releases 5-10
5.3.2 Horizontal and Vertical Dispersion Parameters
for Instantaneous Emission Releases 5-13
5.3.3 Horizontal and Vertical Dispersion Parameters
for Wake Effects 5-16
5.4 Buoyancy-Induced Initial Dilution 5-18
5.5 Virtual Source Distances 5-19
5.5.1 Virtual Distances for Area Sources 5-19
5.5.2 Virtual Distances for Volume Sources 5-20
5.5.3 Virtual Distances for Wake Effects 5-21
5.6 Concentration Calculations 5-24
5.6.1 Cavity Modeling 5-24
5.6.2 Heavy Gas Model - Instantaneous Releases .... 5-26
5.6.3 Heavy Gas Model - Continuous Release 5-28
5.6.4 Dispersion Model for Continuous Releases .... 5-31
5.6.5 Dispersion Model for Instantaneous Releases . . . 5-32
6.0 EXAMPLES 6-1
6.1 Continuous Gaseous Emissions Prom Stacks 6-1
6.1.1 Building Cavity Example 6-1
6.1.2 Near-Wake Example 6-3
6.1.3 Far-Wake Example 6-5
6.2 Fugitive Dust 6-6
6.3 Instantaneous Ejection of Particles from Ducts . . 6-8
6.4 Flare Emissions 6-10
6.5 Continuous Gaseous Releases from Tanks or Pipes . . 6-14
6.6 Instantaneous Gas Releases 6-20
6.7 Continuous Releases of Fugitive Emissions 6-23
6.8 Continuous Gaseous Emissions from Land Treatment . 6-25
6.9 Municipal Solid Waste Landfill 6-27
6.10 Continuous Emissions from an Herbicide 6-29
6.11 Equipment Openings 6-32
6.12 Continuous Gaseous Emissions from Surface
Impoundments 6-34
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Page
6.13 Relief Valve Discharge (Two-Phase) 6-38
6.14 Two-Phase Instantaneous Release 6-44
6.15 Liquid Release from a Pipe 6-47
6.16 Low Volatility Liquid Releases from Tanks 6-49
6.17 High Volatility Liquid Release from a Pipe .... 6-51
6.18 High Volatility Liquid Releases from Tanks .... 6-53
REFERENCES R-l
APPENDIX A EMISSION FACTORS A-l
APPENDIX B GLOSSARY B-l
APPENDIX C FLOWCHARTS FOR WORKBOOK SCENARIOS C-l
APPENDIX D FLOWCHARTS FOR DISPERSION CALCULATIONS D-l
APPENDIX E AVERAGING PERIOD OF CONCENTRATION ESTIMATES E-l
APPENDIX F SELECTED CONVERSION FACTORS F-l
APPENDIX G CALCULATIONAL METHODS FOR DISPERSION PARAMETERS ... G-l
-VI1-
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LIST OF FIGURES
Figure No. Page
2-1 Release Scenarios 2-5
5-1 Horizontal Dispersion Parameter (oy) as a Function of
Downwind Distance and Stability Class (Continuous
Releases) 5-11
5-2 Vertical Dispersion Parameter (az) as a Function of
Downwind Distance and Stability Class (Continuous
Releases) 5-12
5-3 Horizontal Dispersion Parameter (ay) as a Function of
Downwind Distance and Stability Class (Instantaneous
Releases) 5-14
5-4 Vertical Dispersion Parameter (az) as a Function of
Downwind Distance and Stability Class (Instantaneous
Releases) 5-15
-ix-
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LIST OF TABLES
Table No. Page
2-1 Release Scenarios 2-4
2-2 Wind Speed and Stability Class Combinations 2-9
2-3 Calculation Procedures for Use with Various Emission Heights
(Continuous Releases) 2-10
2-4 Approaches for Maximum Concentrations 2-12
3-1 Wind Profile Exponent as a Function of Atmospheric Stability 3-2
3-2 Key to Stability Categories 3-3
3-3 Typical Physical and Chemical Property Parameters Used in
Emission Modeling 3-5
6-1 RVD Model Results: Chlorine Gas Leak 6-17
6-2 RVD Model Results: Chlorine Two-Phase Release 6-41
-xi-
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1.0 INTRODUCTION
This workbook provides a logical approach to the selection of appropriate
screening techniques for estimating ambient concentrations due to various
toxic/hazardous pollutant releases. Methods used in the workbook apply to
situations where a release can be fairly well defined, a condition typically
associated with non-accidental toxic releases. The format of this workbook is
built around a series of scenarios which may be considered typical and
representative of the means by which toxic chemicals become airborne. In
addition, suggestions are provided for modeling less typical cases.
Screening techniques are simplified calculational procedures designed with
sufficient conservatism to allow a determination of whether a source: 1) is
clearly not an air quality threat or 2) poses a potential threat which should
be examined with more sophisticated estimation techniques or measurements.
Screening estimates obtained using this workbook represent maximum short-term
ground level concentration estimates from a meteorological perspective. If
the screening estimates demonstrate that during these conditions the ground
level concentrations are not likely to be considered objectionable, further
analysis of the source impact would not be necessary as part of the air
quality review of the source. However, if screening demonstrates that a
source may have an objectionable impact, more detailed source impact analysis
would be required using refined emissions and air quality models.
Methods used in this workbook should be applied with caution. Techniques
for estimating emissions are evaluated and revised on a continuing basis by
EPA. Thus the user should consult with EPA on the most recent emission models
and emission factors. Meteorological methods presented in this report reflect
guidance published elsewhere, and in particular the Procedures for Evaluating
Impact of Stationary Sources that includes the PC-based model SCREEN (1988b)
and the Guideline on Air Quality Models (Revised) (1986) and Supplement A
1-1
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(1987). The Regional Modeling Contact should be consulted as to the present
status of guidance in air quality modeling.
The workbook is organized into six sections and six supporting
appendices. Section 2 discusses selection of screening techniques and the
general approach to using the workbook. Users are advised to consult this
section both for releases explicitly presented in the workbook and for less
typical releases. The section also considers assumptions, limitations and
conservatism of estimates. Section 3 describes the support data (i.e.,
meteorological data and chemical and physical parameters) needed for making
estimates. Section 4 helps the user identify applicable release scenarios and
determine release and emission rates. In this workbook 18 release scenarios
have been selected to represent situations likely to be encountered.
Section 5 of the report guides the user through all the steps required for
making atmospheric dispersion estimates once atmospheric emissions and
contaminant characteristics are known. Section 6 provides an example of the
emission and associated dispersion estimation methods for each specific
release scenario.
Appendix A discusses currently available sources for obtaining emission
factors that can be used for some of the scenarios. Appendix B provides a
glossary of terms applicable to air toxics modeling. Appendices C and D
contain flow diagrams to be used as a guide in Sections 4 and 5 for selecting
applicable emission and dispersion calculation methods. Appendix E presents
methods for converting concentrations to different averaging times.
Appendix F provides some useful unit conversion factors applicable to the
workbook. Appendix G provides methods for calculating dispersion parameters
as an alternative to using graphical methods.
This workbook supersedes information in EPA Report EPA-450/4-86-11, Some
Applications of Models to Air Toxics Impact Assessments.
1-2
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2.0 SELECTION OF SCREENING TECHNIQUES FOR TOXIC AIR CONTAMINANTS
This workbook attempts to account for many of the scenarios expected to
produce toxic chemical releases to the atmosphere.
2.1 Release Categorization
Selection of appropriate techniques for screening estimates requires
categorization of the toxic chemical release of interest. There are three
overlapping categories which should be considered when defining problems for
screening:
1) Physical State - Gaseous releases to the atmosphere can, in
general, be simulated using techniques developed for criteria air
pollutants unless the gas is dense, is highly reactive, or rapidly
deposits on surfaces. Additional source modeling must be
performed if the release is liquid, aerosol or multi-phased to
determine the state of the material as it becomes available for
dispersion in air.
2) Process/Release Conditions - Knowledge of the circumstances under
which chemicals are released helps to determine both state and
dispersive characteristics. For example, location of a leak in a
pressurized liquefied gas storage tank will determine if a release
is liquid or gas and if source modeling is required prior to
dispersion estimates.
3) Dispersive Characteristics - Techniques for pollutant dispersion
estimates are categorized by terms such as instantaneous versus
continuous, or point versus area or volume releases. To complete
dispersion estimates, this final characterization is required at
some point in concentration calculations.
The primary emphasis in this workbook is to provide the user with screening
techniques for estimating short-term, ground level concentrations of toxic
chemicals released to the atmosphere. However, in order to do this, the
workbook also provides assistance to the user in formulating the release
conditions. In framing each problem, the user must reason the path required
to complete the concentration estimate.
2-1
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2.2 Limitations and Assumptions
Methods included in this workbook are intended to provide simplified,
conservative dispersion estimates for situations which may represent extremely
complex release scenarios. As such, the methods are limited in their
applicability. Some of these limitations are as follows:
Screening techniques provided are intended for use on small to
mid-scale non-accidental releases.
All techniques assume that the toxic air contaminant is non-
reactive and non-depositing. Thus these screening methods are not
applicable for reactive gases and settleable particles.
Conditions resulting in worst case concentrations cannot be
uniquely defined in instances where gases released are denser than
air or where meteorological conditions affect source estimates.
For example, in the case of evaporation, the highest source rates
are related to high wind speeds. High wind speeds, however,
result in more dilution which acts to lower concentrations in air.
Time dependent emissions cannot be simulated with a simple
screening technique. Techniques provided assume steady releases
for a specified period.
Methods are not provided for the following phenomena:
- heavy gas releases influenced by obstructions to flows
- non-vertical jet releases of heavy or passive gases
- the influence of aerosol evaporation, deposition, surface or
radiational cloud heating and exothermic or endothermic
reactions on dispersing clouds
All release calculations assume thermodynamically ideal conditions
for gas and liquid flows.
Pasquill-Gifford dispersion parameters for continuous plumes
(Turner, 1970) are assumed to represent hourly average conditions.
Building wake effects calculations are based on methods used in
the Industrial Source Complex Model (EPA, 1987c) and cavity
calculations are based on methods in Procedures (EPA, 1988b).
Simple rectangular building geometries are assumed in both cases.
When a selection of wake effects is made, calculations are not
performed for receptors within three building heights downwind.
Complex and elevated terrain effects are not considered.
2-2
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Sources in rural and urban settings can use the same procedures
presented here. Dispersion parameters for urban settings are not
presented. They may be obtained from other EPA publications.
Screening techniques are applicable to short-term concentration
estimates.
2.3 Technique Selection and Use
Eighteen most prevalent release scenarios were selected for this workbook
and are grouped in Table 2-1 according to three categories: particulate
matter, gases and liquids. For some of the release scenarios, subcategories
have been added. Table 2-1 also provides a convenient look-up table to
indicate relevant sections in this report that are associated with each
release scenario. Figure 2-1 provides a graphical illustration of each
release scenario. Appendix C guides the user through techniques for
estimating release and emission rates. Appendix D guides the user in
selecting the atmospheric dispersion estimates for these scenarios. The first
step in analyzing any scenario is, therefore, to consult the appropriate
release flowchart in Appendix C.
Steps in using the workbook are as follows:
1. Select the release scenario from Figure 2-1 and Table 2-1 which
most closely resembles the release of concern. Release
descriptions provided in the report sections and noted in the
table provide selection guidance.
2. Follow the instructions given in the report section describing the
release of concern. The flowchart referenced either guides the
user through emission calculations in Section 4 or indicates that
emission estimates can be obtained from a separate source of
emission factors (Appendix A).
3. The final step of the applicable release flowchart indicates the
appropriate dispersion flowchart in Appendix D which guides the
user in determination of ambient concentrations. The user should
proceed to the flowchart keeping in mind the dispersion modeling
category specified for the release scenario of concern in
Table 2-1 (i.e., instantaneous or continuous emissions from a
point, area, or volume source). The dispersion model category is
used at a decision point in the dispersion flowchart.
2-3
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STACK RELEASES OF GASES
OR PARTICLES (4.1.1, 4.1.2)
FUGITIVE DUST EMISSIONS (4.2)
PARTICULATE MATTER
DUCT FAILURES (4.3)
FLARE EMISSIONS (4.4)
GAS LEAKS (4.5)
INSTANTANEOUS
GASEOUS EMISSIONS (4.6)
FIGURE 2-1. RELEASE SCENARIOS
(Section Number in Parentheses)
2-5
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fit
_n
MULTIPLE FUGITIVE EMISSIONS (4.7)
LAND TREATMENT (4.8)
MUNICIPAL SOLID WASTE LANDFILL (4.9>
PESTICIDES/HERBICIDES (4.10)
EQUIPMENT OPENING (4.11)
SURFACE IMPOUNDMENTS (4.12)
FIGURE 2-1. RELEASE SCENARIOS (Continued)
(Section Number in Parentheses)
2-6
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.
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RELIEF VALVE - INSTANTANEOUS (4.14)
LIQUID LEAK - PIPE (4.15)
LIQUID LEAK - TANK (4.16)
HIGH VOLATILITY PIPE LEAKS (4.17)
HIGH VOLATILITY TANK LEAKS (4.18)
FIGURE 2-1. RELEASE SCENARIOS (Continued)
(Section Number In Parentheses)
2-7
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4. The concentrations obtained are representative of the averaging
times identified. Procedures to convert concentrations to other
averaging times are described in Appendix E.
2.4 Determining Maximum Short-Term Ground Level Concentration
In modeling air toxic releases, a reasonable degree of assurance is needed
that the maximum short-term ground level concentration estimate from a
meteorological perspective is obtained. This maximum concentration is
selected from those concentrations calculated using the range of stability
classes and wind speeds in Table 2-2.
The first choice is to use all six stability classes and their associated
wind speeds. To reduce the number of calculations, the second choice is to
use a subset of these meteorological conditions associated with the emission
release condition for the scenario of concern. While these subsets will
generally provide maximum concentration estimates, the user may wish to use
the entire range of conditions in Table 2-2 to provide greater assurance that
the maximum concentration is always calculated.
A. Continuous Passive Releases
Concentration estimates for continuous passive (non-dense)
releases should be made by using the applicable procedures in
Table 2-3.
B. Instantaneous Passive Releases
Concentration estimates for instantaneous passive (non-dense)
releases should be made for the following conditions:
I. Ground level releases: use stability class F and 1 m/s wind
speed
II. Elevated releases: use stability classes A, C, and F each
with 1 m/s wind speeds. Calculate concentrations at the
greater of the distance at which the vertical dispersion
coefficient, az, equals H/V2~ (where H = release height) or the
fenceline distance. Select the maximum of these concentration
estimates.
2-8
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TABLE 2-2
WIND SPEED AND STABILITY CLASS COMBINATIONS
10-m Wind Speed
(m/s)
Stability
Class
A (very unstable)
B (moderately unstable)
C (slightly stable)
D (neutral)
E (slightly stable)
F (moderately stable)
1
*
*
*
*
*
*
2
*
*
*
*
*
*
3 4 5 8 10 15 20
*
* * *
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* * * * * * *
* * *
* *
2-9
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TABLE 2-3
CALCULATION PROCEDURES FOR USE WITH VARIOUS
EMISSION HEIGHTS (CONTINUOUS RELEASES)
Height of Emission Stability Classes Wind Speed (m/s)*
I. Stack Height > 50 m**
II. 10 m < stack height < 50 m**
III. Stack height < 10 m and some
plume rise
IV. Stack height < 10 m and no
plume rise; all ground
level sources
V. Wake effects exist
Cavity effects exist ***
A
C
A
C
F
C
F
F
C
F
-
1 and 3
1, 3, 5, 8 and 10
1 and 3
1, 3, 5, 8 and 10
1, 3 and 4
1, 3, 5, 8 and 10
1, 3 and 4
1
1, 3, 5, 8 and 10
1, 3 and 4
10 for critical speed
1 for dilution in
concentration equation
* Use 10 m wind speed adjusted to stack height using the equation in
Section 3.1.1 with the exponents as shown in Table 3-1.
** For these sources, the user should determine the distance to fenceline,
Xfc, and the distance to final plume rise, Xf. If:
Xfc > Xf final plume rise is used. (The distance to the maximum
concentration is found at oz = H/VT". Where, H = release height,
and az = vertical plume dispersion). For each stability class,
the distance is determined using a wind speed of 1 m/s. The
greater of this distance or the distance to the fenceline is
used in the estimate.
Xfc < Xf transitional plume rise is used, and the calculation,: nust
iterate over several distances from Xfc to Xf, usually at 100 m
intervals.
***
Cavity effects are assumed independent of stability classes.
2-10
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C. Dense Gas Releases
I. For a continuous dense gas release, the maximum ground level
concentration usually occurs under light wind speed, stable
atmospheric conditions, but this may vary depending upon
receptor location relative to initial dilution and
gravitational spreading of the heavy gas cloud (spreading is a
function of initial release size and density). Because of the
complexity of making dense gas calculations, hand calculations
are impractical. The use of the EPA Relief Valve Discharge
(RVD) model is recommended (see Section 5.6.3). This model is
not dependent on stability. After running the model using a
range of wind speeds, the maximum concentration is estimated
at the farther of the plume touchdown distance and the
distance to the fenceline.
II. For instantaneous dense gas releases, a simple model is
provided in Section 5.6.2. As a conservative approximation,
elevated releases should be simulated using the ground level
model. The maximum concentration release is obtained by
assuming Gaussian dispersion and using stability class F with
1 m/s wind speed.
The user should carefully examine each release scenario and use the
appropriate approach from those listed above to determine maximum
concentrations. There is no universal approach to use for all cases.
However, Table 2-4 summarizes approaches that may be used for the scenarios
described in the workbook.
2-11
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-------
3.0 SUPPORT DATA FOR SCREENING ESTIMATES
Simulations for air toxic releases require information on the
meteorological conditions at the time of release as well as physical and
chemical parameters describing the materials being released.
3.1 Meteorological Data
Computational procedures for estimating concentrations require data on
wind speed and direction, temperature and atmospheric pressure. These data
are normally collected at National Weather Service stations and some military
installations on an hourly basis. Stability and turbulence parameters can be
estimated from cloud data as described below. A record of these is available
from the National Climatic Data Center, Asheville, North Carolina.
Meteorological data are sometimes recorded at air quality monitoring sites at
existing plants. The use of the on-site data with proper quality assurance
procedures as described in On-site Meteorological Program Guidance for
Regulatory Modeling Applications (EPA, 1987d) is preferred.
3.1.1 Wind Speed and Direction
Wind speed and direction data are required to estimate short-term peak
concentrations. Wind speed is used to determine (1) plume dilution, (2) plume
rise and (3) mass transfer in evaporation models. These factors, in turn,
affect the magnitude of, and distance to, the maximum ground-level
concentration.
Most wind data are collected near ground level. The wind speed at release
height can be estimated by using the following power law equation:
P
/ h i
u =
* i -
zl
3-1
-------
where:
u = the wind speed (m/s) at release height h (m),
MI = the wind speed at the anemometer height z^ (m), and
p = the stability-related exponent from Table 3-1.
TABLE 3-1
WIND PROFILE EXPONENT AS A FUNCTION OF ATMOSPHERIC STABILITY
Rural
Stability Class Exponent
A 0.07
B 0.07
C 0.10
D 0.15
E 0.35
F 0.55
The wind direction is an approximation for the direction of transport of
the plume. The variability of the direction of transport over a period of
time is a major factor in estimating ground-level concentrations averaged over
that time period.
3.1.2 Stability and Turbulence
Stability categories, as depicted in Tables 3-1 and 3-2, are indicators of
atmospheric turbulence. The stability category at any given time depends upon
thermal turbulence (caused by heating of the air at ground level) and
mechanical turbulence (a function of wind speed and surface roughness).
Stability is generally estimated by a method given by Turner (1970), which
requires information on solar elevation angle, cloud cover, cloud ceiling
height, and wind speed (see Table 3-2).
The solar elevation angle is a function of the time of year and the time
of day, and is presented in charts in the Smithsonian Meteorological Tables
(List, 1968). The hourly weather observations of the National Weather Service
3-2
-------
TABLE 3-2
KEY TO STABILITY CATEGORIES
Surface Wind
Speed at a
Height of 10m
(m/sec)
< 2
2-3
3-5
5-6
> 6
Day
Incoming Solar Radiation**
Strong
A
A-B
B
C
C
(Insolation)
Moderate
A-B
B
B-C
C-D
D
Slight
B
C
C
D
D
Night*
Thinly Overcast
or
> 4/8 Low
Cloud Cover
_
E
D
D
D
< 3/8
Cloud
Cover
_
F
E
D
D
The neutral class (D) should be assumed for all overcast conditions during day
or night.
* Night is defined as the period from one hour before sunset to one hour after
sunrise.
** Appropriate insolation categories may be determined through the use of sky
cover and solar elevation information as follows:
Sky Cover
Solar Elevation
Angle > 60°
Solar Elevation Solar Elevation
Angle < 60° Angle < 35°
But > 35° But > 15°
4/8 or Less or
Any Amount of
High Thin Clouds
Strong
Moderate
Slight
5/8 to 7/8 Middle
Clouds (7000 feet to
16,000 foot base)
Moderate
Slight
Slight
5/8 to 7/8 Low
Clouds (less than
7000 foot base)
Slight
Slight
Slight
3-3
-------
include cloud cover, cloud ceiling height, and wind speed. Methods for
estimating atmospheric stability categories from on-site data are provided in
EPA modeling guidelines.
Friction velocity (u^) represents mechanical turbulence due to wind flow
over the surface and is used in calculating release Richardson number.
Friction velocity is a function of stability, decreasing with increasing
stability. An approximation of friction velocity under neutral stability
conditions and assuming a roughness length of 1 cm is:
u^ = 0.06u
where u is the wind speed (m/s) at a height of 10 m.
3.1.3 Temperature
Ambient air temperature must be known in order to calculate the amount of
rise of a buoyant plume and to calculate evaporation rates.
3.1.4 Atmospheric Pressure
Atmospheric pressure data are used in calculating gas and liquid release
rates from storage and process vessels and pipes.
3.2 Chemical and Physical Parameters
Numerous chemical and physical properties of chemicals and spill
substrates are required to use some of the emission estimation techniques
presented. A list of typical physical and chemical properties and their
typical units as used in this workbook is shown in Table 3-3. The complexity
and diversity of chemical and physical behavior of the many air toxic
substances make it critical that the correct input parameters are obtained.
These parameters can be found in compendiums of physical and chemical
characteristics. Three of the more comprehensive sources of information are:
3-4
-------
TABLE 3-3
TYPICAL PHYSICAL AND CHEMICAL PROPERTY PARAMETERS
USED IN EMISSION MODELING
Parameter Name
Typical Units
Boiling point at ambient pressure
Specific heat of liquid
- at constant pressure
- at constant volume
Specific heat of vapor
- at constant pressure
- at constant volume
Molecular weight
Latent heat of evaporation
Vapor pressure
Vapor density
Liquid density
Specific gravity (S.G.)
Constituent diffusivity
(diffusion coefficient)
- in air
- in water
- in oil
Henry's Law constant
Solubility in water
Net heating value
cal/g-mole °K
cal/g-mole °K
cal/g-mole °K
cal/g-mole °K
g/g-mole
cal/g-mole
atm
cm^/s
cm2/s
cm2/s
atm-m^/g-mole
g-mole/m^
cal/g-mole
3-5
-------
Beilstein, 1987: Handbook of Organic Chemistry, Springer-Verlag, New
York.
Green, D., 1984: Perry's Chemical Engineer's Handbook, Sixth
Edition, McGraw-Hill, New York.
Verschueren, K., 1983: Handbook of Environmental Data on Organic
Chemicals. Van Nostrand Reinhold Company, New York.
The user should be cautioned that a characteristic "constant" used in modeling
may have different values depending on the reference from which the parameter
was obtained.
3-6
-------
4.0 SCENARIOS AND TECHNIQUES FOR RELEASE AND EMISSIONS ESTIMATES
Techniques for estimating air toxics emissions must be capable of treating
a large variety of potential release scenarios. This section is intended to
help the user identify the applicable release scenario and determine release
and emission rates and to guide the user through remaining calculations to
arrive at a concentration estimate. Eighteen release scenarios are presented
in this workbook. Descriptions of similar releases are provided, and the user
is advised to review these descriptions if an obvious choice is not apparent
in Table 2-1.
Since so many varied processes and sources have the potential for toxic
chemical releases, the eighteen scenarios cover a small percentage of possible
release, emission, and dispersion combinations. Techniques in this section
estimate emissions to the atmosphere after providing guidance on release
calculations. In these cases and in all other applications, the
characterization of emissions is a critical step which is best met through a
complete and accurate measurement program. In practical applications,
measured data are seldom available and the user is left to techniques such as
those presented in this section, data from existing inventories, emission
factors, or process specific material balance estimates.
Some of the numerous sources of existing data are permit and registration
files, technical literature, and SARA Title III reporting forms. A new data
source summarizing regulatory data is the National Air Toxics Information
Clearinghouse (NATICH) and Data Base. Information on NATICH is available
through EPA Regional Air Toxics Contacts and:
Pollutant Assessment Branch (MD-12)
U.S. Environmental Protection Agency
Research Triangle Park, NC 27111
(919) 541-0850 FTS 629-0850
4-1
-------
For some sources, mass balances are used to estimate releases when
conservative assumptions concerning quantities of input and output streams are
made. The amounts entering and/or leaving a process can be measured or
estimated. A mass balance can then be performed on the process as a whole or
on the subprocess. For processes where material reacts to form a product or
is significantly changed, use of mass balance may be too difficult for
estimating emissions and the use of emission factors may be more appropriate.
When measured or plant specific data are unavailable, the user is advised
to review emission factors developed for specific processes. Appendix A
provides a description of sources of emission factors.
4-2
-------
4.1 Continuous Particulate and Gaseous Releases from Stacks
Similar Releases; Continuous emissions of particulate matter and gases
from building vents, vertical stacks and pipes, or conventional point sources
when emission flowrates and temperature are known. Combustion sources,
chemical reactors, and some process vents are typical emission sources that
emit pollutants through stacks. These releases may also be due to a process
failure such as a rupture disk release or failure of control equipment.
4.1.1 Particulate Matter
Continuous emissions of particulate from stacks are analyzed beginning
with Flowchart C-l. EPA recommends that emission rates from such sources be
determined through source testing using EPA Reference Methods (40 CFR Part 60
Appendix A) or process calculations. If source-specific emissions are not
available, representative emission factors can be substituted. Emission
factors are available for individual toxic compounds (Appendix A, items 1, 2,
and 3). Otherwise, factors determined by compiling extensive source test
results using EPA Reference Methods are reported in AP-42. Toxic components
of emissions can be determined using the Air Emission Species Manual, Volume
II, Particulate Matter Species Profiles (Appendix A, item 4).
Once emissions of the toxic pollutants are determined, this release
scenario represents a case for which ground level concentration estimates can
be made using specific dispersion calculations outlined in Flowchart D-2
(Appendix D). Specific equations to be applied can be found in the report
sections referenced in each flowchart. For point sources, calculations begin
with determining if cavity or wake analysis is applicable as outlined in
Flowchart D-2. If the plume is in the cavity or the wake region, then cavity
and wake impacts must be determined. If the plume is outside of the cavity or
wake regions, then basic point source techniques are applied as shown in
4-3
-------
Flowchart D-3. Section 6.1 provides an example of this scenario. Maximum
concentrations are obtained as shown in Table 2-3.
4.1.2 Gases
Continuous emissions of gases from stacks are analyzed beginning with
Flowchart C-5. Emission factors are available for individual toxic compounds
(Appendix A, items 1, 2, and 3). Otherwise, total VOC emission rates can be
obtained from AP-42 in a similar manner as discussed in Section 4.1.1. Toxic
components of these emissions can be determined using the Air Emission Species
Manual, Volume I, Volatile Organic Compound Species Profiles (Appendix A).
Dispersion techniques for continuous gaseous emissions (Flowchart D-l) use
similar techniques to modeling particulate emissions except that cloud density
calculations are used to determine if dense gas effects may be applicable. If
the gas is not dense, passive point source techniques apply (Flowchart D-2).
4-4
-------
4.2 Continuous Releases of Fugitive Dust
Similar Releases; Any fugitive dust from process losses, generation by
mechanical action in material handling or windblown dust.
These fugitive dust releases are generalized area emissions originating
from a surface or collection of small, poorly quantified point sources. As
indicated in Flowchart C-2, emissions are either user-specified or calculated
with representative emission factors. Emission factors for fugitive dust
emissions are typically found in AP-42 and are assumed to be independent of
wind speed for this workbook. Toxic components can be determined using the
Air Emission Species Manual, Volume II, Particulate Matter Species Profiles
(Appendix A). Dispersion calculations for a continuous area source release
are outlined in Flowchart D-4. For screening, particle settling is assumed to
be insignificant. Virtual distances are determined for calculation of
dispersion parameters, and concentrations are then calculated for the area
source. Maximum concentration is obtained as shown in Table 2-4.
4-5
-------
4.3 Ducting Failures With Dust Releases
Similar Releases: Instantaneous bursts of dust particles due to duct
failure (e.g., pneumatic conveyor line failures), line disconnection,
isolation joint failure, or other types of equipment openings.
Flowchart C-3 indicates that emission estimation techniques are not
available for duct failures and user specification is required. Limited
information on powder releases is available in the technical literature.
Crude estimates of release amounts can be made based on transfer line rates
and time for equipment shutdown and equipment capacity. Modeling for
dispersion is shown in Flowchart D-6. If possible, the user should attempt to
estimate the initial cloud dimension resulting from dilution due to the
mechanical action of the release. If initial dilution can not be estimated by
the user, conservative concentration estimates can be obtained using an
instantaneous point source simulation (also Flowchart D-6). For screening,
particle settling is assumed insignificant. Maximum concentration is obtained
as shown in Table 2-4.
4-6
-------
4.4 Flare Emissions
Flares are used as a control device for a variety of sources. As such,
flares must comply with requirements specified in 40 CFR 60.18. Once
emissions are vented through the flare, a minimum 98% reduction of all
components of the flare must be achieved. Therefore, the user should
calculate the process emissions that feed into a flare and multiply this
number by 0.02 to achieve a conservative estimate of emissions emitted from
the flare. After emissions are determined (Flowchart C-4), the user should
calculate total plume rise from flares (Flowchart D-7). Flame tip height is
calculated and added to the physical stack height to account for the distance
between the flare outlet and the flame tip. Total heat release rate is
calculated prior to use in buoyant plume rise equations. Continuous point
source dispersion techniques are then used to determine dispersion parameters,
buoyancy induced dispersion, and receptor concentrations (Flowchart D-3).
4-7
-------
4.5 Continuous Gaseous Leaks from Tanks/Pipes
Similar Releases; Continuous gaseous emissions due to visible (usually)
holes or openings in tanks, pipes, or flanges (e.g., at pipe connections,
valves, pumps, and compressors).
Emissions due to continuous gaseous leaks from tanks or pipes can be
estimated using the following procedures, as outlined in Flowchart C-6:
Input:
Pa - atmospheric pressure (dynes/cm^)
?t - tank or pipe pressure (dynes/cm^)
A - hole or pipe area (cm^)
MW - molecular weight (g/g-mole)
Tt - tank absolute temperature (deg. K)
K - ratio of specific heat at constant pressure to specific
heat at constant volume
pv - vapor (gas) density (g/cm^)
Xj - mole fraction of each constituent in vapor
YI - weight fraction of each constituent in vapor
Limitations/Assumptions:
- Does not simulate time dependent release rates
- C
-------
Mean specific heats (calculate for both constant pressure, cp,
and constant volume, cv)
n
cmean
The ratio of specific heats is:
K = C
cv mean
A typical value of K at atmospheric pressure is 1.5.
2) Determine if the maximum (critical) release rate is to be
calculated by evaluating the pressure ratio at the release
point:
If:
use subcritical rate equation
use critical rate equation
£t
A typical value for the right hand side is 2.0.
3) For critical flows, calculate:
Release rate (g/s) .
/K+l
V D. D / 1 \
<3v = Cd A
K Pt Pv / 2
\K+1,
4) For subcritical flows:
Release rate (g/s)
qv = Cd A / KPt Pv ' 2
Output',
Vapor venting rate (qv) in g/s for use in dispersion models.
4-9
-------
After emissions are determined, point source dispersion techniques are
applied (Flowchart D-l), including the determination of plume density and
dense gas concentrations, if applicable. For non-dense gas releases, possible
cavity or wake effects are examined (Flowchart D-2). Note, if the user cannot
obtain reliable data to use in these equations, then the use of emission
factors (Appendix A) is suggested as an alternative.
4-10
-------
4.6 Instantaneous Gaseous Releases from Stacks
Similar Releases: Instantaneous gaseous vent releases or gas leaks and
relief valve or rupture disk discharges which are of short duration (e.g.,
less than 5 minutes). These releases may arise from process upsets, chemical
reactor process failures or equipment opening or purges.
Screening methods are not available for estimating emission rates, plume
rise and downwash effects for this release type (Flowchart C-7). Emissions
estimates for this scenario are generally process specific and must be
specified by the user. Limiting estimates of emissions can be determined by
considering the capacity of the source under consideration. For example, the
maximum amount of gas released from a reactor would be the reactant or product
amount.
Dispersion estimates can be obtained by applying the procedures outlined
in Flowchart D-6. If the release is not dense (based on Richardson number
criteria), instantaneous point source dispersion techniques are applied to
obtain concentration estimates. If, however, the instantaneous puff is
determined to be dense, concentrations are determined after including initial
gravitational spreading. A coarse screening estimate for the effects of dense
instantaneous gaseous releases can be performed assuming that the releases are
undiluted and at ground level as indicated in Flowchart D-6.
Screening techniques are available for simulating only spreading and
dispersion from a low momentum, ground-level dense release. Descent of
elevated heavy gas clouds and high momentum associated with many instantaneous
releases tend to provide significant initial dilution. This dilution acts to
reduce concentrations and density. As a conservative estimate, elevated heavy
gas dispersion from instantaneous releases is assumed to be at ground level.
4-11
-------
4.7 Multiple Fugitive Continuous Gaseous Emission Sources
Similar Releases; Releases from any continuous area or volume source
where the emissions are released uniformly over the area or the area
represents a collection of small sources poorly quantified in terms of
location (e.g., multiple vents on large manufacturing buildings, fugitive VOC
sources in refineries or chemical process manufacturing plants).
Fugitive gaseous emissions resulting from collections of small sources and
gaseous area source emissions of different types (e.g., process equipment,
valves etc.) are modeled using techniques shown in Flowchart C-8. The use of
EPA fugitive emission factors for selected equipment are found in the EPA
report Fugitive Emission Sources of Organic Compounds (Appendix A). For
selected air toxics, fugitive factors are also found in Appendix A (items 1
and 3). Often, areas of fugitive emissions can be specified for elevated
releases such as manufacturing facilities where substantial numbers of hood
and vent sources are found on the roof and fugitive emissions identified in
mass balances are suspected from ventilation sources. In these cases, the
area of release can be considered as a volume source using a characteristic
height such as a building height. Dispersion calculations for continuous area
and volume source releases are outlined in Flowcharts D-4 and D-5,
respectively, with relevant calculations referenced by report section for each
procedure. Receptor concentrations are estimated after determining horizontal
virtual distances and corresponding dispersion parameters.
4-12
-------
4.8 Continuous Gaseous Emissions from Land Treatment
Similar Releases: Landfarms; application of a volatile material to soil.
Land treatment emissions are modeled using the techniques outlined in
release Flowchart C-9. The emissions equation is a simplification of the
Thibodeaux-Hwang Emission Model, assuming ground-level application of the
waste, more rapid diffusion through the oil layer, and vapor-liquid
equilibrium between the oil layer and pore spaces.
Input:
D - diffusivity of organic component in air (cmr/s)
A - land treatment surface area (cm^)
hp - depth of soil penetration by waste sludge (tilling depth)
(cm)
t - elapsed time since waste application (s)
P - vapor pressure of the constituent (atm)
MWoii - average molecular weight of the sludge (g/g-mole)
M - total oil application rate (g/cm^)
ppm - grams of organic component per million grams of waste oil
(g/106g)
R - gas constant (82.06 cm^ atm/g-mole K)
T - gas temperature (K)
Limitations/Assumptions:
- Waste is a sludge consisting of organics in oil.
- Methods are a simplification of the Thibodeaux-Hwang Emission
Model (Thibodeaux and Hwang, 1982).
- Assumes no subsurface injection, slower diffusion of organic
component through air-filled pore spaces than through the oil
layer, and vapor-liquid equilibrium between the air in the pore
spaces and the oil layer.
- Assumes that Raoult's Law applies.
- Effective diffusivity can be assumed to be 40% of pure component
diffusivity.
Procedure:
Determine the average emission rate over the entire area, E, in
g/s:
4-13
-------
0.5
. DP MWoil M \ ,
E = ( ) ' ppm A 2 x 1CT6
I 5 hpRT t
Output:
Emission rate, E (g/s) from a land treatment site
- Land treatment area for use in determining virtual distances
for dispersion
Dispersion of land treatment emissions is simulated as an area
source (Flowchart D-4).
4-14
-------
4.9 Continuous Emissions from Municipal Solid Waste Landfills
Gaseous emissions from municipal solid waste landfills may be greater than
those from properly maintained, capped hazardous waste landfills. Therefore
this section presents how gaseous emissions from municipal solid waste
landfills may be estimated. This information is from the draft background
document for proposal of air regulations for municipal solid waste landfills
(EPA, 1988a). This document explains how emissions can be estimated using
either (1) an emission factor based on the amount of refuse in a landfill or
(2) sampling data (e.g., field measurements of the gas flow rate and
composition). The emission factor is based on measuring the amount of VOC per
ton of landfilled waste using data provided by California's South Coast Air
Quality Management District. The total VOC emissions determined by this
procedure can be speciated using a profile from the Air Emission Species
Manual, Volume I, Volatile Organic Compound Species Profiles (Appendix A,
item 4). There are a number of factors contributing to the variability in
gaseous emissions from municipal solid waste landfills (e.g., waste
composition, landfill moisture content, age of refuse, pH and alkalinity of
landfill, amount of buried waste, climate, and physical and operating
characteristics of landfill). The greatest sources of uncertainty are the
type and amount of waste buried in a landfill. Use of sampling data is
strongly recommended (as described in the EPA background document for draft
proposed regulations for municipal solid waste landfills). However, the use
of an emission factor is considered appropriate as a simple screening tool,
and this approach is described below, as outlined in Flowchart C-10.
Input
M - amount of refuse in place in a landfill (millions of tons)
- either the average annual precipitation at the landfill site
or the state in which the landfill is located
4-15
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Limitations/Assumptions:
- Applicable to municipal solid waste landfills.
- Provides an average VOC emission rate. (To obtain the amount
of individual toxic constituents, the concentration of the
individual toxic constituents is needed. The background
document for the proposed regulations provides the range in
concentration of toxic constituents that has been measured from
landfills nationwide.)
- Emission rates are assumed to be steady-state, with no seasonal
or diurnal variation. However, the effect of precipitation on
emission rate is accounted for using an empirical correlation
based on measured data for 20 landfills. (Refer to background
document for draft proposed regulations for further
information.)
Procedure
1) If the landfill site averages less than 23 inches of
precipitation per year (or, in the absence of local data, if
the landfill is located in the States of AZ, CA, CO, HA, ID,
MT, NV, MM, ND, SD, UT, or WY), then use the following equation
to determine the emission rate, E (g/sec):
E = (0.4 g/s/million tons) M
2) If the landfill site averages 23 inches or more of
precipitation per year (or, in the absence of local data, if
the landfill is not in one of the states noted above), then use
the following equation to determine the emission rate, E
(g/sec):
E = (1.0 g/s/million tons) M
Output
E = Average VOC emission rate (g/s)
Dispersion of emissions from a landfill is simulated as an area source
(Flowchart D-4), involving determination of dispersion parameters based on
virtual distances before concentrations can be calculated at each receptor
location.
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4,10 Continuous Emissions of Pesticides and Herbicides
Emissions resulting from the volatilization of applied pesticides or
herbicides are modeled using area source techniques. Generally, screening
level methods are not available for release estimates; therefore, emission
rates must be user-specified as indicated in emission Flowchart C-ll. The
best sources of information are technical literature searches and contacts
with agricultural research stations. Area source dispersion techniques, as
outlined in Flowchart D-4, are used. These involve determining virtual source
distances, dispersion parameters, and estimated concentrations at each
receptor location.
4-17
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4.11 Instantaneous Emissions Due to Equipment Openings
Similar Releases; Any puff or burst type release with short duration
emissions resulting from the opening of equipment after processing (e.g., coke
ovens or chemical reactors), from routine sampling of product during
processing or gaseous emissions from disconnected lines.
Sourjes of this type are modeled as instantaneous point or volume sources
of gaseous emissions due to the momentum of their release. Emissions can
either be estimated on a source-specific basis by the user or calculated from
representative emission factors, as shown in Flowchart C-12. Emissions
estimates are available in AP-42 for some batch operations. VOC profiles are
also available to identify toxic components (Appendix A, item 4). Simple
estimates of emissions from failed or disconnected transfer lines or similar
sources can be calculated from the gas volume between the break point and
nearest shutoff valve.
Receptor concentrations are calculated as outlined in Flowchart D-6. A
density check determines whether the cloud is negatively buoyant or passive.
If passive, dispersion is simulated as a volume source with initial dimensions
dependent on the circumstances of release. If volume dimensions are not
known, conservative concentration estimates can be obtained by assuming a
point source release.
If the cloud is negatively buoyant, dense gas concentrations are
calculated. Screening techniques are available for simulating only spreading
and dispersion from a low momentum, ground-level release. Descent of elevated
heavy gas clouds and high momentum associated with many instantaneous releases
tend to provide significant initial dilution. This dilution acts to reduce
concentrations and density. A conservative screening estimate for the effects
4-18
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of heavy gas, instantaneous releases can be performed assuming that the
releases are undiluted (i.e., point sources) and at ground level as indicated
in Flowchart D-6.
4-19
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4.12 Evaporation from Quiescent or Aerated Surface Impoundments
Similar Releases; Waste lagoons and other impoundments with emissions
resulting from the evaporation of volatile chemicals from liquid mixtures.
Emission rates from well-mixed aqueous waste in surface impoundments are
modeled using techniques outlined in Flowchart C-13. Branching occurs to
provide techniques for both quiescent and aerated impoundments. Emission
estimates account for volatilization solely, with other removal mechanisms
assumed to be negligible. Inputs, assumptions, and calculation procedures for
emission estimates are summarized below.
Input:
Co - initial concentration of the chemical in the waste (g/m^)
S - chemical solubility in water (g-mole/m^)
Vp - pure component vapor pressure (atm)
H - Henry's Law Constant (atm-m^/g-mole)
A - area of impoundment (m^)
F - fetch (linear distance across the impoundment) (m)
D - depth of waste in impoundment (m)
Q - volumetric flow rate of the waste (m^/s)
t - time after disposal (for impoundments with no outlet flow)
POWR - aerator power (horsepower x number of aerators)
f - fraction of impoundment aerated
Mi - number of aerators
Limitations/Assumptions:
- Equations are simplifications of methods in EPA, 1987a for
quiescent surface impoundments with and without flow and for
aeration basins.
- Simplified by assuming a wind speed of 5 m/s, constituent
diffusivity in water of 10~^ cm2/s, and constituent diffusivity
in air of 0.10 cnr/s.
- Assumes waste is well mixed in impoundment.
- Assumes removal entirely by volatilization, with no loss due to
biodegradation, seepage, or adsorption.
- Assumes waste is aqueous, with no separate organic phase.
4-20
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Reference:
- EPA (1987a)
Procedure:
1) Calculate the equilibrium constant, K6g, using
Keq = 40.9H
If H is not available, then it can be approximated as:
H= VP
2) Determine the gas-phase mass transfer coefficient, kg, in m/s
using
kg = 1.26 x 1CT2 A-°-055
3) Calculate the liquid phase mass transfer coefficient, k]_, in
m/s using the appropriate equation
i) if F/D < 14.0
ki = 2.92 x 10~6
ii) if 14.0 < F/D < 51.2
kx = 6.84 x 10~8 (F/D) + 3.35 x 10~6
iii) if F/D > 51.2
k = 6.85 x 10~6
4) Determine the overall mass transfer coefficient, Kq, in m/s
using:
*,.t 1 + * Vl
1 g eq
5) Determine the equilibrium or bulk concentration in the
impoundment, CT, in g/m^ using
CL =
QCO
Kg A + Q
For aerated impoundments, skip to step 7.
4-21
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6) Calculate the source emission rate, E, from the impoundment
(g/s)
E =
Kg CL
For disposal impoundments with no outlet flow, use the same
steps 1 through 4 above and the following equation:
E = AD co [i - exp <-Kgt/D)]
7) Steps 7 through 11 are for aerated impoundments only. For the
aerated impoundment calculate the turbulent liquid-phase mass
transfer coefficient, k}a:
kla = 0.2623 (POWR / A f)
8) Calculate the turbulent gas-phase mass transfer coefficient,
kga = 0.021 (POWR /Hi )°'4
9) Calculate the overall turbulent mass transfer coefficient, K^:
kla Keq kg<
10) Determine the mass transfer coefficient resulting from the
quiescent and turbulent components:
K = Kt f + (1-f) Kq
11) Emissions are obtained by calculating:
E = K CL A
Output
- source emission rate, E, from the impoundment (g/s)
- area of impoundment, A (m^), for use in virtual distance
calculations
Dispersion from a surface impoundment is simulated as a continuous area
source with initial dimensions equal to those of the impoundment (see
4-22
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Flowchart D-4). Virtual distances are used to determine dispersion parameters
which are input to the continuous point source dispersion equation to
determine receptor concentrations.
4-23
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4.13 Continuous Relief Valve Discharges (Two-Phase Flow)
Similar Releases; High pressure superheated liquid releases, continuous
two-phase releases, two-phase releases from stacks.
A pressure relief event is considered to be the venting of process
equipment (i.e., reactor vessels, columns, storage tanks) through rupture
disks, safety relief valves, manual, or emergency vents. An event may consist
of one pressure relief device discharge or a series of pressure relief device
discharges stemming from the same set of circumstances. Pressure relief
emissions can be either liquids or gases or some combination of the two.
Other two-phase releases can result from tank leaks (see Section 4.18). Pure
vapor continuous discharges are simulated using techniques in Section 4.5.
Parameters for relief valve discharges are specified from plant designs. Steps
in calculating two-phase releases are as follows (Flowchart C-14):
1) Calculate the fraction of liquid flash vaporized:
Applicability:
Calculation of the fraction of liquid flash vaporized in the
depressurization of a pressurized liquefied gas.
Inputs
Ts - storage or line temperature of liquid (deg. K)
TJ-J - boiling temperature at ambient pressure (deg. K)
Cp - specific heat at constant pressure (erg/(g deg. K))
L - latent heat of vaporization (erg/g)
Limitations/Assumptions:
- The method does not allow for aerosol "rain-out".
4-24
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References:
- World Bank (1985)
- Wallis (1969)
Procedure:
The fraction of liquid flashed to vapor is given by:
F = CP '^L^
Output:
Fraction of a release which is vaporized to be used in
calculations of cloud density
2) Calculate the mean density of the mixture:
Applicability:
Cloud density of a release containing a liquid aerosol.
Input:
P! - liquid density (g/cm^)
Pv - vapor density (g/cm^)
F - mass fraction of vapor
Limitations/Assumptions:
- The method assumes suspension of liquid droplets.
Reference:
- Wallis (1969)
Procedure:
Calculate the mean mixture density by:
Pm = l
(F/PV)
4-25
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Output:
Density of a vapor/liquid aerosol cloud (g/cm^)
3) Calculate the two-phase outflow:
Applicability:
Two-phase (vapor/liquid aerosol) release rates resulting from
major leaks in the vapor space of a containment for a
pressurized superheated liquid.
Inputs:
A - Release area (cm^)
Pm - mixture density (g/cm^)
PI - line or storage pressure (dynes/cm^)
Limitations/Assumptions:
Assumes homogeneous flow with components in equilibrium.
Pressure is assumed to drop to atmospheric pressure
immediately on release. This is conservative for higher
pressure releases.
C$, the coefficient of discharge, is assumed to be 0.8.
References:
- World Bank (1985)
Hunsaker and Rightmire (1947)
Procedure:
Calculate the discharge rate: (g/s)
qm = 0.76 A Vp
m
Output :
Total discharge rate, g^, in g/s for the liquid/vapor mixture
4-26
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Screening estimates can be obtained using Flowchart D-l assuming that the
density used is that of the mixture. The mass emission rate is equivalent to
the total release rate from the valve or tank. Density checks are performed
to determine whether the resultant plumes may be affected by gravitational
spreading. The dispersion of the resultant cloud assumes no change of state
from the initial value. Continuous two-phase release events are some of the
most frequent scenarios in which dense gas effects may be indicated.
Estimates of plume density will determine if these effects are important. If
the plume is dense, worst case estimates are made using the RVD model for
which a range of wind speeds should be input. If the plume is not dense,
point source techniques are used to simulate dispersion (Flowchart D-2). If
the plume is in the cavity, different procedures may be necessary depending on
receptor location. If the plume is not in the cavity or wake, normal point
source techniques apply.
Initial dilution of high pressure releases should be considered, but
techniques are not currently available. In addition, neither fallout nor the
evaporation of droplets is not included.
4-27
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4.14 Instantaneous Relief Valve Discharges (Two-Phase)
Similar Releases; This class represents any instantaneous two-phase
pressure relief event from relief valves or pressurized tank lines or vessels.
Relief valve or rupture disk discharges and other two-phased releases are
generally site specific and emissions must be specified as indicated in
Flowchart C-15. Atmospheric dispersion screening estimates are made using
instantaneous dispersion modeling techniques.
Dispersion modeling follows a determination of whether the cloud is
affected by negatively buoyant forces as shown in Flowchart D-6. The density
determination is made based on the mean cloud density (see equation in
Section 4.13), and subsequent simulations of dispersion are based on the total
cloud mass. If the puff is not dense, instantaneous point source techniques
apply.
4-28
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4.15 Low Volatility Liquid Leaks from Pipes
Similar Releases; Continuous low volatility liquid leaks from
connectors, flanges, transfer lines or pumps; finite liquid releases from
disconnected transfer lines.
Liquid releases can occur in the form of finite spills or continuous
leaks. For these screening calculations, it is assumed that finite spills are
small and will form a pool with a 1 cm depth. A steady-state evaporation rate
is estimated and is conservatively assumed to persist throughout the
dispersion averaging period. For continuous releases, Flowchart C-16
indicates that if liquid leak rates are unknown, the leak rate equals the
maximum flow rate in the pipe. The released liquid is then assumed to pool in
an area which is the lesser of the unbounded pool spread area, impoundment
boundary or the area over which the liquid spreads before reaching the
impoundment boundary (i.e., pools not contained in the impoundment). Finally,
the steady state emission rate is calculated. Equations for these
calculations are provided below.
Input:
MW - molecular weight (g/g-mole)
Ac - area for confined releases (m^)
Tp - pool surface temperature (deg. K) assumed equal to ambient
temperature
u - wind speed (m/s)
qi - liquid release rate (g/s)
P - vapor pressure (dynes/cm^) at surface temperature Tp
V - finite liquid release volume (m^)
Limitations/Assumptions:
The model is steady state and applicable to single phase
releases.
- Phase change of superheated liquids is not considered.
- Evaporation begins after pool formation.
4-29
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- Minimum pool depth is 1 cm.
- Pool spreading reaches a steady-state when the liquid release
rate equals the evaporation rate.
References:
- NOAA (1988)
Procedure:
1) Calculate the intermediate parameter B consolidating terms of
the model:
A u°-78 MWO-67 p
B = 1.54 x 1(T4
TP
2) Estimate liquid release amount or rate. For finite releases
use a known volume or estimate the released volume as the
volume of the disconnected or failed transfer line. For
continuous liquid leaks, assume that the liquid release rate,
qj, is equal to the mass flow rate in the pipe or line.
3) Calculate the area of the pool. For finite releases:
A = 100 V
where: V = liquid release volume (nH)
Proceed to step 4.
For continuous releases, the area of the evaporating pool is
calculated as the smaller of the impoundment area or the area
at which evaporation across the pool equals flow into the pool.
A = min.
4) The steady state emission rate qv (g/s) is given by:
qv =BAO-94
Output:
- Steady state emission rate (g/s)
- Pool area (m^)
4-30
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Dispersion from the pooled liquid is simulated using continuous area
source techniques (Flowchart D-4). Low volatility liquids are expected to
pool in a ground level area source from which emissions are generated by
evaporation. Pool dimensions are used to determine virtual distances and
dispersion parameters for area source modeling of passive releases.
4-31
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4.16 Low Volatility Liquid Leaks from Tanks
Similar Releases: Low volatility liquid leaks from containment or
reactor vessels (i.e., leak below the liquid level).
As indicated in release Flowchart C-17, liquid release rates from a tank
are computed and used as input to the evaporation model, where pool spread
area and a steady state emission rate are estimated. Equations for the
calculation are given below.
1) Calculate Liquid Release Rate
Input:
P! - liquid density (g/cm^)
A - hole or puncture area (cm^)
Pa - atmospheric pressure (dynes/cm^)
Pt - tank pressure (dynes/cm^)
H - height of the liquid column above the hole (cm)
C<3 - coefficient of discharge
Limitations/Assumptions:
- Does not simulate time dependent release rates for tanks
with decreasing pressure.
- The coefficient of discharge varies between 0.6 and 1.0 as a
function of release geometry and Reynolds number. For
screening purposes, assume C^ = 0.8.
References:
- Environmental Protection Service (1985)
- Hunsaker and Rightmire (1947)
Procedure:
Calculate the liquid flow rate by:
4-32
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Flow rate (g/s)
= 0.8 A P 1960
H + 2 (Pt ~ Pa>
2) Proceed to apply the Evaporation Model from Section 4.15 for
continuous releases beginning in step 1 using the value for
q derived above.
4-33
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4.17 High Volatility Liquid Leaks from Pipes
Small pipe leaks of highly volatile liquids are assumed, for screening
purposes, to boil off instantaneously, resulting in a plume which is simulated
with a continuous point source dispersion model. Emission rate (g/s) is
assumed to equal the pipe flow rate, as indicated in release Flowchart C-18.
Dispersion of the plume is simulated with a continuous point source
dispersion model (Flowchart D-2). For screening estimates, it is assumed that
the leak is in the cavity zone and plume rise calculations are not required.
In addition, due to the small leak size, it is assumed that the release has
low momentum and is passive rather than negatively buoyant (i.e., not a dense
gas).
4-34
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4.18 High Volatility Liquid Leaks from Tanks
Small tank leaks of highly volatile liquids are simulated using an
equation to calculate liquid release rates and an assumption of instant
evaporation, resulting in an emission rate equal to the release rate (see
release Flowchart C-19). The emission rate calculation procedures are given
below. (Note, in the special case of pressurized releases from moderate leaks
below the liquid level of the tanks, techniques used are those in
Section 4.13.)
Applicability:
Calculation of release rate of a liquid from a storage tank or
other vessel with a small leak below the liquid level.
Input:
PI - liquid density (g/cm^)
A - hole or puncture area (cnr)
Pa - atmospheric pressure (dynes/cm^)
Pt - tank pressure (dynes/cnr)
H - height of the liquid column above the hole (cm)
C(j - coefficient of discharge
Limitations/Assumptions:
- Does not simulate time dependent release rates for tanks with
decreasing pressure.
- The coefficient of discharge varies between 0.6 and 1.0 as a
function of release geometry and Reynolds number. For
screening purposes, assume C^ = 0.8.
References:
- Environmental Protection Service (1985)
- Hunsaker and Rightmire (1947)
Procedure:
Calculate the liquid flow rate by:
4-35
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Flow rate (g/s)
= Cd A P! 1960
H + 2 (?t ~ Pa>
Output :
Liguid discharge rate in g/s for use in the dispersion model.
Dispersion of the plume is conservatively simulated with a continuous
point source dispersion model (Flowchart D-2). For screening estimates, it is
assumed that the leak is in the cavity zone and plume rise calculations are
not reguired. In addition, due to the small leak size, it is assumed that the
gas release has low momentum and is passive rather than dense.
4-36
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5.0 ATMOSPHERIC DISPERSION ESTIMATES
This section provides screening techniques and equations to determine
receptor concentrations resulting from toxic releases. Methods are provided
to examine and estimate the impacts of cloud density, plume rise, initial
dilution, and atmospheric dispersion on downwind concentrations. This section
is designed to be used in conjunction with the dispersion flowcharts in
Appendix D. Prior to using this section, the user should estimate emissions
using the release scenario descriptions and emission estimating techniques of
Section 4 along with the flowcharts in Appendix C. The final step of the
appropriate release scenario flowchart (Appendix C) will direct the user to
the first step of the appropriate dispersion flowchart and subsection below.
Dispersion estimates for continuous and instantaneous emissions are
provided by the techniques described in this section. Averaging times
represented in the estimates are determined by the dispersion parameters
used. It is assumed that continuous estimates will result in hourly average
concentrations and that instantaneous estimates represent peak concentrations
averaged over periods of less than a minute. The user is directed to
Appendix E if approximations to other averaging periods are required.
5-1
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5.1 Cloud Densities
5.1.1 Calculations to Determine the Relative Density of Instantaneous
Releases
Applicability:
All instantaneous releases
Input
pv - density of the gas or vapor (g/cm^)
mn - mass of each constituent (g)
Mn - molecular weight of each constituent (g/g-mole)
u - wind speed (m/s)
Ts - temperature of the material released (deg. K)
Ta - ambient temperature {deg. K)
Ms - molecular weight of material released (g/g-mole)
Vi - volume released (m^) (calculated using density and release
amount)
pa - density of air (g/cm^)
Limitations/Assumptions:
It is assumed that neutrally and positively buoyant releases
will be simulated with passive dispersion models.
Releases which are negatively buoyant may disperse as passive
materials if atmospheric turbulent energy exceeds or dominates
buoyancy effects.
References:
Havens and Spicer (1985)
Briggs (Randerson, 1984)
Procedure:
1) Density calculations begin with an estimate of molecular
weight for gas mixtures
MS = £mn
£ (mn/Mn)
2) If:
Ms 28.9
5-2
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dispersion is not affected by negative buoyancy effects and
passive techniques can be used.
3) If the release Richardson number
Ri = 2,722 / MS Ta - A Vi1/3 > 30
28.9 Ts / u2
then effects of the negative buoyancy should be considered.
Otherwise passive techniques can be used. Note that the
following density ratios may be substituted , for the
parenthetical expression above:
Pv - Pa \ or /Pv _
Pa / \ Pa
5.1.2 Calculations to Determine the Relative Density of Continuous Releases
Applicability:
All continuous releases
Input:
mn - mass of each constituent (g)
MH - molecular weight of each constituent (g/g-mole)
u - wind speed (m/s)
Ts - temperature of the material released (deg. K)
Ta - ambient temperature (deg. K)
d - effective diameter (m)
V - volume emission rate (m^/s) specified or calculated from
mass release rate and density
Ms - molecular weight of material released (g/g-mole)
Limitations/Assumptions:
- It is assumed that neutrally and positively buoyant releases
will be simulated with passive dispersion models.
- Releases which are negatively buoyant may disperse as passive
materials if they are small enough such that atmospheric
turbulent energy exceeds or dominates buoyancy effects.
References:
- Havens and Spicer (1985)
- Briggs (Randerson, 1984)
5-3
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Procedures:
1) Density calculations begin with an estimate of molecular weight
for gas mixtures
Ms =
E (mn/Mn)
2) If:
Ms 28.9
dispersion is not affected by negative buoyancy effects and
plume rise and passive modeling procedures should be applied.
Otherwise, proceed to Step 3.
'3) The release Richardson number is calculated:
Ri = 2,722
Ms Ta _ , \ V
28.9 Ts / u3d
where V is the volume release rate which is either specified or
calculated from the mass release rate. The volume rate is
obtained by multiplying the mass release rate by the molar
volume at the release temperature and dividing by the molecular
weight.
V =
_ gv
where MV is the molar volume (m^/mole) and gv is the mass
release rate (g/s). Alternatively, if the vapor (gas) density
is known, the volume rate is the mass rate divided by the
density.
The diameter, d, in the equation is a scale length or effective
diameter. For gaseous releases, it is taken to be stack or
vent diameter. For high volatility liquid releases, it is
taken to be the diameter of a circular plane situated
perpendicular to the direction of material transport by the
wind away from the leak or
d = _ _
The parenthetical terms in the Richardson number equation can
be replaced by the density ratios
/ N
or / Pv -1
Pa I Pa
5-4
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if densities of the vapor (gas) and air are known.
If Ri > 30, the effects of the negative buoyancy should be
considered. Otherwise, plume rise and passive modeling
procedures should be applied.
5-5
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5.2 Plume Rise Calculations
5.2.1 Mean Molecular Weight for Mixtures of Gases
Applicability:
Plume rise must be calculated for a complete effluent stream.
This calculation provides a mean molecular weight to be used in
determining buoyancy flux.
Input:
mn - mass of each constituent (g)
Mn - molecular weight of each constituent (g/g-mole)
Procedure:
Compute the mean molecular weight in (g/g-mole) by the following
Mc = E m"
(mn/Mn)
5.2.2 Flare Plume Rise
Applicability:
Continuous flares
Input:
V - volume release rate from the flare (m^/s)
fi - volume fraction of each component of the flare gas
hs - stack height (m)
HJ - net heating value of each component (cal/g-mole)
Limitations/Assumptions:
- Plume rise for continuous flares must be calculated using
special techniques to account for radiational heat losses and
flame bending in the wind.
- 55 percent of the total heat output of the flare is radiated
and unavailable for plume rise.
- The flare flame is assumed to be tilted 45 degrees from
vertical.
5-6
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- Source height for the flare consists of the physical stack
height and the height from flare outlet to flame tip.
References:
- Beychok (1979)
- Leahey and Davies (1984)
Procedures:
The EPA regulates the design of flares used as control devices in
40 CFR 60.18. In the regulation, minimum values of net heating
value for the combusted gas and exit velocity for flares, and air
and steam assisted flares are specified. It should be confirmed
prior to calculating flare plume rise that the flare is permitted
under this regulation.
1) Calculate the total heat release rate from the flare gas
combustion by:
n
Qt (cal/s) = 44.64 V E fj_ Hi
where the summation is over the n components of the flare gas
stream, fi is the volume fraction, and Hi is the net heating
value of each component.
2) Calculate the vertical flame tip height, hf (meters)
hf = 4.56 x 10~3 Qt°'478
and the effective release height before plume rise (meters) as
hse = hs + hf
3) Calculate buoyancy flux (m^/s3) based on the heat release rate:
F = 1.66 x 10~5 Qt
Output
- hse, flare flame tip height (m)
- F, buoyancy flux
5-7
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5.2.3 Buoyancy Plume Rise
Applicability:
Plume rise for continuous sources with buoyant releases
Input:
mn - mass of each constituent (g)
Mn - molecular weight of each constituent {g/g-mole)
F - buoyancy flux (m^/s^)
x - downwind distance (m)
hs - stack height (m)
Ta - ambient temperature (K)
u - wind speed (m/s)
Ts - stack gas temperature (K)
Ms - mean molecular weight of effluent (g/g-mole)
Vs - stack exit velocity (m/s)
d - diameter of stack (m)
Reference
- Briggs (Randerson, 1984)
Procedures
1) If calculating plume rise for a flare, use the flux determined
in Section 5.2.2 and start with Step 3. For other sources,
compute the mean molecular weight by:
MS = l mn
Mn
2) If calculating plume rise from a flare, use the flux determined
in Section 5.2.2 and start with Step 3. For other sources,
calculate the buoyancy flux, F (mVs^), using either a mixture
or single component molecular weight and the following:
F = 2.45 V.d'f (TS/MS) " (Ta/28'9>
(Tg/Mg)
If F is negative (or zero), the plume is assumed to be passive
and release height, hs, should be- used as the effective plume
height, H, in dispersion calculations (i.e.. Ah = 0 and
continue with Step 6).
If F is positive, the plume is buoyant and plume rise can be
calculated with the procedures that follow.
5-8
-------
3) Determine the distance to final plume rise (meters):
neutral and unstable conditions:
(49 F0.625 for F < 55 m4/s3
119 F°-4 for F > 55 m4/s3
stable conditions:
xf = 2.0715 u s-°-5
where
A0
9.81 _
Az
s =
A0
and equals 0.02 K/m and 0.035 K/m for E and F stability,
Az
respectively.
4) If the receptor distance, x, is less than Xf, calculate plume
rise (meters) for neutral and unstable conditions as:
F0.33 X0.667
Ah = 1.6
u
For x > xf, substitute x = xf in the above equation.
5) Estimate the effective plume height (meters) as
H = hs + Ah
For flares, the stack height hs is defined as hse.
5-9
-------
5.3 Dispersion Parameters
5.3.1 Horizontal and Vertical Dispersion Parameters for Continuous
Emission Releases
Applicability:
Parameters estimating the horizontal and vertical dispersion of a
plume for use in Gaussian dispersion models.
Input:
x - downwind distance (m) for point and area sources
Xy - horizontal virtual distance plus receptor distance (m), for
use with area and volume sources
xz - vertical virtual distance plus receptor distance (m), for
use with volume sources
A-F - stability class
Limitations/Assumptions:
- The parameters were developed from data collected over 10 min.
periods but have been used extensively to provide concentration
estimates for one hour averaging periods
Reference:
- Turner (1970)
Procedure:
1) Figures 5-1 and 5-2 provide estimates of dispersion parameters
versus downwind distance (i.e., source-receptor distance) for
each stability class. Mote that downwind distance in the
figures is given in kilometers.
2) Virtual point source distances for each stability class can be
determined by first locating the value of the dispersion
parameter for each stability class and then locating the
initial distance on the x axis.
3) Dispersion parameters for modeling can be determined by
selecting the value of the dispersion parameter for each
receptor distance and stability.
5-10
-------
10000
5000
0.5 1 5 10
DOWNWIND DISTANCE, km
50 100
FIGURE 5-1. HORIZONTAL DISPERSION PARAMETER (cfy) AS A
FUNCTION OF DOWNWIND DISTANCE AND STABILITY CLASS
(Continuous Releases; Turner, 1970)
5-11
-------
5000
1000
0.1
0.5 1 5 10
DOWNWIND DISTANCE, km
50 100
FIGURE 5-2. VERTICAL DISPERSION PARAMETER (
-------
Output:
°y az ~ horizontal and vertical continuous plume dispersion
parameters for each stability class and receptor
distance (m)
5.3.2 Horizontal and Vertical Dispersion Parameters for Instantaneous
Emission Releases
Input:
x - downwind distance to the receptor (m)
A-P - stability class
Limitations/Assumptions:
- Parameters are given for three stability classes. It is
assumed that the unstable class includes PG classes A - C, the
neutral category includes PG class D, and the stable class
includes PG classes E and F.
- Dispersion parameters represent instantaneous peaks of one
minute duration.
Reference:
- Petersen (1982)
Procedure:
1) Figures 5-3 and 5-4 provide instantaneous dispersion parameters
relative to the puff center for each stability class and
downwind distance (i.e., source-receptor distance). Note that
downwind distance in the figures is given in kilometers.
2) Horizontal dispersion parameters, represented by ax and av, are
assumed to be equal and are given by 0r in the figures.
3) Dispersion parameters can be determined by selecting a value
for each downwind distance and stability class.
Output:
- Vertical, crosswind, and downwind dispersion parameters, az,
Oy, and ox, for each downwind tosition of the puff (m)
5-13
-------
5000
1000
500
100
£ 50
0)
*
0)
E
10
0.5
0.1
' /
X
X
inn
Illl
_
X X
' / =
/
x
X
0.01
0.05 0.1 0.5 1 5 10
DOWNWIND DISTANCE, km
50 100
FIGURE 5-3. HORIZONTAL DISPERSION PARAMETER (
-------
5000
1000
500
100
CO
k
0)
*-
0)
E
0.5
0.1
0.01 0.05 0.1 0.5 1
5 10
50 100
DOWNWIND DISTANCE, km
FIGURE 5-4. VERTICAL DISPERSION PARAMETER (dz) AS A
FUNCTION OF DOWNWIND DISTANCE AND STABILITY CLASS
(Instantaneous Releases; Petersen, 1982)
5-15
-------
5.3.3 Horizontal and Vertical Dispersion Parameters for Wake Effects
Applicability:
Continuous emission point sources
Inputs:
hs - stack height (m)
hfc - building height (m)
W - building width (m)
L - building length (m)
hjn - momentum plume rise (m) at x = 2h^ (see Section 5.6.1)
A-F - stability
Limitations/Assumptions:
For stack to building height ratios (hg/h^) < 1.5, EPA recommends
the use of the Schulman and Scire method in the ISC model.
However, this method is not simple enough to use in this
workbook. The Huber and Snyder technique is used here.
Reference:
- Huber and Snyder technique derived from the ISC model (EPA,
1987c)
Procedure :
1) Determine the maximum projected building width:
hw = (L2 + W2)1/2
2) Determine if the plume will be affected by the wake by
comparing the plume height (hs + hn,) to the depth of the wake
region:
If:
Wake effects are not significant and need not be considered.
Proceed to Flowchart D-3.
3) Determine if the receptor is located in the cavity region:
If:
x < 3 ha
5-16
-------
where:
ha =
for squat buildings (hw >
for tall buildings (hw < hj-,)
then the downwash concentrations cannot be estimated with
screening techniques. Otherwise, proceed with the wake
analysis.
4) Calculate plume dispersion parameters:
0.7 ha + 0.067 (x - 3ha) for 3ha < x < 10 ha
°z =\
az (x + xvz) for x > 10ha
If:
then
1.2 h
0.35 ha + 0.067 (x - 3ha) for 3ha < x < 10ha
(x +
for x > 10h
Otherwise
is obtained from Section 5.3.1.
Xyy and xvz are the horizontal and vertical virtual distances
for wake effects as calculated in Section 5.5.3.
5) Use dispersion parameters from 4) to calculate concentrations
using Section 5.6.4.
Output:
and oz for wake effects.
5-17
-------
5.4 Buoyancy-Induced Initial Dilution
Applicability:
Turbulent motion of buoyant plumes and associated entrainment of
air provides initial dilution of dispersing clouds. Pasguill
(1976) provides a method for computing the effect of this dilution
by modifying the dispersion parameters used in continuous sources
models.
Input:
aZ' °y ~ Dispersion parameters used in modeling (m) from
Section 5.4.1(a)
Ah - Distance dependent buoyant plume rise (m)
Limitations/Assumptions:
For small sources the effect may be negligible.
- For most elevated sources, neglect of this step results in a
less conservative result (i.e., lower predicted concentrations).
Procedure:
1) Calculate the modified horizontal dispersion parameter
ay =
-------
5.5 Virtual Source Distances
5.5.1 Virtual Distances for Area Sources
Applicability:
Continuous or instantaneous releases of pollutants from area
sources or initial clouds of pollutants which cover a significant
near-source area.
Input:
W - area source width (m)
A-F - stability class
Limitations/Assumptions:
- An area source can be represented by a large plume or puff with
initial spreading or dispersion similar to that which would
occur with a point source located at some distance upwind of
the source.
- Centerline concentrations near the area source will be very
conservative.
Reference:
- Turner (1970)
Procedure:
1) Determine the width of the area source and estimate the initial
horizontal dispersion parameter by:
oyo = W / 4.3
2) Determine the downwind distance corresponding to the initial
horizontal dispersion parameter using either Figure 5-1 (for
continuous releases) or 5-3 (for instantaneous releases). This
distance is the virtual point source distance, xv. Convert
this distance to meters.
3) For both instantaneous and continuous sources, dispersion can
be simulated by using the distance, in meters:
Xy = {xr + xv)
where xr is the distance to each receptor from the area source
center. In the calculation of dispersion parameters, the
distance xv is used for determining ay.
5-19
-------
4) In comparisons of the source/receptor distance relative to
position of maximum concentrations, the receptor distance, xr,
should be used.
Output:
- Virtual point source distance and total distance to be used in
determining plume spreading parameters for area sources.
5.5.2 Virtual Distances for Volume Sources
Applicability:
Continuous/instantaneous ground level releases of pollutants
which, as a result of release characteristics or initial
dispersion, can be represented as a cloud with significant volume
at the start of dispersion away from a source location.
Input:
W - cloud or source width
Hv - cloud depth (m)
A-F - stability class
Limitations/Assumptions:
- A pollutant cloud with initial spreading or dilution can be
represented by a point source located sufficiently far upwind
such that the simulated puff or plume is bounded by the initial
cloud dimensions.
- Centerline concentrations within 10 side widths of the volume
source will be conservative.
Reference:
- Turner (1970)
Procedure:
1) Use the techniques of Section 5.5.1 to determine a horizontal
virtual source distance, xv, for use with the volume source.
2) Determine the depth of the volume source and the initial
vertical dispersion parameter by:
azo = Hv / 2.15
5-20
-------
3) From Section 5.3.1 (for continuous releases) or 5.3.2 (for
instantaneous releases), determine the downwind distance
corresponding to the initial vertical dispersion parameter.
This distance is the vertical virtual point source distance.
xvz-
4) For both instantaneous and continuous sources, dispersion
estimates are made by determining the vertical and horizontal
parameters using the distances
Xy = (Xj- + X^)
for the horizontal parameter, and
for the vertical parameter.
Output :
- Virtual source distances and total distances for determining
plume and puff spread parameters to be used in dispersion
models for volume sources.
5.5.3 Virtual Distances for Wake Effects
Applicability:
Continuous emission point sources
Inputs:
hs - stack height (m)
h]-, - building height (m)
W - building width (m)
L - building length (m)
t^ - momentum plume rise (m) at x = Zhj-) (see Section 5.6.1)
u - wind speed (m/s)
A-F - stability
Limitations/Assumptions:
For stack to building height ratios (hg/hj^) < 1.5, EPA recommends
the use of the Schulman and Scire method in the ISC model.
However, this method is not simple enough to use in this
workbook. The Huber and Snyder technique is used here.
5-21
-------
Reference:
- Huber and Snyder technique derived from the ISC model (EPA,
1987c)
Procedure:
1) Determine the maximum projected building width:
1^ = (L2 + W2)1/2
2) Determine if the plume will be affected by the wake by
comparing the plume height {hs + 1^) to the depth of the wake
region:
If:
2.5 hb
1.5 hw + hb
, or
Wake effects are not significant and need not be considered.
Proceed to Flowchart D-3.
3) Determine if the receptor is located in the far wake region:
If:
x > 10 ha
where:
hb for squat buildings (hy, >
hy, for tall buildings (h^ <
then proceed with step 4. Otherwise, wake effects virtual
distances are not needed (proceed with near wake calculations
in Section 5.3.3).
4) Calculate the horizontal and vertical plume
parameters at x = 10ha:
oy = 0.82 ha
dispersion
= 1.2 h,
where:
ha =
for squat buildings (h^, >
for tall buildings (h^, <
5-22
-------
5) Determine the downwind distance corresponding to the enhanced
horizontal dispersion parameter using either Figure 5-1
(continuous releases) or 5-3 (instantaneous). Similarly, use
Figures 5-2 or 5-4 to determine the distance corresponding to
the enhanced vertical dispersion parameters for the stability
category of interest.
6) Use the horizontal virtual distance, xvy, and the vertical
virtual distance, xvz, when calculating wake effects dispersion
parameters for the far wake using Section 5.3.3.
Output:
and xvz for wake effects
5-23
-------
5.6 Concentration Calculations
5.6.1 Cavity Modeling
Applicability:
Concentrations from continuous point sources trapped within the
recirculation zone in the lee of a building.
Input:
hfc - height of building (m)
Lfc - lesser dimension (height or projected width of building)(m)
Ta - ambient air temperature (K)
Ts - stack exit temperature (K)
vs - stack exit velocity (m/s)
d - stack diameter (m)
uc - critical wind speed (m/s)
hs - stack height (m)
W - crosswind building dimension (m)
A - cross-sectional area of building normal to wind (m2)
u - wind speed (m/s)
L - alongwind building dimension (m)
Limitations/Assumptions:
- The model simulates cavity concentrations with a uniform
distribution.
Reference:
- EPA, 1984 (Regional Workshop on Air Quality Modeling:
Summary Report)
Procedure:
1) Compare the stack height to the cavity height. Calculate the
cavity height hc (m):
hc = hb + 0.5(Lb),
If the stack height is greater than or equal to the cavity
height, no further cavity analysis is required. Proceed to
perform the wake effects analysis (Section 5.5.3). If the
stack height is less than the cavity height, proceed to Step 2.
2) Estimate the momentum plume rise for neutral atmospheric
conditions. First compute the momentum flux, Fm (m4/s2):
=
5-24
-------
3) Next, compute the momentum plume rise hm (m)
3Fm*x
b2 u^
where: b = (1/3 + uc/vs).
x = downwind distance (m). Use x = 2hj-,.
Assume a critical wind speed uc = 10 m/s
4) Compute the plume height, Hp (m):
^p = hg + hj^
If the plume height is greater than or equal to the cavity
height calculated in Step 1, then no further cavity analysis is
required. Proceed to the wake effects analysis
(Section 5.5.3). If the plume height is less than the cavity
height, proceed to Step 5.
5) Estimate the downwind extent of the cavity. Compute the cavity
length (xr), measured from the lee side of the building (m):
For short buildings (L/h^Z):
1.0 +
where: a = -2.0 + 3.7 (L/hb)-°-33 and
b = -0.15 + (0.305) (L/hb)-°-33
For long buildings (L/hj-, > 2):
xr = 1.75{W)
1.0 + 0.
25(W/hb)
Next, compare the cavity length to the closest distance to the
plant property line. Consider only plant property to which
public access is precluded. If the cavity does not exceed this
distance, then it may be assumed that cavity effects will not
impact ambient air, and no further cavity analysis is
required. Proceed to the wake effects analysis. If the cavity
extends beyond plant property, proceed to step 6.
6) Estimate impacts within the cavity. "Worst case" concentration
impacts (C) can be estimated by the following approximation
(g/m3):
5-25
-------
where:
Q = emission rate (g/s),
A = cross-sectional area of building normal to wind
(n)2) equals W x h^ and
u = wind speed (m/s).
For u, choose the lowest wind speed likely to result in
entrainment of most pollutants into the cavity. If no data are
available from which the minimum speed can be estimated, assume
a worst case wind speed of 1 m/s.
Since both the cavity concentration and cavity length
calculations depend on building orientation relative to the
wind, it is advisable to repeat the calculations using two
orientations of the building; one with the minimum dimension
alongwind and the other with maximum dimension alongwind and
choosing the highest concentration.
5.6.2 Heavy Gas Model - Instantaneous Releases
Applicability:
The model described is applicable to instantaneous ground level
releases of dense gases in flat terrain. The model neglects
evaporation of condensation of drops and vapors in clouds, heat
and mass exchange with underlying surface, radiation flux and
chemical reactions.
Inputs:
Ro - initial cloud radius (m)
u - wind speed (m/s)
Vo - initial cloud volume (m^)
Pa - density of air (g/m^)
Pq - density of gas (g/m^)
xr - downwind receptor distance (m)
Limitations/Assumptions:
Gases which are heavier than air disperse under the influence of
buoyant and turbulent forces. Models for simple cases of gases
much heavier than air on flat surfaces have been developed and
perform well in the nearfield where buoyant forces clearly
dominate spreading due to atmospheric turbulence. Additional
research is required for determining concentrations in the
transition zone between dominance by buoyant forces and turbulent
mixing.
5-26
-------
The model presented consists of two components, a negatively
buoyant cylindrical spreading model and a passive dispersion
model. The initial spreading of the dense cloud is controlled by
gravitational effects, and the cloud essentially slumps under the
influence of gravity. Slumping is terminated by entrainment of
air. This results in a dense gas cloud which hugs the ground as
it travels downwind. Eventually the cloud of dispersing gas
becomes dilute, and atmospheric turbulence dominates the cloud
growth. Van Ulden (1974) recommends ending the slumping phase
when the frontal velocity becomes less than twice the friction
velocity. At this point, the maximum spread radius is
calculated. This radius is approximate since the simple spreading
model of Van Ulden neglects any entrainment during the slumping
phase.
To approximate dilution due to entrainment, a relationship is
defined which estimates the volume to be added to the cloud as a
result of edge entrainment. This total volume is then used to
calculate the cloud height at the termination of spreading. The
entrainment parameter for edge spreading is given by Cox and
Carpenter (1980). Gravitational spreading is assumed to occur
instantaneously and to be centered at the initial source
location. The resultant cloud is assumed to be cylindrical with
dimensions given by the maximum spread radius and height
determined from the spread volume.
Dispersion under the control of atmospheric turbulence is assumed
to be represented by a passive Gaussian volume source model.
Virtual distances are calculated using expressions similar to
those presented in Section 5.5.2.
Procedure:
Steps in the heavy gas simulation are shown in Flowchart D-6.
1) The maximum spread radius (m) is given by the following
equation using information on the initial cloud size, density,
and wind speed:
14.7 Pg-Pa V0
_ / _ - _
u V pa
V0 is determined from the initial mass released and cloud
density.
2) The total cloud volume (V-p) is calculated based on the maximum
radius and an approximate equation representing the final cloud
volume due to edge entrainment (V):
V = V0
Rmax
1.2
\ Ro
VT = V0 + V
5-27
-------
3) Cloud height can then be calculated assuming a cylindrical
shape:
VT
H =
In the case of very heavy gases, the potential exists for
unrealistically thin clouds to be formed. To bound
calculations in this instance, a minimum depth of 5 cm is
specified. If the calculated cloud height is less than 5 cm,
this height is used in further calculations and the maximum
radius is recalculated.
4) The emission amount (g) for passive calculations is obtained by
multiplying the initial cloud density by the initial cloud
volume.
Qt = v0 Pg
5) Virtual distances for passive dispersion estimates are
calculated with dispersion parameters using:
xvz = (9.3 H) i-64
X~~ t V -4- V J« 1? \
z ~~ *xr r vz *Tnax'
and
xvy = (23.26 Rmax'
X / *r ^ V \
y ~ vAr ~ Avy'
Calculations proceed as in Section 5.6.5 using the mass
emission amount (Qt)' wind speed, virtual distances and
stability.
5.6.3 Heavy Gas Model - Continuous Release
Applicability:
The model described is applicable to short-term ambient
concentration estimates resulting from continuous elevated dense
gas releases occurring at a release height of 10 m or more in flat
terrain. Concentration estimates are applicable for downwind
distances of no more than one kilometer.
5-28
-------
Inputs:
Q - contaminant emission rate (kg/s)
vs - stack exit velocity (m/s)
Ts - stack exit temperature (K)
hs - stack height (m)
d - stack diameter (m)
u - wind speed at stack height (m/s)
y - contaminant concentration in stack exit gas (percent volume)
po - exhaust gas density (kg/rn-^)
M - exhaust gas molecular weight (g/ g-mole)
Mc - contaminant molecular weight(gXg-mole)
x - receptor distance (m)
p - wind speed profile exponents (see section 3.1)
pa - ambient air density (kg/irH)
Ta - ambient temperature (K)
C(x) - centerline ground level contaminant concentration at
downwind distance, x (ug/m^)
Limitations/Assumptions
The Relief Valve Discharge (RVD) model is a screening technique
applicable to denser-than-air gaseous releases. The name of the
model would imply that it is only applicable to pressure relief
valves. However, this is not the case and the model is applicable
to screening analysis of elevated dense gas releases. The model
is based on wind tunnel data and empirical relationships developed
by Hoot, Meroney, and Peterka (1973) using heavy gas tracers in a
non-turbulent environment. Since the wind tunnel experiments most
closely represent stable atmospheric conditions, the
concentrations that occur under unstable or neutral conditions may
be significantly overestimated. The model simulates plume rise
and descent under the control of buoyant forces to provide an
estimate of plume trajectory to the touchdown point and
concentration at the point of touchdown. The dense gas plume
rises at first due to initial upwind momentum from the stack, but
then sinks due to its excess density. Eventually the plume
centerline strikes the ground surface. For screening
calculations, the maximum concentration should be that calculated
at the point of plume touchdown. If this estimate indicates a
problem, more refined techniques such as the DEGADIS model should
be used.
Procedure:
The RVD model is available on a PC compatible diskette from:
Source Receptor Analysis Branch, MD-14, USEPA, RTP, NC 27711. The
use of this computer model is suggested for accuracy. The
following is an abbreviated version of the RVD model for use as an
illustrative tool only.
1) Calculate the release Richardson number R^ as described in
Section 5.1.2. If R^ > 30, then dense gas effects are
considered. Proceed to step 2 below.
5-29
-------
2) Calculate Froude number, Fr:
Po \\°-5
Fr = u
Po-Pa
9.8 d
3) Calculate plume rise, Ah:
u pay
4) Calculate dilution ratio at maximum plume height:
1.85
R = 5.67 x 10'
-
yd2uM\/Ah
5a) Calculate plume molecular weight at maximum plume height:
.. _ M + 29 (R-l)
MH =
5b) Calculate density difference at maximum plume height:
A =
- 1
29
If A < 0.005, use continuous dispersion model for non-dense
gases. Section 5.6.4.
6) Calculate distance at plume touchdown, XT:
XT =
"
+ 0.56 d Fr
/VsPa\
0.5
7) Calculate concentration at plume touchdown,
-1.95
C(xT) = 3.1 x 109
ud2 \ d
8) Downwind concentrations after plume touchdown depend on
critical distance, xc:
. K ,-1.538
'2.045 x 105 Mc\
xc = XT
C(xT)
5-30
-------
If xc < XT, let xc = XT-
If receptor distance x < xc:
\-0.65
C(x) = C(xT)
(-T
\XT/
If receptor distance x > xc:
-0.65 -1.7
C(x) = C(xT)
M /
*T/ \xc
Model results should be reviewed first to determine if the
plume touches down within 1 kilometer. If it does, then an
estimate of touchdown distance and concentration is available.
If touchdown is not indicated within the first kilometer, the
model is performing beyond its scope.
5.6.4 Dispersion Model for Continuous Releases
Applicability:
Simulations of dispersion from continuous point sources and
continuous area or volume sources through application of virtual
point source techniques. May also be applied to sources with wake
effects due to building downwash.
Inputs:
Q - emission rate (g/s)
°z' °y ~ continuous vertical and horizontal dispersion
parameters (m)
u - wind speed (m/s)
Limitations/Assumptions:
- Method provides centerline maximum concentrations with downwind
distance for specific input conditions.
Reference:
- Turner (1970)
Procedure:
Concentrations in g/m^ are provided at each downwind distance, x,
by the following equation.
5-31
-------
C (x) =
ir oz Oy u
exp -0.5
H
The effective dispersion parameters incorporate, where necessary,
initial dispersion due to wake effects as well as area or volume
source releases. Concentrations estimated represent one hour
average values.
5.6.5 Dispersion Model for Instantaneous Releases
Applicability:
Simulations of dispersion from instantaneous point sources and
continuous volume or area sources through application of virtual
point source techniques.
Input:
Qt - release amount (g)
H - effective source height (m)
az' ay ~ instantaneous horizontal and vertical dispersion
parameters (m)
Limitations/Assumptions:
- Maximum ground level concentrations are provided for the center
of each instantaneous puff at selected downwind locations.
- Full surface reflection is assumed.
- Crosswind and downwind dispersion are assumed to be equal.
- The downwind position of the puff is determined by multiplying
wind speed times travel time.
Reference:
- Petersen (1982)
Procedure:
1) Use of the PUFF model (Petersen, 1982) will simplify
calculations.
2) Hand calculations are provided by:
0.127 Qt
C = exp (-0.5 (H/az)2)
5-32
-------
The technique is strictly valid only if travel time to the
receptor from the source exceeds the release duration.
Otherwise, an unrealistically high concentration will result.
5-33
-------
6.0 EXAMPLES
This section provides examples for the release scenarios identified in
Table 2-1 and Section 4. These examples illustrate the solution to
mathematical equations used in the text and may not represent the maximum
short-term ground level concentration estimate from a meteorological
perspective. To obtain these maximum concentrations, the user should follow
the procedures outlined in Section 2.4.
6.1 Continuous Gaseous Emissions from Stacks
Scenario: Hydrocyanic Acid (HCN) is released from a vent stack. Hourly
maximum concentration estimates are required.
Discussion; This example represents a continuous stack release of a vapor
or, similarly, particulate matter at near ambient conditions. The only
difference for particulate matter is that plume density checks would not be
performed. In the example, flow is under the influence of a nearby building.
Emission rates are determined as specified in Section 4.1.2 and shown in
Flowchart C-5. Emission rates must be calculated from process parameters or
determined from representative emission factors. In this case, emissions are
specified. Emission factors are also available (see Appendix A). Dispersion
procedures are outlined beginning in Flowchart D-l.
6.1.1 Building Cavity Example
Source Parameters
MHCN = 27 9/g-mole
Ta = 298 K
Ts = 298 K
d = 0.1 m
V =0.14 m3/s
xrec = distance to property line = 25 m
Q = 9.3 x 10~4 g/s
6-1
-------
hg = 16 m
hjj = height of building = 19 m
L = alongwind building dimension = 19 m
W = crosswind building dimension = 19 m
Ljj = lesser dimension = 19 m
v = stack exit velocity = 4V/(ird2) =17.8 m/s
Sample Estimates
1) (Flowchart D-l and Section 5.1.2). Because HCN (molecular
weight 27) is the primary constituent (besides air) in the gas
stream, and its concentration is low compared to air, the
overall molecular weight of the gas stream can be approximated
by that of air. Alternatively, mean density calculations would
show the gas stream to have a mean molecular weight which is
slightly less than air.
Ms = 28.9 g/g-mole
2) Determine if dispersion is affected by negative buoyancy
(Section 5.1.2).
Ts 298 Ta
Ms 28.9 28.9
Negative buoyancy is not a factor and point source passive
dispersion techniques apply (Flowchart D-2).
3) Determine if plume is in the cavity (Section 5.6.1)
Compare stack height to cavity height (Note that LJ-, = L = W
for this square building):
hc = 19 m + 0.5 (19 m) = 28.5 m
hs = 16 m < hc; therefore, the stack release height is in
the cavity
Calculate the momentum flux and plume rise (Note that stack
exit velocity is obtained from the volumetric flow rate and
the stack diameter):
/ 298 K \ / m \2 2 . ,
Pm = / H 17.8 ] (O.lm) /4 = 0.79 m4/s2
298 K / \ sec
3 (0.79 m4/s2) 2 (19 m)
(0.33 + 10 m/s/17.8 m/s)2 (10 m/s)2
1/3
= 1.0 m
6-2
-------
* Compute plume height and determine if plume is in cavity:
Hp = 16 m + 1 m = 17 m
Hp < hc; therefore plume is in cavity
4} Determine if the receptor is in cavity.
Compute the downwind extent of the cavity
= 19 m/19 m = 1; therefore use the equation for
short buildings:
[-2.0 + 3.7 (19/19)-°-33] 19
xr = = 28.0 m
1.0 + [-0.15 + 0.305 (19/19)-0-33] (19/19)
For receptor = 25 m, receptor is in cavity region. Use cavity
model (Section 5.6.1). The cavity calculation would not be
necessary if the receptor were outside of the plant boundary.
A = (hb) (L) = 19 m x 19 m = 361 m2
9.3 x 10~4 g/s
1.5 (361 m2) (1 m/s)
9.3 x 10~4 g/s CQ o
C = = 1.7 x 10~6 g/m3 = 1.7 ug/m3
6.1.2 Near-Wake Example
In the previous example cavity calculations were required. If in the
example
hs = 28 m
and
x = 100 m,
an alternative path of calculations is necessary, resulting in a near-wake
region concentration estimate. Prom step 3 above:
hs = 28
Hp = 28 + 1.2 = 29.2
!!._, > 23.5 (the cavity height). Therefore, the plume is not in the
cavity and the user should follow Section 5.5.3.
6-3
-------
Sample Estimates
1) liw = ((19)2 + (19)2)1/2 = 26.9 m
Since: hs+hm=28m+l.lm=29.1m
and:
(2.5 (19 m), or
1.5 (19 m) + 19 m,
wake effects are significant.
2) Compare receptor distance to the wake region.
x = 100 m < 10 (19 m)
therefore, near wake equations are used.
3) In Section 5.3.3, all parameters through step 2 are calculated.
In step 3:
x = 100 m > 3 (19),
therefore, the receptor is located in the near wake region.
4) Calculate plume dispersion parameters:
x = 25 m < 10 (19)
therefore:
oz = 0.7 (19 m) + 0.067 (100 m - 3 (19 m)) = 16.18 m
and,
hs + hn, = 29.1 m < 1.2 (28 m)
therefore:
ay = 0.35 (19 m) + 0.067 (100 m - 3 (19 m)) = 9.53 m
5) Calculate concentration using Section 5.6.4:
9.3 x 10~4 g/s
C (100 m) = exp
3.14 (9.53 m) (16.18 m) 1 m/s
= 3.81 x 10~7 g/m3 =38.1 ug/m3
6-4
-0.5
29.1
,16.18,
2
-------
6.1.3 Far-Wake Example
The far-wake region is defined as receptor distances beyond 10 ha or, in
this example, x > 190 m. This section demonstrates a calculation for a
receptor at 200 m from the source.
Sample Estimates
1) The far-wake determination is made in Section 5.5.3, Step 3. In
Step 4, dispersion parameters are calculated along with virtual
distances:
oy = 0.82 (19 m) = 15.6 m
az = 1.2 (19 m) - 22.8 m
giving virtual distances using Figures 5-1 and 5-2 (stability F):
Xyy = 440 m
xvz = 2200 m
2) In Section 5.3.3, check the location of the receptor relative to
the cavity zone. The dispersion parameters are determined using
virtual distances, x^, xvz (stability F):
ay (200 m + 440 m) = 22 m
az (200 m + 2200 m) = 24 m
3) Calculate concentration (Section 5.6.4):
9.3 x 10~4 g/s
C (200 m) =
3.14 (22 m) 24 m (1 m/s)
exp
-0.5
'29.2
24 m
= 2.68 x 10~7 g/m3 =26.8 ug/m3
6-5
-------
6.2 Fugitive Dust
Scenario; Concentration estimates at the fenceline are required for
arsenic emissions resulting from wind erosion from a pile of flyash at a
secondary lead smelter blast furnace.
Discussion; This example demonstrates calculation of particulate
emissions from storage piles and use of particulate matter profiles to study a
specific toxic chemical. Maximum concentration estimates are normally
obtained using the procedures described in Section 2.4. Worst case estimates,
in this case, use conservative assumptions and deviate from those discussed in
Section 4.2 since maximum emissions are wind speed dependent.
Source Parameters
Ash pile: height -3m
diameter - 10 m
Distance to boundary 100 m
Sample Estimates
I) The fugitive dust scenario is presented in Section 4.2, and
estimates follow Flowchart C-2. Fugitive emissions for this
scenario are not directly available. Emissions factors for
aggregate storage are available in AP-42 as are particulate matter
profiles (Appendix A). For this example, the profiles indicate
that arsenic makes up 0.3 percent of fine particles (less than 2.5
microns) emissions mass. The aggregate storage emission factor
for windblown dust is:
(365-p)
E (kg/day/hectare) = 1.9 (s/1.5) (f/15)
235
where:
s - percent silt content
p - number of days with more than 25 mm of precipitation
f - percent of time wind exceeds 5.4 m/s
Since the factor is not directly applicable, conservative
assumptions are made that 20 percent of wind exceeds 5.4 m/s, no
days have precipitation in excess of 25 mm and the silt content is
50 percent. The calculated emission rate is 131.2 kg/day/hectare
6-6
-------
or 0.00015 g/(s m^). Since 0.3 percent of this mass is arsenic,
the emission rate is 4.55 x 10~^ g/(s m^) over 78.5 m^ (irr^ area)
or 3.58 x 10~5 g/s.
2) The pile height is relatively low (3 m) in height, and
conservative estimates will result if area source techniques
(Flowchart D-4) are used. The initial horizontal plume spread
parameter is given by (Section 5.5.1):
<7y = 10 m/4.3 = 2.3 m
3) Meteorological conditions resulting in maximum concentrations for
a ground level area source are low wind speed (1 m/s), stable (F)
conditions. These are assumed for conservatism and provide a
virtual distance from Figure 5-1 of approximately:
xv = 74 m
4) Using this distance with the minimum receptor distance, the
horizontal and vertical dispersion parameters (Section 5.3.1) are
determined from Figures 5-1 and 5-2:
Oy (174 m) = 6.6 m
oz (100 m) = 2.3 m
5) Maximum estimated concentration is obtained from Section 5.6.4
(the exponential term equals unity because H is zero):
3.58 x 10~5 g/s -TOO
C = = 7.5 x ID"7 g/m3 = 0.75 ug/m3
3.14 (6.6 m) 2.3 m (1 m/s)
6-7
-------
6.3 Instantaneous Ejection of Particles from Ducts
Scenario; A failure of a pneumatic conveyor system carrying
3,3-dichlorobenzidine powder from a spray dryer lasted 5 minutes. Estimates
are required for 15-minute average concentrations at receptors greater than
100 m downwind of the source.
Discussion: The scenario represents a class of possible releases from
various types of gas-solid conveyance systems or reactor failures. Common
causes of this type of release are duct failures due to abrasion or failure of
flexible connectors. Short duration events can be simulated as instantaneous
passively dispersing puffs (i.e., all mass was released instantaneously
(within one minute)). The effect of this assumption is a conservative
concentration estimate. In general, powders emitted by this type of release
will consist of relatively large particles (order of 10 microns) which would
be subject to gravitational fallout. Since the screening techniques neglect
deposition and fallout, conservative concentration estimates are expected.
Source Parameters
release height = 10 m
conveyance rate = 2 kg/s
duct diameter = 0.305 m
Sample Estimates
1) Section 4.3 and Flowchart C-3 indicate that duct failure emissions
are typically user estimated and that dispersion calculations
follow point source procedures in Flowchart D-6 (a point source is
assumed because no indication of initial dilution dimensions are
provided in the problem). The release scenario would result in an
initially high rate of emissions which decreases rapidly as line
pressure decreases, as in a pipeline blowdown. One example of a
user calculation is given by assuming that total emissions (0,^)
consist of that material which would normally be conveyed in a 5
minute period, i.e.:
/60 s\ _
Qt = (5 minutes) j J 2 kg/s = 6.0 x 105 g
\rnin /
6-8
-------
2) Since the release is at 10 m, the distance to maximum ground level
concentration (Section 2.4) is approximately that at which az =
H/>5~or 7 m. Distances to maximum concentration for each
stability are determined from Figure 5-4:
Stability Distance (m) az (m) ov (m)
y
unstable 30 73
neutral 240 7 10
stable 3500 7 30
(unstable) (100) (10) (10.5)
The maximum concentration occurs where the product Oy az2 is a
minimum which, for this case, is under unstable conditions. Since
the distance to maximum concentration for unstable conditions is
within the minimum receptor or fenceline distance (100 m),
dispersion parameters for 100 m were also determined.
3) With the unstable dispersion parameters considered, the maximum
concentration occurs with the minimum product of oz Oy2, or during
neutral conditions. Prom Section 5.6.5:
0.127 Qt .
C = exp (-0.5 (H/oz)2)
az 0y2
0.127 (6.0 x 105 g) ,
= exp (-0.5 (10 m/7 m)2)
7 m (10 m)2
= 39.2 g/m3
The resultant peak 15 minute average concentration at the receptor
can be found using techniques in Appendix E:
(900 s) (1 m/s)
N = = 45 giving A = 1
2 (10 m)
A - 0.5
F = = 0.028
(0.3989) 45
The concentration is then:
Cavg = °-028 <7-85
6-9
-------
6.4 Flare Emissions
Scenario; A gas is sent to an elevated flare to be burned. For
simplicity, it is assumed that the flare is permitted. The gas is a mixture
with one toxic component. The gas stream is made up of methane, ethane,
carbon dioxide and benzene. Maximum one-hour concentrations are required for
benzene assuming 98% reduction efficiency of the flare.
Discussion; Flare problems are done in two parts, an emission calculation
and dispersion modeling. Toxic emissions for permitted flares are reduced to
2% of the potential emissions based on a required control efficiency of 98%.
Flare problems are similar to stack examples except that there are buoyancy
flux reductions associated with radiative heat losses and a need to account
for flame length in estimating plume height. Estimates of concentrations
require calculations of heat flux, flame length, plume rise and dispersion.
Source Parameters
Fi = gas composition (volume fraction):
- methane - 0.50
- ethane - 0.098
- carbon dioxide - 0.40
- benzene - 0.002
hs = flare height - 32 m
V = flow rate to flare - 6.58 nrVs
Sample Estimates
The flare scenario is presented in Section 4.4 and Flowchart C-4. Steps
in calculations are as follows:
1) Determine the emission rate of benzene from the volume fraction,
molar volume, flow rate, and molecular weight. The volume of
benzene is the fraction times flow rate:
V(benzene) = 0.002 (6.58 m3/s) = 0.013 m3/s
6-10
-------
Mass emission rate after controls is given by determining the
number of moles in the benzene fraction and multiplying times
molecular weight (the gas is assumed to be at standard conditions)
considering the control efficiency:
(0.013 m3/s) (78.1 g/g-mole) (0.02)
Q = = 0.9 g/s
0.0224 m3/g-mole
2) Calculate the total heat release from the flare (Section 5.2.2).
In this example, carbon dioxide is not combustible and is assumed
not to affect flame heat. Total heat generated by the flame is
determined using mole fractions, molar flow rate, and heats of
combustion for methane, ethane, and benzene (see references of
phys i ca1 cons tant s).
Qt = (44.64 g-mole/m3) 6.58 m3/s [0.5(191,760 cal/g-mole) +
0.098 (341,260 cal/g-mole) + 0.002 (780,922 cal/g-mole)]
= 3.84 x 107 cal/s
3) Compute the flame tip height, Flowchart D-7 (Section 5.2.2):
. _ 0.478
hf = 4.56 x 10~3 (3.84 x 107)
= 19 m
4) Calculate the effective release height before plume rise:
hse = 32 m + 19 m = 51 m
5) Determine the buoyancy flux from the heat release rate
(Section 5.2.3):
F = 1.66 x 10~5 (3.84 x 107)
= 637.4 m4/s3
6) Buoyancy plume rise calculations begin at step 3 of
Section 5.2.3. The distance to final plume rise for unstable
conditions and F > 55 mVs3 is:
xf = 119 (637.4)°'4= 1575 m
The high buoyancy of this release makes determination of the maximum
concentration very complex. The distance to final plume rise at 1568 m would
in most applications be well beyond the facility fenceline distances. As a
6-11
-------
result, the potential exists for the maximum concentration to occur during the
transitional plume rise stage which makes the concentration calculations very
complex. That is, plume rise is a function of wind speed, downwind distance,
and stability, concentration is a function of dispersion parameters, wind
speed and height of release, and dispersion parameters are a function of
stability and downwind distance. The method of determining maximum
concentration available in Section 2.4 requires numerous iterations to
determine the maximum concentration. In this instance, it is recommended that
refined modeling, such as ISC, be used to determine maximum concentrations.
An alternative to refined modeling or iterative solutions is the very
conservative assumption that the maximum plume height equals the stack height
modified by the flame length.
The following steps are included to demonstrate calculations for a single
receptor distance (1 km) and arbitrary meteorological condition (5 m/s and B
stability) typically required in an iteration to determine maximum
concentration.
7) The receptor distance is less than xf and is used to estimate a
transitional plume rise (step 4 of Section 5.2.3):
(637.4)°-33(1000)°-67
Ah = 1.6 = 276 m
5 m/s
Effective plume height is given by:
H = 276 m + 51 m = 327 m
8) Dispersion parameters (Flowchart D-3 and Section 5.3.1) for
receptors at 1000 m are:
Oy (1000 m) = 158 m
az (1000 m) = 110 m
6-12
-------
9) The effects of buoyancy induced dispersion (Section 5.4) are
calculated by:
2 ?,°-5
oy = [(158 m) + (276/3.S)2] = 177 m
2 . 0.5
az = [(110 m) + (276/3.S)2] = 135 m
10) One-hour concentrations can then be calculated at the receptor by
(Section 5.6.4):
0.9 g/s r ,,
exp[-0.5(327 m/135 m)2]
3.14 (5 m/s)(135 m)(177 m)
= 1.28 x 10~7 g/m3 = 0.128 ug/m3
6-13
-------
6.5 Continuous Gaseous Releases from Tanks or Pipes
Scenario; In this example chlorine gas is released from the vapor space
of a pressurized tank through a 2.8 cm diameter hole.
Discussion:
This scenario represents a continuous gaseous release from a pressurized
vessel. The eguations used in the calculations are shown in Section 4.5.
Source Parameters
Chlorine gas:- molecular weight = 70.9 g/g mole
- temperature = ambient = 283 K
- pressure = 6.89 x 10& dynes/cm2
- ratio of specific heats = 1.35
minimum receptor distance = 100 m
Sample Estimates
Calculations for this release are guided by Section 4.5 and
Flowchart C-6. Steps in the calculations are as follows:
1) Release calculations:
Because the release is single component, mean density and mean
specific heat ratios are not required. Calculations begin with a
determination of the emission rate from the leaking tank.
Selection of the equation for release rate depends on the ratio of
tank to atmospheric pressure:
_ = (6.89 x 106 dynes/cm2 / 1.01 x 106 dynes/cm2)
pa
= 6.8
to be compared to the conditional value:
1.35
(1.35 + 1}\(1'35 1}
= 1.86
Since the pressure ratio is greater than the conditional value of
1.86 generated using the specific heat ratio of 1.35, the critical
flow equation is used after calculating vapor density by:
6-14
-------
M Pt
Pv =
R* T
70.9 g/g-mole .(6.89 x 106 dynes/cm2)
(8.31 x 107 dyne-cm/g-mole K) 283 K
and:
= 0.0208 g/cm3
1.35+1
qv = 0.8 (6.16 cm2)/ (1.35)6.89 x 106 dynes/cm2 (0.0208 g/cm3)/ 2 ^1.35-1
,1.35-fl/
= 1261 g/s
Prior to density determination (Flowchart D-l), the volume flow
rate and exit velocity from the tank must be determined. Volume
flow rate can be determined from the vapor density at tank
conditions or, as in this case, from the molecular weight and
molar volume:
1261 g/s m3 1.01 x 106 dynes/cm2 283
v _ . 0.0224 K
70.9 g/g-mole g-mole 6.89 x 106 dynes/cm2 273
=0.06 m3/s
Exit velocity is calculated from the leak area
0.06 m3/s
A).028\2
Vs = =97.5 m/s
3.14
2) Chlorine is substantially more dense than air which can be
confirmed in the first step of Flowchart D-l (Section 5.1.2). In
the second step, a determination is made of whether the density is
sufficient such that buoyant effects will dominate turbulent
mixing in the atmosphere. This is done using the Richardson
number:
70.9 \ 0.06 m3/s
Ri = 2722 f -11
28.9 / (I m/s)3 0.028 m
= 8,477
The value is well in excess of 30 indicating the importance of
heavy gas modeling and the RVD model (Section 5.6.3) is used.
6-15
-------
Table 6-1 provides results for this example beginning with a listing of
model inputs for the version available in August, 1988. The second portion of
the output identifies those cases in which the model is applicable. In this
section, a "0" indicates that the release is passive and the model is
inapplicable, a "1" indicates that the gas is influenced by gravitational
effects and a "2" indicates that the meteorological condition identified is
not likely to occur. The determination of whether the gas is affected by
gravitational effects is made based initially on Richardson number for which a
table is presented. Model results are given in two forms, a table showing
plume rise, touchdown distance, and touchdown concentration for each
meteorological condition and a table of concentrations at specified
receptors. In this example both of these tables are reviewed to determine the
maximum concentration. Since the fenceline is at 100 m, a review of touchdown
distances in excess of 100 m indicates that the maximum concentration is 8.22
g/m^ and occurs at 125 m from the source within stability classes E and F and
2 m/s winds. A review of the table giving the post-touchdown concentration
confirms that this concentration exceeds any fenceline value.
6-16
-------
TABLE 6-1
RVD MODEL RESULTS: CHLORINE GAS LEAK
:hlorine Leak Example
08-05-1988
Input Data
Pollutant emission rate (kg/sec) = 1.261
Exit gas velocity (m/sec)= 97.6
Exit Temperature (K)= 283
Stack Height (m) = 5 Diameter (m) = .028
Pollutant Concentration (volume %) = 100
Exhaust Gas Density (kg/m3) = 3.045529
Exhaust Gas Molecular Weight = 70.9
Exhaust Gas Mass Flow Rate (kg/sec) = 1.261
Pollutant Molecular Weight = 70.9
Molar Volume (m3/mole) = 2.328003E-02
Release duration (sec) = 900 Av. Time (sec) = 900
Wind Speeds (m/sec) = 1.0 2.0 4.0 6.0 8.0
Distance (m) = 100
Ambient Temperature (K) = 283 283 283 283 283 283
Wind Speed Profile Exponents = .15 .15 .2 .25 .3 .3
(Friction Velocity) / (Wind Speed at z=10m)
".06 0.06 0.06 0.06 0.06 0.06
10.0
Dense Gas Behavior
Stability Class
123456
Wind
Speed
1.0
2.0
4.0
6.0
8.0
10.0
(0=Non-Dense Behavior l=Dense Gas Behavior
2=Combinations that cannot occur)
Release Richardson Numbers
1
1
2
2
2
2
1
1
1
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
2
1
1
2
2
2
1
1
2
2
2
2
Wind
Speed
1.0
2.0
4.0
6.0
8.0
10.0
Stability Class
2 3
64534.0
8066.7
1008.3
298.8
126.0
64.5
64534.0
8066.7
1008.3
298.8
126.0
64.5
66809.8
8351.2
1043.9
309.3
130.5
66.8
69165.8
8645.7
1080.7
320.2
135.1
69.2
71604.9
8950.6
1118.8
331.5
139.9
71.6
71604.9
8950.6
1118.8
331.5
139.9
71.6
6-17
-------
TABLE 6-1
RVD MODEL RESULTS: CHLORINE GAS LEAK
Dense Plume Trajectory
Stability Wind Plume Touchdown
Class Speed Rise Distance
(m/sec) (m) (m)
Touchdown
Concentration
(ug/m3) (ppm)
1 1.0
1 2.0
2 1.0
2 2.0
2 4.0
3 1.0
3 2.0
3 4.0
3 6.0
3 8.0
3 10.0
4 1.0
4 2.0
4 4.0
4 6.0
4 8.0
4 10.0
5 2.0
5 4.0
6 1.0
6 2.0
9.2
7.3
9.2
7.3
5.8
9.3
7.4
5.9
5.1
4.7
4.3
9.4
7.5
6.0
5.2
4.7
4.4
7.6
6.0
9.6
7.6
65.85
140.79
65.85
140.79
305.06
63.41
135.50
293.39
464.18
644.90
833.93
61.07
130.41
282.18
446.24
619.76
801.20
125.52
271.40
58.81
125.52
0.11042E+08
0.77962E+07
0.11042E+08
0.77962E+07
0.53969E+07
0.11231E+08
0.79366E+07
0.54998E+07
0.43939E+07
0.37293E+07
0.32747E+07
0.11422E+08
0.80792E+07
0.56044E+07
0.44804E+07
0.38045E+07
0.33420E+07
0.82239E+07
0.57106E+07
0.11616E+08
0.82239E+07
0.38158E+04
0.26940E+04
0.38158E+04
0.26940E+04
0.18649E+04
0.38809E+04
0.27426E+04
0.19005E+04
0.15183E+04
0.12887E+04
0.11316E+04
0.39470E+04
0.27918E+04
0.19366E+04
0.15482E+04
0.13147E+04
0.11548E+04
0.28418E+04
0.19734E+04
0.40141E+04
0.28418E+04
6-18
-------
TABLE 6-1
RVD MODEL RESULTS: CHLORINE GAS LEAK
Concentrations at Specific Receptor Distances
Stability Wind Distance Concentration
Class Speed
(m/sec) (m) (ug/m3)
1.0 100.0 0.54271E+07 0.1875E+04
1.0 100.0 0.54271E+07 0.1875E+04
1.0 100.0 0.51773E+07 0.1789E+04
1.0 100.0 0.49390E+07 0.1707E+04
1.0 100.0 0.47117E+07 0.1628E+04
6-19
-------
6.6 Instantaneous Gas Releases
Scenario; An instantaneous chlorine discharge results from failure of a
transfer line from a pump. An instantaneous chlorine cloud is formed for
which an estimate of the peak concentration at 100 m from the release is
required.
Discussion: This scenario represents any instantaneous gas release which
may result from a vent, stack, pipe or compressor failure, relief valve or
similar case. Dispersion estimates require characterization of the cloud
primarily to determine if calculations should be made using a dense or passive
model. Calculations are guided by Section 4.6 and Flowchart C-7.
Source Parameters
- release temperature - 283 K
- molecular weight - 70.9 g/g-mole
- ambient temperature - 283 K
- transfer line: - diameter - 2 cm
- length in release region -3m
Sample Estimates
1) Because the release is single component, calculations of mean
values do not apply. Begin by calculating the mass released from
the known volume. In other cases, release mass may be known and
volume will be determined. In this example, it is conservatively
assumed that all gas in the isolated pipe segment is available
instantaneously as an emission input to a dispersion model. From
volume considerations:
Vi = 3 m (3.14 (0.02/2)2) = 0.0009 m3
The release mass is determined using either a known density or, in
this case, the molar volume and molecular weight.
(0.0009 m3)(70.9 g/g-mole) 283 K
Qt = = 2.95 g
0.0224 m3/g-mole 273 K
2) Calculate Richardson number as a check on puff density. The
Richardson number is given by:
/70.9 g/g-mole 283 K \ .0009 °-33
Ri = 2,722 ( 1_ - 1] = 391
\28.9 g/g-mole 283 K / (1 m/s)2
6-20
-------
which indicates dense gas modeling should be used.
3) Steps in the heavy gas simulation are given in Flowchart D-6
(Section 5.6.2):
14.7 (1.45 (0.0009 m3))0'5
Rmax = = 0.53 m
1 m/s
where 1.45 is the ratio of the density difference to the density
of air. This is identical to the parenthetical expression in Step
2.
Initial conditions for releases of the type must be assumed. In
this case it is assumed that the gas released forms a hemisphere.
An initial radius for the cylindrical heavy gas model is
determined by equating the hemispherical and cylindrical volumes
and assuming the radii are equal:
irr3 =
2
so that h = r. Then, for the model
3
2 , /3 Vn \0.33
V0 = ir R0J and Ro =|
2 TT
The initial radius is:
3 (0.0009 m3)0'33"
3.14
= 0.077 m
The entrained volume is:
, /0.53 m \L2
V =0.0009m3/ ) = 0.009m3
\ 0.077 m/
VT = 0.0009 m3 + 0.009 m3 = 0.0099 m3
and:
0.0099 m3
H = = 0.0112 m
3.14 (0.53 m)2
6-21
-------
4) H is less than 5 cm so Rmax is recalculated:
= 2.52V0.0099 m3 = 0.25 m
5) The release is sufficiently small such that the initial spread
radius is trivial relative to turbulent spreading and calculations
can be completed using the simple Gaussian puff model without
considering virtual distances.
6) Dispersion parameters for this instantaneous release are
determined using Figures 5-3 and 5-4 (Section 5.3.2) at a distance
of 100 m for a stability of F (as defined in Section 2.4). Peak
concentration is then estimated as indicated in Section 5.6.5:
0.127 (2.95 g)
C = = 0.325 g/m3
(1.2 m)2 0.8 m
6-22
-------
6.7 Continuous Releases of Fugitive Emissions
Scenario; The maximum hourly average concentration estimate is required
for ambient ethylene dichloride at a fenceline receptor 100 meters downwind
from a production facility.
Discussion: Normal production of ethylene dichloride in vinyl chloride
plants results in fugitive emissions from storage and vents. Specific sources
of the emissions cannot be specified. As a result, simulations make use of
emission factors to provide average emissions plantwide. These emissions are
used in a continuous ground level area source dispersion model.
Source Parameters
area of emissions at the plant - 100 m x 100 m
production rate - 204,000 Mg/yr in continuous operation over the year
Sample Estimates
1) Section 4.7 and Flowchart C-8 guide estimates of emissions.
Emissions are obtained from emission factors published by EPA
(1987b). Plant-wide emissions are calculated from the production
rate and an emission factor from various fugitive sources. The
emission factor per production unit is given by:
chlorination vent 0.0216 kg/Mg
column vents 0.06 kg/Mg
process storage vents 0.0003 kg/Mg
process fugitive 0.265 kg/Mg
Total 0.3469 kg/Mg
Total emissions for the plant are given by:
g = 204 x 103 Mg/year (0.3469 kg/Mg) = 70.77 x 103 kg/year
= 2.24 g/s
2) Dispersion estimates are provided by calculations using continuous
area source equations (Flowchart D-4). Fugitive emissions are
generally assumed to be dilute and therefore not dense. The
virtual source distance (Section 5.5.1) using the width of the
square plant area is:
oy = W/4.3 = 100 m/4.3 = 23.2 m
6-23
-------
From Figure 5-1 , under stable (F) conditions:
xv = 620 m
and:
Xy = (620 + 100 m) = 720 m
for receptors at 100 m.
3) Concentrations are calculated using Section 5.6.4:
C = _ exp (-0.5 (H/oz)2)
TT az (100 m) ay (720 m) u
Since the source is at ground level, H = 0 and the exponential term
2.24 g/s , .
C = _ = 1.24 x 10~2 g/m
3.14 (2.3 m) 25 m (1 m/s)
6-24
-------
6.8 Continuous Gaseous Emissions from Land Treatment
Scenario: Sludge containing 1000 ppm benzene is applied to a one acre
land treatment site at a rate of 1 lb/ft2 and filled to a depth of 8 inches.
Determine the one-hour average concentration of benzene 200 m downwind during
the first one hour after application. The following input data are known or
assumed:
D =0.027 cm2/s
P = 0.1244 atm
MWoji = 142 g/g-mole (assumed to be decane)
M =1 lb/ft2 = 0.489 g/cm2
hp = 8 in = 20.3 cm
R =82.06 cm3 atm/g-mole k
T = 298 K
t = 3600 sec
ppm = 1000 g benzene/10^ g oil
A =1 acre = 40,468,564 cm2
xr = 200 m fenceline distance
Discussion; Simulations of emissions from land treatment are handled
using the procedures specified in Section 4.8 and release Flowchart C-9. The
resultant emission rate, along with the area of the land treatment operation,
is then used to determine virtual distance, dispersion rates, and receptor
concentration, as outlined in Flowchart D-4.
Sample Estimates
/
1) Average emissions over first hour after application are estimated
using the eguation of Section 4.8:
^(0.027 cm2/s)(0.1244 atm)(142 g/g-mole)(0.489 g/cm2)^
E = (.
5(20.3 cm)(82.06 cm3 atm/g-mole K)(298 K)(3600 s),
(1000 ppm) (40,468,564 cm2) 2 x 10~6
E = 0.414 g/s
6-25
-------
2) As indicated in Flowchart D-4, dispersion from land treatment
emissions is treated as a continuous, passive area source. The
virtual horizontal distance (Section 5.5.1), based on the
equivalent square dimension of the facility, is used to determine
horizontal dispersion.
estimate ayO
W = (40,468,564 cm2)0-5 = 6361 cm = 63.6 m
CTyo(W) =63.6 m/4.3 = 14.8 m
determine the horizontal virtual distance xv, using avo and
Figure 5-1 (assume stable conditions to ensure peak
concentrations from the ground level source)
xv = .41 km = 410 m (F stability)
calculate receptor plus virtual distance, xv
xy = 200 m + 410 m = 610 m
3) Calculate dispersion parameters (Section 5.3.1) and receptor
concentration (Section 5.6.4)
determine av at the modified horizontal receptor distance, xv
(Figure 5-1)
ay (610 m) = 21 m (F stability)
determine az at the receptor distance (Figure 5-2)
oz (200 m) = 4 m (F stability)
determine receptor concentration
0.414 g/s
C = !
ir (21 m)(4 m)(l m/s)
C = 0.00157 g/m3
6-26
-------
6.9 Municipal Solid Waste Landfill
Scenario: Hourly concentration estimates are required for emissions of
perchloroethylene from a municipal landfill in Ohio.
Discussion; Concentration estimates from landfills are determined using
either emission factors, an emissions model or site-specific measurements
(Flowchart C-1Q). In this example, measurements are not available and the VOC
emission model in Section 4.9 is used. Once VOC emissions are calculated, VOC
emissions profiles (Appendix A, item 4) are used to determine what fraction of
the total is perchloroethylene. Dispersion calculations use a continuous
ground-level area source.
Source Parameters
- Amount of Refuse - 3.8 million tons
- Landfill area - 3 hectares (30,000 m2)
- Distance to nearest offsite receptor (100 m)
Sample Estimates
1) From Flowchart C-10 and Section 4.9, emissions are calculated as:
E(g/s) = (1 (g/s)/million tons) 3.8 million tons = 3.8 g/s
From the emissions profiles, perchloroethylene constitutes 0.3
percent of total VOC emissions or 0.011 g/s.
2) Dispersion estimates use the virtual point source, area approach
with initial horizontal dispersion (Section 5.5.1) given by:
ayo = (30,000 m2)°-5/4.3 = 40.3 m
which, under stable conditions, results in a virtual source
distance of approximately 1,200 m using Figure 5-1.
3) Dispersion parameters (Section 5.3.1) are obtained from
Figures 5-1 and 5-2:
Oy (100 m + 1,200 m) = 44 m
az (100 m) = 2.3 m
6-27
-------
4) The maximum estimated concentration (Section 5.6.4) is calculated
from:
0.011 g/s _ ,
C = = 3.46 x 10~5 g/m3
3.14 (44 m) (2.3 m) 1 m/s
6-28
-------
6.10 Continuous Emissions from an Herbicide
Scenario; 2,4-DB butoxy ethanol ester, a restricted herbicide, is applied
to a farm field of four acres. Maximum post-application one-hour average
concentrations are to be estimated. The property boundary is located 100 m
from the edge of the study field.
Discussion; Pesticide/herbicide dispersion estimates can usually be
obtained by using an emission factor and a ground-level, area source
dispersion model. Emission factors are often difficult to obtain but may be
available from the technical literature or state agricultural agencies.
Unless emissions estimates are sensitive to meteorological conditions, maximum
short-term calculations will be controlled by dispersion. The worst
dispersive conditions for ground level area sources occur under low wind
speed, stable conditions.
Source Parameters
field size - 14 acres (approx. 56,660 m^)
Sample Estimates
Section 4.10 and Flowchart C-ll indicate that emission factors must be
specified since the evaporation rates of herbicides vary widely based on
composition and sources for emission factors are not generally available. The
following provides an example from a typical data source. The best emissions
estimates for 2,4-DB butoxy ethanol ester were found in results of inventory
and flux studies performed by the California Department of Food and
Agriculture, a typical data source. In one study, they determined that half
of the applied pesticide consisted of the active ingredient. Losses through
evaporation were approximately equal for the first two months after
6-29
-------
application at a rate of approximately one third of the applied material per
month. Biodegradation and sequestration were not found to be significant. In
the sample study, data were provided on total regional application,
application rate and acreage of application. An emission factor of
approximately 1.2 Ib active ingredient per acre per month or 1.87 x 10"^
g/h-m2 was also determined.
1) Total emissions for this example are given by the emission factor
times the field area:
g = 56,660 m2 (1.87 1CT4 g/h-m2} = 10.6 g/h = 2.9 x 10~3 g/s
2) Flowchart D-4 indicates that concentration estimates for this
example are made with passive area source dispersion equations.
Worst case meteorology for area source estimates is represented by
low wind speeds, stable (F) conditions. For this example a
virtual source distance (Section 5.5.1) for a square field is
given from:
W = (56,660 m2)0-5 = 238 m
resulting in:
cryo = 238 m / 4.3 = 55 m
From Figure 5-1, the distance at which a stable horizontal
dispersion parameter is 55 m is:
xv = 1700 m
To determine the centerline concentration at 100 m from the field
edge:
Xy \Xj-* T X^)
= (219 m +1700 m) = 1919 m
where xr includes the distance from the field center to the edge
summed with the distance to the property line (100 m)
3) The horizontal dispersion parameter is determined from Figure 5-1
(Section 5.3.1).
oy (1919 m) = 62
m
6-30
-------
The vertical dispersion parameter is determined at the receptor
distance from Figure 5-2.
0Z (219 m) = 4.4 m
4) Concentration (Section 5.6.4) is given by (with H = 0):
Q
C =
ir oz(219 m) oy{1919 m) u
2.9 x 1CT3 g/s , .
= 3.39 x 1CT6 g/m3
3.14 (4.4 m) (62 m) 1 m/s
6-31
-------
6.11 Equipment Openings
Scenario: A common source of emissions due to equipment openings is found
in the production of coke where opening of the ovens at the completion of
processing results in a near instantaneous release. One toxic component of
the emissions is toluene. It is desired to estimate a 15 minute average
concentration at distances beyond 50 m downwind of the source.
Discussion: Emissions from coke ovens result primarily from charging and
discharging operations and fugitive losses which occur on a continuous basis.
The example presented is for the near instantaneous emissions which result
from discharging the completed coke through the oven doors. Sample
simulations are based on the impact of a single furnace although in real
applications total emissions from a battery of ovens over time would be more
typical.
Simulations require determination of an emission factor for the oven and
the total emissions based on oven capacity. Dispersion estimates are made
assuming that the release is instantaneous and within the wake cavity formed
by the oven battery.
Source Parameters
oven door dimensions: - height = 5m
- width = 40 m
coke production per oven = 20 Mg on an 18-hour cycle
Sample Estimates
1) Section 4-11 and Flowchart C-12 guide the user through typical
calculations for equipment openings. To begin, the emissions are
estimated using emission factors (EPA, 1987b). Total toluene
emissions from coke production are 0.48 Ib/ton (0.24 g/kg).
Emissions from door openings must be approximated. From AP-42,
coke pushing emissions account for approximately three percent of
VOC emissions. Coke pushing emissions are then given by
multiplying the emission factor times the production rate:
6-32
-------
Q = 0.24 g/kg coke (0.03) 20,000 kg coke = 144 g toluene
2) Flowchart D-6 shows the path for dispersion estimates if the
release amount is known. Because the release is heated and
insufficient data on component density are available, the cloud is
treated as passive for this example. Estimates of volume
dispersion parameters require some assumptions on the initial
release dimensions (Section 5.5.2). For this example, the coke
oven door dimensions are assumed:
oz = Hv / 2.15 = 5 m/ 2.15 = 2.3 m
oy = W / 4.3 = 40 m/ 4.3 = 9.3 m
From Figures 5-3 and 5-4, under stable conditions:
Xyy = 994 m
xvz
and:
xvz = 542 m
xv = (994 + 50 m) = 1044 m
xz = (542 + 50 m) = 612 m
for receptors at 50 m.
3) Concentrations are calculated using Section 5.6.5 (with H = 0):
Oy = 9.7 m
oz = 2.5 m
0.127 (144 g) _
C = = 0.078 g/m3
2.5 m (9.7 m )2
4) From Appendix E, the 15 minute average (assuming a 1 m/s wind
speed) is given by:
(900) (1 m/s)
N = = 46 giving A = 1
2 (9.7 m)
and
A - 0.5 _
F = = 2.73 x 10~2
46 (0.3989)
and the average peak concentration is:
Cavg = 2.73 x ID"2 (0.078 g/m3) = 2.13 x 10~3 g/s
6-33
-------
6.12 Continuous Gaseous Emissions from Surface Impoundments
Scenario; One-hour concentration estimates of benzene are desired at a
receptor located 200 km downwind of a surface impoundment. The following
known data are relevant to the simulation:
Source Parameters
C0 = 1000 g/m3
H = 5.5 x 10~3 atm-m3/g-mole
A = 1500 m2
F = 43.7 m
D = 1.8 m
Q = 0.0016 m3/s
xr = 200 m fenceline
Discussion; Estimates of emissions from impoundments are determined using
the procedures specified in Section 4.12 and Flowchart C-13. The first
equilibrium constant equation is used because the Henry's Law Constant for
benzene in water is available. The resultant emission rate, along with the
area of the surface impoundment, is then used to determine horizontal virtual
distance, dispersion rates, and receptor concentrations, as outlined in
Flowchart D-4.
Sample Estimates
Emission Calculations - Quiescent Case
1) Determine the equilibrium constant with the first form of the
equation:
Keq = (40.9)(5.5 x 10~3) - 0.225
2) Calculate the gas phase mass transfer coefficient
kg = {1.26 x 10-2)(1500)-°-055 = 0.008 m/s
3) Determine the liquid phase mass transfer coefficient
determine the k^ equation to use based on the fetch to depth
ratio:
6-34
-------
43.7 m
= 24.3
D 1.8 m
Based on the criteria in Section 4.12, the second k^ equation
applies:
. 43.7 , ,
ki = 6.84 x 1CT8 + 3.35 x 1CT6 = 5.01 x 1CT6 m/s
1.8
4) Calculate the overall mass transfer coefficient
1 1 s
Kg=|
= 5.0 x 10~6 m/s
5.01 x 10~6 (0.008)(0.225) )
5) Determine the bulk concentration in the impoundment:
(0.0016 m3/s)(1000 g/m3) ,
CL = = 175.8 g/m3
(5.0 x 10~6 m/s)(1500 m2) + (0.0016 m3/s)
6) Calculate the area source emission rate:
E = (5.0 x 10~6 m/s)(175.8 g/m3)(1500 m2) = 1.32 g/s
Emission Calculations - Aerated Case
An aerated impoundment example is presented as a modification to the
quiescent case. Assume the impoundment has a single 75 horsepower aerator
(i.e., POWR =75), aerating half the impoundment. Repeat steps 1 through 6.
7) Estimate the turbulent liquid-phase and gas-phase mass transfer
coefficients.
kla = 0.2623 (75/U500 m2(0.5» = 0.0262
kga = 0.021 (75/1)0-4 = 0.1181
8) The overall turbulent and complete mass transfer coefficients are
given by:
/i i
Kt ={ + = 0.0132 m/s
\0,0262 0.225 (0.1181)
6-35
-------
K = 0.0132 (0.5) + (1-0.5) 5.0 x 10~6 = 0.0066 m/s
9) Emissions are calculated as:
E = (0.0066 m/s) 175.8 g/m3 (1500 m2)
E = 1,740 g/s
Aeration provides a significantly higher emission rate.
Dispersion Calculations
As indicated in Flowchart D-4, dispersion from surface impoundment
emissions is treated as a continuous, passive area source. The virtual
horizontal distance, based on the equivalent square dimension of the
impoundment, is used to determine horizontal dispersion while vertical
dispersion is handled as with a point source. Concentrations are estimated
with the continuous point source Gaussian equation.
The following demonstrates dispersion for the quiescent example. For the
aerated impoundment, dispersion estimates will differ only by the area source
emission rate.
1) Determine the horizontal virtual distance for the quiescent
impoundment (Section 5.5.1):
estimate avo:
W = (1500 m2)0-5 = 38.7 m
38.7
4.3
ayo = = 9 m
Determine the horizontal virtual distance, xv, using ovo and
Figure 5-1 (assume stable conditions to ensure peak
concentrations from the ground level source):
xv = .24 km = 240 m (F Stability)
calculate receptor plus virtual distance, xv
X = 200 m + 240 m = 440 m
6-36
-------
2) Determine dispersion parameters (Section 5.3.1) and calculate
receptor concentration (Section 5.6.4):
determine ay, at the modified horizontal receptor distance, xy
(Figure 5-1)
ay (440 m) = 16 m (F Stability)
determine az at the receptor distance (Figure 5-2)
az (200 m) = 4 m (F Stability)
determine receptor concentration (where H = 0)
1.32 g/s
C =
ir (16 m)(4 m)(l m/s)
C = 6.6 x 10~3 g/m3
6-37
-------
6.13 Relief Valve Discharge (Two-Phase)
Scenario; The relief valve scenario represents estimates of the maximum
1-hour concentration resulting from a two-phase mixture of chlorine and
suspended chlorine droplets.
Discussion; Two-phase releases can result from both relief valve
discharges and liquid releases from pressurized tanks. Release estimates for
relief valves must be specified, while tank releases can be calculated. The
example shows the use of the RVD model for a heavy gas. Simulation using RVD
requires careful definition of all input parameters to represent the density
of the liquid/vapor mixture. As such, it is possible to use the one-phase RVD
model to simulate aerosol (droplet) dispersion by providing as input the
"equivalent" molecular weight of the aerosol mixture. However, the accuracy
of such a simulation has not been evaluated.
Source Parameters
Chlorine gas: - molecular weight =70.9 g/g mole
- temperature = 249 deg. K
- release rate = 3840 g/s
- exit velocity = 30.4 m/s
- exit diameter = 20 cm
- release height = 10 m
- fraction of release in liquid phase = 20 percent
nearest receptor distance = 50 m
ambient temperature = 283 deg. K
R* = 8.31 x 107 dyne cm /(g-mole K)
Sample Estimates
The methods for estimating concentrations for a two-phase release are
presented in Section 4.13 and Flowchart C-14. Due to design differences in
chemical plant processes, a generic method of obtaining emissions and release
parameters is not available and these parameters must be supplied by the user.
6-38
-------
1) Because the flashed liquid fraction is specified for this example,
the first step in calculations is a determination of mean
parameters of the released gas liquid/stream. The mean density in
this example is given by:
Pm =
(0.2/0.0034) + (0.8/3.214)
= 0.01693 g/cm3
where (from the ideal gas law):
70.9 g/g-mole (1.01 x 106 dynes/cm2) _
pv = = 0.0034 g/cmj
(8.31 x 107 dyne cm/(g-mole K)) 249 K
and from references on physical parameters:
Pl = 3.214 g/cm3
2) An equivalent molecular weight must also be calculated:
Pm R* T
MWe =
P
- / ., dyne-cm \
(0.01693 g/cm3) / 8.31 x 107 \ (249 K)
y g-mole K /
1.01 x 106 dynes/cm2
= 346.8 g/g mole
Volume release rate can be estimated using the mean density and
mass rate.
q 3840 g/s _ _ _
V = = _ = 2.27 x 105 cm3 = 0.227 m3
pm 0.01693 g/cm3
3) Dispersion calculations begin with a check of the importance of
buoyant effects (Flowchart D-l). A ratio test (Section 5.1.2,
Step 2) indicates the plume is dense relative to air. The
Richardson number test provides the same result:
Ms Ta
Ri = 2722 / -1
28.9 Ts
V
/346.S g/g-mole 283 deg.K \ 0.227 m3
= 2722 / , __ -1\
\ 28.9 g/g-mole 249 deg.K / (1 m/s)3 0.2 m
= 39,046
6-39
-------
Table 6-2 provides RVD results (Section 5.6.2) for this example beginning
with a listing of model inputs. The second portion of the output identifies
those cases in which the model is applicable. In this section, a "0"
indicates that the release is passive and the model is inapplicable, a "1"
indicates that the gas is influenced by gravitational effects and a "2"
indicates that the meteorological condition identified is not likely to
occur. The determination of whether the gas is affected by gravitational
effects is made based initially on Richardson number for which a table is
presented. Model results are given in two forms, a table showing plume rise
and touchdown distance and concentration for each meteorological condition and
a table of concentrations at specified receptors. In this example both of
these tables are reviewed to determine the maximum concentration. Since the
fenceline is at 50 m, a review of touchdown distances in excess of 50 m
indicates that the maximum concentration is 4.26 g/m^ and occurs in stability
classes B through E with 4 m/s winds at 71 m. A review of the table giving
post-touchdown concentrations confirms that this concentration exceeds any
fenceline value.
6-40
-------
TABLE 6-2
RVD MODEL RESULTS: CHLORINE TWO-PHASE RELEASE
Chlorine Relief Valve Example
08-16-1988
Input Data
.2
Pollutant emission rate (kg/sec) = 3.84
Exit gas velocity (m/sec)= 30.6
Exit Temperature (K)= 249
Stack Height (m) = 10 Diameter (m) =
Pollutant Concentration (volume %) = 100
Exhaust Gas Density (kg/m3) = 17.08235
Exhaust Gas Molecular Weight = 349.9
Exhaust Gas Mass Flow Rate (kg/sec) = 3.84
Pollutant Molecular Weight = 70.9
Release duration (sec) = 900 Av. Time (sec) = 900
Release pressure (atm) = 4
Wind Speeds (m/sec) = 1.0 2.0 4.0
Distance (m) = 50
Ambient Temperature (K) = 283 283 283 283
Wind Speed Profile Exponents = .15 .15 .2
(Friction Velocity) / (Wind Speed at z=10m) =
0.06 0.06 0.06 0.06 0.06 0.06
6.0
283
.25
8.0
283
.3 .3
10.0
Dense Gas Behavior
Stability Class
123456
Wind
Speed
1.0
2.0
4.0
6.0
8.0
10.0
(0=Non-Dense Behavior l=Dense Gas Behavior
2=Combinations that cannot occur)
Release Richardson Numbers
1
1
2
2
2
2
1
1
1
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
2
1
1
2
2
2
1
1
2
2
2
2
Wind
Speed
l.C
2.C
Stability Class
2 3
4.0
6.0
8.0
10.0
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
38897
4862
607
180
76
38
.9
.2
.8
.1
.0
.9
6-41
-------
TABLE 6-2
RVD MODEL RESULTS: CHLORINE TWO-PHASE RELEASE
Dense Plume Trajectory
Stability Wind Plume Touchdown
Class Speed Rise Distance
(m/sec) (m) (m)
Touchdown
Concentration
(ug/m3) (ppm)
1
1
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
5
5
6
6
1.0
2.0
1.0
2.0
4.0
1.0
2.0
4.0
6.0
8.0
10.0
1.0
2.0
4.0
6.0
8.0
10.0
2.0
4.0
1.0
2.0
13.4
12.6
13.4
12.6
10.0
13.4
12.6
10.0
8.7
7.9
7.4
13.4
12.6
10.0
8.7
7.9
7.4
12.6
10.0
13.4
12.6
13.27
32.83
13.27
32.83
71.84
13.27
32.83
71.84
114.43
159.78
207.45
13.27
32.83
71.84
114.43
159.78
207.45
32.83
71.84
13.27
32.83
0.11362E+08
0.62434E+07
0.11362E+08
0.62434E+07
0.42608E+07
0.11362E+08
0.62434E+07
0.42608E+07
0.33715E+07
0.28414E+07
0.24811E+07
0.11362E+08
0.62434E+07
0.42608E+07
0.33715E+07
0.28414E+07
0.24811E+07
0.62434E+07
0.42608E+07
0. 11362E+08
0.62434E+07
0.39263E+04
0.21575E+04
0.39263E+04
0.21575E+04
0.14723E+04
0.39263E+04
0.21575E+04
0.14723E+04
0.11651E+04
0.98188E+03
0.85737E+03
0.39263E+04
0.21575E+04
0.14723E+04
0.11651E+04
0.98188E+03
0.85737E+03
0.21575E+04
0.14723E+04
0.39263E+04
0.21575E+04
5-42
-------
TABLE 6-2
RVD MODEL RESULTS: CHLORINE TWO-PHASE RELEASE
Concentrations at Specific Receptor Distances
Stability Wind Distance Concentration
Class Speed
(m/sec) (m) (ug/m3) (ppra)
1 1.0 50.0 0.11922E+07 0.4120E+03
1 2.0 50.0 0.30538E+07 0.1055E+04
2 1.0 50.0 0.11922E+07 0.4120E+03
2 2.0 50.0 0.30538E+07 0.1055E+04
3 1.0 50.0 0.11922E+07 0.4120E+03
3 2.0 50.0 0.30538E+07 0.1055E+04
4 1.0 50.0 0.11922E+07 0.4120E+03
4 2.0 50.0 0.30538E+07 0.1055E+04
5 2.0 50.0 0.30538E+07 0.1055E+04
6
6
1.0 50.0 0.11922E+07 0.4120E+03
2.0 50.0 0.30538E+07 0.1055E+04
6-43
-------
6.14 Two-Phase Instantaneous Release
Scenario; An instantaneous pressurized chlorine discharge results in a
flash vaporization forming an instantaneous cloud of 65 percent vapor and 35
percent suspended droplets. An estimate of the peak concentration at 100 m
from the release is required.
Discussion: Release estimates for two-phase instantaneous releases are
beyond the scope of this workbook and data must be specified (Section 4.14 and
Flowchart C-15). Techniques followed for dispersion estimates require
characterization of the cloud dilution and density prior to estimates using a
dense or passive model. Evaporation of droplets in the cloud cannot be
simulated using the workbook. As a result, cloud density is assumed to change
only by dilution.
Source Parameters
- released mass - 50 kg
- molecular weight - 70.9 g/g-mole
- ambient temperature - 283 K
- chlorine boiling point - 239 K
Sample Estimates
1) Calculate the density of the initial cloud assuming the cloud
volume is attributable only to vapor and the temperature is the
boiling temperature of chlorine. The vapor mass is 65 percent of
total mass (50 kg) or 32.5 kg. The cloud volume is calculated
from the mass of vapor using the molar volume:
32,500 g , 239 K
V = 0.0224 m3/g-mole
70.9 g/g-mole 273 K
= 9.0 m3
and the mean density can be calculated as in Section 4.13 or by
simple volume relationships:
pm = mass/cloud volume
pm = 50,000 g/9.0 m3 = 5556 g/m3
6-44
-------
2) Mean molecular weight for an instantaneous release (Section 5.1.1)
is a parameter which would normally be specified. A simple
calculation for this example can be performed by relating th«
total cloud density (vapor + droplets) and the cloud molar volume
to obtain the mean molecular weight;
, , 239 K
Ms = 5556 g/mj (0.0224 nv-Vg-mole) = 109 g/g-mole 8
273 K
Richardson number is given by:
/109 g/g-mole 283 K
Ri = 2,722|
- 1
(9.0 m3)
.9 g/g-mole 239 K / (1 m/s)2
which indicates dense gas modeling should be used.
0.33
= 19,481
3) Steps in the heavy gas simulation are given in Section 5.6.2 and
Flowchart D-6:
14.7
1 m/s
(3.46)(9.0 m3)
0.5
= 82 m
where 3.46 is the ratio of the density difference to the density
in air. This is identical to the parenthetical expression in the
Richardson number (Step 2).
Initial conditions for releases of this type must be assumed. In
this case, it is assumed that the gas released forms a
hemisphere. An initial radius for the cylindrical heavy gas model
is determined by equating the hemispherical and cylindrical
volumes:
irr3 = irr2h
so that h =
r. Then for the model
vo =
ir R03 and Ro =i
'3 Vn\0'33
2 ir
The initial radius is:
3 (9 m3)10-33
2 3.14
= 1.62 m
6-45
-------
The entrained volume is:
r
1.62 ml
VT = 9.0 m3 + 998.6 m3 = 1,007.6 m3
_ / 82 m i _
V = 9.0 m3 / \ = 998.6 m3
and:
1,007.6 m3
H = = 0.05 m
3.14 (82 m)2
4) Instantaneous dispersion estimates are calculated for using a
horizontal virtual point source approach for an area source since
the cloud depth is small (i.e., xz is insignificant):
Xyy = (23.26 82 m)1-12 = 4721 m
Xy. = (100 m + 4721 m) = 4821 m
5) Dispersion parameters (Section 5.3.2) are obtained for F stability
and used to determine concentration (Section 5.6.5):
0y (4821 m) = 38 m
crz (100 m) = 0.8 m
0.127 50,000 g _
C = =5.5 g/m3
(38 m)2 0.8
m
6-46
-------
6.15 Liquid Release from a Pipe
Scenario; Concentrations are estimated from a transfer pipe failure
between unsymmetrical dimethyIhydrazine tanks resulting in an unconfined
liquid pool.
Discussion; This scenario represents cases where liquid is released from
pipes on the ground and, due to its low volatility, pooling occurs.
Evaporation of the liquid results in formation of vapor for which continuous
area source techniques can be used to simulate dispersion. It is assumed for
screening that flow in the pipe is continuous and frictionless. The pool
evaporation rate is assumed to reach a steady state after spreading such that
the evaporation rate equals the pipe flow rate.
Source Parameters
- hydrazine (UDMH): density - 0.786 g/cm3
molecular weight - 60.1 g/g-mole
vapor pressure - 19.88 x 104 dynes/cm2
- flow rate - 0.0001 m3/s
- ambient temperature - 283 K
- minimum receptor distance - 100 m
Sample Estimates
1) The path through calculations is presented in Section 4.15 as
shown in Flowchart C-16. Estimates are calculated separately for
pool spreading and evaporation and subsequent dispersion. The
liquid release rate is given by:
qi = 0.786 g/cm3 (100 cm3/s) = 78.6 g/s
2) Evaporation is calculated using an intermediate parameter of the
evaporation model to determine the area at which the evaporation
rate equals the release rate:
1.54 x 10~4 (1 m/s)°'78 (60.1 g/g-mole)0'67 (19.88xl04 dyne/cm2)
B =
283 K
= 1.683
6-47
-------
78 6
i /o.b _
A =( I = 58.8 m2
1.683
^
Since the spill is continuous and unconfined, no check is required
for determining maximum area (Section 4.15).
3) The areaoof the evaporating pool is used in area source dispersion
calculations using Flowchart D-4. The virtual source distance
(Section 5.5.1) is obtained assuming a square pool:
W = J58.8 m2 = 7.67 m
ay = W/4.3 = 7.67 m/4.3 = 1.78 m
From Figure 5-1, under stable conditions,:
and:
xv = 70 m
xy = (100 + 70 m) = 170 m
for receptors at 100 m.
4) Dispersion parameters from Section 5.3.1 are used for F stability
to determine concentration in Section 5.6.4 (with H = 0).
cry = (170 m) = 7 m
oz = (100 m) = 2.3 m
78.6 g/s _
C = = 1.55 g/m3
3.14 (7 m) 2.3 m (1 m/s)
6-48
-------
6.16 Low Volatility Liquid Releases from Tanks
Scenario; Maximum concentrations are estimated from a leak in an
unpressurized tank containing unsymmetrical dimethylhydrazine (UDMH) .
Discussion; This example represents a scenario where liquid is released
from a storage tank and pools on the ground. Evaporation of the liquid
results in a plume of vapor for which continuous area source techniques can be
used to simulate dispersion.
Source Parameters
UDMH: - density - 0.786 g/cm3
- molecular weight - 60.1 g/g-mole
- vapor pressure - 19.88 x 10^ dynes/cm^
- tank pressure = atmospheric pressure
leak: - 100 cm below liquid level
- 8 cm2 area
meteorology - wind speed - 2 m/s
- temperature - 283 deg. K
- stable (F)
receptor distance - 100 m (fenceline)
impoundment area - 2500 m2
Sample Estimates
The path through calculations is presented in Section 4.16 and
Flowchart C-17. Estimates are calculated separately for release from the
tank, pool spreading and evaporation and subsequent dispersion.
1) Using the release model, the liquid release rate is calculated as:
= 0.8 (8 cm2) 0.786 g/cm3 /1960 (100 cm) + 0
= 2227 g/s
2) Evaporation is calculated using an intermediate parameter of the
evaporation model to determine the area at which the evaporation
rate equals the release rate (as described in Section 4.15):
6-49
-------
n 1.54 x 10~4 (1 m/s)°-78 (60.1 g/g-mole)0-67 (1.988 x 105 dyne/cm2)
a
283 K
= 1.683
/2227\1'°* ,
A =/ \ = 2037 m2
I 1.683 I
The area of the evaporating pool is the smaller of the impoundment
area (2500 m2) and the area at which evaporation across the pool
equals flow into the pool (2037 m2). Therefore, the vapor
emission rate equals the liquid release rate:
qv = q1 - 2227 g/s
Alternatively, qv can be calculated as in Section 4.15.
3) Dispersion calculations follow using Flowchart D-4. The virtual
source distance using the width of a square impoundment is
(Section 5.5.1):
W = /2037 = 45.1 m
oyo = W/4.3 = 45.1 m/4.3 = 10.5 m
From Figure 5-1, under stable (F) conditions,:
xv = 280 m
and: xv = (280 + 100 m) = 380 m
4} Dispersion parameters are determined for stability F in
Section 5.3.1. Concentrations are calculated using Section 5.6.4
(with H = 0):
ay (380 m) = 14 m
az (100 m) = 2.3 m
2227 g/s -
C = = 22.01 g/m3
TT (14 m) (2.3 m) (1 m/s)
6-50
-------
6.17 High Volatility Liquid Release from a Pipe
Scenario: Aminomethane liquid is released from a small pipe, and a
concentration estimate is required for a site with a minimum receptor distance
of 100 m downwind.
Discussion; The high volatility liquid release is intended to represent
calculations for materials which when released immediately volatilize and are
airborne. Estimates of the liquid release rate are required, but the material
can be considered gaseous at the source. A continuous dispersion model can
then be used to estimate downwind dispersion. For pipe releases, the
screening technique is to assume that the volume rate of release equals the
emission to the atmosphere and is determined by the flow volume in the pipe.
Source Parameters
- liquid characteristics - density - 0.69 g/cm^
- molecular weight - 31 g/g-mole
- boiling point - -6.3 deg. C
- liquid mass flow rate - 9,8 g/s
- wind speed - 1 m/s
- ambient temperature - 283 K
Sample Estimates
1) A technique for estimating emissions is provided in Section 4.17
and Flowchart C-19. The release rate is assumed to equal the pipe
flow rate:
qi = 9.8 g/s
In this example, it is assumed that aminomethane has a
sufficiently high vapor pressure that evaporation from a small
leak is instantaneous and no pooling results. Such an
instantaneous vaporization results in a vapor cloud which is
roughly at the boiling point of the liquid. This assumption could
be checked using techniques presented in Section 4.15.
2) Simulations of dispersion follow Flowchart D-l for passive
continuous releases. A check of plume density shows that the
release at the boiling temperature is slightly more dense than air
(Section 5.1.2). Calculation of the Richardson number as a
6-51
-------
density check requires a determination of the volume release rate
using the molar volume and molecular weight:
9.8 g/s (0.0224 m3) (267 K)
V = !
31 g/g-mole g-mole (273 K)
= 6.9 x 10~3 m3/s
and effective diameter:
/4 (6.9 x 10~3)
d =/ . = 0.094 m
\ 3.14 (1 m/s)
The Richardson number is given by:
(31 g/g-mole) (283 K) \ 0.0069 m3/s
Ri = 2722 / - 1
28.9 g/g-mole (267 K) / (1 m/s)3 (0.094 m)
= 27
A value of less than 30 confirms that the release is essentially
passive.
3) Since, in this scenario, no pooling occurs, point source
dispersion calculations (Flowchart D-2) can be used to calculate
concentration. No information is available on the setting of the
leak relative to the tank dimensions or other structures so that
the estimate cannot evaluate the effects of downwash. In the
absence of this information, the most conservative assumption is
to consider the source a continuous ground level point source. In
addition, because the release is not heated, no buoyancy plume
rise occurs (Section 5.2.3).
4) Point source dispersion calculations (Flowchart D-3) begin with
specification of dispersion parameters (Section 5.3.1) for F
stability:
Oy (100 m) = 4 m
oz (100 m) = 2.3 m
5) Because there is no plume rise, buoyancy induced dispersion
(Section 5.4) does not apply. Concentration is estimated by
(Section 5.6.4 with H = 0):
9.8 g/s .
C = _ = 0.339 g/m3
3.14 (4 m) (2.3 m) 1 m/s
6-52
-------
6.18 High Volatility Liquid Releases from Tanks
Scenario: Aminomethane liquid is released from a minor tank leak' and a
maximum concentration estimate is required for a fenceline receptor 100 m
downwind.
Discussion; The high volatility liquid release is intended to represent
calculations for materials which, when released, immediately evaporate (no
pooling results). In this example, it is assumed that aminomethane has a
sufficiently high vapor pressure so that evaporation from a small leak is
instantaneous and no pooling results. Estimates of the liquid release rate
are required, but the material can be considered gaseous at the source. This
scenario shares many of the calculations found in example Section 6.17.
Differences lie only in the release and emission calculations. In both,
steady-state release estimates are used in a dispersion model. In a refined
analysis, the examples would show different release rates as pressure in the
transfer line or tank is reduced. A continuous dispersion model can then be
used to estimate downwind dispersion.
Source Parameters
liquid characteristics - density - 0.69 g/cm^
- molecular weight - 31 g/g-mole
- boiling point 6.3 deg. C (267 deg. K)
tank pressure - 2.03 x 10^ dynes/cm^
atmospheric pressure - 1.01 x 10^ dynes/cm^
ambient temperature - 283 deg. K
release depth - 100 cm below liquid level
release area - 0.01 cm^
wind speed - 2 m/s
stability - neutral
Sample Estimates
1) Techniques to estimate emissions are provided in Section 4.18 and
Flowchart C-19. The release rate is calculated by:
6-53
-------
= 0.8(0.01 cm2) 0.69 g/cm3
/ 1960 (100 cm) + 2(2.03 x 106 - 1.01 x 106) dynes/cm2
> 0.69 g/cm3
= 9.8 g/s
2) Simulations of dispersion follow Flowchart D-l for continuous
releases. No information is available on the location of the leak
relative to the tank dimensions or other structures so that the
estimate cannot evaluate the effects of downwash. In the absence
of this information, the most conservative assumption is to
consider the source a continuous ground level point source.
A check of plume density shows that the release (31 g/g-mole) at
the boiling temperature is slightly more dense than air (28.9
g/g-mole). Thus, a check of the Richardson number ('Section 5.1.2)
must be made.
The volume rate is calculated at ambient temperature using the
molar weight and gas molar volume at ambient temperature:
9.8 g/s / m3 \ /267 K
V = I 0.0224 1 (
31 g/g-mole \ g-mole / V 273 K
= 6.9 x 10~3 m3/s
An equivalent diameter is calculated:
4V
d =]
iru
4 (6.9 x lO-3)
= / =0.094
\ (3.14) 1 m/s
Then,
(31 g/g-mole)(283 deg. K) \ 0.0069 m3/s
Ri = 2722 / -1] = 27
28.9 g/g-mole (267 deg. K) / (1 m/s)3 0.094 m
Since Ri < 30, the release is essentially passive.
3) As with Example 6.17, no downwash, plume rise,, or buoyancy induced
dispersion apply. Therefore, once dispersion parameters are
determined (Flowchart D-3, Section 5.3.1), receptor concentration
can be calculated (Section 5.6.4):
<7y (100 m) = 4 m
6-54
-------
o~ (100 m) = 2.3 m
C =
9.8 g/s
3.14 (2.3 m) (4 m) 1 m/s
= 0.339 g/m3
6-55
-------
REFERENCES
Beilstein, 1987: Handbook of Organic Chemistry. Springer-Verlag, New York.
Beychok, M., 1979: Fundamentals of Stack Gas Dispersion, Irvine, CA
Cox, A. and R. Carpenter, 1980: Further Development of a Dense Vapour
Dispersion Model for Hazard Analysis. Heavy Gas and Risk Assessment, S.
Hartwig (ed.) D. Reidel Publishing, Dordrecht, Holland.
Environmental Protection Service, 1985: Introduction Manual, Technical
Information for Problem Spills (TIPS), Technical Services Branch. Ottawa,
Canada
Fingas, M. , I. Buist, R. Belore, D. Mackay, and P. Kawamura, 1986: The Input
of Spilled Chemicals into the Environment. Hazardous Materials Spills
Conference, St. Louis.
Green, D., 1984: Perry's Chemical Engineers Handbook. McGraw-Hill, New York.
Havens, J. and T. Spicer, 1985: Development of an Atmospheric Dispersion
Model for Heavier-than-Air Gas Mixtures. U.S. Dept. of Transportation
CG-D-23-85.
Hoot, T., R. Meroney, and J. Peterka, 1973: Wind Tunnel Tests of Negatively
Buoyant Plumes. EPA 650/3-74-003.
Hunsaker, J. and B. Rightmire, 1947: Engineering Applications of Fluid
Mechanics. McGraw-Hill, New York.
Leahey, D. and M. Davies, 1984: Observations of Plume Rise from Sour Gas
Flares, Atm. Envir., 18:917-922
List, R. , 1968: Smithsonian Meteorological Tables. Smithsonian Institute,
Washington, D.C.
National Oceanographic and Atmospheric Administration, 1988: ALOHA - Areal
Locations of Hazardous Atmospheres, Technical Appendix, Hazardous
Materials Response Branch, Seattle, WA.
Pasguill, F., 1976: Atmospheric Diffusion (2nd ed.). John Wiley & Sons, New
York.
Petersen, W. , 1982: Estimating Concentrations Downwind from an Instantaneous
Puff Release. U.S. EPA, ESRL.
Slade, D., 1968: Meteorology and Atomic Energy, U.S. Atomic Energy Commission
(T10-24190).
Thibodeaux, L. and S. Hwang, 1982: Landfarming of Petroleum Wastes - The
Modeling Problem, Environmental Progress, 1(46).
Turner, D., 1970: Workbook of Atmospheric Dispersion Estimates, Office of Air
Programs Publication AP-26. U.S. Environmental Protection Agency.
R-l
-------
U.S. Environmental Protection Agency, 1986: Compiling Air Toxic Emission
Inventories. EPA-450/4-86-010.
U.S. Environmental Protection Agency, 1987a: "Hazardous Waste Treatment,
Storage and Disposal Facilities (TSDF) - Air Emission Models", Draft
Report, U.S. EPA OAQPS, April, 1987.
U.S. Environmental Protection Agency, 1987b: Emission Factors for Equipment
Leaks of VOC and HAP. EPA-450/3-86-002.
U.S. Environmental Protection Agency, 1987c: Industrial Source Complex (ISC)
Dispersion Model User's Guide, Second Edition (Revised), Volume 1 and 2.
EPA-450/4-88-002a and 002b.
U.S. Environmental Protection Agency, 1987d: On-Site Meteorological Program
Guidance for Regulatory Modeling Applications, 1987. EPA-450/4-87-013.
U.S. Environmental Protection Agency, 1988a: Air Emissions from Municipal
Solid Waste Landfills - Background Information for Proposed Standards and
Guidelines, Office of Air Quality Planning and Standards (Preliminary
Draft).
U.S. Environmental Protection Agency, 1988b: Procedures for Evaluating Impact
of Stationary Sources, Braft Report, September 1988.
f - , . - - *",' *
Van Ulden, A. 1974: On Spreading of a Heavy Gas Released Near the Ground, 1st
International Loss Prevention Symposium, The Hague/Delft.
Verschueren, K., 1983: Handbook of Environmental Data on Organic Chemicals.
Van Mostrand Reinhold Company, New York.
Wallis, G., 1969: One Dimensional Two-Phase Flow, McGraw-Hill, New York.
World Bank, 1985: Manual of Industrial Hazard Assessment Techniques. Office
of Environmental and Scientific Affairs, Washington.
R-2
-------
APPENDIX A
EMISSION FACTORS
-------
APPENDIX A
EMISSION FACTORS
One alternative for estimating air toxic emissions from sources is through
the use of emission factors. Emission factors have been developed for a
number of processes and pollutants. Emission factors provide an estimate of
emissions as a function of source activity such as process rate or some other
operating parameter. Emission factors are intended to be used for making
preliminary estimates of toxic air emissions. As sach, they represent generic
factors whose applicability to a specific source may be questionable. These
factors will not likely provide exact estimates of emissions from any
particular source. The source of an emission factor must be carefully
evaluated to determine whether it is applicable to a particular facility.
Emission factors are available for both area and point sources. Some sources
of emission factors applicable to air toxics emissions are presented below.
1) U.S. Environmental Protection Agency. Locating and Estimating Air
Emissions from Sources of (Substance). EPA 450/4-84-007a-q.
EPA has underway a program to compile and publish emission factors for
various air toxics. To date, sixteen reports have been published as part of
this program. The substances covered by this series include: acrylonitrile,
carbon tetrachloride, chloroform, ethylene dichloride, formaldehyde, nickel,
chromium, manganese, phosgene, epichlorohydrin, vinylidene chloride, ethylene
oxide, chlorobenzenes, PCBs, POM, and benzene.
2) U.S. Environmental Protection Agency. Survey of (Substance) Emission
Sources.
A second series of reports on specific air toxics has been developed by
EPA as part of the National Emissions Standards For Hazardous Air Pollutants
(NESHAPS) program. The substances covered by this series include:
trichloroethylene (EPA 450/3-85-021), perchloroethylene (EPA 450/3-85-017),
ethylene oxide (EPA 450/3-85-014), chloroform (EPA 450/3-85-026), ethylene
dichloride (EPA 450/3-84-018), methylene chloride (EPA 450/3-85-015), and
carbon tetrachloride (EPA 450/3-85-018).
A-l
-------
3) U.S. Environmental Protection Agency. Preliminary Compilation of Air
Pollutant Emission Factors for Selected Air Toxic Compounds.
EPA-450/4-86-010a, 1987.
This preliminary report presents emission factors of air toxic pollutants
for a variety of sources with varying activity levels. This listing gives
little technical detail concerning the derivations or applicability of any of
the factors therein. This preliminary report is currently being updated and
expanded and a data management system is being developed to allow for easy
access of the factors. The updated factors are associated with pollutant
names and CAS numbers, process descriptions and SIC codes, emission source
descriptions and SCC codes, notes on the derivation of the factors and on
control measures associated with the factors, and references. The emission
factors can be used to obtain guick, rough estimates of air toxic emissions.
More detailed data on the emission sources can be obtained from the Notes and
References Sections listed in the emission factor tables. The primary
limitation of using just the emission factors listed in this compilation is
that their accuracy in application to a given source is not shown. More
accurate emissions estimates may reguire evaluation of the application of
available test data to specific source characteristics.
4) U.S. Environmental Protection Agency. Compilation of Air Pollutant
Emission Factors, Fourth Edition. AP-42, September 1985. Air
Emissions Species Manual, Volume I, Volatile Organic Compound Species
Profiles. EPA 450/2-88-003a, 1988. Air Emissions Species Manual,
Volume II, Particulate Matter Species Profiles. EPA-450/2-88-003b,
1988.
Another tool for estimating air toxic emissions involves the use of VOC/PM
factors presented in AP-42 and species profiles presented in Volumes I and II
of the Air Emissions Species Manual. AP-42 contains emission factors for
total VOC and PM rather than for a single chemical compound. These factors
can be used with profiles contained in the Species Manual to estimate releases
of specific toxic compounds based on the total amount of VOC or PM released
from a source. The Species Manual shows the percent by weight and percent by
volume of specific chemicals in emissions from specific chemicals in emissions
from specific processes. The VOC profiles were obtained from the 1980 VOC
Data Manual, new VOC profiles developed from readily available existing data,
and new VOC profiles developed from original data as part of the VOC
speciation field sampling program. The PM profiles were obtained from the
1984 Source Composition Library and from the literature. In addition to the
VOC and PM profiles, profile assignments linking the profiles to source
categories are presented in the Species Manual. Species profiles for VOCs and
PM are developed from generic sources and may not be representative of
emissions from an individual facility.
5) U.S. Environmental Protection Agency. Fugitive Emission Sources of
Organic Compounds - Additional Information on Emissions, Emission
Reductions, and Costs. EPA-450/3-82-010, 1982.
This document contains the data and methodologies which EPA believes most
accurately characterize average synthetic organic chemical manufacturing
industry eguipment leak emission rates of VOC, effectiveness of control
techniques, and control costs for selected equipment used in the processing of
organic chemicals. The emission factors (on Page 1-4) can be used to estimate
VOC emissions from any industrial plant which has the selected equipment and
handles organic chemicals.
A-2
-------
APPENDIX B
GLOSSARY
-------
APPENDIX B
GLOSSARY
Accident - In this workbook, an unplanned release event not considered in the
design of a facility; a major or catastrophic release (e.g., release from a
tank rupture disk).
Boiling - Vaporization resulting from heat transfer to liquids with boiling
temperatures below ambient or spill surface temperatures.
Cavity - An aerodynamic recirculation zone formed in the lee of an obstruction
(e.g., building) in a wind field.
Continuous Release - A release for which the discharge rate is not time
varying or instantaneous; a dispersing plume for which sampling at a receptor
observes little or no variability due to a time change of emissions (see
instantaneous).
Dense gas - Gas mixtures that are denser than the surrounding air. A dense
gas may rise at first due to its initial upward momentum from a stack, but
then sinks due to its excess density. In the context of this workbook, a
release can result in dense gas effects if its release Richardson number is
> 30.
Depositing - Removal from a plume of gases or particles by chemical or
physical interactions with surfaces or by precipitation.
Emissions - A release of gas, aerosols or particulate matter to the
atmosphere; a gaseous release is an emission while a liquid release must
evaporate before being emitted.
Evaporation - Vaporization resulting from mass transfer; i.e., turbulent
interactions between the atmosphere and the liquid surface,
Fugitive emissions - Emissions resulting from a source for which quantifying
release parameters, emission rates, and locations is difficult. Emissions
estimates may result from emission factors or mass balance calculations and
typically represent a number of small disperse emission sources at a facility.
Instantaneous release - 1) For a liquid, a release which occurs within a very
short time. An instantaneous liquid release may result in formation of an
instantaneous gaseous or two-phase cloud if the liquid is highly volatile or a
pool from which emissions may be continuous or highly time dependent. 2) For
a gas, a release is considered instantaneous for modeling if:
- the travel time to a receptor is long relative to the length of the
release
- the averaging or sampling time is long relative to the release time
An instantaneous model is used if the concentration estimate at a receptor is
significantly affected by the upwind and or downwind edge of the cloud. For
this workbook, releases of less than five minutes duration are arbitrarily
considered instantaneous.
B-l
-------
Neutral - 1) A stability class in which mixing is controlled by mechanical
turbulence; 2) A passive or tracer release, see passive release.
Passive release - Emissions to the atmosphere which disperse as a tracer,
i.e., as a massless point in space which in no way influences the environment
in which it disperses; sometimes called a neutral buoyancy release as opposed
to releases for which buoyancy is important in dispersion.
Pool, pooling - Accumulation of released liquid in a puddle on a surface;
pools may be unconfined or spreading, confined by a berm or dike or
steady-state for which evaporation equals input to the pool and spreading is
halted.
Reactive - Dispersing pollutants which chemically interact with surfaces or
other chemicals resulting in a transformation to another chemical. Reactions
are important in estimating pollutant losses or formation as well as in
estimating energy balances in plumes.
Release - Chemicals or pollutants leaving containment, stacks or vents.
Slumping - Initial spread of a dense gas characterized by vertical sinking and
horizontal spreading resulting from (negative) buoyant or gravitational forces
acting on the cloud.
Two-phase release - Releases consisting of a vapor and suspended liquid
droplets resulting from the violent flash vaporization of superheated liquids
as they rapidly depressurize.
Volatile - A liquid subject to high vaporization due to its high vapor
pressure. In this workbook, the term high volatility is intended to represent
liquids that pool after release requiring an evaporation model to simulate
emissions. If a doubt exists, the liquid can be simulated using an
evaporation model. An assumption of rapid evaporation of a high volatility
liquid is a conservative assumption.
Wake - The entire zone of wind field disturbance caused by an obstruction in
the flow.
B-2
-------
APPENDIX C
FLOWCHARTS FOR WORKBOOK SCENARIOS
-------
FLOWCHRRT C-l
CONTINUOUS PRRTICULRTE EMISSIONS FROM STRCKS
NO
YES
USE
REPRESENTATIVE
EMISSION FRCTOR
(RPPENDIX fl)
USE SOURCE
SPECIFIC
EMISSION RRTE
c-i
-------
FLOXCHRRT C-2
CONTINUOUS FUGITIVE DUST EMISSIONS
NO
USE
REPRESENTRTIVE
EMISSION FFICTOR
(flPPENDIX fl)
USE SOURCE
SPECIFIC
EMISSION RfiTE
C-2
-------
FLOWCHRRT C-3
INSTRNTRNEOUS PRRTICULRTE EMISSIONS FROM DUCT FRILURES
USE SOURCE
SPECIFIC
EMISSION RRTE
C-3
-------
FLOWCHflRT C-4
CONTINUOUS FLflRE EMISSIONS
USE SOURCE
SPECIFIC
EMISSION RfiTE
FLOWCHflRT
0-7
C-4
-------
FLOWCHRRT C-5
CONTINUOUS GRSEOUS EMISSIONS FkOM STRCKS
NO
YES
USE
REPRESENTflTIVE
EMISSION FRCTOR
(RPPENDIX R)
USE SOURCE
SPECIFIC
EMISSION RRTE
C-5
-------
FLOWCHRRT C-6
CONTINUOUS GRSEOUS LEflKS FROM TRNKS/PIPES
MULT I
COMPONENT
RELERSE
YES
DETERMINE MERN
DENSITY RND
MEflN SPECIFIC
HERT RRTIO
O*-
YES
CRLCULRTE
CRITICRL FLOW
EMISSION RRTE
CRLCULRTE
SUBCRITICRL
FLOW EMISSION
RRTE
[ FLOWCHRRT )
C-6
-------
FL01ICHRRT C-7
INSTRNTRNEOUS GRSEOUS EMISSIONS FROM STflCKS/VENTS
USE SOURCE
SPECIFIC
EMISSION RflTE
FLOWCHflRT
C-7
-------
FLOWCHRRT C-8
MULTIPLE FUGITIVE CONTINUOUS GflSEOUS EMISSIONS
USE
REPRESENTRTIVE
EMISSION FRCTOR
(flPPENDIX fl)
USE SOURCE
SPECIFIC
EMISSION RRTE
RRE
EMISSIONS
FROM FIN RRER
OR VOLUME
9
C-8
-------
FLOWCHRRT C-9
CONTINUOUS 6RSEOUS EMISSIONS FROM LRNO TRERTMENT
DETERMINE
EMISSION RRTE
FLOWCHRRT
C-9
-------
FLOWCHRRT C-10
CONTINUOUS GRSEOUS EMISSIONS FROM LRNDFILLS
SITE
SPECIFIC
EMISSION
RflTES
KNOWN
USE SOURCE
SPECIFIC
EMISSION RRTE
LRNDFILL
IN DRY
CLIMRTE
CflLCULRTE
MOIST CLIMRTE
EMISSION RRTE
CflLCULRTE DRY
CLIMflTE
EMISSION RRTE
C-10
-------
FLOWCHRRT C-ll
PESTICIDE/HERBICIDE VOLflTILIZRTIQN
USE SOURCE
SPECIFIC
EMISSION RflTE
FLOWCHRRT
0-4
C-ll
-------
FLOWCHRRT C-12
INSTRNTRNEOUS GflSEOUS EMISSIONS DUE TO EQUIPMENT OPENINGS
USE
REPRESENTATIVE
EMISSION FflCTOR
(flPPENDIX fi)
USE SOURCE
SPECIFIC
EMISSION RfiTE
C-12
-------
FLOWCHRRT C-13
SURFflCE IMPOUNDMENT EMISSIONS
CRLCULRTE
EQUILIBRIUM
CONSTflNT
DETERMINE GRS
flND LIQUID
MfiSS TRflNSFER
COEFFICIENTS
DETERMINE
OVERflLL MfiSS
TRflNSFER
COEFFICIENT
CflLCULflTE
EQUILIBRIUM
CQNCENTRRTION
IS
THE
IMPOUNDMENT
RERRTED
9
CRLCULRTE
TURBULENT GRS
flND LIQUID
MflSS TRRNSFER
COEFFICIENTS
NO
CRLCULRTE
OVERRLL
TURBULENT
MflSS TRflNSFER
COEFFICIENT
DETERMINE
EMISSION RRTE
CRLCULRTE
COMBINED
QUIESCENT/TURB.
MRSS TRRNSFER
COEFFICIENT
C-13
-------
FLOWCHRRT C-14
CONTINOUS RELIEF VflLVE DISCHRRGE (TWO-PHRSE)
RELIEF VRLVE
PRESSURIZED TflNK
USE SOURCE
SPECIFIC
EMISSION RRTE
CflLCULRTE
FLflSH
FRRCTION flND
TWO-PHflSE
OUTFLOW
CRLCULRTE
MEflN DENSITY
PRRRMETERS
FLOWCHRRT
0-1
C-14
-------
FLOWCHRRT C-15
INSTflNTRNEOUS RELIEF VRLVE DISCHRRGE (TWO-PHRSE)
USE SOURCE
SPECIFIC
EMISSIONS
CflLCULflTE
MEfiN DENSITY
C-15
-------
FLOWCHRRT C-16
EMISSIONS DUE TO L0« VOLflTILITY LIQUID LERKS FROM PIPES
DETERMINE
MflSS TRRNSFER
COEFFICIENT
CONTINUOUS/
INSTRNTRNEOUS
RELERSE
LIQUID
RELERSE RRTE
KNOWN ?
INSTRNTflNEOUS
YES
flSSUME
RELERSE RflTE
EQUflLS PIPE
FLOW RRTE
USE SOURCE
SPECIFIC
RELERSE RflTE
RSSUME
RELERSE RflTE
EQUflLS PIPE
SECTOR VOLUME
DETERMINE SPILL
RREfl BflSED ON
LIQUID RELERSE
RflTE
USE SOURCE
SPECIFIC
RELERSE
RMOUNT
DETERMINE SPILL
RRER BRSED ON
LIQUID RELERSE
RMOUNT
DETERMINE
EVRPORRTION
RflTE
FLOWCHRRT
D-4
C-16
-------
FLOKCHRRT C-17
EMISSIONS DUE TO LEflKS OF LOW VOLflTILITY LIQUIDS FROM TRNKS
DETERMINE
LIQUID
RELERSE RRTE
DETERMINE
EVflPORflTION
RflTE RND
SPILL RREfl
FLOWCHflRT
D-4
C-17
-------
FLOWCHRRT C-18
EMISSIONS DUE TO LEflKS OF HIGH VOLRTILITY LIQUIDS FROM PIPES
fiSSUME
EMISSION RRTE
EQUflLS PIPE
FLOW RflTE
FLOWCHRRT
0-2
C-18
-------
FLOWCHRRT C-19
EMISSIONS DUE TO LEftKS OF HIGH VOLRTILITY LIQUIDS FROM TRNKS
CflLCULRTE
LIQUID
RELERSE RRTE
RSSUME
EMISION RRTE
EQUflLS LIQUID
RELERSE RRTE
C-19
-------
APPENDIX D
FLOWCHARTS FOR DISPERSION CALCULATIONS
-------
FLOHCHRRT D-l
DETERMINflTION OF DISPERSION CLflSS FOR CONTINUOUS SOURCES
DETERMINE
DENSITY OF
CONTINUOUS
RELERSE
(5.]
.2)
2-STEP
DENSITY/
RICHRRDSON
NUMBER CHECK
CRLCULflTE
CONCENTRflTIONS
(5.6.3)
POINT, RRER
OR VOLUME
SOURCE ?
D-l
-------
FLOKCHRRT D-2
CONTINUOUS POINT SOURCE CRLCULRTIONS
FLOWCHRRT
D-2
RECEPTOR
IN CflVITY ?
PLUME IN
CflVITY ?
(5.6.1)
DETERMINE
WRKE IMPRCTS
(5.5.3)
ESTIMflTE NOT
RVRILRBLE
CflVITY (x < 3ha)
NEflR (3ha < x < IDha)
K)
RECEPTOR
LOCflTION
PLUME IN
*flKE
FflR (x > lOha)
DETERMINE
VIRTUflL
DISTRNCES
(5.5.3)
DETERMINE
BUOYRNT PLUME
RISE (5.2.3)
FLOWCHRRT
D-3
CRLCULRTE
CRVITY
IMPRCTS
(5.6-1)
END
ESTIMRTE
DISPERSION
PRRRMETERS
(5.3-3)
CRLCULRTE
CONCENTRRTIONS
(5.6.4)
END
D-2
-------
FLOWCHRRT D-3
CONTINUOUS POINT SOURCE DISPERSION CRLCULflTIONS
( FLOWCHRRT ]
V D-3 I
DETERMINE
DISPERSION
PRRRMETERS
(5.3-1)
DETERMINE
BUOYRNCY
INDUCED INITIRL
DILUTION (5.4)
CRLCULRTE
CONCENTRRTIONS
(5.6.4)
END
D-3
-------
FLOWCHflRT D-4
CONTINUOUS flREfl SOURCE DISPERSION CRLCULRTIONS
FLOWCHflRT
D-4
DETERMINE
HORIZONTflL
VIRTURL
DISTRNCES
(5.5.1)
DETERMINE
DISPERSION
PRRRMETERS
(5.3.1)
CRLCULRTE
CONCENTRflTIONS
(5.6.4)
END
D-4
-------
FLOWCHRRT D-5
CONTINUOUS VOLUME SOURCE DISPERSION CRLCULRTIONS
DETERMINE
VOLUME SOURCE
VIRTURL
DISTRNCES
(5.5.2)
DETERMINE
DISPERSION
PRRRMETERS
(5.3.1)
CRLCULRTE
CONCENTRRTIONS
(5.6.4)
END
D-5
-------
FLOWCHRRT D-6
INSTRNTRNEOUS DISPERSION CRLCULRTIONS
IS \ PRRTICULRTE
ELERSE\ MRTTER
"GRSEOUS OR
PRRTICULflTE
DETERMINE
DENSITY OF
INSTRNTRNEOUS
RELERSE
(5.1.1)
YES
DETERMINE CLOUD
VOLUME, HEIGHT,
SPRERD RRDIUS,
MRSS (5.6.2)
POINT OR
VOLUME
SOURCE
9
VOLUME
DETERMINE
DISPERSION
PflRflMETERS
(5.3.2)
DETERMINE
VIRTURL
DISTRNCES
(5.5.2)
CRLCULRTE
CONCENTRRTIONS
(5-6.5)
DETERMINE
DISPERSION
PRRRMETERS
(5.3.2)
END
D-6
-------
FLOWCHRRT D-7
CONTINUOUS FLRRE CRLCULRTIONS
FLOWCHRRT
D-7
CRLCULRTE
FLRRE FLfiME
TIP HEIGHT
(5.2.2)
CRLCULRTE
BUOYRNCY
PLUME RISE
(5.2.3)
( FLOWCHRRT )
D-7
-------
APPENDIX E
AVERAGING PERIOD OF CONCENTRATION ESTIMATES
-------
APPENDIX E
AVERAGING PERIOD OF CONCENTRATION ESTIMATES
The purpose of this appendix is to provide some simplified techniques for
concentration averaging from instantaneous and continuous equations provided
in the workbook. Methods presented are applicable to ground-level and
elevated emissions of passive gases and particulate matter.
Instantaneous Estimates
Methods provided for instantaneous concentration estimates (i.e., puff
releases with a duration of 5 minutes or less) in Section 5.6.5 represent peak
concentrations at the centroid of an expanding puff transported in the wind.
Petersen (1982) provides equations for estimating peak average concentrations
over time periods of up to one hour. The method is as follows:
C(mean) = C(instantaneous) x F
where the correction factor (F) represents the mean height of the area of
the Gaussian puff which traverses a receptor in the sampling time (T). It is
given by:
F = (A - 0.5)/(0.3989 N)
where:
A = the area under the Gaussian distribution within N standard
deviations, as found in Figure E-l
N = the number of standard deviations from the peak defined as:
N = TU
2ar
T - averaging time in seconds
u - transport wind speed (m/s)
ar - instantaneous horizontal plume dispersion parameter at the
receptor distance (m) (from Figure 5-4)
Continuous Estimates
To obtain the estimate of the maximum concentration for a longer averaging
time, multiply the 1-hour maximum concentration by the given factor:
Averaging Time Multiplying Factor
3 hours 0.9 (±0.1)
8 hours 0.7 (±0.2)
24 hours 0.4 (±0.2)
E-l
-------
FIGURE E-l
AREA UNDER NORMAL CURVE
4.0
C/5
O
UJ
C
O
CC
<
O
z
<
C/J
tx
O
cr
UJ
CD
2
Z
3.0
2.0
1.0
50 60 70 80
90 95
98 99
99.8 99.9
99.99
E-2
-------
The numbers in parentheses are recommended limits to which one may diverge
from the multiplying factors representing the general case. For example, if
aerodynamic downwash or terrain is a problem at the facility, or if the
release height is very low, it may be appropriate to increase the factors up
to the limits specified in parentheses. Conversely, if the stack is
relatively tall and there are no terrain or downwash problems, it may be
appropriate to decrease the factors. For averaging times in between the
values listed above, use the multiplying factor for the shorter averaging
time. For example, if a 4-hour average concentration is needed, use the
multiplying factor for the 3-hour averaging time (0.9).
T6~db±ain the estimated maximum concentration for a shorten averaging time
than 1-hour, use thex'l-hour\ maximum concentration for any desired averaging^
time between 30 and7 60 minutes. For/ averaging \times less than\30 minutes, a
specific procedure/can not be recommended. "
"~
t
C
/ }°'2
*-> i ^//%/. i
<<3,'^^ JZJ^vT/
E-3
-------
APPENDIX F
SELECTED CONVERSION FACTORS
-------
APPENDIX F
SELECTED CONVERSION FACTORS
Pressure
1 ATM = 1.013 x 106 dynes/cm2
1 millibar = 1000 dynes/cm2
1 mm Hg = 1333.224 dynes/cm2
1 lb/in2 = 68,947.6 dynes/cm2
1 in. Hg = 33,863.9 dynes/cm2
1 Pascal = 10 dynes/cm2
Volume
,3 = 106 cm3
i3 = 103 liters
1
1
1 cu ft = 28.317 liters
1 liter = 103 cm3
1 m3 = 35.315 cu feet
1 gal = 3,785 cm3
Mass Release Rate
1 g/s = 7.9367 Ib/hr
1 t/yr = 2.8766 x 10~2 g/s
It/day = 10.500 g/s
Concentration
1 cal (g) = 3.9685 x 10~3 BTU
1 BTU = 251.634 cal
1 BTU = 1.0543 x 1010 ergs
1 BTU = 1054 Joules - (N - m)
Heat Rate
-6
1 cal/s = 1.102 x 10~D BTU/h
Flow
1 m3/h = 3600 m3/s
Area
1 m2 = 104 cm2
1 ft2 = 0.0929 m2
1 hectare = 104 m2
1 acre = 4046.86 m2
Conversions with parts per million by
volume
ug/m3 = (ppm) 40.87 MW
ppm = (ug/m3)
0.02447
MW
P /To
Po \ T
Po / T
P \ To
F-l
-------
-------
APPENDIX G
CALCULATIONS METHODS FOR DISPERSION PARAMETERS
-------
APPENDIX G
CALCULATIONAL METHODS FOR DISPERSION PARAMETERS
Dispersion parameters presented graphically in Section 5.3.1 and 5.3.2 are
statistical fits to observed experimental data. Dispersion parameters may be
derived from these figures or from equations presented in this appendix.
Instantaneous dispersion parameters are derived from quasi-instantaneous
releases (Slade, 1968). The parameters are of the form:
a = a xb
where x is the downwind distance in meters and the coefficients are given by:
STABILITY HORIZONTAL VERTICAL
a b a b
Unstable (A-C) 0.14 0.92 0.53 0.73
Neutral (D) 0.06 0.92 0.15 0.70
Stable (E-F) 0.02 0.89 0.05 0.61
Pasquill-Gifford dispersion parameters for continuous sources can be
calculated using downwind distance in kilometers using Figures 5-1 and 5-2 or
techniques from ISC (EPA, 1987c) given in Tables G-l and G-2:
G-l
-------
TABLE G-l
PARAMETERS USED TO CALCULATE PASQUILL-GIFFORD oy
oy (meters) = 465.12 (x) tan (TH)
Pasguill
Stability
Category
A
B
C
D
E
P
TH = 0.01745
c
24.17
18.33
12.50
8.33
6.25
4.17
-------
TABLE G-2
PARAMETERS USED TO CALCULATE PASQUILL-GIFFORD oz
Pasguill
Stability
Category x (km)
A* <.10
0.10 - 0.15
0.16 - 0.20
0.21 - 0.25
0.26 - 0.30
0.31 - 0.40
0.41 - 0.50
0.51 - 3.11
>3.11
B* <.20
0.21 - 0.40
>0.40
C* All
D* <.30
0.31 - 1.00
1.01 - 3.00
3.01 - 10.00
10.01 - 30.00
>30.00
CTz
a
122.800
158.080
170.220
179.520
217.410
258.890
346.750
453.850
**
90.673
98.483
109.300
61.141
34.459
32.093
32.093
33.504
36.650
44.053
(meters) = a x"
b
0.94470
1.05420
1.09320
1.12620
1.26440
1.40940
1.72830
2.11660
**
0.93198
0.98332
1.09710
0.91465
0.86974
0.81066
0.64403
0.60486
0.56589
0.51179
* If the calculated value of a_ exceeds 5000 m, a- is set to 5000 m.
** i
is equal to 5000 m.
G-3
U S. GOVERNMENT PRINTING OFT ICE I')-
-------
TABLE G-2
(Continued)
PARAMETERS USED TO CALCULATE PASQUILL-GIFFORD az
Pasguill
Stability
Category x (km)
E <.10
0.10 - 0.30
0.31 - 1.00
1.01 - 2.00
2.01 - 4.00
4.01 - 10.00
10.01 - 20.00
20.01 - 40.00
>40.00
F <.20
0.21 - 0.70
0.71 - 1.00
1.01 - 2.00
2.01 - 3.00
3.01 - 7.00
7.01 - 15.00
15.01 - 30.00
30.01 - 60.00
>60.00
°z
a
24.260
23.331
21.628
21.628
22.534
24.703
26.970
35.420
47.618
15.209
14.457
13.953
13.953
14.823
16.187
17.836
22.651
27.074
34.219
(meters) = a x13
b
0.83660
0.81956
0.75660
0.63077
0.57154
0.50527
0.46713
0.37615
0.29592
0.81558
0.78407
0.68465
0.63227
0.54503
0.46490
0.41507
0.32681
0.27436
0.21716
G-4
-------
ADDENDUM
"A Workbook of Screening Techniques for Assessing Impacts of Toxic
Air Pollutants" September 1988
Page l-l: The title and reference of the publication
"Procedures for Evaluating Impact of Stationary Sources"
(1988b) is now "Screening Procedures for Estimating the Air
Quality Impact of Stationary Sources," Erode, 1988. EPA-450/4-
88-010.
Page 2-9: Table 2-2 is taken from Appendix A of Erode (1988)
referenced above.
Page 6-17: Model output shown is based on a preliminary
version of RVD run on 8/16/88. Results may differ from those
obtained when using the final version of RVD.
Page 6-41: Same comment about RVD model output as above.
Page R-2: The reference "U.S. Environmental Protection
Agency, 1988b: Procedures of Evaluating Impact of Stationary
Sources," Draft Report, September 1988 is now "Erode, R. W.,
1988: Screening Procedures for Estimating the Air Quality
Impact of Stationary Sources," Draft for public comment,
August 1988. EFA-450/4-86-010.
Page E-3: Delete the last paragraph. Replace with the
following paragraph:
To obtain the estimated maximum concentration, Ct,
for a shorter averaging time than 1 hour adjust the
1-hour maximum concentration by the following ratio:
Ct = (60 min/t)°-2
where:
t - averaging period of interest (less
than 60 minutes)
December 8, 1988
------- |