GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
GBH Enterprises, Ltd.
Process Safety Guide:
GBHE-PGP-020
GAS DISPERSION
A Definitive Guide to Accidental Releases
of Heavy Gases
Process Information Disclaimer
Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

Process Safety Guide: Gas Dispersion
CONTENTS
0 INTRODUCTION
0.1 AIMS
0.2 SCOPE
0.5 FURTHER INFORMATION
1 METEOROLOGICAL PARAMETERS WHICH AFFECT DISPERSION
1.1 ROUGHNESS LENGTHS (Zo)
1.2 WIND SPEEDS
1.3 ATMOSPHERIC STABILITY
1.3.1 General
1.3.2 Pasquill - Gifford Methods of Characterizing Atmospheric Stability
1.3.3 Monin-Obukhov Length Methods of Representing Atmospheric
Stability
TABLES
1.1 TYPICAL ROUGHNESS LENGTHS
1.2 KEY TO PASQUILL - GIFFORD STABILITY CATEGORIES
1.3 METHOD OF ESTIMATING LEVEL OF INCIDENT RADIATION
1.4 EXAMPLE PASQUILL-GIFFORD STABILITY ANALYSIS
FIGURES
1.1 THE EFFECT OF ATMOSPHERIC STABILITY ON PLUME DISPERSION
1.2 RELATIONSHIP BETWEEN PASQUILL-GIFFORD STABILITY
CATEGORY AND MONIN-OBUKHOV LENGTH
2 AIR QUALITY STANDARDS
2.1 WHAT ARE AIR QUALITY STANDARDS?
2.2 WHAT AIR QUALITY STANDARDS EXIST
2.2.1 General Background
2.2.2 United States
2.2.3 European Union
2.2.4 The Netherlands
2.2.5 Japan
2.2.6 Taiwan
2.2.7 United Kingdom Air Quality Strategy

2.2.8 Non-Governmental Organizations
2.2.9 Occupational Exposure Limits
2.2.10 General Comparison
2.2.11 Air Quality Standards for Odor Impacts
2.3 WHAT IS THE LAW - AND WHAT ISN’T
2.4 FUTURE DEVELOPMENTS
3 MODEL COMPARISON AND SELECTION
3.1 CLASSIFICATION OF DISPERSION MODELING PROBLEMS
3.2 WHAT MODELS ARE AVAILABLE?
3.3 DESCRIPTION OF AVAILABLE MODELS
3.3.1 General
3.3.2 ADMS (Atmospheric Dispersion Modeling System)
3.3.3 ALOHA (Areal Locations of Hazardous Atmospheres)
3.3.4 DISP2
3.3.5 ISC (Industrial Source Complex)
3.3.6 PHAST (Process Hazard Assessment Tools)
3.3.7 Other Models
3.3.8 Summary of Model Applications
3.4 COMPARISON OF MODEL RESULTS
3.4.1 General
3.4.2 Buoyant gas releases
3.4.3 Dense Gas Dispersion
3.5 RECOMMENDATIONS
TABLES
3.1 COMMONLY USED DISPERSION MODELS
3.2 SUMMARY OF MODEL APPLICATIONS

FIGURES
3.1 CATEGORIZATION OF DISPERSION MODELING
PROBLEMS
3.2 BUOYANT GAS RELEASE: ADMS RESULTS
3.3 BUOYANT GAS RELEASE : DISP2 RESULTS
3.4 BUOYANT GAS RELEASE: ISC RESULTS
3.5 BUOYANT GAS RELEASE: PHAST RESULTS
3.6 SINGLE PHASE DENSE GAS RELEASE UNDER STABLE
ATMOSPHERIC CONDITIONS
3.7 CATASTROPHIC DENSE GAS RELEASE UNDER
UNSTABLE ATMOSPHERIC CONDITIONS
3.8 SINGLE PHASE DENSE GAS RELEASE: ALOHA
RESULTS
3.9 TWO PHASE DENSE GAS RELEASE: PHAST RESULTS
4 STACK DESIGN
4.1 INTRODUCTION
4.2 STACK DESIGN
4.2.1 Stage A: Preceding Design Work
4.2.2 Stage B: Estimate Mass Emission Rates
4.2.3 Stage C: Identify Acceptable Process Contributions
4.2.4 Stage D: Identify Significant Pollutants
4.2.5 Stage E: Initial Stack Design
4.2.6 Stage F: Model On-site Concentrations
4.2.7 Stage G: Model Off-site Concentrations
4.2.8 Stage H: Assess Results
4.3 FURTHER CASE STUDY
FIGURE
4.1 FLOW CHART FOR STACK DESIGN

5 DENSE GAS DISPERSION
5.1 INTRODUCTION
5.2 MODELING METHODOLOGIES
5.2.1 Instantaneous Catastrophic Releases
5.2.2 The Dispersion of a Continuous Dense Gas Plume
5.3 POINTS TO NOTE
5.4 VALIDATION WORK
5.5 DISPERSION MODELS AVAILABLE
5.5.1 DISP2
5.5.2 HGSYSTEM5
5.5.3 ALOHA
5.5.4 PHAST
5.5.5 EFFECTS
5.5.6 GASTAR
5.5.7 LORIMAR Model
FIGURES
5.1 CLOUD SHAPE AS A FUNCTION OF TIME
5.2 BEHAVIOR OF A DENSE GAS PLUME WITH VERTICAL MOMENTUM
6 SOURCE TERMS
6.0 INTRODUCTION
6.1 SOURCE CHARACTERISTICS AND HOLE SIZES
6.1.1 Ammonia Storage Tank Example
6.1.2 Estimation of Hole Sizes
6.1.3 Inventories and Time Dependent Behavior
6.2 THE DISCHARGE OF GASES THROUGH HOLES
6.2.1 Compressible Choked Flow
6.2.2 Compressible Unchoked Flow
6.2.3 Incompressible Flow
6.2.4 Discharge Coefficients

6.3 TWO-PHASE RELEASES
6.3.1 Catastrophic Releases of a Liquefied Gas
6.3.2 Two-Phase Releases Arising from Guillotine Failures of Pipe work
6.4 LIQUID POOL SPREADING AND EVAPORATION
6.5 SOURCE TERMS FOR ENVIRONMENTAL RELEASES
6.5.1 General
6.5.2 A Real Example
6.5.3 A Source Data Checklist for Environmental Applications
6.6 REFERENCES
FIGURES
6.1 POSSIBLE RELEASE SCENARIOS FROM A LIQUEFIED AMMONIA
STORAGE TANK
6.2 COMPARISON OF PLUME CHARACTERISTICS vs. TARGET
DISTANCE
6.3 DIAGRAMMATIC REPRESENTATION OF PSEUDO SOURCE
DIAMETER
6.4 EVAPORATION RATE OF CHLORINE FROM AN INSTANTANEOUS
10 TONNE SPILL
7 BUILDING WAKE EFFECTS
7.1 WHY ARE BUILDING WAKE EFFECTS IMPORTANT?
7.2 HOW DO BUILDINGS INFLUENCE ATMOSPHERIC
DISPERSION?
7.3 SCIENTIFIC UNDERSTANDING OF BUILDING WAKE EFFECTS
7.4 THE BUILDINGS MODULE IN ADMS: PRINCIPLES
7.5 THE BUILDINGS MODULE IN ADMS: APPLICATION
7.5.1 When Should the Buildings Module be Used?
7.5.2 Points to Note About Using the Buildings Module
7.5.3 Interpreting the Results of the Buildings Module

FIGURES
7.1 THE INFLUENCE OF A BUILDING WAKE ON PLUME
DISPERSION
7.2 SCHEMATIC DIAGRAM OF TURBULENT ZONES USED IN
ADMS BUILDING MODULE
7.3 EFFECT OF BUILDING WIDTH ON WAKE DISPERSION
7.4 AREAS OF CONCERN DUE TO BUILDING EFFECTS
7.5 THE BUILDINGS MODULE OF ADMS (STABLE CONDITIONS)
8 MODELING THE DISPERSION OF OXIDES OF NITROGEN
8.1 GENERAL
8.2 ASSESSING NOx LEVELS
8.2.1 Approach 1
8.2.2 Approach 2
8.2.3 Approach 3
8.2.4 Approach 4
8.2.5 Suggested Method
8.3 EXAMPLE: DISPERSION OF NOx FROM A BOILER HOUSE
FIGURE
8.1 SAMPLE NO2 NOx RATIO CALCULATION
9 THE COMPLEX TERRAIN MODULE IN ADMS
9.1 WHAT IS THE COMPLEX TERRAIN MODULE?
9.2 HOW DOES THE COMPLEX TERRAIN MODULE OF ADMS
WORK?
9.2.1 Wind Flow
9.2.2 Dispersion Calculations
9.3 WHEN AND HOW SHOULD THE COMPLEX TERRAIN MODULE
BE USED?

9.4 WHAT IS THE EFFECT OF USING THE COMPLEX TERRAIN
MODULE?
9.4.1 Conclusions - Terrain Elevations
9.4.2 Conclusions - Variations in Surface Roughness
9.4.3 Conclusions - Buoyant Releases
TABLES
9.1 COMPARISON OF REPRESENTATIVE CONCENTRATIONS FOR
RELEASES UPWIND OF HILL
9.2 COMPARISON OF REPRESENTATIVE CONCENTRATIONS FOR
RELEASES DOWNWIND OF HILL
FIGURES
9.1 WIND FLOW AROUND A HILL (SIDE VIEW)
9.2 WIND FLOW AROUND A HILL UNDER STABLE ATMOSPHERIC
CONDITIONS (PLAN VIEW)
9.3 TOPOGRAPHY OF THE RUNCORN AREA
9.4 MODELED CONCENTRATIONS DUE TO EMISSIONS FROM 60 m
STACKS UPWIND OF HILL B STABILITY / 2 m/s
STACKS DOWNWIND OF HILL B STABILITY / 2 m/s
STACKS UPWIND OF HILL D STABILITY / 5 m/s
STACKS DOWNWIND OF HILL D STABILITY / 5 m/s
STACKS UPWIND OF HILL F STABILITY / 2 m/s
STACKS DOWNWIND OF HILL F STABILITY / 2 m/s

10 THE DEPOSITION MODULE OF ADMS - A BRIEF GUIDE
10.1 INTRODUCTION
10.2 DEPOSITION MODELING METHODOLOGY USED IN ADMS
10.3 DEFAULT AND RECOMMENDED INPUTS USED IN ADMS
10.3.1 Wet Deposition
10.3.2 Dry Deposition
10.4 RECOMMENDATIONS FOR USING DEPOSITION MODULE
10.5 EXAMPLE APPLICATION OF THE DEPOSITION MODULE
TABLES
10.1 WET DEPOSITION COEFFICIENTS
10.2 DRY DEPOSITION VELOCITIES FOR GASEOUS COMPOUNDS
10.3 DISTANCES AT WHICH DEPOSITION PROCESSES HAVE A
SIGNIFICANT EFFECT ON AIR CONCENTRATIONS
FIGURES
10.1 PARTICULATE DRY DEPOSITION VELOCITIES AS A FUNCTION OF
PARTICLE DIAMETER
11 EXAMPLE GAS DISPERSION CALCULATIONS FOR
ENVIRONMENTAL APPLICATIONS USING ADMS
11.1 INTRODUCTION
11.2 SOURCE DATA
11.3 EXAMPLE CALCULATIONS
11.3.1 EXAMPLE ONE - CONTINUOUS EMISSIONS
11.3.2 EXAMPLE TWO - MULTIPLE STACK CALCULATION
11.3.3 EXAMPLE THREE - ODOR DISPERSION CALCULATION
11.3.4 EXAMPLE FOUR - DISPERSION AROUND A BUILDING
11.3.5 EXAMPLE FIVE - ANNUAL AVERAGE STATISTICAL
CALCULATION FOR AN AREA SOURCE
11.3.6 EXAMPLE SIX - DISPERSION OF PARTICULATES FROM
A PRILLING TOWER
11.4 ACCURACY OF ADMS-2

11.5 CHOICE OF WIND AND WEATHER CONDITIONS FOR DESIGN
11.6 RUN TIMES
11.6.1 GENERAL
11.6.2 RUNNING BATCH FILES
11.7 WIND AND WEATHER DATA
11.8 SUMMARY OF ROUGHNESS LENGTHS (Z O)
11.9 CALCULATION TRENDS
FIGURES
11.1 OUTPUT FROM X-Y PLOTTING OPTION
11.2 THE DISPERSION OF SULFUR DIOXIDE FROM A 40 M STACK
11.3 SAMPLE ADMS LINE PLOT : PLUME HEIGHT (M)
11.4 SAMPLE ADMS LINE PLOT : MAXIMUM CONCENTRATION IN
PLUME
11.5 THE DISPERSION OF THE OXIDES OF NITROGEN FROM A
PLASTICS WORKS
PLASTICS WORKS
PLASTICS WORKS
PLASTICS WORKS
11.9 ANNUAL AVERAGE CONCENTRATION OF THE OXIDES OF
NITROGEN - BOTH STACKS AT 40 m
11.10 MEAN GROUND-LEVEL CONCENTRATION - EXAMPLE THREE
11.11 THE DISPERSION OF ETHYL ACRYLATE FROM A 15 m HIGH
STACK - 98th PERCENTILE OF CONCENTRATION
FLUCTUATIONS - 5 m/s NEUTRAL ATMOSPHERIC STABILITY

STACK - 2 m/s UNSTABLE ATMOSPHERIC CONDITIONS -
98th
PERCENTILE OF SHORT TERM CONCENTRATIONS
STACK - 2 m/s UNSTABLE ATMOSPHERIC CONDITIONS -
98th
PERCENTILE OF CONCENTRATION FLUCTUATIONS
11.14 EFFECT OF WIND DIRECTION ON CONCENTRATION -
EXAMPLE FOUR
11.15 DISPERSION OF SO2 FROM A SULFURIC ACID RECOVERY
PLANT - EXAMPLE FOUR
11.16 DISPERSION OF SO2 FROM A SULFURIC ACID RECOVERY
PLANT - 30 m STACK - EXAMPLE FOUR
11.17 ANNUAL AVERAGE BENZENE CONCENTRATIONS FROM A
SMALL LAGOON
11.18 THE DISPERSION OF PARTICULATES FROM A PRILLING
TOWER - EXAMPLE SIX
11.19 TOTAL ANNUAL DEPOSITION RATE FROM THE PRILLING
TOWER (µg/m2
s)
11.20 MAXIMUM 24 HOUR MEAN PARTICULATE CONCENTRATION
FROM A PRILLING TOWER
12 DISPERSION MODELING OF ODOROUS RELEASES
12.1 ODOR EMISSIONS - CHARACTERIZATION AND
MEASUREMENT
12.2 AVERAGING TIMES
12.2.1 Concentration Fluctuations
12.2.2 Change in Mean Wind Direction
12.2.3 Accounting for Dependence on Averaging Time
12.3 ODOR THRESHOLDS
12.4 ODOR DISPERSION MODELING
12.5 EXAMPLE ODOR DISPERSION MODELING STUDY

FIGURES
12.1 INSTANTANEOUS AND AVERAGED PLUME DISPERSION
12.2 ACTUAL CONCENTRATIONS OF RELEASED MATERIAL
12.3 ACTUAL AND MEASURED CONCENTRATIONS OF RELEASED
MATERIAL
12.4 STATISTICAL DESCRIPTIONS OF MEASURED
CONCENTRATIONS
12.5 WIND DIRECTION ENVELOPES FOR SHORT AND LONG-TERM
MEANS
12.6 EXAMPLE STUDY : SITE DIAGRAM
TABLES
12.1 APPROPRIATE AVERAGING TIMES
12.2 EXAMPLE STUDY: PLANT ODOROUS RELEASES
12.3 EXAMPLE STUDY: MODELED CONCENTRATIONS
13 BIBLIOGRAPHY
14 GLOSSARY
APPENDICES
APPENDIX A WIND GENERATION OF PARTICULATES
APPENDIX B TABLE OF PROPERTY VALUES FOR SPECIFIC
CHEMICALS
DOCUMENTS REFERRED TO IN THIS PROCESS SAFETY GUIDE

0 INTRODUCTION
0.1 AIMS
This Process Safety Guide has been written with the aim of assisting process
engineers, hazard analysts and environmental advisers in carrying out gas
dispersion calculations. The Guide aims to provide assistance by:
• Improving awareness of the range of dispersion models available within
GBHE, and providing guidance in choosing the most appropriate model for
a particular application.
• Providing guidance to ensure that source terms and other model inputs
are correctly specified, and the models are used within their range of
applicability.
• Providing guidance to deal with particular topics in gas dispersion such as
dense gas dispersion, complex terrain, and modeling the chemistry of
oxides of nitrogen.
• Providing general background on air quality and dispersion modeling
issues such as meteorology and air quality standards.
• Identifying personnel within GBHE's Alliance Network with expertise and
experience of dispersion modeling.
• Providing example calculations for real practical problems.
0.2 SCOPE
The gas dispersion guide contains the following Parts:
1 Fundamentals of meteorology.
2 Overview of air quality standards.
3 Comparison between different air quality models.
4 Designing a stack.
5 Dense gas dispersion.

6 Calculation of source terms.
7 Building wake effects.
8 Overview of the chemistry of the oxides of nitrogen.
9 Overview of the ADMS complex terrain module.
10 Overview of the ADMS deposition module.
11 ADMS examples.
12 Modeling odorous releases.
13 Bibliography of useful gas dispersion books and reports.
14 Glossary of gas dispersion modeling terms.
Appendix A : Modeling Wind Generation of Particulates.
APPENDIX B TABLE OF PROPERTY VALUES FOR SPECIFIC
CHEMICALS
The two models referred to by name are the currently preferred models for dense
gas dispersion (PHAST) and neutral/buoyant gas dispersion (ADMS).

1 METEOROLOGICAL PARAMETERS WHICH AFFECT DISPERSION
1.1 ROUGHNESS LENGTHS (Zo)
The roughness length is a parameter which quantifies the effect ground
roughness has on the turbulent flow properties of the wind - the higher the
roughness length, the more turbulent the wind flow.
For an elevated stack, the higher the roughness length, the more rapidly the
plume centerline concentration decreases with distance. However, the higher
the roughness length, the more rapidly the plume spreads in the vertical
direction, counteracting the effect of roughness on plume centerline
concentrations. Hence it is not possible to generalize the effect surface
roughness has on ground level concentrations.
For a ground level release of a heavier-than-air gas cloud, the higher the surface
roughness, the more rapid is the dispersal rate of the cloud Estimating roughness
lengths can be difficult - rarely is the terrain uniform around a source - in general,
consider the roughness of the ground upwind of the source. Typical values are
as given in Table 1.1 below:-
TABLE 1.1 TYPICAL ROUGHNESS LENGTHS

GBHE suggests that if in doubt one should choose a roughness length of
h/30 where h is the average height of the obstacles e.g. if the typical size of the
roughness elements is 9-10 m, use Zo = 0.3 m. This is only a simple rule of
thumb.
Commercial programs, can only accept roughness length inputs of 0.01, 0.1 and
1 m - use a roughness length of 0.1 m for an industrial site.
1.2 WIND SPEEDS
Wind speed varies as a function of height and ground roughness. In general,
whenever a wind speed is quoted, it refers to the speed at a height of 10 m,
although sometimes data from the US or from a small local weather station may
be measured at a height of 2 m.
The velocity profile as a function of height is dependent on atmospheric stability.
Models such as the US-EPA models, commercial programs assume a power law
velocity profile:-
where n is a function of roughness and atmospheric stability; z is the height
above the ground (m), and uz is the velocity at height z m.
More widely used is a log-law relationship based originally upon Prandtl mixing
length theory for the turbulent boundary layer over a flat surface:-
where L mo is the Monin-Obukhov length. Y is a function that takes into account
the effect of atmospheric stability - usually found empirically. u* is a term known
as the friction velocity defined as √(τ/ρa), where τ is the surface shear stress
and ρa is the air density. k is the von Karman constant, which has a value close
to 0.4.
The effect of the variation in wind speed as a function of height does have a
significant effect on gas dispersion modeling. For example, the advection
velocity of a dense gas box-type model is usually taken to be the wind speed at
half the height of the gas cloud. As more air is entrained into the cloud, its height
increases and hence the bulk velocity of the cloud increases.

1.3 ATMOSPHERIC STABILITY
1.3.1 General
The term atmospheric stability describes the degree of stratification of the
atmosphere, which plays a vital part in the dispersion of atmospheric pollutants.
On hot sunny days with cloudless skies, the ground absorbs radiation from the
sun at a faster rate than the air above it. The ground then re-radiates and
convects heat back into the atmospheric boundary layer setting up large scale
convective motions. These cause rapid plume spreading in the vertical direction
and large scale plume meandering. This rapid spreading brings elevated plumes
down to ground level. For elevated stacks, the highest ground level
concentrations occur in low wind speed, unstable atmospheric conditions.
During cold winter evenings and nights with little or no cloud cover, the ground is
at a lower temperature than the air above it and heat is transferred from the air to
the ground. This sets up a stratified layer of colder air close to the ground which
dampens out atmospheric turbulence. Gaseous effluent from elevated stacks
form narrow pencil-shaped plumes which rarely strike the ground. Hence, stable
conditions, in general, give low ground level concentrations from elevated stacks.
However, stable conditions would give the worst case conditions if the plume
directly impacted an adjacent plant structure or hill nearby. Low wind speed,
stable atmospheric conditions always give the worst case scenario for
catastrophic releases of a heavier than air gas cloud and for any ground level
release.
In practice, for at least 60% of the time in the USA, there is neutral atmospheric
stability where the effect of heat transfer from the ground into the plume is
negligible. In this case, mechanical turbulence generated by the wind flow in
addition to turbulence generated by the initial momentum of the plume, control
the dispersion rate. Neutral conditions usually prevail when the wind speed
exceeds 5 m/s. The effect of atmospheric stability on plume dispersion is
illustrated in Figure 1.1.

There are two commonly applied ways of characterizing atmospheric stability:-
Pasquill -Gifford stability scheme and Monin-Obukhov length scaling. The
former methodology is used by many models, including DISP2 and PHAST but
there is increasing tendency for the latest dispersion models such as UK-ADMS,
to adopt the latter approach.
1.3.2 Pasquill - Gifford Methods of Characterizing Atmospheric Stability
Pasquill - Gifford stability analyses assign a letter in the range A to G in order to
characterize atmospheric stability. The most unstable atmospheric conditions,
characteristic in the USA of a few really hot summer afternoons, are represented
by the letter A; neutral conditions by the letter D and stable conditions by F. A
few modelers in Northern Latitudes use G conditions to represent really stable
conditions (e.g. winter evenings in Norway).
The actual choice of stability category is governed by wind speed and cloud
cover and is defined in Tables 1.2 and 1.3.

A typical wind speed/direction/Pasquill-Gifford atmospheric stability analysis is
shown in Table 1.4.

TABLE 1.4 EXAMPLE PASQUILL-GIFFORD STABILITY ANALYSIS

Typically, the atmospheric stability categories in the USA occur with the following
probabilities:
A-stability < 1%
B-stability 1-2%
C-stability around 10%
D-stability 50-70%
E-stability 10-20 %
F-stability 5-15%
G-stability <2%
In general the further the meteorological station is from the sea, the higher is the
frequency of stable and unstable conditions. Also, note that in the categorization
of atmospheric stability category in Table 1.2, there is no link between
temperature and stability category. In the USA we automatically associate F-
stability conditions with cold weather - in fact, the definition of atmospheric
stability is linked with cloud cover and incident radiation levels.
In the Far East, cloudless skies at night often occur far more frequently than in
the USA. This can lead to F-stability frequencies of 30%, even though
temperatures do not fall below freezing.
1.3.3 Monin-Obukhov Length Methods of Representing Atmospheric
Stability
Many gas dispersion models developed since 1990 have adopted Monin-
Obukhov length scaling methods. The Monin-Obukhov length (Lmo) is defined
by:-
where ρa is the air density (kg/m3); Ta is the air temperature (K); u* is the friction
velocity as defined above; k is the von Karman constant (0.4); H is the surface
heat flux (W/m2) - the heat flow from the ground into the atmosphere rather than
the incident radiative heat flow.

The Monin-Obukhov length is a parameter for the ratio of the mechanical
turbulent energy to that produced by buoyancy. It is an extremely awkward
parameter to use since in neutral atmospheric conditions (Pasquill - Gifford
stability category D), the surface heat flux is zero and hence L mo is infinite.
Consequently many models use as an input the reciprocal of the Monin-Obukhov
length. Note that the L mo is negative in unstable conditions and positive in stable
conditions.
For the gas dispersion practitioner, the Monin-Obukhov length is very difficult to
measure. To estimate u*, it is necessary to take measurements in order to
quantify the velocity profile of the wind flow with height above the ground.
Additionally the surface heat flux would have to be measured. In practice,
standard values for the Monin-Obukhov length are used. Also, because the
friction velocity is dependent on the ground roughness, the Monin-Obukhov
length is both a function of roughness length and atmospheric stability
category.
The following Figure 1.2, derived from Golder (1972) enables a direct
comparison to be made between Pasquill Gifford stability category and Monin-
Obukhov lengths.
Typical values of the reciprocal of the Monin-Obukhov length for a roughness
length of 0.1 m are:-
1 m/s A-stability - 0.5 m-1
2 m/s B-stability - 0.075 m-1 (or possibly as high as -0.1 m-1)
5 m/s C-stability - 0.01 m-1
5 m/s D-stability 0.00 m-1
3 m/s E-stability 0.01 m-1
2 m/s F-stability 0.05 m-1

2 AIR QUALITY STANDARDS
2.1 WHAT ARE AIR QUALITY STANDARDS?
Air quality standards are limits on concentrations of pollutants in the air.
The limits are usually set on the basis of the health effects of the particular
pollutants. In some cases, there are also limits designed to protect
vegetation (for example, World Health Organization guidelines for ozone).
In other cases, where pollutants interact, standards have been set for two
pollutants in combination: for example, European Union limits on smoke
and sulfur dioxide. The limits are designed to ensure that there would be
no significant adverse effects to the most vulnerable in society arising from
exposure to the pollutant at levels below the air quality standard.
Air quality standards are set by international or national governments.
Recommendations are also made by interested bodies, notably the World
Health Organization. The standards are used by licensing agencies such
as the United States Environmental Protection Agency, or the
Environment Agency/Scottish Environmental Protection Agency
in the UK. These bodies would use the standards to determine whether
pollution levels in their areas are acceptable. This will feed into their
readiness or otherwise to license new or existing processes, and may also
be used to limit the contribution that each individual process can make to
off-site levels of air pollution. Air quality standards apply to environmental
levels of pollutants from all sources in combination, rather than to
emissions from a single source, or works.
Air quality standards need the following components:
• Identification of the pollutant (for example, sulfur dioxide, or "particulate
matter which passes through a size selective inlet with a 50%
collection efficiency cut-off at 10 microns ( PM10)").
• A numerical concentration (for example, 100 parts per billion by
volume (ppb), or 50 micrograms per cubic meter (μgm-3
)).
• An averaging time for the numerical concentration (for example, 15-
minute mean, or running 24-hour mean).
• An acceptable level of compliance (for example, 99th
percentile, or
complete compliance) - see Box 1.

Additionally, air quality standards may have other relevant information,
such as an indication of their status (for example, legislative limit, or
government objective), details of their applicability (for example,
appropriate for use in sensitive areas, or particular designated planning
zones), and specification of the conditions to which the standards refer to
enable conversion between units (for example, 20°C, 760 mmHg
pressure)
Once all this information is known, it is possible to investigate measured
pollution levels to determine whether compliance with a quality standard
has been achieved. An example is given in Box 2. When compliance or
non-compliance has been established, it is also necessary to consider the
status of the standard to determine how significant this result is. For
example, could non-compliance result in prosecution for the company, or
significant expenditure in the period leading up to the implementation of an
objective?

As well as looking at measured pollution levels, it is also possible to
consider the results of dispersion models in the light of air quality
standards. This would enable a similar assessment to be carried out at
locations where measurements have not yet been carried out, for future
years at existing plants, or for new plants and developments. This kind of
assessment is very useful in obtaining licenses to operate new plant, and
in planning the extent of investment that will be necessary to meet
forthcoming air quality standards.
In the next sections, we will consider the various types of standards that
exist; what the standards actually are, and how they should be applied in
various situations.

2.2 WHAT AIR QUALITY STANDARDS EXIST?
2.2.1 General Background
There are two complementary approaches to regulating air pollution
emissions. You can either place limits on emissions (that is, what goes
up), or place limits on ambient concentrations (that is, what comes down) -
or both. Placing limits on emissions is an attractive approach, because it
enables the regulator to ensure that emissions from each source are
appropriately limited, and measurement is relatively straightforward. In
principle, this approach avoids the need to work backwards from high
ambient levels of air pollution to establish which sources should be
controlled.
The disadvantage is that careful specification and enforcement of
emissions controls is required to restrict levels of pollutants in air to
acceptable levels. The lack of overall controls of air pollution impacts in
the UK culminated in the smogs of the 1950s and 1960s, when as many
as 4,000 additional deaths were caused by air pollution within a few days.
Nowadays, emissions from individual sources of pollutants (including road
vehicles) are regulated. However, the lack of overall controls on emissions
of oxides of nitrogen and volatile organic compounds (VOCs)
particularly from road traffic results in high levels of ozone and
photochemical smog in many parts of the world.
2.2.2 United States
The United States has specified air quality standards since the
introduction of the Clean Air Act in 1970. Recently, revisions have been
made to the air quality standards for ozone and fine particulate matter
(July 1997). The current standards are given in Box 3. Many other
countries adopt the USEPA standards for use where there are no local
standards.

2.2.3 European Union
The European Union has specified ambient air quality standards for pollutants in
a series of directives in the 1980s. These are now implemented into
environmental legislation throughout Europe. The European Union standards are
given in Box 4.

For some pollutants, the existing standards comprise mandatory "limit
values" and discretionary "guide values". The limit values are mandatory
standards for application throughout member states, whereas the guide
values are intended to contribute to the long-term protection of the
environment, particularly in setting up specific environmental improvement
projects. These would not generally be directly relevant to all businesses
in Europe, although they may influence the policy of regulatory bodies.
In some European countries, additional standards have been specified.
These include The Netherlands, where standards have additionally been
specified for carbon monoxide and benzene. The Dutch standard for
benzene is an annual mean concentration of 10 µgm-3.
The European Union has recently implemented a directive known as the
"Air Quality Framework Directive". This directive lays down a mechanism
of establishing a sliding scale of air quality standards. Two levels can be
specified for a pollutant, the first for immediate application and the second
for application at a specified future date. In the intervening period, the
standard is progressively tightened towards the second more stringent
level. At the time of writing, proposed standards for sulfur dioxide, nitrogen
dioxide, PM10 and lead have been published (see Box 5). The link
between levels of sulfur dioxide and particulates (see Box 4) has not been
carried through into this new generation of air quality standards.

2.2.4 The Netherlands
Ambient air quality standards have been specified in the Netherlands which
exceed the current requirements deriving from the EU directives. The relevant
standards are given in Box 6.
2.2.5 Japan
Air quality standards are specified in the Basic Law for Environmental
Control. The standards were set between 1969 and 1978. The standards
of relevance to select chemical companies are summarized in Box 7.

2.2.6 Taiwan
Air quality standards are based on the Release of Air Quality Standard in
Taiwan. The standards of relevance to select chemical companies are
summarized in Box 8.
2.2.7 United Kingdom Air Quality Strategy
Recent developments in air quality policy in the UK are highly significant in
the development of air quality standards. A government advisory panel
known as the Expert Panel on Air Quality Standards (EPAQS) has made
recommendations for standards for 8 pollutants, with several more due to
be produced by the end of 1998. These recommendations do not have
any legal basis, but they have formed the basis of the UK air quality
strategy objectives.

The Environment Act 1995 provided for the preparation of a national air quality
strategy and guidance on its implementation. In 1996, the Government consulted
on the air quality strategy for the UK. In March 1997, this air quality strategy was
published in its final version. The EPAQS recommendations were used in this
document as objectives to be achieved by 2005 - see Box 9. The UK air quality
strategy and the air quality objectives contained in it will be very influential in
guiding Environment Agency and Local Authority thinking on air quality.
Regulations implementing the air quality objectives have been made under the
Environment Act, and commenced in December 1997, specifying that
compliance is to be achieved by 2005.
The Environment Act also introduced a program of "Local Air Quality
Management" in which local authorities are required to assess their air quality. If
it appears that the statutory air quality objectives will not be met by 2005, then a
local air quality management plan should be devised and implemented to ensure
that the objectives will be met. This may include additional controls on industrial
emissions and traffic pollution, although the plan should ensure that the burdens
on various sectors are "proportionate".
2.2.8 Non-Governmental Organizations
The World Health Organization published an influential set of air quality
guidelines in 1987 ("Air Quality Guidelines for Europe", WHO European Office,
Copenhagen). These were, in general, relatively stringent guideline values for
levels of air pollutants, and included guidelines for pollutants not covered in
legislation. Guidelines were specified to protect not only human health, but also
components of the natural environment. These guidelines have been used by
select chemical companies as objectives for ambient air quality for pollutants
which do not have air quality standards (for example, vinyl chloride and toluene).
The guidelines are also used by some countries in place of specific local air
quality standards (for example, Pakistan). The guidelines are due to be updated
during 1998, but the process has currently stalled due to financial difficulties
within the WHO. The draft guidelines are given in Box 10.

The World Bank has also specified air quality standards for use in
assessing projects which it funds - see Box 11. These have been adopted
for use in some countries including Pakistan

2.2.9 Occupational Exposure Limits
Occupational exposure limits exist for a very wide range of pollutants.
These are specified to protect the health of employees in the workplace.
Ambient air quality guideline values are frequently derived from these
occupational exposure limits for pollutants which do not have any specific
air quality standards or WHO guidelines. These occupational limits
themselves should not be used directly for ambient air, as they are
appropriate to fit and healthy adults (making no allowance for sensitive
members of the population such as children and those suffering from
respiratory disease), and they are specified on the basis that exposure
takes place during working hours only.
With suitable adjustments to allow for these constraints, however, ambient
air quality guideline values can be derived from the occupational exposure
limits. This is achieved by dividing the occupational exposure limit by a
specified factor to give the ambient air quality standard. A range of factors
have been used for this purpose in the past, ranging from one twenty-fifth
to one hundredth.
In the UK, the Environment Agency has issued guidance on how this
conversion should be addressed in a recent publication (Technical
Guidance Note (Environmental) E1, "Best Practicable Environmental
Option Assessments for Integrated Pollution Control", 1996). It indicates
that "environmental assessment levels" for pollutants can be
determined as follows:
• Hourly mean concentration:
2% of the 15-minute maximum exposure limit (MEL: these are
occupational exposure limits for carcinogens) or 10% of the 15-
minute occupational exposure standard for materials where no
MEL has been specified (i.e., non-carcinogens).
• Annual mean concentration:
0.2% of the 8-hour maximum exposure limit or
1% of the 8-hour occupational exposure standard.
The guidance note indicates that individual processes should be a "priority
for control" if they contribute more than 2% of the environmental
assessment level for a given pollutant.

This is a highly restrictive constraint, and in practice, a process that
contributes less than 10% of the environmental assessment level will
generally be considered acceptable.
Further guidance on acceptable off-site concentrations is given in Box 3 of
Part 4.
2.2.10 General Comparison
In general, newer air quality standards tend to be more stringent than
older standards, and guidelines tend to be more stringent than regulatory
limits. The least stringent standards are standards specified during the
1970s and 1980s such as the US National Ambient Air Quality Standards,
and the existing set of EU standards. Standards based on occupational
health guidelines also tend to be relatively lax. For example, the hourly
average environmental assessment level for use in the UK based upon
10% of the occupational health standard for nitrogen dioxide would be 500
ppb. In contrast, the UK Air Quality Strategy objective for hourly mean
nitrogen dioxide concentrations is 150 ppb.
Newer standards such as the UK Air Quality Strategy objectives, the EU
daughter directive proposals and the US standards for ozone and PM2.5
are more stringent than the existing legislative standards, and cover a
wider range of pollutants. The UK Air Quality Strategy objectives are
similar to the WHO guidelines of 1987 in most respects, although
for particulate matter, new information has led to a significantly tighter
objective. The WHO guidelines also cover a wider range of pollutants. It
may be expected that the revised WHO guidelines to be issued during
1998 will be more stringent than the 1987 document.
2.2.11 Air Quality Standards for Odor Impacts
Odor impacts are likely to become an increasingly important driver of limits
on air pollution emissions. In many countries, process operators are
required to ensure that there is no off-site odor.
Odor impacts can be forecast, or estimated from process emissions data,
but the procedure is very uncertain, and because a large number of
safeguards must be built in, the assessments are of necessity very
stringent in terms of acceptable release conditions.

A reasonable standard for off-site Odor would be that hourly average
concentrations of an odorous chemical should not exceed 2.5% - 5% of
the Odor threshold. Odor thresholds are discussed in Box 12. This
standard would provide a sufficient safety margin to protect against the
uncertainty in Odor threshold measurements, short-term fluctuations in
concentration that can give rise to transient Odor, and the variability in
human response to different Odors. The Odor standard is likely to be
much more stringent than the corresponding health-based guidelines,
reflecting the fact that Odor is generally significant at lower concentrations
than health effects, and also reflecting the additional safety margin in the
Odor standard. It should be noted that for a few chemicals such as
ethylene dichloride, the health impacts occur at concentrations below the
Odor threshold.
2.3 WHAT IS THE LAW - AND WHAT ISN’T
A clear distinction should be made between air quality standards which
comprise legal limits in particular countries, and other recommendations
and guidelines which are not limits. In practice, air quality standards are
frequently exceeded in many parts of the world - particular problems
surround standards for ozone and fine particulate matter. This does
not translate into legal action against emitters of pollution. Process
operators would be affected by air quality standards under the following
circumstances:
• A new process is highly unlikely to be permitted if emissions will
lead to a contravention of an air quality standard.

• In some countries (for example, China), different air quality
standards apply in different planning zones. Thus, the locations
where chemical industries may be located could be restricted by
the more stringent standards in some areas.
• Continued operation of a process may be at risk in an area where
air quality standards are frequently exceeded. Under these
circumstances, the process operator may be required to reduce
emissions to enable the air quality standards to be met.
Legislative air quality standards currently applicable in various parts of the
world are as follows:
USA: National Ambient Air Quality Standards (see Box 3)
European Union: Directives 80/779, 82/884, 85/203 as enacted in
individual Member States (see Box 4)
Netherlands: Legal and non-legal air quality standards (see Box 6)
Japan: Basic Law for Environmental Control (see Box 7)
Taiwan: Release of Air Quality Standard in Taiwan (see Box 8)
Air quality standards are progressively tightening. The EU is due to
propose a range of new and progressively tightening standards for air
quality over the coming year. New limits for nitrogen dioxide, smoke,
particulate matter and lead have been specified (see Box 5). These will be
made under the "Framework Directive", and will eventually have legal
force. In the period between the standards being adopted by the EU and
their implementation in individual member states, they should be treated
as if they were legal limits. For design of new plant in the EU at any time,
the new limits should also be treated as if they had legal force to ensure
that plant design is adequate. In the UK, the new standards are unlikely to
lead to a significant additional burden on industry, over and above the
burden imposed by the new UK air quality objectives.
There are now objectives for air quality in the UK, specified as part of the
UK air quality strategy. These objectives are shown in Box 9. The
objectives will be reviewed during 1998, and may be tightened, and/or
brought into line with any new European air quality standards.

The objectives will have legal force, but the onus will be on local
authorities to implement air quality management plans to achieve
compliance by 2005 rather than on individual process operators. Thus,
local authorities and/or the Environment Agency are likely to require action
to be taken in any areas where it is likely that the objectives will not
be achieved. These objectives may impact select chemical companies as
the regulatory bodies assess their requirements for reductions in the
impact of industrial air pollution to meet the air quality objectives by 2005.
Select chemical companies operating in the UK may need to be prepared
to undertake independent assessments of the impact of their air pollution
emissions in order to ensure that any additional regulatory burden is
appropriate and proportionate (see Box 13 for an example).
Apart from these legislative and proposed air quality standards and
objectives, a number of other guidelines for ambient air quality may be
used. These do not have legal force. They would be used where
businesses are releasing compounds for which there are no other air
quality standards. This covers a wide range of Select chemical companies
process emissions, whereas combustion emissions would generally be
covered by the air quality standards and objectives. The World Health
Organization standards and the application of occupational health
standards in ambient air quality assessments is described in Section 2.2
above.
National Air Quality Standards do not apply in plant areas to which the
public cannot gain unrestricted access. In these areas, Occupational
Exposure Standards (OESs) and Maximum Exposure Limits (MELs) are
appropriate.

Materials with a Maximum Exposure Limit present serious concerns about
possible health effects in workers. In practice, MELs have most often been
allocated to chemicals for which there is no clearly defined safe
concentration level and for which there is no doubt about the seriousness
of the hazard posed by the substance. Usually MELs are defined for
chemicals which are carcinogenic or can cause occupational asthma.
OESs are set at levels below which it is believed (based on current
scientific knowledge) that the substance would not damage the health of
workers exposed to it day after day.
For listings of OESs and MELs, see either the UK Health and Safety
Executive’s EH 40 document - “Occupational Exposure Limits”, which is
published annually.
The HSE provides the following guidance on how to apply OESs and
MELs:
“Applying OESs:- if exposure to a substance that has an OES is reduced
at least to that level, then adequate control has been achieved. If this level
is exceeded, the reason must be identified and measures to reduce
exposure to the OES put into action as soon as reasonably practicable.
Applying MELs:- Exposure should be reduced as far below the MEL as
reasonably practicable and should never exceed the MEL when averaged
over the appropriate reference period.”
2.4 FUTURE DEVELOPMENTS
In general terms, the most significant future development is the
progressive tightening of air quality standards around the world. One
example is the recent introduction of a tighter ambient air quality standard
for ozone, and a new standard for PM2.5 in the USA.
New air quality standards have been drafted by the European Commission
(see Box 5). A further standard for ozone is expected to be published by
the end of 1998, with proposals for polycyclic aromatic hydrocarbons and
some heavy metals to follow. These represent a considerable tightening of
standards in comparison to current air quality standards in Europe.

The planned revisions to the World Health Organization air quality
guidelines are unlikely to have as profound an impact as the original 1987
guidelines. Many of the considerations adopted by the WHO have been
taken on board by bodies such as the US EPA and European Union in
setting air quality standards.
A significant future development in the UK will be the implementation of
the air quality strategy up to 2005. This may well lead to additional
constraints on industrial emissions in some areas where the UK air quality
objectives would not otherwise be met. These constraints may be
implemented through limits on emissions agreed between individual
process operators and the regulator (Environment Agency and/or Local
Authority). The current set of air quality objectives (see Box 9) are under
revision, with revised targets and/or dates to be published during 1998.
Again in the UK, the implementation of Technical Guidance Note E1 may
lead to tighter restrictions on emissions of pollutants not covered by the air
quality strategy. This is because of restrictions on the contribution of
individual processes to ambient levels of air pollutants. The guidance
indicates that those pollutants contributing more than 2% of the
Environmental Assessment Level off-site will become "a priority for
control". It will not be possible to apply this process in practice because of
the large number of industrial processes which will become "priorities for
control". A value of 10% of the EAL is generally considered to be
acceptable. However, the Guidance Note does indicate a significant shift
in Environment Agency policy.
Odor issues are likely to become an increasing driver for restrictions on
emissions. This reflects some success in dealing with emissions of the
health effects of pollutants, and also sustained public awareness and
concern regarding air pollution. There is very little formal guidance on the
assessment of odor emissions, but it is likely that plants which have
known odor problems are likely to come under increasing pressure to
control the emissions. If this cannot be achieved via process
improvements, investment in end-of-pipe control equipment may be
required.
Finally, aesthetic effects may well become more important. Already, local
authorities are often unwilling to allow new tall stacks to be constructed
because of their visual impact. In the next few years it is likely that industry
will be under pressure to reduce the visual impact of large plumes of water
vapor from vents.

3 MODEL COMPARISON AND SELECTION
3.1 CLASSIFICATION OF DISPERSION MODELING PROBLEMS
Dispersion modeling problems are commonly categorized as “safety” or
“environmental.” - see Figure 3.1. “Safety” issues involve the assessment
of the consequences of unplanned releases which may present a
significant direct hazard to the health of individuals located either on or off-
site. Because the majority of chemicals used by chemical businesses are
heavier than air, these are usually dense gas releases. Storage at low
temperature also tends to result in releases of gases which are denser
than air. These are seen as safety issues because the effects are
potentially serious, and the release will only take place over a short period.
In contrast, “environmental” issues generally arise from continuous or
intermittent releases of material of similar density to air (“neutral”), or
lighter than air (“buoyant”). Occasionally, continuous releases may be
more dense than the air. These are generally planned releases of material
arising from normal process operations. Any effects of these releases tend
to be most significant off-site. As well as short-term toxicity effects, the
assessment of environmental releases also takes into account the effects
of long-term exposure to released materials. In some cases, consideration
is given to effects on the natural environment, as well as on the human
population. For the purposes of dispersion modeling, there is some
overlap between the two categories of problem.

FIGURE 3.1 CATEGORIZATION OF DISPERSION MODELING PROBLEMS
The range of potential release scenarios means that a large number of
dispersion modeling tools have been designed to assess their
consequences. The aim of this Part of the guide is to provide guidance on
selecting the appropriate tool for a particular problem. The appropriate
model(s) to use for a particular application is dependent on the initial
density, duration and location of the release.

3.2 WHAT MODELS ARE AVAILABLE?
To deal with the situations for which dispersion modeling is required, a
range of modeling tools have been developed. For the purposes of this
guide, a “model” is defined as a computational code which provides an
airborne concentration of material given a set of release conditions, a set
of meteorological conditions, and a location relative to the source.
These have been developed to varying specifications over and above the
minimum model definition. For example, some models contain algorithms
for calculating loss rates of material, given some assumptions regarding
the quantity of material, the size and location of a leak, etc. Some models
permit highly flexible specification of the locations at which concentrations
are to be calculated, or permit the use of long-term meteorological data to
calculate long-term mean concentrations of material. A number of
commonly-used models are listed in Table 3.1, together with an indication
of the type of situations in which they can be applied, and their
functionality.
TABLE 3.1 COMMONLY USED DISPERSION MODELS

3.3 DESCRIPTION OF AVAILABLE MODELS
3.3.1 General
As indicated in Table 3.1, the dispersion models listed in the table all have
advantages and disadvantages associated with their use. The aim of this
section is to set out the pros and cons of each model, and to provide some
practical guidance in using each model. Finally, Table 3.2 summarizes the
type of problem for which each model should be used.
The use of dispersion models is regulated to varying extents in different
countries of the world. In some countries, a specific model needs to be
used in a specific way; in other countries, the applicant is free to use any
appropriate model. Some examples are as follows:
• Germany: Dispersion modeling to be carried out as laid down in TA
Luft regulations. These specify the dispersion equations to be used,
and appropriate values for many of the inputs.
• UK: ADMS is preferred by the Environment Agency for regulatory
applications, but no formal guidance exists.
• Netherlands: EFFECTS is the preferred model for dense gas
releases, and PLUIM for buoyant/neutral releases.
• USA: A variety of different models are approved by the US EPA for
various situations, as laid down in Appendix W to the 40th
Congressional Federal Register part 51. For modeling point source
emissions in non-complex terrain, ISC is recommended (section 4.1
of Appendix W; see the USEPA web site for further details:
www.epa.gov). For dense gas dispersion modeling, any appropriate
model is permitted.
Attempts are currently under way to harmonize the approach to dispersion
modeling across national boundaries, but many countries (e.g. Hungary)
insist on the use of a national dispersion model.

3.3.2 ADMS (Atmospheric Dispersion Modeling System)
This model is produced and developed by Cambridge Environmental
Research Consultants on behalf of the Environment Agency, Health and
Safety Executive, and a consortium of industry and government bodies
including select chemical companies. The model flexibility is enhanced
with a number of additional modules for dealing with specific cases, as
listed in Table 3.1.
The model is designed for neutral density and buoyant releases. ADMS
can also be used for releases of dense gases from elevated sources
providing the plume does not slump to ground level. This can be a matter
of judgment, but some indication can be gained from consideration of the
plume centerline height, and/or by considering near-source results
from a dense gas dispersion model such as PHAST.
The model is straightforward to use, with a series of screens providing
rapid data entry. The model is supplied with a range of example source
and meteorological data files, which can be used as a basis for compiling
inputs for other applications. The file "r91a-g.met" is particularly useful, as
it provides a set of 7 meteorological conditions representative of the range
of conditions encountered in temperate regions.
ADMS has a straightforward x-y plotting program, and can provide contour
plots via a link to the SURFER package. The program can be linked to a
GIS system if required, to facilitate data input and results presentation.
The program uses state-of-the-art understanding of meteorology to
represent the atmospheric boundary layer. Output is provided in a set of
separate ASCII text files, which can be imported into other applications
if required. Percentile concentrations can be obtained provided the
appropriate meteorological data is used: this is useful for obtaining
predictions in terms of air quality standards and objectives. ADMS is the
preferred model for regulatory applications in the UK. In view of its
technical merits and the wide range of problems it can deal with, it is
also recommended for use outside the UK in situations where no other
model is specified by the regulatory authority.
ADMS only permits modeling to be carried out for a limited number of
receptors (maximum grid size: 31 x 31 x 2 receptors). This may be a
restriction for some applications. Model run times can be very long when
long-term meteorological data is being used, particularly where building
effects or complex terrain are incorporated into the model.

3.3.3 ALOHA (Areal Locations of Hazardous Atmospheres)
This model is a user-friendly version of the US Coastguard/University of
Arkansas model DEGADIS. It is extremely user-friendly, and enables even
a novice user to set up appropriate meteorological and source inputs
rapidly. The tank source options are particularly user-friendly with
graphical images to assist the source specification. It is probably best
used as an emergency response tool, with more complex planning cases
being handled by a more flexible model such as PHAST. It may also be
appropriate for use in Risk Management Planning in the US.
The model can provide indoor concentrations of pollutants, based on
certain assumptions relating to air exchanges in the building. The model
can handle a variety of source types including mixed aerosol/vapor
releases arising due to a tank rupture, and liquid puddles. Because
ALOHA is set up to model releases from a relatively simple set of cases in
an emergency situation, more complex cases cannot easily be modeled.
The major disadvantage of ALOHA for planning purposes is that receptors
must be specified individually, and the model re-run for every receptor.
The model only allows for a one-hour run time, and so concentrations are
not predicted at locations where the maximum concentration from a
release would not have been reached one hour after the release. The
model has also been found to reset parameters without warning (for
example, changing units from mgm-3 to ppm), and frequently clears the
values of modeling parameters which have already been entered. This
can occur for example when attempting to edit the source details if the
wrong type of source is selected in error.
3.3.4 DISP2
DISP2 was developed by a European chemical company, and has two
components. The BURST model gives concentrations arising from a short
emission period, and the PLUME model gives concentrations arising from
a continuous emission. It has been shown to be robust in handling a wide
range of cases for both environmental and safety applications. However,
it cannot handle two phase releases. The model is not used outside of the
European chemical company that developed it, and there may be
problems in justifying its use to regulatory authorities. It should, in general,
not be used for new applications as there are externally validated models
available which can cover most of the situations for which DISP2 was
designed.

DISP2 is straightforward to operate. Study inputs are entered via a series
of screens in a logical sequence. The "mass fraction" option for burst
releases (i.e., instantaneous catastrophic releases) of mixtures should not
be used, as there is an error in the software associated with this option.
The model is limited to three values for surface roughness (0.01 m, 0.1 m
and 1 m). This can be a restriction in the common situation of a release in
a typical urban/industrial area where a value of around 0.3 - 0.5 m would
be appropriate. Using a surface roughness of 0.1 m could underestimate
the influence of the terrain on dispersing emissions from a ground level
release. Data for each run is saved in a file named
"c:windowstempanalysis.lis": the data in this file is overwritten each time
the model is run, and must be extracted between runs if required. More
information is given in the model problem file (*.prb)
A maximum of 21 receptors downwind of the source is permitted. A single
source, wind direction and downwind line of receptors is considered in
each model run, although the model does provide off-axis concentration
isopleths if required. The meteorological data entry can be unclear: the
available conditions are specified using codes such as "B2." The default
setting is for the number 2 in this context to refer to Force 2 on the
Beaufort scale, rather than 2 ms-1.
Care needs to be taken when considering dense gas releases to ensure
that two-phase effects are not significant.
STACK2 is a multi-source neutral/buoyant release dispersion model. It
should not be used for new applications as both ADMS and ISC can be
used to carry out all the calculations that are possible with STACK2.
3.3.5 ISC (Industrial Source Complex)
ISC is available as ISCST (Short Term) and ISCLT (Long term) and was
developed by the US-EPA. This model is very widely used throughout the
world for environmental modeling applications. It is prescribed for
regulatory use in the USA. The core of the model is now some 20 years
old, and it contains some major shortcomings - for example, the inability to
specify the terrain surface roughness (see below). The model is due to be
replaced by a new version currently known as “Aermod” within the next
12-18 months. The use of ISC is prescribed for regulatory calculations in
the US and some other countries. Where the use of ISC is not prescribed,
it would generally be preferable to use ADMS in view of the more
advanced modeling methods and greater flexibility of ADMS.

ISC is not straightforward to use. In principal, a logically ordered input
(.dat) file is written, and the model produces an output .lst file. However,
the user interface requires a number of apparently unrelated inputs to
compile an appropriate .dat file, and it can be difficult to provide a
combination of inputs that is acceptable for the model. For example, a
parameter "RUNORNOT" must be set to "RUN" for the model to proceed.
Some processing of the output .lst file is necessary to produce contours
which can be incorporated into a graphical plotting or numerical analysis
package. This can be time consuming and is a potential source of error.
The model is very flexible in terms of the number and type of sources that
can be included. Also, concentrations can be modeled at a very large
number of receptors. The model runs relatively quickly, which is useful for
producing long-term statistics based on measured meteorological data. A
serious disadvantage is that the effects of surface roughness can only be
incorporated by running the model in "urban" or "rural" modes, and it is not
clear what values of Zo these modes correspond to. As a rough guide,
"rural" is likely to correspond to Zo ~ 0.1 m, and "urban" is likely to
correspond to Zo ~ 1.0 m.
Some example meteorological data is provided with the model; however, it
is not straightforward to provide data in the correct format for the model to
use.
User friendly versions of the US-EPA models are produced by various
Consultants in the USA. File driven versions of models, available free of
charge from the US-EPA Bulletin Board, are very difficult to use.
3.3.6 PHAST (Process Hazard Assessment Tools)
This model has been developed by DNV Technica. It is probably the most
sophisticated general purpose hazard assessment software package
currently available - for example, it covers high momentum jet releases at
a range of angles, catastrophic dense gas releases, pool evaporation, two
phase releases, fires and explosions. The model has an extensive
physical properties database. The two main drawbacks are firstly its cost -
with an annual maintenance fee of around; secondly, there are a number
of bugs in the program. Some of these are inconvenient, but others could
give rise to serious errors in executing the model.

GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases

GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases

Similar to GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases (20)

More from Gerard B. Hawkins

More from Gerard B. Hawkins (15)

Recently uploaded

Recently uploaded (20)

GAS DISPERSION - A Definitive Guide to Accidental Releases of Heavy Gases