Understanding Atmospheric Dispersion of Accidental Releases

55 Pages·1995·2.3 MB·English

by  George E. Devaull

Understanding
Atmospheric Dispersion
of Accidental Releases
A CCPS Concept Book
George E. DeVaull
John A. King
Ronald J. Lantzy
David J. Fontaine
Center for Chemical Process Safety
of the
American Institute of Chemical Engineers
345 East 47th Street, New York, NY 10017
© Copyright 1995
American Institute of Chemical Engineers
345 East 47th Street
New York, New York 10017
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ISBN 0-8169-0681-5
PRINTED IN THE UNITED STATES OF AMERICA
DISCLAIMER: It is sincerely hoped that the information presented in this document will lead to an even more
impressive safety record for the entire industry; however, the American Institute of Chemical Engineers, its
consultants, CCPS subcommittee members, their employers, their employers' officers and directors, and the
authors disclaim making or giving any warranties or representations, express or implied, including with respect
to fitness, intended purpose, use or merchantability and/or correctness or accuracy of the content of the information
presented in this document. As between (1) the American Institute of Chemical Engineers, its consultants, CCPS
subcommittee members, their employers, their employers' officers and directors, and the authors and (2) the user
of this document, the user accepts any legal liability or responsibility whatsoever for the consequence of its use
or misuse.
Preface
As introduction to the subject of this document, dispersion from accidental
releases encompasses many different aspects. It includes the methods of classical
air pollution modeling used in estimating the dispersion of dilute air pollutants,
but also encompasses the more complex physics of dense-vapor and high-mo-
mentum releases, boiling and evaporating liquids, multiphase flow, vessel
blowdown, and aerosol transport.
For the newly initiated, such a broad-based array of topics can be over-
whelming. The purpose of this document is to present a basic description of
accidental release problems in the context of a technical narrative. The material
is intended only as a basic introduction and the interested reader is encouraged
to consult the cited references for further information. We also note that, in some
aspects of accidental release physics, current understanding is only rudimentary,
and further work may be needed toward development of practical engineering
models.
We also recognize that vapor dispersion modeling does not stand alone in
evaluating process hazards. Such analysis must be used within a more compre-
hensive context of overall risk analysis and risk management. The American
Institute of Chemical Engineers (AIChE) has a long history of involvement with
loss control and process safety for the petrochemical and chemical industry.
Through its strong ties with engineers, operators, safety professionals, and
academia, the AIChE has enhanced communication and fostered improvement
in the high safety standards of the industry. AIChE publications and symposia
have become a prime information resource for the chemical engineering profes-
sion on the causes of accidents and means of prevention.
The Center for Chemical Process Safety (CCPS) of the AIChE was char-
tered to develop and disseminate technical information for use in the prevention
of major chemical accidents. In pursuit of this charter, the CCPS has published
Guidelines, Workbooks, and Conference Proceedings on process safety, risk
analysis, hazard evaluation, and vapor dispersion. The CCPS is supported by
over 80 world-wide industrial sponsors in the chemical process industry who
provide the necessary funding and professional guidance to its technical com-
mittees. A partial list of available CCPS publications is given opposite the title
page of this book.
Organization of This Book
This volume is intended as a brief introduction to the various topics addressed
in accidental releases and atmospheric dispersion. A list of references cited
within this text is given at the end of this document for individuals who desire
more detailed information.
The topics addressed in this primer include a basic introduction in Chapter
1 and a discussion of turbulence and meteorology in Chapter 2. In Chapter 3 the
problem of release rate estimates is covered. Following this, in Chapter 4, the
problem of liquid pool evaporation is covered. Momentum or buoyancy domi-
nated jet releases are covered in Chapter 5, and the problem of low-momentum
heavy-gas releases is covered in Chapter 6. In Chapter 7, the problem of passive
dispersion, where ambient turbulence dominates the dilution process, is dis-
cussed. Chapter 8 includes discussion a number of special complications that
can arise in a dispersion analysis. Chapter 9 is a discussion of toxic vapor hazard
levels. Chapter 10 includes information on the availability and use of computer
models for accidental release air dispersion modeling. Each chapter is intended
as a topical overview of the problem of interest. References are included for
further information on the covered subjects.
Acknowledgments
This book was authored by George E. DeVaull (Shell Oil Company), John A.
King (Shell Oil Company), Ronald J. Lantzy (Rohm & Haas), and David J.
Fontaine (Chevron). Significant contributions in content and in technical review
by members of the CCPS Vapor Cloud Subcommittee are acknowledged. The
committee was co-chaired by Ronald J. Lantzy and Ronald D. Myers (Rohm &
Haas), vice-chaired by Gib R. Jersey (Mobil), and, in addition to the authors,
included (in alphabetical order), Douglas N. Blewitt (Amoco), Sanford G.
Bloom (Martin Marietta), C. K. Cheng (Monsanto), Donald J. Connolley
(AKZO Chemicals), Seshu Dharmavaram (du Pont), Ebrahim Esmaili (Exxon),
Jerry R. Foster (Union Carbide), David Guinnup (U.S. EPA), William J. Hague
(Allied-Signal), Gene K. Lee (Air Products and Chemicals), David McCready
(Union Carbide), Malcolm L. Preston (ICI Engineering), Jerry M. Schroy
(Monsanto), Kenneth W. Steinberg (Exxon), and David Winegardner (Dow).
William J. Minges, Bob G. Perry, Jack Weaver, and Lester H. Wittenberg of
CCPS provided staff support. Comments and suggestions from David J. Wilson
(University of Alberta) and John L. Woodward (DNV Technica) are acknow-
ledged. The support and contributions of Larry V. Csengery, Dan C. Baker, A.
Bruce Krewinghaus, and Judy C. Moorad of Shell Oil Company in preparation
of this book are greatly appreciated.
Nomenclature
Variable names are listed here; they are also defined when first used in the text.
A cross sectional or surface area
c pollutant concentration
CD empirical discharge coefficient
Cp specific heat
d liquid droplet diameter
Dj jet diameter
Fro initial densimetric Froude number Fro = wj/(Djg Ipj - pool/poo) 2
Fr Froude number Fr = MJ/(Djg)!/
g gravitational constant
h vertical height or depth
H sensible heat release
HQ sensible heat flux
k von Karman constant ~ 0.40 (in the atmosphere)
L Monin-Obukhov length L = (PaCp^3) / (*g//o),
p total pressure
q pollutant emission release rate
/?WW(T) autocorrelation function of velocity w as a function of time lag T
p density
pa air density
poo ambient density
pj jet density
s entropy
t time
TLw, TL,V, TL, w Lagrangian (frame of reference moving with the fluid) integral
time scales for w, v, and w
^i,w, Ti,, ti,w Eulerian (fixed frame of reference) integral time scale for
v
u, v, and w
T^C shear stress, normal to z, in x direction
T temperature
7 ambient temperature
a
Tas adiabatic saturation temperature
6 initial jet release angle
o surface tension
Cy, GZ spatial standard deviations of a plume cross-section in the y and z
directions, respectively
a , a , (Tw standard deviation of w, v, or w velocity
M v
W friction velocity
514
WOO ambient velocity
wj initial jet velocity
w, v, w velocity vectors in the downstream, cross stream, and vertical
_ direction, respectively
w mean value for w
V volumetric flowrate
2
Wed droplet Weber number Wed = pa" d/G
x, y, z spatial coordinates in the downstream, cross stream, and vertical
direction, respectively
Zi inversion height, overall depth of the thermal boundary layer
ZO aerodynamic surface roughness height
Contents
Preface ................................................................................ vii
Nomenclature ...................................................................... xi
1. Introduction ................................................................. 1
1.1 Purpose ............................................................................ 1
1.2 Release/Dispersion Scenario Overview ........................... 1
1.3 Hazards ............................................................................ 3
2. Meteorology ................................................................. 6
2.1 The Atmosphere ............................................................... 6
2.2 Turbulence in the Atmosphere ......................................... 6
2.3 Mechanically Generated Turbulence ................................ 8
2.4 Vertical Density Stratification and Buoyancy .................... 8
2.5 Atmospheric Stability Classifications ................................ 10
2.6 Similarity Scaling in the Atmospheric Boundary
Layer ................................................................................. 12
2.7 Changes over Time in the Atmospheric Boundary
Layer ................................................................................. 13
3. Source Estimates – Leaks and Ruptures .................. 15
3.1 Leaks and Small Holes ..................................................... 15
3.2 Phase Changes in Released Fluids ................................. 16

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v
vi Contents

3.3 Aerosol Formation in Liquid or Flashing Liquid
Releases ........................................................................... 18
3.4 Transient Vessel Inventory Loss ...................................... 18
3.5 Catastrophic Vessel Failures ............................................ 19
4. Sources – Liquid Pools .............................................. 20
4.1 Boiling Liquid Pools .......................................................... 20
4.2 Evaporation of Volatile Liquids ......................................... 21
4.3 Evaporation of Relatively Nonvolatile Liquids ................... 21
4.4 Multicomponent Mixture Spills .......................................... 22
5. Buoyant and Dense-Gas Jet Releases ...................... 23
5.1 Jet Length Scales ............................................................. 23
5.2 Momentum and Buoyancy ................................................ 24
5.3 The Effect of Wind and Ambient Turbulence .................... 25
6. Low-Velocity Dense-Gas Releases ............................ 28
6.1 Source Specification ......................................................... 28
6.2 Source Area Region ......................................................... 28
6.3 Stably-Stratified Region .................................................... 30
6.4 Passive Dispersion Region ............................................... 30
7. Passive Dispersion ..................................................... 31
7.1 The Mechanics of Turbulent Dispersion ........................... 31
7.2 Passive Dispersion from Elevated Releases .................... 32
7.3 Near-Ground Passive Dispersion ..................................... 34
7.4 Dispersion Averaging Times ............................................. 34
8. Complex Flow Considerations ................................... 35
8.1 Building Wakes and Stack Downwash ............................. 35
8.2 Gravity-Driven Flows and the Effects of Terrain ............... 36

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