API PUBL 4628

Abstract

<b><u>Scope:</u></b> <span id="LabelScope"> <br> The purpose of this Guidance Manual is to provide methodologies for consequence analysis purposes. That is, given a potential or after-the-fact set of circumstances for the accidental release of a chemical fluid into the atmosphere (the "scenario"), what are the appropriate methods to estimate spatial and time dependent concentrations of the material over a particular geographical area? <br> Scenarios may be generated within the hazard analysis parts of overall risk assessment studies. Techniques such as HAZOPS, fault tree, and "what if" analyses are used to discover potential hazardous situations which lead to an accidental release. Concentrations predicted for an identified release scenario may then be used to estimate possible impact of the shock waves and thermal damage from vapor cloud explosions, or potential toxic effects. The impact of toxic dispersed vapor clouds requires interpretation by specialists such as toxicologists and industrial hygienists. Thus the person or persons doing the source/release and dispersion modeling must interact "upstream" with those doing hazard analyses and with those "downstream" who interpret health or flammability aspects. <br> The modeling methodologies presented and recommended are intended for use in risk assessment studies for refinery/chemical plants during design or operation as well as for emergency response planning purposes. Also, available quantitative or semi-quantitative methods for mitigating a release after or during its occurrence are discussed. The product of any modeling exercise is an estimate of concentrations of the released material over a potentially affected geographic area so that possible toxic and/or flammability impact can be estimated. Modeling procedures are recommended on the basis of applicability to the particular situation being considered, required accuracy of results, simplicity, and availability of computer codes. If comparable modeling methods are available, they are discussed and used selectively in the example release scenarios. The calculational procedures and computer programs discussed are in the public domain. To limit the extent of the demonstrative simulation work, only SLAB, HGSYSTEM and DEGADIS program systems were used. <br> The release/dispersion scenarios described and exemplified are hypothetical; that is, they do not describe any particular accident that has occurred, nor a known situation for which an accident is liable to occur. However, the treatment and examples are realistic, for they have been drawn from the experience and knowledge of many technical personnel working in industry, government and academia. Calculational procedures used and recommended are well-accepted and are state-of-the-art in terms of readily available methods in the public domain; for example, the calculations for initial fluid release amount/flow rate and its concomitant physical state use standard chemical engineering methods, including thermodynamic and physical property estimation. Additionally, the turbulent jet and dense gas atmospheric dispersion models are based on atmospheric boundary layer theory and other physical principles; the models have been extensively evaluated against small and large scale experiments. <br> This Manual does not cover the origination of release scenarios, nor the interpretation of concentration estimates, except to demonstrate the concepts. These factors should be established separately for each individual modeling application. Effects criteria, such as flammability limits and references to toxic materials, toxicities, or toxic concentrations levels, (concentration of concern), are used only to impart a sense of reality to the various examples and should not be used without review. Estimation and/or interpretation of possible toxic effects of model-predicted chemical concentrations are outside the scope and purpose of this Manual. The sidebar suggests a method for selection of flammability limit concentrations if no other information is available and/or for screening purposes. <br> The scope of the manual does not include model development and evaluation. Use of a specific modeling program for a type of application does not imply that program is particularly recommended. In general, problems encountered by the author in using a program were made known to the program author. <br> Manual Organization <br> Chapter 2 of the Manual provides an overview of phenomena which must be considered in defining release/dispersion scenarios. The phenomena involved are discussed qualitatively so that the applicability of modeling methods described in later chapters can be appreciated. Eight representative scenarios are described for the purpose of previewing types of problems, which will be analyzed, simulated and discussed in Chapter 6. <br> Chapter 3 provides working methods for estimating the rates and physical states of released substances on entering the atmosphere. Because chemical engineering thermodynamics is extensively used, a working overview to this subject provides the basic concepts involved, as well as references to sources of physical and thermodynamic data required for the calculations. Calculational methods follow for releases from process equipment (valves, openings in vessel walls, sudden vessel failure, etc.). The releases may form "clouds" consisting of vapor, and vapor plus liquid aerosols. In some cases, evaporating pools of liquids may be formed on water or ground surfaces. The best, practical, calculational methods are presented for the current state-of-the-art. The results of these calculations are needed as parameters for the atmospheric dispersion models. <br> Chapter 4 provides an brief overview of meteorological phenomena which affect vapor cloud transport and dispersion as well as descriptions of associated characterization parameters used in dispersion modeling. Recommendations are made for selection of parameter values. <br> Chapter 5 describes mitigation of accidental releases into the atmosphere from the standpoint of modeling after-the-fact actions with estimates of their effectiveness. This information can be used with dispersion modeling to help in the selection and design of mitigation measures. <br> Chapter 6 presents the purposes, statement, analysis, simulation methods and results for eight release scenarios. Recommendations are made within each presentation for selection and analysis of modeling parameters for the problem at hand. <br> The Nomenclature (follows the Table of Contents) lists the symbols with their generalized units-of-measure used in the Manual. The SI units-of-measure system is used for all equations and examples. The three modeling program systems use various mixtures of the metric system units; conversions to and from SI units by the user are implied. <br> Appendix I summarizes the suggested default values for selected modeling parameters. <br> Appendix II is an overview of the features and input data requirements of the three modeling program systems, SLAB, HGSYSTEM and DEGADIS. These are used for demonstrating the modeling techniques and typical results for the demonstration scenarios in Chapter 6. User's Manuals should be consulted for detailed descriptions of program theory, operation, data requirements, file formats, and results generated. <br> Appendix III contains listings of germane modeling program input and intermediate result files referenced in the time dependent release simulation of Scenario 7 of Chapter 6. <br> Quick References <br> In addition to the Table of Contents and the Subject Index, several techniques have been used to aid the reader in locating particular information or methods to use in modeling applications. Chapters 3 through 5 each have a Quick Reference text box showing the page and/or equation where a particular subject or algorithm is most directly discussed. <br> In the scenario descriptions of Chapter 6, a text box summarizes the major Release Attributes by which the scenario's type and principal parameters may be readily identified. A RECAP summarizes the parameters used in the modeling. Additional methods, noted in the introduction of that chapter, are also used to aid quick referencing. <br> Equations which are primarily used for calculations have the symbol &#9830; appended to the equation number. <br> Conventions <br> Within each chapter, tables, figures and equations are each numbered sequentially from the top of the chapter. If reference to these items is made from one chapter to another, the chapter number is prefixed. For example, Figure 3-12 is the 12th figure in Chapter 3. Pages are numbered according to Chapter number-Page number. To avoid confusion, the tables and figures for each scenario in Chapter 6 are prefixed by Sn, where n is the scenario number and S designates "Scenario." For example, Figure S6-2 denotes the second figure in Scenario 6. <br> References, listed after Chapter 8, are presented according to chapter number. Within a chapter, a particular reference is made according to author [reference number]. If a citation is made fiom within one chapter to a reference listed under another chapter heading, the chapter number is prefixed. <br> Terminology <br> Not all of the terminology used in this field is precisely defined or used. In this manual, "substance" is used to denote a pure chemical, chemical mixture, or other material which can or might be released to the atmosphere. "Fluid" is used for a substance capable of flowing, which may be gas, liquid, a gas/liquid mixture or aerosol suspension, or even a suspension containing solids in a flowing stream. <br> It is common practice to use the term "vapor cloud" for any visible or invisible body of substance moving with, and/or through, the atmosphere. Such a cloud may actually contain released liquid (in addition to its vapor) or even suspensions of solid particles. Although "plume" may most often refer to elevated releases from stack, it is used herein also for ground level or vapor clouds. "Gas blanket" is sometimes used to describe a surface level, dense vapor cloud in the early stages of its release. These terms are used more or less synonymously in this manual to mean the bounded, fluid entity of released substance (mixing with air) flowing through the atmosphere. <br> The symbol "ppm" refers to parts-per-million by volume [mole] in the gas, unless otherwise stated. Also, if at all possible, standard mathematical symbols as in the disciplinary literature are used for the parameters and variables. However, the same symbol or letter is often used in this text for many different items; this can be very confusing. Therefore, to help minimize possible misinterpretations, some different-than-standard symbols are used. <br> Subject Index Development <br> The subject index is based upon Chapters 1 - 6 and Appendix II. Not every appearance of a particular word or phrase is listed; a usage which is of minor importance does not appear in the index. The Nomenclature, References, Appendix I and Appendix III are not indexed. <br> In Chapter 6 demonstration scenario simulations, the words/phrases pressure, temperature, wind speed and stability class are not indexed because they appear a large number of times in the text for each scenario. <br> Flammability Criteria <br> Published flammability limits are determined in the laboratory by means of experiments in which the fuel/air concentrations are uniform and precisely known. Thus measured, the lower flammability limit ("LFL") for most hydrocarbons ranges from about 2% to 6% by volume. So, to be conservative, a LFL of 2% can be taken as a nominal, global value for the purposes of this Guidance Manual. <br> Actually, the concentration at any point and time in a real cloud, such as that being considered here, will be highly variable. There will be "fingering," "holes," and other inhomogeneities caused by terrain, structures and general turbulence. <br> In a number of field tests, it has been found that peak concentrations are generally about twice that of average concentrations in vapor clouds. On this basis, the LFL of 1% should be considered for general use. However, dispersion models do not predict concentrations more accurately than a factor of 2 at best. For these reasons, a range of concentrations for the lower flammability limits must be considered. Therefore, for these purposes, if a model calculates a released material concentration less than 0.5%v, the corresponding real cloud will be taken as non-flammable. <br> Also, an upper flammability limit need not be considered, as it must be for a closed system. With a cloud, there is always an "edge" in which a flammable mixture exists, thus the cloud can be ignited if the edge encounters an ignition source.