Environmental Protection Agency Washington, D. The many contributors addressed the challenging task of reviewing, analyzing and summarizing the myriad existing models and empirical data on flammable chemicals and their hazards. A major consideration in this work was the need to express the hazards of flammable chemicals as correctly and simply as possible for use by risk managers. Finally, the Environmental Protection Agency Science Advisory Board provided comments that were helpful in preparation of the final draft of this document. Methodologies for Modeling B-l B. I General Description B-l B.
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This module presents several procedures to predict the fate and transport of spilled hazardous materials. The discussion is general and it stresses downwind toxic and explosive hazards. EPA B Specific characteristics for all regulated hazardous materials are also included in the appendices of this module to enable the efficient use of these procedures.
A discussion is also provided that considers mixtures of materials and how these mixtures may be more hazardous than individual material losses. Much of the material in this report section is summarized from the recent EPA guidance document.
This Act directed the EPA to issue regulations requiring facilities that handle, manufacture, store, or use large quantities of very hazardous chemicals to prepare and implement programs to prevent the accidental release of those chemicals. These facilities also must be prepared to mitigate the consequences of any releases that do occur. This regulation requires these facilities to prepare a risk management system, including analyses of potential toxic and explosive conditions if such material is lost to the environment.
The summarized material presented in this module refers to the worst-case scenario procedures included in the guidance document. This summary is not a substitute for the complete report for regulated facilities, of course, but is presented here as a currently accepted evaluation procedure that is suitable for evaluating transportation accidents involving hazardous materials.
The results obtained using these methods are expected to be conservative i. Identify the toxic gas and quantity released. Step 3: Determine distance to endpoint.
Select the appropriate table based on the density of the released substance, the topography of the site urban or rural , and the duration of the release. Identify the toxic liquid and quantity released. Step 2: Determine release rate. Estimate the volatilization rate for the toxic liquid and the duration of the release. Select the appropriate reference table based on the density of the released substance, the topography of the site rural or urban , and the duration of the release.
Estimate distance to the endpoint from the appropriate table. Identify the appropriate flammable substance and quantityreleased. Step 2 : Determine distance to endpoint. Estimate the distance to the required overpressure endpoint of 1 psi for a vapor cloud explosion of the flammable substance. Estimate the distance to the endpoint from the quantity released. The procedure provides two choices for topography, urban and rural.
Thus, if the site is located in an area with few buildings or other obstructions e. If the site is in an area with many obstructions, even if it is in a remote location that would not usually be considered urban, urban conditions should be assumed. Gases liquefied by refrigeration alone that would form a pool one centimeter or less in depth upon release must be modeled as gases. Table A-1 in Appendix A of this module lists the endpoint for each toxic gas. These endpoints are used for air dispersion modeling to estimate the consequence distance and are considered critical levels of the contaminants.
For toxic liquids, it is assumed that the total quantity of the toxic liquid in a vessel is spilled. For toxic liquids carried in pipelines, the quantity potentially released from the pipeline is assumed to form a pool. The total quantity spilled is assumed to spread instantaneously to a depth of one centimeter 0. The release rate to air is estimated as the rate of evaporation from the pool.
Table A-2 lists the endpoint for air dispersion modeling for each regulated toxic liquid the endpoints are specified in 40 CFR part 68, Appendix A, and are considered to be critical levels of the contaminants. The procedure uses an endpoint for a vapor cloud explosion as an overpressure of 1 pound per square inch psi. This endpoint is the threshold for potentially serious injuries to people as a result of property damage caused by an explosion e. Simple release-rate equations are provided, and the factors to be used in these equations are given for each substance in Tables A-1, A-2, and A-3 of Appendix A of this module.
These estimated release rates are used in the next part of this module to predict dispersion distances to the toxic endpoint for regulated hazardous gases and liquids. Gases liquefied under pressure should be treated as gases. Gases liquefied by refrigeration that would form a pool one centimeter 0. The evaporation rate from such a pool would be equal to or greater than the rate for a toxic gas, which is assumed to be released over 10 minutes.
Therefore, treating liquefied refrigerated gases as gases rather than liquids in such cases is reasonable. Unmitigated Releases of Toxic Gas. If a toxic gas that is liquefied by refrigeration alone is released into an area where it will be contained by dikes to form a pool more than one centimeter 0. If the gas liquefied by refrigeration would form a pool one centimeter 0. If the material would be released in a diked area, first compare the diked area to the maximum area of the pool that could be formed to see if the pool depth is less or greater than one centimeter.
Equation 1. If the dikes prevent the liquid from spreading out to form a pool of maximum size one centimeter in depth , use the following equation:. Equation 2. After the release rate is estimated, estimate the duration of the vapor release from the pool in the diked area the time it will take for the pool to evaporate completely by dividing the total quantity spilled by the release rate. The duration of a chlorine or sulfur-dioxide release, liquefied by refrigeration alone, is not needed for the analyses for critical distances.
A refrigerated tank contains 50, pounds of liquid chlorine at ambient pressure. A diked area around the chlorine tank is ft 2 and is sufficient to hold all of the spilled liquid chlorine. The evaporation rate at the boiling point is determined from equation 2. For this calculation, the wind speed is assumed to be 1. The release rate is:. Assume the total quantity in a vessel or the maximum quantity from ruptured pipes is released into the pool.
Passive mitigation measures e. To estimate the critical distance using this method, the evaporation duration the duration of the release and the release rate must be known. The calculation methods presented here apply to substances that are liquids under ambient conditions or gases liquefied by refrigeration alone. It is assumed that these liquids form pools deeper than one centimeter upon release. Gases liquefied under other conditions under pressure or a combination of pressure and refrigeration or gases liquefied by refrigeration alone that would form pools one centimeter or less in depth upon release are treated as gas releases, rather than liquid releases.
The procedures above are used for those releases. Releases of Toxic Liquids from Pipes. When considering a liquid release from a broken pipe, the maximum quantity that could be released assuming that the pipe is full must be estimated. The time needed to stop pumping the liquid also needs to be calculated as part of the release.
The quantity in the pipe in pounds is the volume released divided by the Density Factor DF times 0. DF values are listed in Table A Assume the estimated quantity in pounds is released into a pool and use the method and equations described below to determine the evaporation rate of the liquid from the pool.
Unmitigated Releases of Toxic Liquids. If no passive mitigation measures are in place, the liquid is assumed to form a pool one centimeter 0. The release rate to air from the pool the evaporation rate is calculated as discussed below for releases at ambient or elevated temperature.
A tank contains 20, pounds of acrylonitrile at ambient temperature. The total quantity in the tank is spilled onto the ground in an undiked area, forming a pool. Assume the pool spreads out to a depth of one centimeter.
The release rate from the pool QR is calculated from Equation 3. For the calculation, the wind speed is assumed to be 1.
If the temperature is elevated, calculate the release rate of the liquid from the following equation:. A tank contains 20, pounds of acrylonitrile at an elevated temperature. The release rate from the pool is calculated from Equation 4. For the calculation, the wind speed factor for 1. Mixtures Containing Toxic Liquids. If the partial pressure of the hazardous substance in the mixture is known, it is possible to estimate an evaporation rate.
In this case, estimate a pool size for the entire quantity of the mixture, assuming an unmitigated release. If the density of the mixture is known, use it in estimating the pool size.
Otherwise, assume the density is the same as the pure regulated substance in most cases, this assumption is unlikely to have a large effect on the results. The molecular weight of acrylonitrile, from Table A-2, is Using Equation 5, calculate the mole fraction of acrylonitrile in the solution as follows:. Equation 5. Before calculating the evaporation rate for acrylonitrile in the mixture, the surface area of the pool formed by the entire quantity of the mixture is needed.
The quantity released is 50, pounds and the Density Factor for acrylonitrile is 0. Now calculate the evaporation rate for acrylonitrile in the mixture from Equation 6 using the VP m and A calculated above:.
Equation 6. The following discussion presents a simple method of estimating the release rate from spills of water solutions of several substances. Oleum a solution of sulfur trioxide in sulfuric acid also is discussed. The vapor pressure and evaporation rate of a substance in a solution depends on its concentration in the solution.
If a concentrated water solution containing a volatile toxic substance is spilled, the toxic substance initially will evaporate more quickly than water from the spilled solution. The vapor pressure and evaporation rate will decrease as the concentration of the toxic substance in the solution decreases.
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