The potential for accidental release of hazardous chemicals, fires, and explosions, poses a risk that must be addressed on several levels. The EPA Risk Management Program (RMP, Rule 112r) requires hazard assessment and emergency response programs.
To assure that you have adequate resources and that plans are well organized for any contingency, you need to plan for emergency response. Should such an accident occur, you need real-time data to guide the response activities. CHARM software helps you meet all of these needs.
CHARM can perform a number of simulations that are not available in standard air dispersion models. The most recent version of the CHARM® software has been extended to allow multiple releases; multiple species; particle formation, coagulation, evaporation / condensation, deposition, and interaction with gases; and chemical reactions.
Some examples of these non-standard uses are described below.
Liquid Release with Terrain Influenced Flow
CHARM has the capability to allow a liquid release to follow terrain contours and evaporate as it flows. The emission rate into the atmosphere from the amorphous source is time varying as the surface temperature and wind speed can change with location and time.
The model can visually display the location of the liquid with depth indicated by color. An example of such a display is shown in Figure 1. In the figure the liquid release is assumed to occur at the top of a ridge (a small red S marks the spot) with a steep slope to the left of the figure and a ditch to the right. The liquid depth is indicated by color. Blue is the deepest liquid and white is shallower. The liquid is deepest in the ditch. Some liquid goes down the steep slope and pools at the bottom.
Cooling Tower Droplet Deposition
Cooling towers can create droplets that can coagulate, evaporate, and be deposited on the ground. CHARM is capable of handling all these processes. In some cases the droplets formed may have toxic material mixed with the water, for example, sulfuric acid. A specified droplet size distribution for the source was entered into the model. The droplets were allowed to evaporate, coagulate, and deposit on the ground.
Water Curain Scavenging Efficiency
CHARM was used to model a ground release of ammonia vapor with a source of water droplets higher than the release site and downwind. To test the effect of different flow rates a number of water emission rates are used. One simulation is performed with only the ammonia emission to determine the non-mitigated case. Most scenarios were run with 50% humidity. One scenario used 95% humidity to investigate the effects of evaporation on the droplets.
A chemical reaction that allows ammonia vapor to be absorbed by particles was defined in the chemical database. The assumed reaction for the analysis is:
NH3(g) + H2O(l) -->NH4+ + OH-
The reaction rate is assumed as:
Reaction Rate = 4 π D r
Where D = binary diffusion coefficient of ammonia in air;
r = radius of particle absorbing the ammonia.
Multiple Buildings
In most modeling that takes buildings into account, only one building upwind is assumed to effect the dispersion. The CHARM Software allows for multiple buildings at arbitrary locations with respect to the release site. Multiple buildings near each other can have a much different effect on the wind field affecting the dispersion of the released material than a single building. Buildings downwind can have split or deflect the released material.
CHARM operates in one of two modes: 1) Planning, 2) Emergency Response
Planning Mode
In the Planning Mode, CHARM software guides you through a list of scenarios that describe a particular release and the present meteorological conditions. The release and meteorology descriptions can be input via a summary sheeet or tabbed dialogs. To modifiy information on the summary sheet, double click on it.
When you select the Plume display, CHARM software requests a release time to use in the calculations. This enables a simulation of the release. The various graphic displays can be altered with a variety of options, and the release information can be stored in a scenario file for use with CHARM software's emergency response mode.
Emergency Response Mode
The potential release scenarios saved in the Planning Mode can be used in a real-time Emergency Response Mode. The scenarios are linked with icons on a map. You can define maps with the CHARM Editor for use in CHARM displays. With icons on a map, a user can define a release by point and click on a map. This capability makes graphical input available.
In the Emergency Response Mode, CHARM software quickly accesses stored input and can combine that information with real-time meteorological data, if available, to give real-time predictions of where the impact is going. The CHARM software can be set up to start with a map/icon interface. The map icons can be selected to show release scenarios and other maps.
CHARM software generates concentration, radiation, and overpressure footprint displays, plots, tables, and site information. In Emergency Mode the impacts are automatically updated as new meteorological data becomes available. Emergency response procedures for a particular chemical release, local sources of help, and important phone numbers stored in the planning mode can be retrieved quickly in the emergency response mode.
CHARM comes in three versions.
CHARM for flat terrain - desktop application
CHARM for flat terrain - Web based version (no software to download)
CHARM for complex terrain
The flat terrain versions lends itself to fast what if analysis. Input parameters can be easily changed with the effects seen quickly. The input required for a scenario to be run in the flat terrain version is a release description and meteorological parameters.
The complex terrain version is designed for more detailed analysis. In addition to defining the release and meteorological parameters, the complex version also requires a grid defining the region to be analyzed.
Feature / Capability
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CHARM
Flat Terrain |
CHARM
Complex
Terrain |
Web
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| Flat terrain puff model |
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| Complex terrain grid model |
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| Single source scenarios only |
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| Multiple sources and multiple species |
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| Calculates concentration calculations |
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| Calculates thermal radiation impacts from BLEVEs, pool fires, and jet fires |
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| Calculates explosion overpressures from mechanical and vapor cloud explosions |
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| Source terms calculated from observable data input |
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| Impact history at specified locations |
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| 3D display of impact areas |
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| Import and display base maps |
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| Editor for adding / editing chemicals |
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| Editor for adding maps |
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| Time varying emission rates |
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| Two-phase releases |
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| Releases from containment, evaporating pools |
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| User-defined release rates |
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| Time varying meteorology |
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| Available real-time meteorology (data acquisition system required) |
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| Import maps automatically from Yahoo |
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| Vertical cross-sections of downwind plume centerline |
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| Sanpshot / instantaneous 3D displays |
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| Replay impact progress |
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| Flyby in 3D display |
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| Create movies |
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| Vertical cross-sections can be displayed for the region along any user drawn line. |
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| Effects of 3D terrain on the wind field and dispersion over an area |
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| Trajectory analysis using 3D winds |
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| Averaging time included in all concentration results |
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| All outputs can be displayed in 3-D. This includes fires and explosions as well as concentration impacts |
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| Editor is capable of defining grids by importing DEM and land use data available over the Internet from the USGS and the Globe Project. |
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| Receptor height for plan views can be either in MSL altitude or altitude above surface. |
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| Maps can be superimposed over the 3-D terrain. |
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| A graphics program is available to show terrain, impacts, and maps using DirectX for more realistic displays. |
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| The advection/dispersion calculation is done over the 3 dimensional grid using a finite difference solution. |
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| A scenario and its calculations can be saved in a separate set of files so recalculation is not required. |
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| Multiple release sites, each with a different species, can be modeled in one run. |
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| Multiple meteorological sites. |
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| Liquid releases can be allowed to flow with the terrain when forming pools. |
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| A source can be particles with a user-defined size distribution. Concentrations and deposition amount can be displayed. |
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| Particles can coagulate, evaporate/condense, be created from chemical reactions, and fall to the ground. |
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| Convective heat transfer from the surface into the plume is considered. |
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| Chemical reactions can be calculated. Reaction rates can vary with temperature, solar radiation, particle size, or keyed to air and water vapor. |
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| Chemical reactions can be edited by the user. |
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| The 3-D calculation grid can contain nested grids. |
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| The nested grids can be user defined or automatically created by CHARM. |
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| The 3-D grid can represent rooms, piping, or other objects not attached to the ground. |
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| Temperature displays |
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Development of the CHARM® Software began in 1981. At that time, personal computers were very limited in their capabilities. Because of the computer memory and speed restrictions, the first versions of CHARM contained a puff model for advection and dispersion calculations. A puff model simulates a continuous release as a series of discreet puffs. The number of puffs was restricted.
As computers became more capable, so did the model. Initially, the model was a DOS program. In 1990, the model was moved to the Windows environment. Throughout its development, continuous improvements and enhancements have been added as users requested them. Around the year 2000 a hybrid version of the model was made to try to simulate the effects of complex terrain on the wind field and dispersion of a release. It didn’t take long to discover that a hybrid was not the approach to take.
Once computers were capable of running in excess of about 1 GHz, a true complex terrain version of the model was developed to take advantage of the advancements. Rather than a puff model, the complex terrain version uses a 3 dimensional grid to perform the simulation. Two versions of CHARM then existed in parallel. The flat terrain version is fast and so can be used for screening and rapid “what if” simulations. The complex terrain version is slower but useful for detailed simulations. Besides considering terrain effects it can be used inside and around buildings. It will also calculate the dynamics of particles, and air chemistry.
Additonal information regarding specific features of the CHARM model are available by clicking on the following links: