As mentioned earlier, risk is defined as a function of incident occurrence, fre- quency, and consequence. Consequence analysis is the quantitative estimation of the consequence of a chemical process incident—an estimate of the magnitude of the potential harm to people, the environment, or property. Because there is a wide range of potentially harmful impacts of chemical process incidents, there is a number of different tools which may be useful in analyzing these impacts. In this discussion, the consequence analysis tools described will be limited to those commonly used to estimate the potential for injury or fatality to people as an imme- diate result of exposure to harmful materials or energy. However, it is recognized that there is a wide variety of other potential consequences of incidents and a correspondingly wide variety of tools used to understand these consequences.
Consequence models can be quite complex and can only be described in general terms in this discussion. A number of publications by the Center for Chemical Process Safety [20–24] describe specific types of consequence analysis models in detail. Les [25] also provides a detailed description of incident consequence mod- els. There are also a number of public domain and proprietary commercial com- puter-modeling systems available for chemical release consequence analysis.
Source Models
Source models quantitatively estimate the magnitude, rate, duration, physical state (solid, liquid, gas, or a combination), and temperature or other physical condition of a chemical release based on the physical and chemical parameters associated with a particular release scenario.
Most source models are well developed in chemical engineering theory and are essentially the same as the models used for similar material flow scenarios used to design plant equipment. These include single-phase and multiphase flow models for flow-through holes, orifices, and pipes, which are readily adapted to describe flow from a leaking pipe or vessel. Two-phase flashing flow models are based on technology developed by the Design Institute for Emergency Relief Sys- tems (DIERS) [26]. Two-phase or flashing jet release models must also consider the formation of fine aerosols in the discharge and the potential for the small drops
to remain suspended in the atmosphere rather than ‘‘raining out’’ into an evaporat- ing pool on the ground. For a discharge of material from a reactive system, the models required to fully understand the system may be quite complex and data from reaction calorimetry tests may be required.
If a release is wholly or partially in the form of a liquid, it will form a pool on the ground. The evaporation of vapor from this pool is another potential source term for atmospheric dispersion models, which estimate the downwind concentra- tion of the vapor. The first step in estimating evaporation from a liquid pool is to estimate the size of the pool. Pool size models consider the momentum of the liquid stream entering the pool, gravity spreading resulting from the depth of the pool, and the liquid physical properties (e.g., viscosity, surface tension, and surface wetting properties). Physical constraints such as dikes and containment systems may also determine the size of a pool of spilled liquid.
There are three major pool evaporation situations, which are typically modeled: boiling liquid pools, volatile liquid pools, and relatively nonvolatile liquid pools.
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Boiling liquid pools occur when the pool liquid boils at a temperature below that of its surroundings (the ground and atmosphere). In this case, vapor genera- tion is controlled by heat transfer into the liquid pool, both from the ground and from the surrounding atmosphere. The vapor release rate is determined from an estimate of the total heat transfer into the pool and the heat of vaporization of the liquid.•
Volatile liquid pools exert a significant vapor pressure but are at a temperature below the liquid boiling point. Evaporation models for volatile liquid pools consider both heat transfer into the pool and mass transfer rates into the atmo- sphere from the pool surface.•
The evaporation of relatively nonvolatile liquid pools is primarily determined by mass transfer at the surface of the pool. Because the evaporation rate is low, the pool temperature will be essentially the same as the temperature of the surroundings after any initial temperature differences equilibrate. Evaporation models are based on standard methodologies for estimating convective mass transfer from a liquid into a gas.Vapor Cloud Dispersion
Vapor cloud dispersion models estimate the area covered by the vapor cloud from a chemical release as it disperses in the atmosphere, and they estimate the vapor concentrations at specific locations in the cloud. Some of the data required for a vapor cloud dispersion model include the following:
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Characteristics of the release, including rate, total quantity released, location of the release•
Characteristics of the release (phase, direction, velocity, composition, tempera- ture, pressure)•
Atmospheric conditions, including wind speed, atmospheric stability, tempera- ture, pressure•
Characteristics of the surface, including surface roughnessSome of the complex vapor cloud dispersion models may require additional infor- mation to characterize the release, the atmospheric conditions, and surface condi- tions.
Vapor cloud dispersion models consider three typical types of behavior:
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Neutrally buoyant gases (having a density close to the density of air)•
Positively buoyant gases (having a lower density than air)•
Dense or heavy (negatively buoyant) gases Two major types of releases must also be considered:•
Instantaneous (puff releases)•
Continuous releases (plumes)The CCPS [20] describes vapor cloud models in detail, including all of the major types of dispersion models and release types. The CCPS [22] provides a more condensed summary of some of these models. The output of these models describes the concentration of the released material in both time and space as the vapor cloud travels downwind.
Fires
Incident consequence analysis may require consideration of one or more of several different types of fire: