Consequences of a Release

Introduction Gas dispersion (or vapor dispersion) is used to determine the consequences of a release of a toxic or flammable material. Typically, the calculations provide an estimate of the area affected and the average vapor concentrations expected. In order to make this determination, one must know the release rate of the gas (or the total quantity released) and the atmospheric conditions (wind speed, time of day, cloud cover). 

Risk is composed of both consequence and probability. Thus an estimate of the consequences of a release provides only half the total risk assessment. It is possible that a particular release incident might have high consequences, leading to extensive plant mitigation efforts to reduce the consequence. However, if the probability is low, the effort might not be required. Both the consequence and the probability must be included to assess risk. 

Modify the process to reduce the consequences of a release for example, use refrigerated rather than pressurized storage, which results in higher liquid release and lower vapor release rates (from the same hole size) and lower vapor generation rates from a pool dilute hazardous materials with inert or less hazardous material to reduce partial pressure or reduce operating pressures to reduce release rates. 

Refrigeration is a prerelease technique that can contribute to reducing the consequences of a release. It is useful when there are process reasons that require the use of a hazardous or toxic material at reduced temperatures, ideally, below its atmospheric boiling point. In such cases the refrigeration requirement can reduce the potential consequences of a release by reducing the system pressure, which affects the rate of flow, and the quantity of the... 

As was discussed in Section 3.2, if there are process reasons for refrigeration, it can also be effective in reducing the consequences of a release. In this example the release of ammonia from a vertical storage vessel has been chosen... 

Scenarios for terrorist acts that involve the release of chemical substances into ambient air are considered among the most likely, as this is an easy way for a terrorist to achieve dispersion, affect a large population, and gain attention. Therefore, models currently available or that undergoing development primarily focus on identifying the consequences of a release into air. 

Range of potential consequences Depending on the scenario, hazards of the material, the material conditions, and the location, the consequences of a release can range from a liquid spill, vapor dispersion downwind, immediate ignition and localized fire, delayed ignition resulting in a flash fire or explosion, ora BLEVE. 

The modeling of vapor releases is complex and requires the resources of skilled analysts. Recognizing this, the EPA developed look-up tables where a facility would simply identify its particular situation and have a rough idea as to what the consequences of a release might be. These are particularly useful for Program (Tier) One and Two companies, which often have similar technologies and chemicals, and which do not have the resources to conduct a full-scale risk analysis. The look-up tables are generally considered to be conservative, i.e., their forecast as to vapor dispersions are likely to be worse than a forecast obtained from simulations. 

The next step in addressing inherent safety is to replace a hazardous material with one that is less hazardous. Thus the consequences of a release are fundamentally less dangerous. 

As a consequence of a release atmospheric dispersion occurs. The dispersion is airborne if the material is lighter than air. However, if is heavier than air it is dispersed as a dense gas. Additionally issues such as minimum released mass or minimum mass flow rates and the release temperature are considered in deciding on one of the two kinds of dispersion. A combination of both kinds of dispersion is possible. For example, refrigerated ammonia is initially dispersed as a dense gas and after being warmed by heat transfer from the surrounding air and diluted as an airborne gas. It should be noted that the release of any dense gas becomes airborne after a certain distance due to dilution of the gas with air (vid. Sect. 10.5). 

Choice of the stability class allows relatively easy manual calculation, and many of the results are available in the form of standard charts [70]. This is a simplified approach to a complex phenomenon, and modem computing power makes it possible to take into account variations in wind velocity, terrain, and gas density. Many specialized models and commercial systems are now available [71], and some are used in plants for real-time modeling. Near-field variability remains a difficult problem, and inclusion of features of the terrain and the presence of obstacles such as buildings can have a major influence on the calculated spread of a plume and on the predicted consequences of a release [72]. 

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