The Use of CFD to Evaluate the Interactions Between Multiple-Leak Sources and to Assess Effectiveness of Integral Modeling Techniques in a Multiple-Leak Scenario

In many practical applications within the chemical industry, it is of vital importance to know the concentration profile that may occur from the dispersion of a material. For example, effect footprints for the release of a toxic material are needed to properly assess risk to both a facility and its surrounding communities and are useful for emergency response planning. Likewise, knowing the concentration gradient of a flammable gas can affect, among many other things, the layout of a facility to prevent or lessen the consequence of explosion and fire scenarios or for the placement of detection systems.

There are two main tools for the modeling of dispersion scenarios. Computational fluid dynamics (CFD) is a useful tool for accurately assessing the dispersion of materials, whether toxic, flammable, pollutant, or otherwise. It has found extensive use in academic as well as industrial settings when scenarios and geometries are well-defined because it takes into account the surroundings in the release environment. However, it is relatively slow and computationally expensive to use.

Also used extensively in the chemical industry are integral consequence modeling tools (such as Phast) which are generally more user-friendly and have the advantage of delivering results in a more time-efficient manner, but cannot take into account complex geometry considerations. A particularly interesting problem to consider in practice is that of multiple simultaneous release sources. This could be caused by a common failure of two units causing concurrent release, a domino effect where an event occurs that causes another event leading to multiple releases, or even an intentional event such as a terrorist attack. While CFD has the capability to resolve the possible interactions between the multiple release sources, integral modeling does not.

A simple current approach to the problem of multiple-source scenarios is to model them independently and superimpose the results upon one another. This may be a good approximation in some cases, but it does not take into account the complex interactions that may take place between the sources. The practical question then becomes whether it is acceptable in certain cases to model a certain set of simultaneous releases as an independent set of sources or whether it could be possible to model them as a single combined source.

This study aims to utilize CFD modeling of a set of varying scenarios to classify multiple-release dispersion scenarios into one of three different categories:

·               Releases that are independent and may be modeled as such

·               Releases that may be combined and modeled as one source

·               Releases that must be considered together due to inherent interactions between them

Based upon criteria such as material, distance between sources, dispersion rate and duration, weather conditions, and geometry considerations, a general methodology for classifying multiple-release scenarios into these three different categories will be proposed so that these releases can ultimately be modeled in the appropriate manner.