Fire and explosion analysis

The paradigm shift that The Cullen Report emphasized was to move toward a performance and risk-based approach to offshore safety management through the use of upgraded and much more thorough Safety Cases. The report also stressed the importance of Formal Safety Assessments that address issues such as fire and explosion analysis and the design of escape routes. 

Fire and explosion analysis is a complex topic, and one that uses sophisticated software. Specialist support is invariably needed to carry out this work. Nevertheless, professional understanding to do with fires and explosions is stiU limited. For example, the initial report from the Bimcefield explosion and fire that occurred in... 

Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

Once the source modeling is complete, the quantitative result is used in a consequence analysis to determine the impact of the release. This typically includes dispersion modeling to describe the movement of materials through the air, or a fire and explosion model to describe the consequences of a fire or explosion. Other consequence models are available to describe the spread of material through rivers and lakes, groundwater, and other media. 

Many sophisticated models and correlations have been developed for consequence analysis. Millions of dollars have been spent researching the effects of exposure to toxic materials on the health of animals the effects are extrapolated to predict effects on human health. A considerable empirical database exists on the effects of fires and explosions on structures and equipment. And large, sophisticated experiments are sometimes performed to validate computer algorithms for predicting the atmospheric dispersion of toxic materials. All of these resources can be used to help predict the consequences of accidents. But, you should only perform those consequence analysis steps needed to provide the information required for decision making. 

Consequence Phase 3 Develop Detailed Quantitative Estimate of the impacts of the Accident Scenarios. Sometimes an accident scenario is not understood enough to make risk-based decisions without having a more quantitative estimation of the effects. Quantitative consequence analysis will vary according to the hazards of interest (e.g., toxic, flammable, or reactive materials), specific accident scenarios (e.g., releases, runaway reactions, fires, or explosions), and consequence type of interest (e.g., onsite impacts, offsite impacts, environmental releases). The general technique is to model release rates/quantities, dispersion of released materials, fires, and explosions, and then estimate the effects of these events on employees, the public, the facility, neighboring facilities, and the environment. 

Complete and accurate written documentation of chemicals properties, process teclinology, and process equipment is essential to the PSM program and to a process hazards analysis (PrHA). This information serves many users including the PrHA team. The needed chemical information includes fire and explosion characteristics, reactivity hazards, safety and health hazards and the corrosion and erosion effects. Current material safety data sheet (MSDS ) information helps meet this requirement, but must be supplemented with process chemistry information regarding runaway reactions, and over-pressure hazards. 

This study investigated risks to the public from serious accidents which could occur at the industrial facilities in this part of Essex, U.K. Results are expressed as risk to an individual and societal risk from both existing and proposed installations. Risk indices were also determined for modified versions of the facilities to quantify the risk reduction from recommendations in the report. Nine industrial plants were analyzed along with hazardous material transport by water, road, rail and pipeline. The potential toxic, fire and explosion hazards were assessed for flammable liquids, ammonia, LPG, LNG, and hydrogen fluoride (HE). The 24 appendices to the report cover various aspects of the risk analysis. These include causes and effects of unconfined... 

Figure 9.1. Procedure for calculating the fire and explosion index and other risk analysis information. From Dow (1994) reproduced by permission of the American Institute of Chemical Engineers. 1994 AIChE. All rights reserved.

The first step in the procedure is to conceptually divide the process into separate process units. A process unit is a single pump, a reactor, or a storage tank. A large process results in hundreds of individual units. It is not practical to apply the fire and explosion index to all these units. The usual approach is to select only the units that experience shows to have the highest likelihood of a hazard. A process safety checklist or hazards survey is frequently used to select the most hazardous units for further analysis. 

The ideal solution is to perform a risk analysis for each item in a facility to determine the probable maximum fire and explosive range the location may produce. The calculations and expense to accomplish such a task today does not appear to justify a unilateral application to every piece of equipment at a facility. Consequentially the use of a spacing table for a facility design provides for an economical and expedient solution. This is especially important when several options on the layout of the facility are available. However in some instances the use of risk analysis may demonstrate less spacing is necessary that what a spacing chart requires. 

The first task in applying a spacing table to a facility is to ensure it corresponds to the philosophy of protection adopted by the company. Where limited space is available to provide the required spacing, an examination of the equivalent fire and explosive barriers or active fire suppression system should be confirmed. This analysis should be accepted by the company as part of the design risk analysis. 

Several qualitative approaches can be used to identify hazardous reaction scenarios, including process hazard analysis, checklists, chemical interaction matrices, and an experience-based review. CCPS (1995a p. 176) describes nine hazard evaluation procedures that can be used to identify hazardous reaction scenarios-checklists, Dow fire and explosion indices, preliminary hazard analysis, what-if analysis, failure modes and effects analysis (FMEA), HAZOP study, fault tree analysis, human error analysis, and quantitative risk analysis. 

Dow s Fire and Explosion Risk Analysis Program provides a step-by-step, objective evaluation of the realistic fire, explosion, and reactivity potential of process equipment and its contents. The procedure allows calculation of the damage that would probably result from, and the areas which could be exposed to, fire or explosion generated in the process unit being evaluated. Management can then decide unit spacing needed to protect people from injury and to keep potential property and equipment damage to acceptable levels. 

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