Accident Hazard Analysis

MORT Safety Assurance Systems by WilMam G. Johnson This text serves well both for incident causation model building and for incident investigation. The accident investigation chapter states that while accident investigation has always been a major element in safety, pre-accident hazard analysis is preferable (p. 347). 

The purpose of hazard analysis and risk assessment ia the chemical process industry is to (/) characterize the hazards associated with a chemical facihty (2) determine how these hazards can result in an accident, and (J) determine the risk, ie, the probabiUty and the consequence of these hazards. The complete procedure is shown in Figure 1 (see also Industrial hygiene Plant safety). 

Hazard analysis does have limitations. First, there can never be a guarantee that the method has identified all of the hazards, accident scenarios, and consequences. Second, the method is very sensitive to the assumptions made by the analysts prior to beginning the procedure. A different set of analysts might well lead to a different result. Third, the procedure is sensitive to the experience of the participants. Finally, the results are sometimes difficult to interpret and manage. 

An important part of hazard analysis and risk assessment is the identification of the scenario, or design basis by which hazards result in accidents. Hazards are constandy present in any chemical faciUty. It is the scenario, or sequence of initiating and propagating events, which makes the hazard result in an accident. Many accidents have been the result of an improper identification of the scenario. 

The Rohm and Haas Major Accident Prevention Program (Ren-shaw, 1990 Berger and Lantzy, 1996 Hendershot, 1991a) is based on potential accident consequence analysis and uses checklists based on inherently safer design principles to identify ways to eliminate or reduce hazards. 

A critical assembly is a split bed on which fissionable material used to mock up up a separated reactor core that is stacked half on each half. One half is on roller guides so that the two halves may be quickly pulled apart if the neutron multiplication gets too high. Use the Preliminary Hazards Analysis method described in section 3,2.1 to identify the possible accidents that may occur and the qualitative probabilities and consequences. List the initiators in a matrix to systematically investigate the whole process. Don t forget human error. 

Hazards analysis techniques fall in two broad categories. Some techniques focus on hazards control by assuring that the design is in compliance with a pre-existing standard practice. These techniques result from prior hazards analysis, industry standards and recommended practices, results of incident and accident evaluations or similar facilities. Other techniques are predictive in that they can be applied to new situations where such pre-existing standard practices do not exist. 

You can quickly identify these plant sections by reviewing process flow diagrams and valving arrangements. Isolation points are defined by control valves or powered block valves that can be remotely activated. Process hazard analysis techniques help you identify the maximum credible accident scenarios. (Note that manual valves should not be considered reliable isolation points unless they are located to be accessible following a major accident. However, remotely-activated valves can only be considered reliable isolation points if there are adequate reliability engineering and maintenance programs in place.)... 

A fault tree is a grapliic teclmique used to analyze complex systems. The objective is to spotlight conditions tliat cause a system to fail. Fault tree analysis attempts to describe how and why an accident or otlier undesirable event lias occurred. It may also be used to describe how and why an accident or otlier undesirable event could take place. Thus fault tree analysis finds wide application in hazard analysis and risk assessment of process and plant systems. ... 

One approach is to compare the risks, calculated from a hazard analysis, with risks that are generally considered acceptable such as, the average risks in the particular industry, and the kind of risks that people accept voluntarily. One measure of the risk to life is the Fatal Accident Frequency Rate (FAFR), defined as the number of deaths per 108 working hours. This is equivalent to the number of deaths in a group of 1000 men over their working lives. The FAFR can be calculated from statistical data for various industries and activities some of the published values are shown in Tables 9.8 and 9.9. Table 9.8 shows the relative position of the chemical industry compared with other industries Table 9.9 gives values for some of the risks that people accept voluntarily. 

One of the most important elements of the PSM Rule is the process hazard analysis (PrHA). It requires the systematic identification of hazards and related accident scenarios. The PSM Rule allows the use of different analysis methods, but the selected method must be based on the process being analyzed. The PSM Rule specifies that PrHAs must be completed as soon as possible within a 5-year period. However, one-fourth of the PrHAs must have been completed by May 26, 1994, with an additional one-fourth completed each succeeding year. The highest risk processes were to be done first. A schedule for PrHAs must be established at the outset of a process safety management (PSM) program to give priority to the highest risk processes. PrHAs must be reviewed and updated at least every 5 years. 

Under the PSM Rule, the PrHA element requires the selection and application of appropriate hazard analysis methods to systematically identify hazards and potential accident scenarios associated with highly hazardous chemicals. The components of a PrHA are summarized and explained below. 

Topic 5 Hazard Analysis and Categorization Topic 11 Analysis of Accident Conditions... 

Cox, R. A., "An Overview of Hazard Analysis," in Proceedings of the International Symoisum on Prevention of Major Chemical Accidents, p. 1.37, Center for Chemical Process Safety/AIChE, New York, NY (1987). 

The concept of a safety case comes from the requirements of the European Union/European Community (EU/EC) Seveso Directive (82/501/EC) and, in particular, regulations that the United Kingdom and other member states used to implement that directive. United Kingdom regulations (Control of Industrial Major Accident Hazards [CIMAH], 1984 replaced by Control of Major Accident Hazards Involving Dangerous Substances [COMAH] in 1999) require that major hazardous facilities produce a safety report or safety case.64 The requirement for a safety case is initiated by a list of chemicals and a class of flammables. Like the hazard analysis approach (Section 8.1.2), experts identify the reactive hazards of the process if analysis shows that the proposed process is safe, it may be excluded from additional regulatory requirements. 

Job Safety Analysis (JSA)—procedure that systematically identifies (1) job steps, (2) specific hazards associated with each job step, and (3) safe job procedures associated with each step to minimize accident potential. Also called job hazard analysis. 

An excellent attempt to formalize such an investigative procedure has been presented by Richardson (2) who summarized hazards analysis as essentially an accident investigation before it happens. ... 

The hazard analysis of any industrial process impacts on risk assessment. Risk assessment involves the estimation of the frequency and consequences of a range of hazard scenarios and of individual and societal risk. The risk assessment process is shown in Figure 3.1. The risk criterion used in hazard analysis is the fatal accident rate (FAR). The FAR is defined as the number of fatalities per 108h exposure. The actual FAR in the U.K. was 3.5 in the chemical industry in 1975. No doubt the ideal FAR value should be zero, which is difficult to achieve in practice. 

After the serious hazards have been identified with a HAZOP study or some other type of qualitative approach, a quantitative examination should be performed. Hazard quantification or hazard analysis (HAZAN) involves the estimation of the expected frequencies or probabilities of events with adverse or potentially adverse consequences. It logically ties together historical occurrences, experience, and imagination. To analyze the sequence of events that lead to an accident or failure, event and fault trees are used to represent the possible failure sequences. 

As described earlier, a HAZOP hazard and operability study) is a technique used to identify process hazards and obstacles to the efficient operation of industrial facilities. It provides a systematic approach to quantifying hazard potentials. A HAZAN hazard analysis) is a technique for quantitative evaluation after a serious hazard has been identified or after an accident has occurred. 

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