Probability assessment hazard analysis

Hazard analysis (exposure assessment) the process of determining releases or event probabilities. For landfill leachate, this could include determination of the the routes via which the leachate could reach sensitive receptors (e.g. people, groundwater), and the characteristics of these receptors... 

The software safety assurance process starts with the Hazards Analysis Risk Assessment (HARA). This step aims to determine safety-relevant systems functions, the safety requirements of these functions (maximum tolerable failure probabilities) as well as the potential demands for additional safety functions. The different safety standards generally agree on common HARA techniques [6]. 

Hazard analysis (HAZAN) is a quantitative way of assessing the likelihood of failure. Other names associated with this technique are risk analysis, quantitative risk assessment (QRA), and probability risk assessment (PRA). Keltz [44] expressed the view that HAZAN is a selective technique while HAZOP can be readily applied to new design and major modification. Some limitations of HAZOP are its inability to detect every weakness in design such as in plant layout, or miss hazards due to leaks on lines that pass through or close to a unit but cany material that is not used on that unit. In any case, hazards should... 

In this study detailed fault trees with probability and failure rate calculations were generated for the events (1) Fatality due to Explosion, Fire, Toxic Release or Asphyxiation at the Process Development Unit (PDU) Coal Gasification Process and (2) Loss of Availability of the PDU. The fault trees for the PDU were synthesized by Design Sciences, Inc., and then subjected to multiple reviews by Combustion Engineering. The steps involved in hazard identification and evaluation, fault tree generation, probability assessment, and design alteration are presented in the main body of this report. The fault trees, cut sets, failure rate data and unavailability calculations are included as attachments to this report. Although both safety and reliability trees have been constructed for the PDU, the verification and analysis of these trees were not completed as a result of the curtailment of the demonstration plant project. Certain items not completed for the PDU risk and reliability assessment are listed. 

The terminology used varies considerably. Hazard identification and risk assessment are sometimes combined into a general category called hazard evaluation. Risk assessment is sometimes called hazard analysis. A risk assessment procedure that determines probabilities is frequently called probabilistic risk assessment (PRA), whereas a procedure that determines probability and consequences is called quantitative risk analysis (QRA). 

The purpose of hazard analysis and risk assessment in the chemical process industry is to (/) characterize the hazards associated with a chemical facility (2) determine how these hazards can result in an accident, and (3) determine the risk, ie, the probability and the consequence of these hazards. The complete procedure is shown in Figure 1 (see also INDUSTRIAL HYGIENE Plant SAFETY). 

Metal Emission Limits. Limits for metals, both carcinogenic and noncarcinogenic, are based on an adjusted stack height. Failure to meet these limits requires risk assessments using site specific factors and modeling to establish limits for each metal. The assessments are based on the probability of developing adverse health effects or cancer, based on an inhalation exposure pathway to maximum exposed individuals located near the incinerator (see Hazard ANALYSIS AND RISKASSESSL nt). 

A risk assessment is an analysis that addresses both (a) the probability of a hazards-related incident occurring and (b) the severity of harm or damage that may result. Effective application of hazard analysis... 

To be effective, safety professionals must understand hazards, risks, the relationship between hazards and risks, and hazards analysis and risk assessment techniques. In the use of hazard analysis and risk assessment matrices, judgments of incident probability and consequence will often be made on a subjective basis. And such systems can be made to work. They should be considered more art than science. 

Risk is determined by assessing its two components the severity of harm or damage resulting from a hazard-related event and the probability that the event could occur. Table 15.1 presents a sample Risk Assessment Matrix, illustrating how these two factors are combined to obtain a risk level. A review of the many published risk assessment matrices appears in chapters titled A Primer On Hazard Analysis And Risk Assessment and Risk Scoring Systems in this authors book titled Innovations In Safety Management. 

Several methods relating to hazards analysis and risk assessment exist. They are generally divided into qualitative and quantitative (lEC 61508 lEC 61511). The choice of specific method depends on accident scenario being considered and available data. When a risk evaluated for scenario considered is high, it is necessary to reduce it to an acceptable level using protection layers, each of specified reliability, expressed often as the probability of failure on demand (PFD) (LOPA 2001). 

In a statement indicating risk level, both probability of occurrence and severity of outcome must be included. After determining the severity of expected damage or harm through a hazard analysis, estimating the probability of an incident or exposure occurring is the additional and necessary step in concluding a risk assessment. 

A written communication accompanies the analysis, explaining the assumptions made and the rationale for them. Comments would then be made on the assignment of responsibilities for the remedial actions to be taken and when. A Hazard Analysis and Risk Assessment Worksheet (formerly called a Preliminary Hazard Analysis Worksheet) appears in Addendum A at the end of this chapter, courtesy of A-P-T Research, Inc. That form, and other similar forms, require entry of severity, probability, and risks codes before and after countermeasures are taken. 

Some who oppose the use of qualitative risk assessment techniques do so because the outcomes are not stated in absolutely assured, precise numbers. Such accuracy is not attainable because incident probability data are lacking and the severity of event outcomes is a best estimate. Expecting such results is unrealistic. Fortunately, recognition continues to grow that hazard analysis and risk assessment methods, although qualitative, add value to safety decision making. 

For Step 2—Assess the Risks—a Risk Assessment Matrix is provided. Its configuration is unusual and it does not duplicate well. The terminology used in the matrix for probability and severity, and for the risk gradings, is identical to that in Table 10 in Chapter 8, A Primer on Hazard Analysis and Risk Assessment, with one exception. In the Handbook, M is the designation for Medium Risk, rather than Moderate Risk. We here include Table 10 from Chapter 8 as Table 3. 

In applying The Safety Decision Hierarchy, the goal in the problem identification and analysis phase is to identify and analyze the hazards and assess the risks. Hazard and risk problems cannot be intelligently addressed until the hazards are analyzed and assessments are made of the probability of incidents or exposures occurring and the possible severity of their consequences. Chapter 8, A Primer on Hazard Analysis and Risk Assessment, is a resource for this problem identification and analysis phase. 

A system is a part of the universe within a certain domain in space and time. What is an environment Outside the frontier of the system is the environment [1], Here, system shall have an identity, that is, deterministic. There shall be an external boundary to the system. An external boundary is determined by what aspect of system performance is of concern. This is stated here because for quantitative hazard analysis, boundary definition is extremely important. Also, the interface part needs to be considered (See Fig. V/3.0-l). The process definition for qualitative risk analysis is Qualitative Risk Analysis assesses the priority of identified risks using their probability of occurring, the corresponding impact [...] as well as other factors such as the time frame and risk tolerance [..On the contrary, quantitative risk analysis (QRA) as per DNV is Typically, a QRA can be defined as the formal and systematic approach of identifying potentially hazardous events, estimating the likelihood and consequences of those events, and expressing the results as risk to people, the environment or the husiness. ... 

I vo components must be addressed in developing a risk assessment— probability of occurrence and severity of outcome. Hazard identification and analysis establish severity—the probable harm or damage that could result if an incident occurs. To convert a hazard analysis into a risk assessment, a probability of occurrence factor must be added. Then, risk levels can be established (e.g.. Low, Moderate, Serious, High) and priorities can be set. 

Procedures set forth in the Guidelines establish that, in the risk assessment process, both the expected severity of consequences of an incident resulting from a hazard s potential being realized and the probability of its occurrence be evaluated. In a logical sequence, the guideline indicates that after the hazard analysis and risk assessment are concluded, decisions are to be made about whether the risk is acceptable or whether further mitigation is necessary. 

There is a reality in Browning s observations System safety literature at the time he wrote his book was loaded with governmental jargon, and it easily repelled the uninitiated. It made more of the highly complex hazard analysis and risk assessment techniques requiring extensive knowledge of mathematics and probability theory than it did of concepts and purposes. 

Again, a PHL is developed (use Appendix C as a starting point). The PHL is divided into hazard categories. The functional tree is created. Then the actual facility hazard analysis is started. Each hazard is assigned a severity and probability level, and the other portions of the hazard analysis worksheet are completed. Then a system safety assessment is performed and the worksheet results are analyzed. 

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