Qualitative safety analysis

Following the completion of the first draft design of the planned installation, the necessary requirements derived from experience must be established. Applicable laws and technical rules must be observed. In the event that requirements based on experience cannot adequately guarantee system safety, special attention must be devoted to failure effect on the environment. Qualitative analysts will be continued step by step until unequivocal assessment is possible. In the event that qualitative analysis does not furnish a clear statement concerning system safety, and if the expected danger for the environment due to the behavior of the system under consideration requires it, quantitative analysis should follow qualitative analysis. As a rule, quantitative analysis concerns only particularly critical subsystems indicated in the scope of qualitative analysis. 

A series of methods [4-17], [4-181, (4-19j were developed for qualitative safety analysis and found to be successful. The choice of the analysis procedure, the special methods included, the application sequence, and the required extent of work depend on the project. The following must be considered  

Due to safety requirements ascertained with respect to the plant, essential system changes might result. In the event that, with due consideration for necessary changes, the planning of the plant is completed. analysis after planning completion takes place. The completed planning documents are checked to see whether the technical design observes specifications and can attain the safety aim. 

Here it should be pointed out that inspection programs for production, start-up, and current operation are derived from safety analysis. In the following the individual operational steps will be explained and applied to an example. For this a leaching solution settling tank is selected as subsystem of a petrochemical plant. This system will be described first. 

The most important operational data of the system under investigation are given in Table 4.8. 

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Figure 4.12 Sequence of qualitative safety analysis of a technical process plant from plant planning to initial operation.

For this reason, more and more standards and guidelines for the development of safety-relevant systems demand safety analyses for the system and the software as part of a rigorous development process. Examples of this are lEC 61508 [1], lEC/TR 80002 [2], MISRA safety analysis guidelines [3], and ISO 26262 [4]. ISO 26262 is a committee draft for the development of road vehicles. It defines requirements on the development of electrical and electronic systems and particularly requirements on the development of software, which include qualitative safety analysis for software architecture as well as for software unit design. However performing a qualitative safety analysis technique such as failure mode and effect analysis (FMEA) or fault tree analysis (FTA) on software architectmal design is a complex task. One reason for this is that safety analyses do not fit well with software architectural design and do not... 

Hierarchical component-based abstraction is a new topic for FTA and a challenge for safety analyses of software architectures. Because of this, hierarchical abstraction has also been proposed by other approaches, but not for qualitative safety analysis such as FTA. In the Rich Component Model (RCM) [19], formal state machines are used to model the failure behavior of components. To abstract in the (black-box) specification from the realization (gray-box specification), all or only some combinations (e.g.. 

The following qualitative safety analysis methods are listed ISO 26262, Part 9, clause 8 ... 

Safety analysis can be generally divided into two broad categories, namely, quantitative and qualitative analysis (Wang and Ruxton (1997)). Depending on the safety data available to the analyst, either a quantitative or a qualitative safety analysis can be carried out to study the risk of a system in terms of the occurrence probability of each hazard and its possible consequences. 

Qualitative safety analysis is used to locate possible hazards and to identify proper precautions that will reduce the frequencies or consequences of such hazards. Generally this technique aims to generate a list of potential failures of the system under consideration. Since this method does not require failure data as an input to the analysis, it relies heavily on engineering judgement and past experience. 

AIChemE (2001) Guidelines for Chemical Processes Qualitative Risk Analysis, 2nd edn (Center for Chemical Process Safety, American Institute of Chemical Engineers, New York). 

There are many methods of safety analysis reviews that are available and can be applied to a facility or project design to overcome human errors and the various failures of the process system. The methods may be either qualitative or quantitative in nature. 

The site-level health and safety manager was given notice from corporate for the need to conduct the qualitative risk analysis of the plant s hazardous material transportation operations. The safety manager was provided with a questiotmaire regarding all chemicals of concern, shipping quantities, modes of transit, and the route characteristics (similar to the checklist found in Table 4.2). This information was transmitted back to corporate for analysis, which included a corporate-level comparison to the other XYZ Chemical facihties risk results. Based on the information collected at the site level, the following was determined and reviewed by corporate for this facility ... 

Hazard Analysis—The determination of material, system, process, and plant characteristics that can produce undesirable consequences, followed by the assessment of hazardous situations associated with a process or activity. Largely qualitative techniques are used to pinpoint weaknesses in design or operation of the facility that could lead to accidents. The Safety Analysis Report (SAR) hazard analysis examines the complete spectrum of potential accidents that could expose members of the public, on-site woikers, facility workers, and the environment to hazardous materials. 

Fundamental for the protection measures is the classification of plants according to zones, which is done on the basis of a hazard analysis (cf. [42]), for which the qualitative methods of safety analysis of Chap. 9 are used. 

Stated that fault tree analysis searches for the conditions of plant failure and therefore may be regarded as the antithesis of the design process, which aims at identifying the conditions for functioning. Hence, it proves useful for identifying design weaknesses in both its qualitative and quantitative parts. Eventually, the synthesis of design and safety analysis leads to a better and safer plant. 

With that scheme, the analyst identifies first which systems are needed given the PSA scenario. Then from the list of required systems, he determines which actions are plausible that could prevent these systems to work. That method is theoretically the more exhaustive and the more justified. We use it explicitely in EDF s PSA at the step of what we call AQS, in French Analyse Qualitative des s6quences, that means Qualitative Sequence Analysis (Cucciniello Vidal, 2000). As Cucciniello and Vidal said The qualitative analysis of sequences (AQS) aims at characterizing in the accidental situations of reference, defined by the initiators selected and the states of the reactor considered, the missions of the frontal systems, I C and human like their sequence. It is built starting from a detailed structuring of the functions of safety and course of accidental control APE [i.e. French symptom based procedures]. After the characterization of the initiators, the method breaks up into three stages ... 

A qualitative safety review methodology, primarily utilized in project management, to identify hazards in activities or systems, their probability of occurrence, and determine if protection measures are adequate. It is similar to a lob Safety Analysis. It is sometimes called a Danger Analysis, Safety Verification, or Preliminary Danger Analysis. 

An evaluation of both the frequency and the consequences of potential hazardous events to make a logical decision on whether the installation of a particular safety measure can be justified on safety and loss control grounds. Frequency and consequences are usually combined to produce a measure of risk, which can be expressed as the average loss per year in terms of injury or damage arising from an incident. The risk calculations of different design alternatives can be compared to determine the safest and most economical options. Calculated risk may be compared to set criteria that have been accepted by society or required by laws. See also Qualitative Risk Analysis. 

Provides options for management personnel and others to carry out either qualitative or quantitative reliability/safety analysis... 

Theoretical system analysis may be of a quantitative or a qualitative nature. In each case, quantitative analysis must be preceded by a qualitative analysis in order to discover relevant problems and recognize the areas where qualitative analysis must be complemented by quantitative analysis. Even then, a certain restriction will be necessary because of the scarcity of the possibilities at one s disposal, e.g., mathematical models, data, and computers. The instrument of quantitative theoretical system analysis is mathematical simulation. Due to its importance for modern safety analysis, this will be treated separately in the following section. 

The scope of safety analysis ranges from a rough qualitative appraisal of sources of hazards, via detailed qualitative analysts, to quantitative analysis with figures concerning the expected frequency and consequences of incidents. The scope must be governed by the definition of the task. Here the following must be taken into account ... 

Within the overall aim it is the task of quantitative safety analysis to ascertain the frequency or occurrence probability of undesired events leading to incidents. Safety analysis will, in the case of problematic results of qualitative analysis, necessarily inspire the question of whether it should be continued in quantitative form. The question arises in particular when new technical equipment and processes are used. Quantitative safety analysis starts with knowledge of the logic structure of the system to be examined, as has already been ascertained in the course of qualitative analysis. A condition for execution is the presence of sufficient data—information about the behavior of the individual system components and parts. The information must be arranged in such a way that reliability characteristics (failure probabilities, failure rates) and maintenance characteristics (rates of repairs) can be derived. It is only when it is certain that sufficient data are available that quantitative analysis is possible. 

M. Rousand, Preliminary Hazard Analysis, NTNU/Willey, October 2005. AuthorAnonymous, J.F. Shortle, Applying Qualitative Hazard Analysis to Support Quantitative Safety Analysis for Proposed Reduced Wake Separation ConOps, George Mason University, Fairfax, VA M. Allocco, FAA, Washington, DC. 

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