Safety analysis methods

In this section, four computer codes with a 1-D model for deterministic safety analyses of the Super LWR are introduced. They are summarized in Table 6.7. 

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Some Safety Analysis Methods for Process Plant Design... 

Several safety analysis methods have been developed already. Some of them are internationally known and proved, some have been used and developed more or less inside companies. Information requirements of the methods are different, also the results produced vary. Thus different safety methods are suitable for different stages of process development, design and operation (Fig. 3). Some safety analysis methods are discussed below in more detail. 

Another widely used safety analysis method in process industry is the Hazard and Operability Analysis, better known as Hazop (Kletz, 1992). The conventional Hazop is developed to identify probable process disturbances when complete process and instrumentation diagrams are available. Therefore it is not very applicable to conceptual process design. Kletz has also mentioned a Hazop of a flowsheet, which can be used in preliminary process design, but it is not widely used. More usable method in preliminary process design is PIIS (Edwards and Lawrence, 1993), which has been developed to select safe process routes. 

Other possible preliminary safety analysis methods are concept safety review (CSR), critical examination of system safety (CE), concept hazard analysis (CHA), preliminary consequence analysis (PCA) and preliminary hazard analysis (PHA) (Wells et al., 1993). These methods are meant to be carried out from the time of the concept safety review until such time as reasonably firm process flow diagrams or early P I diagrams are available. 

Hazard and Operability Analysis (Hazop) (Kletz, 1992) is one of the most used safety analysis methods in the process industry. It is one of the simplest approaches to hazard identification. Hazop involves a vessel to vessel and a pipe to pipe review of a plant. For each vessel and pipe the possible disturbances and their potential consequences are identified. Hazop is based on guide words such as no, more, less, reverse, other than, which should be asked for every pipe and vessel (Table 1). The intention of the quide words is to stimulate the imagination, and the method relies very much on the expertise of the persons performing the analysis. The idea behind the questions is that any disturbance in a chemical plant can be described in terms of physical state variables. Hazop can be used in different stages of process design but in restricted mode. A complete Hazop study requires final process plannings with flow sheets and PID s. 

Table 1. The elements included into some safety analysis methods. 

Limitations of the Existing Safety Analysis Methods in Conceptual Process... 

The Dow and Mond Indices and Hazop presented in Chapter 4 are widely used for the safety evaluations of process plants. They cover well those risks and hazards existing on a chemical plant. However a lot of detailed information is needed to complete those analysis. In the early stage of process design many of the required process details are still unknown. Therefore the presented safety analysis methods are not directly applicable in their full mode. 

In Table 3 there have been presented the information requirements of the safety analysis methods in Chapter 4. It can be seen by comparing the information available (Table 2) and information requirements (Table 3) that the inherent... 

Table 2. Safety analysis methods in the different phases ofprocess plant design. 

Table 3. Information required for safety analysis methods. 

There seems to be plenty of evaluation methods for inherent safety. Unfortunatelly they are not directly suitable safety analysis tools to be used with novel design systems in preliminary process design. Most existing safety analysis methods need detailed process information and are not directly applicable in early design stages. On the other hand all methods are not suitable for computerized use with optimization and simulation tools. 

The comparison of the safety of equipment is not straightforward. It depends on several features of both process and equipment themselves. It can be evaluated from quantitative accident and failure data and from engineering practice and recommendations. Experience has been used for layout recommendations and for the development of safety analysis methods such as the Dow E F Index (Dow, 1987). Statistics contain details, causes and rates of failures of equipment and data on equipment involved in large losses. 

SAFETY ANALYSIS METHODS—SOFTWARE DEVELOPMENT QUESTIONS... 

Failure Modes and Effects Analysis (FMEA) and its variants have been widely used in safety analyses for more than thirty years. With the increase of application domain of software intensive systems there was a natural tendency to extend the use of (originally developed for hardware systems) safety analysis methods to software based systems. 

The aforementioned procedure is carried out iteratively using the safety analysis methods described in Chap. 9. As a result all safety relevant information should be available as required for the licensing procedure and the start of the detailed planning. 

Just like any other safety analysis method, the TOR has its strengths and weaknesses. Its main strength is the involvement of line personnel/people in the analysis, and its main weakness is its after-the-fact process. 

Part II of this Basic Guide to System Safety presents and briefly discusses some of the more common system safety analytical tools used in the performance of the system safety function. Through example analyses of hypothetical mechanical and/or electrical systems, the reader should become familiar with each type of system safety analysis method or technique discussed. However, it must be understood that it is not within the limited scope of this volume to provide a detailed explanation of each of these methods and/or techniques. The intention is to merely introduce the reader to the various tools associated with the system safety process. The value of each concept in the analysis of hazard risk will vary according to the individual requirements of a given organization or company. 

This Safety Guide identifies the key recommendations for carrying out the safety assessment and the independent verification. It provides detailed guidance in support of Ref. [1], particularly in the area of safety analysis. However, this does not include all the technical details which are available and reference is made to other IAEA publications on specific design issues and safety analysis methods. 

In the case of our exemplar, lAT, preliminary safety activities have started to identify the types of evidence that will be considered suitable to support the claims made in the safety case to an acceptable degree of confidence. The identified types of evidence include evidence that are produced by processes common in systems adhering to older standards (such as 00-55) as well as more novel safety analysis methods. Part of the novelty of lAT derives from its SoS characteristics, and therefore novel safety analysis techniques are required. 

Event Tt ee Analysis A system safety analysis method, similar to fault tree analysis, used to examine different system or operational responses to various positive or negative conditions which occur during system operation. 

Fault (or Functional) Hazard Analysis A system safety analysis method, usually an extension of the failure mode and effect analysis that evaluates the overall effect of functional failures on other subsystems or the overall system itself. 

Preliminary Hazard Analysis (PHA) A system safety analysis method used to formally evaluate and document the hazard risks associated with a new or modified system. 

As Chapter 3 demonstrates, there are numerous safety analysis methods, almost as many different styles as there are industries. However, they all seem to have one aspect in common, the identification of hazards and recommended controls. After understanding the macroscopic view of safety—safety management system—the next step is to add depth to the understanding of what safety is and how to achieve it. The purpose of this chapter is to detail a few of the most commonly used techniques in hazard analysis and to demonstrate their practical applications. It is hoped that you will walk away with a good understanding of how and when to apply these tools. The safety techniques are not difficult to learn and actually are quite easy to pick up and apply immediately. 

Wang, X Ruxton, T. 1997. A review of safety analysis methods applied to the design process. Journal of Engineering Design, 8, 131-152. 

When it comes to the estimation of human reliability, the available methods are not as straightforward as those applied for technological systems. Problems are brought on by the fact that human behavior is influenced by lots of different factors that are hard to describe using formal methods (mainly factors that belong to the area of psychology, medicine or social science). This makes the performance of human operators barely predictable and therefore traditional safety analysis methods inapplicable. 

The proposed safety analysis method is based on simulation of failures. Component models that only reflect normal operation have to be enhanced accordingly. At system level, control or reconfiguration logics have to be implemented that define how a system reacts to the occurrence of failures. The pursued modelling approach is structure invariant, i.e. the DAE structure (equation 1) remains always the same. Failures are represented by model parameter (values in matrices A, B) changes. 

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

It is felt that many of the interpretation problems illustrated above could be avoided or mitigated if the results of qyplication of the safety analysis methods were given more formal semantics expressed in terms of a common formal model. The benefits of such step could be threefold ... 

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