Typical failure analysis procedures

As at this stage, accurate modeling of failure after hrst damage is very complicated and not very accurate. For general laminates, failure theories used in practice conservatively equate hnal failure with failure of the first ply in the laminate. The procedure requires determination of the stresses or strains in each ply in the ply axes and uses one of the many failure criteria. The steps followed in a typical failure analysis are as follows ... 

Availability, in general, is defined as the ability of the plant/equipment to perform its required function over a stated period of time. Maintainability is the probability that a failed item can be restored to operation effectiveness within a given period of time when repair action is performed as per the specified procedure (Smith, 2011). Software is available for performing RAM studies. For smaller projects, spreadsheets can be used. Reliability and process safety are interlinked, and so combined RAM and safety (RAMS) studies can be performed with the RAMS software (Sikos and Klemes, 2010). It considers many factors affecting the plant performance such as equipment performance, redundancy, demand requirements and logistics. RAM analysis is based on statistical failure data such as mean time between failures (MTBF), mean time to repair (MTTR), mean time to failure (MTTF) and mean down time (MDT). Wherever possible, failure data available within the company should be used for RAM/RAMS study. If not, typical failure data available in the literature/software can be used. 

Whereas the operation of batch reactors is intrinsically unsteady, the continuous reactors, as any open system, allow for at least one reacting steady-state. Thus, the control problem consists in approaching the design steady-state with a proper startup procedure and in maintaining it, irrespective of the unavoidable changes in the operating conditions (typically, flow rate and composition of the feed streams) and/or of the possible failures of the control devices. When the reaction scheme is complex enough, the continuous reactors behave as a nonlinear dynamic system and show a complex dynamic behavior. In particular, the steady-state operation can be hindered by limit cycles, which can result in a marked decrease of the reactor performance. The analysis of the above problem is outside the purpose of the present text ... 

The preparation of the polarographic sample becomes more complicated in the analysis of drugs [211], e.g., of seeds or barks. A typical procedure can be exemplified with the determination of cinnamaldehyde in cinnamon bark. A quantity of about 2-5 g of this bark is pulverised and extracted with chloroform for 10 hr in a Soxhlet apparatus. The solvent is then distilled off and the residue dissolved in ethanol. An aliquot of this is mixed with an aqueous solution of the supporting electrolyte and polaro-graphed. A similar procedure proved successful in the determination of carvone in Semen carvi it differs only in the application of ethanol for the extraction. This extraction in the determination of cuminaldehyde in the Roman caraway seeds was a failure because electroactive impurities were extracted simultaneously. For this reason, it was necessary to make use of steam distillation. 

Cause analysis is usually divided into three types (1) direct causes (2) contributing causes and (3) root causes. The direct cause of an incident is the immediate event or condition that caused the incident. Contributing causes are events or conditions that collectively increase the likelihood of the direct cause but that are not the main factors causing the incident. Root causes are the events or conditions underlying the root cause. Corrective measures for root causes will prevent the recurrence of the incident. In simple cases, root causes include materials or equipment deficiencies or their inappropriate handling. More complex examples are management failures, inadequate competencies, omissions, nonadherence to procedures, and inadequate communication. Root causes can be typically attributed to an action or lack of action by a group or individual. 

In most equilibrium-based analytical methods, the success or failure of a determination is not affected by the reaction mechanism, provided that the reaction is either quantitative or the measured parameter at equilibrium is linearly proportional to the initial concentration of the species of interest. This is not the case in reaction-rate methods. Any development of a kinetic method should include, if possible, a complete study of the reaction mechanisms involved in the procedure. (Unfortunately, some reactions, such as catalytic reactions, are so complicated that complete elucidation of the mechanism is impossible.) It should also include a detailed study of the effects of typical sample-matrix components, which can act as catalysts, induce side-reactions, alter the activity of the reactants, and so on. The rates and rate constants for chemical reactions are very sensitive to low concentrations of such spectator species hence, samples containing the same true initial composition of the species of interest but coming from different sources can very often give quite different apparent concentrations. Unless the experimenter is aware of the total reaction mechanism and of all possible factors that can affect either the activation energy or the reaction path, erroneous analytical results can be obtained. A detailed investigation of the simultaneous, in situ, analysis of binary amine mixtures illustrates this point. (Most systems, by the way, are less error-prone than this one.) The rate constants for the reaction of many individual organic amines with methyl iodide in acetone solvent... 

The magnitude of risk from some event depends on the product of how often the analyst thinks an event will occur and how seriously the event impacts on the overall process. Therefore, it is. incumbent on the scientist to develop a quantitative sense of where the risks in an analysis exist, and how serious they are. The best systems analyst cannot perform this function only the person who the is most knowledgeable about the analytical procedure can function as the risk assessor. This person is normally the research chemist who developed the methodology and not the analyst who may run the procedure routinely. He or she is most familiar with the emerging methodology and has a basis (whether it be historical, intuitive or reasoned) to assign a factor of risk to the individual components of the analysis. Typical mechanisms for risk assessment studies include either the use of a "Fault Tree", which uses lists of major failures and associated minor failures which might cause them, or a "Failure Modes and Effects Analysis Model" (21) which uses lists of the ways a system can fail and the results of each failure. For this study, the "Failure Modes and Effects Analysis Model" was chosen. 

Most companies have an accident/incident analysis process that identifies the proximal failures that led to an incident, for example, a flawed design of the pressure relief valve in a tank. Typical follow-up would include replacement of that valve with an improved design. On top of fixing the immediate problem, companies should have procedures to evaluate and potentially replace all the uses of that pressure relief valve design in tanks throughout the plant or company. Even better would be to reevaluate pressure relief valve design for all uses in the plant, not just in tanks. 

A What-If analysis can be organized in one of two ways. The first is to divide the facility into nodes, rather like a HAZOP, except that the nodes are typically bigger and more loosely defined. The second approach is to organize the analysis by major items of equipment rather like an FMEA, and then to discuss the different types of failure mode for each. These two approaches are discussed below. Guidance to do with utilities, batch processes, operating procedures, and equipment layout is also provided. 

Test qualification of items in seismic categories 1 and 3 should be carried out when failure modes cannot be identified or defined by means of analysis or earthquake experience. Direct qualification by testing makes use of type approval and acceptance tests. Low impedance (dynamic characteristic) tests should be limited to identify similarity or to verify analytical models. Code verification tests should be used for the generic verification of analytical procedures, which typically use computer codes. The methods of testing depend on the required input, weight, size, configuration and operational characteristics of the item, plus the characteristics of the available test fadlify. 

Sneak circuit analysis was standardized by Boeing in 1967 and is a formal analysis conducted on every possible combination of paths that a process (most typically electrical circuits, though it could also apply to process flows) could take. The intent is to identify all the paths in the circuit that are designed in and not created due to failures. In other words, the analyst tries to find sneak paths, timing, or procedures that could yield an undesired effect. These sneak or latent paths are found in systems... 

The first phase in the analysis of the reliability of a stracture, equipment, or system is aimed at identifying the hazards associated to its operation and the mechanisms of failure to which it can be exposed during operation. The analysis is typically qualitative, based on expert judgment but driven by a systematic framework of procedures for organizing the expert knowledge. The output of the analysis is a list of the hazards and failure mechanisms, with technical information on the consequences they can provoke to the function provided by the structure, equipment, or system. 

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