Failure conditions analysis approaches

After each process is separately understood, interactions can be put together to provide a life prediction of the durability of the component under actual service conditions. One approach is to use multiple regression analysis to develop predictive equations for failure times in which several parameters (e.g., temperature, relative humidity, and stress) are treated as independent variables.6... 

The Process Hazards Analysis team takes a systematic approach to identify potential process hazards and to document them [51]. The Hazardous-Operation Analysis (Haz-Op) is a method by which the process procedures, process and instrument diagrams, and process flow diagrams are evaluated for operability and safety. Fault-Tree Analysis (FTA) is also a method, which investigates the assessment of what-if scenarios and failure conditions. The outcomes of this analysis are recommendations for the col-... 

The fundamental problem involved is always one of data quality, and consequently the back analysis approach must be applied with care and the results interpreted with caution. Back analysis is of use only if the soil conditions at failure are unaffected by the failure. For example, back-calculated parameters for a first-time slide in stiff overconsolidated clays could not be used to predict subsequent stability of the sliding mass, since the shear strength parameters will have been reduced to their residual values by the failure. In such cases, an assumption of c = 0 and the use of a residual friction angle ip is warranted (Bromhead, 1992). If the three-dimensional geometrical effects are important for the failed slope under consideration and a two-dimensional back analysis is performed, the back-calculated shear strength will be too high and thus unsafe. 

The literature offers several methods and models to analyze survival data under the circumstance of different usage conditions. Cox (1972) introduced the Proportional Hazards Model (PHM) to allow for the influence of covariates. This approach is one of the most widely used, particularly in survival analysis. Bendell et al. (1991) discuss the application of this model especially in the context of reliability data. Case studies involving the PHM in the area of reliability were done by Kumar et al. (1992), Bendell et al. (1986) and Jardine et al. (1989). Alternatives to the PHM can be found in Oakes (1995), Kordonsky Gertsbakh (1997) and Duchesne (1999) which follow the idea of multiple times scales to take more than one explanatory variable into account. Another possibility arises if there is some knowledge about the physical-failure mechanism. This approach is usually used with acceler-... 

It must be beared in mind that all model-based analysis methods capture only those phenomena that are covered in the modelling approach. Then again, the automation ensures that 1 relevant failure conditions (at least in so far as modelled) are considered, whereas a manually conducted analysis might be errorneous or incomplete. 

Of these, the failure logic can easily be the most demanding. The simplest approach is to stop or hold on any abnormal condition, and let the process operator sort things out. However, this is not always acceptable. Some failures lead to hazardous conditions that require immediate action waiting for the operator to decide what to do is not acceptable. The appropriate response to such situations is best determined in conjunc tion with the process hazards analysis. 

The use of a model of human error allows a systematic approach to be adopted to the prediction of human failures in CPI operations. Although there are difficulties associated with predicting the precise forms of mistakes, as opposed to slips, the cognitive approach provides a framework which can be used as part of a comprehensive qualitative assessment of failure modes. This can be used during design to eliminate potential error inducing conditions. It also has applications in the context of CPQRA methods, where a comprehensive qualitative analysis is an essential precursor of quantification. The links between these approaches and CPQRA will be discussed in Chapter 5. 

Process Hazard Analysis (PHA) can be defined as the application of a systematic method to a process design in order to identify potential hazards and operating problems. It determines the causes and consequences of abnormal process conditions that arise from equipment failure, human error or other events. The goal is to determine whether opportunities exist to reduce the risks of the toll s hazards and then to implement warranted action items. The AJChE CCPS guideline Guidelines for Hazard Evaluation Procedures, Second Edition with Worked Examples is a good resource for fully detailed approaches to process hazard analysis. It provides an introduction to hazard evaluation as well as guidance on ... 

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 ... 

This approach is illustrated by the development of event trees and fault tree analysis. In fault tree analysis, the probability of an accident is estimated by considering the probabihty of human errors, component failures, and other events. This approach has been extensively applied in the field of risk analysis (Gertman and Blackman 1994). THERP (Swain and Guttman 1983) extends the conditioning approach to the evaluation of human reliability in complex systems. 

The influence of inelastic stress-strain behavior on crack tip conditions and failure is often neglected in numerical analysis, but may be considered using some computational approaches. For this task, approaches based on configurational mechanics (Mueller and Maugin, 2002) have proven quite powerful (Naser et al., 2007,2009 Timmel et al., 2009). 

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