Failure Modes Effects Analysis limitations

For MEA designers or fuel cell stack engineers, a polarization curve is an immensely useful practical analysis tool. It allows for a comparative assessment of sources of voltage losses in the cell, fuel cell failure modes, critical or limiting current densities, as well as impacts of degradation and water management. For materials scientists, the polarization curve entails useful information on performance effects... 

In the FMECA procedure [2,3,256], an exhaustive list of the equipment is first made. Every item on the list is then reviewed for possible ways in which it can fail (the failure modes are open, closed, leaks, plugged, on, off, etc.). The effects of each failure mode are then recorded and a criticality ranking of every item of equipment is calculated. A limitation of this procedure is that combinations of failures which may cause an incident are not really identified. Failure modes and effects analysis (FMEA) is the same procedure without the criticality analysis. 

Other examples of inductive tools that have limited application in incident investigation include failure mode and effects analysis (FMEA), hazard and operability study (HAZOP), and event tree analysis (ETA). These are detailed in the CCPS book, Guidelines for Hazard Evaluation Procedures... 

The process hazards analysis is conducted by an experienced, multidisciplinary team that examines the process design, plant equipment, operating procedures, and so on, using techniques such as hazard and operability studies (HAZOP), failure mode and effect analysis (FMEA), and others. The process hazards analysis recommends appropriate measures to reduce the risk, including (but not limited to) the safety interlocks to be implemented in the safety interlock system. 

Identification can be as simple as asking what-iP questions at design reviews. It can also involve the use of a checklist outlining the normal process hazards associated with a specific piece of equipment. The major weakness of the latter approach is that items not on the checklist can easily be overlooked. The more formalized hazard-assessment techniques include, but are not limited to, hazard and operability study (HAZOP), fault-tree analysis (FTA), failure mode-and-effect analysis (FMEA), safety indexes, and safety audits. 

The failure mode and effect analysis (FMEA) is generally applied to a specific piece of equipment in a process or a particularly hazardous part of a larger process. Its primary purpose is to evaluate the frequency and consequences of component failures on the process and surroundings. Its major shortcoming is that it focuses only on component failure and does not consider errors in operating procedures or those committed by operators. As a result, it has limited use in the chemical process industry. 

Three hazard analysis techniques are currently used widely Fault Tree Analysis, Event Tree Analysis, and HAZOP. Variants that combine aspects of these three techniques, such as Cause-Consequence Analysis (combining top-down fault trees and forward analysis Event Trees) and Bowtie Analysis (combining forward and backward chaining techniques) are also sometimes used. Safeware and other basic textbooks contain more information about these techniques for those unfamiliar with them. FMEA (Failure Modes and Effects Analysis) is sometimes used as a hazard analysis technique, but it is a bottom-up reliability analysis technique and has very limited applicability for safety analysis. 

A formal hazard analysis of the anticipated operations was conducted using Preliminary Hazard Assessment (PHA) and Failure Modes and Effects Analysis (FMEA) techniques to evaluate potential hazards associated with processing operations, waste handling and storage, quality control activities, and maintenance. This process included the identification of various features to control or mitigate the identified hazards. Based on the hazard analysis, a more limited set of accident scenarios was selected for quantitative evaiuation, which bound the risks to the public. These scenarios included radioactive material spills and fires and considered the effects of equipment failure, human error, and the potential effects of natural phenomena and other external events. The hazard analysis process led to the selection of eight design basis accidents (DBA s), which are summarized in Table E.4-1. 

Fault tree analysis is used primarily as a tool for conducting system or subsystem hazard analyses, even though qualitative or top-level (that is, limited number of tiers or detail) analyses may be used in performing preliminary hazard analyses. Generally, FTA is used to analyze failure of critical items (as determined by a failure mode and effects analysis or other hazard analysis) and other undesirable events capable of producing catastrophic (or otherwise unacceptable) losses. 

In order to properly execute a failure mode and effect analysis, certain detailed data must be made available to the analyst. These data typically include, but certainly are not limited to, the following fundamental information for each system, subsystem, and its components (TAl 1989) ... 

Critical items List The purpose of the FMEA is to identify and evaluate failure modes and the possible system effects of those failures. Since the potential for undesirable effects must be eliminated or controlled, the FMEA also provides recommended actions that must be taken to accomplish this goal. As part of this analysis process, the FMEA identifies any and all items within the system that, if a failure were to occur, would have a critical effect on the operation of that system. Therefore, to facilitate evaluation and analysis of these system effects, a critical items list is developed. The list provides detailed descriptive information on each item. It will explain its overall function within the system, as well as the function of any components that may make up that item. The failure mode determined as critical is then listed along with the potential effect(s) of such a failure. If an item on the critical items list is to be accepted as is, then acceptance rationale must be provided. Such rationale may include an explanation of any existing or planned design limitations that will prevent the failure during actual system operations, or the provision of excessive factors of safety that will render such fail-ure(s) extremely improbable. Another area for evaluating acceptance is the history, or lack thereof, and any known failures of systems similar in nature and operation. 

The PHA (Figure 6.4) is perhaps the most critical analysis which will be performed because it is usually the first in-depth attempt to isolate the hazards of a new or, in some cases, modified system. The PHA will also provide rationale for hazard control and indicate the need for further, more detailed analyses, such as the Subsystem Hazard Analysis (SSHA) and the System Hazard Analysis (SHA). The PHA is usually developed using the system safety techniques known as Failure Modes and Effects Analysis (FMEA) (Chapter 9) and/or the ETBA. Data required to complete the PHA include, but is not necessarily limited to, any available data having to do with the following ... 

Using this method to build reliability models of complex mechanical systems can reduce complexity and solve the limit samples problem. It can also effectively reflect the dynamic characteristic of reliability, the feature of multiple failure modes and reliability logical relationships between different system levels of complex mechanical systems, which provides a good foundation for the future reliability design and analysis. 

From the above, it is clear that to make a case based upon in-service experience, some judgements must be made both about the application with a view to determining whether any special features or operating regime are employed and about the likelihood that failures would be expected to be observed, identifled and repented. This involves a failure modes and effects analysis (FMEA) to support limited arguments that failures would be expected to be revealed and/or that failures would not be expected to result in a dangerous failure mode. 

Failure Mode and Effect Analysis, FMEA, is a hazard identification technique in which all known fiiilure modes of conqwnents or features of a q stem are considered in turn and undesired outcomes are noted. The system has had limited use in the chemical industry in Europe as it is tedious and does not readily identify conqxrsition chan. Data for reliabilify studies can be very difficult to obtain. 

Failure Mode and Effect Analysis (FMEA) is intended to identify failures which have significant consequences affecting the system performance in the application considered [1]. FMEA assumes that the system structure has been identified down to the level where the primary failure modes are avail le. From this level, FMEA determines secondary failure modes which may occur on the higher levels of the structure hierarchy. FMEA can be applied in a limited way during conception, planning and definition phases and more fully in the design and development... 

In this paper, we propose and investigate formal concepts that aim to overcome this bias. They support the construction of FMEA tables solely based on the system model and the failure modes, i.e., without requiring the set of effects as input. More concretely, given a system specification in the Architecture Analysis and Design Language (AADL), we show how to derive relations that characterize the effects of failures based on the state transition system of that specification. We also demonstrate the benefits and limitations of these concepts on a satellite case study. 

The acceptance criterion for GSI 022 is that new plants shall minimize the consequences of inadvertent boron dilution events by meeting the intent of SRP Section 15.4.6. Specifically, when performing a safety analysis to evaluate the consequences of an inadvertent boron dilution, plant designers should consider (1) design limits for maximum RCS pressure and minimum DNBR, (2) moderate frequency events in conjunction with a single failure or operator error and their possible effects on fuel integrity and radiological dose calculations, (3) and time limits specified for each mode of plant operation, if operator action is required to terminate an inadvertent boron dilution. 

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