FMEA/FMECA

We previously encountered failure modes and effects (FMEA) and failure modes effects and criticality analysis (FMECA) as qualitative methods for accident analysis. These tabular methods for reliability analysis may be made quantitative by associating failure rates with the parts in a systems model to estimate the system reliability. FMEA/FMECA may be applied in design or operational phases (ANSI/IEEE Std 352-1975, MIL-STD-1543 and MIL-STD-1629A). Typical headings in the F.Mld. A identify the system and component under analysis, failure modes, the ef fect i>f failure, an estimale of how critical apart is, the estimated probability of the failure, mitigaturs and IHissihiy die support systems. The style and contents of a FMEA are flexible and depend upon the. ilitcLiives of the analyst. 

This FMEA/FMECA shows failure rates that are both demand and time dependent. Adding the demand failure rates gives a train failure rate of 5. 1 E-3/demand. The sum of the time dependent failure rates is 3Ei-10/hr. A standby system such as this, does not exhibit its operability until it is actuated for which the probability is needed that the train has failed since the last use Val " are considered to be part o ng envelope and... 

The assembly process (Figure 10-1) brings together all of the assessment tasks to provide the risk, its significance, how it was found, its sensitivity to uncertainties, confidence limits, and how it may be reduced by system improvements. Not all PSAs use fault trees and event trees. This is especially true of chemical PSAs that may rely on HAZOP or FMEA/FMECAs. Nevertheless the objectives are the same accident identification, analysis and evaluation. Figure 10-1 assumes fault tree and event tree techniques which should be replaced by the equivalent methods that are used. 

To identify the hazards of the EUC in all modes of operation, the event sequences leading to the hazards, and the EUC risks associated with the hazards have to be analyzed (methods are well known like FTA, FMEA, FMECA, etc.)... 

Single point failures from the FMEA/FMECA (see Chapter 5). 

Often too much reliance is placed on the FMEA/FMECA, while ignoring threats that can arise from outside the system (e.g. common cause failures, human error, multiple failures, etc.). 

The most careful and consolidated risk analysis methods, as FTA, ETA, FMEA/FMECA, have serious problems with the possibility of finding the data which analysis are based on. 

A system design or condition such that the failure of a component, subsystem, or system, or input to it, will automatically revert to a predetermined safe static condition or state of least critical consequence. The opposite of fail-safe is fail to danger. See also Failure Mode Failure Mode and Effects Analysis (FMEA/FMECA). 

Failure Mode and Effects Analysis (FMEA/FMECA)... 

Assesses the likelihood of occurrence of potential misuse modes and their effect on safety before and after corrective actions. See also Failure Mode and Effects Analysis (FMEA/FMECA). 

The hazard identification and evaluation of a complex process by means of a diagram or model that provides a comprehensive, overall view of the process, including its principal elements and the ways in which they are interrelated. There are four principal methods of analysis failure mode and effect, fault tree, THERP, and cost-benefit analysis. Each has a number of variations, and more than one may be combined in a single analysis. See also Cost-Benefit Analysis Failure Mode and Effects Analysis (FMEA/FMECA) Fault Tree Analysis (FTA) THERP (Technique for Human Error Rate Probability). 

FMEA is an analytical method used to identify potential problems in the product and in its process of development. It is an inductive method used for identification of hazards of a system with single point failure. When criticality analysis is added with FMEA it is known as failure mode effect and criticality analysis (FMECA). It was used as early as 1950 in reliability engineering. FMEA/FMECA is mainly used for manufacturing, product development, etc. 

The basic objective of FMEA/FMECA is early identification of catastrophic, critical and other potential failures so that these can be eliminated or mitigated through design and manufacturing process at the earliest possible time. There are three kinds of FMEA/FMECA, namely, functional FMEA/FMECA, design FMEA/FMECA (DFMEA/DFMECA), and process FMEA/FMECA (PFMEA/PFMECA) (see Clause... 

The aim of functional FMEA/FMECA is to improve the design of the system. Similarly, DFMEA/DFMECA always looks to see that there is improvement in the design of subsystems/components. The objective of PFMEA/PFMECA is to improve the manufacturing process. Thus it transpires that the major objective of FMEA/ FMECA shall include ... 

Successful FMEA/FMECA is helpful in identifying the potential failure mode from experience with a similar product or production process or on common physical failure logic. FMEA/FMECA is an iterative process and supports maintainability, safety, and logistic analysis. Also it is carried out at various stages of manufacturing of the product, so it is important to coordinate and ensure that at no time or stage effort is duplicated in the same program. 

So, it is seen that FMEA/FMECA needs to address the following issues/information Items Functions Failure (all)... 

In this connection one may note that ISO 9001 2000 para 8.5.3 requires a documented procedure for preventive action. It also demands that preventive actions must be in proportion to the effect of the failure. The FMEA/FMECA approach satisfies ISO 9001 para 8.5.3, so for ISO 9001 2000 FMEA/FMECA may be applied, but it is not a specific ISO 9001 requirement, since the basic purpose of FMEA or FMECA is the same and the procedures are similar. However, one should note that there are some differences between them, which are listed in Table IV/2.0.3-l. There are different types of FMEA/FMECA, discussed next. 

There are three types of FMEA/FMECA functional FMEA/FMECA, DFMEA/ DFMECA, and PFMEA/PFMECA. Apart from these there are two other types of FMEA service FMEA and SWFMEA. In service FMEA the focus is on service issues. SWFMEA (discussed separately in later clauses) focuses on software issues. Functional FMEA/FMECA is also known as concept FMEA/FMECA or system FMEA/ FMECA. Some literature shows two types of FMEA/FMECA in the sense that one is functional and other is hardware FMEA/FMECA (where both PFMEA/PFMECA and DFMEA/DFMECA are considered under hardware FMEA/FMECA). These different types are a result of changes in analysis pattern and assessment, but the basic concepts/approaches are the same. 

Functional (/system/concept) FMEA/FMECA This actually determines the correct failure step. It is done at the conceptual stage when design is not frozen to fix hardware. Naturally, at this stage there will be functional analysis. The following points may be noted about this type of FMEA/EMECA ... 

Types of FMEA/FMECA. (A) Relation amongst FMEA/EMECA types, (B) design FMEA/ FMECA concept, (C) process EMEA/FMECA concept, and (D) uses of EMEA/EMECA. 

As discussed, at different stages of the production cycle there will be an FMEA exercise. Naturally, the details with which FMEA/FMECA at different stages will be carried out will vary and accordingly the results will also vary. 

Where could FMEA/PWIECA he applied and what is it for These are very pertinent questions, which are answered when one goes through the application area of FMEA/ FMECA. There are a few distinct advantages or benefits from FMEA/FMECA. However, there are limitations too. All these are discussed next to get a good appreciation for the system ... 

Application of FMEA/FMECA FMEA/FMECA contributes to improve product and production process to achieve better quality, better reliability, enlarged efficiency with increased safety, and enhanced customer satisfaction at comparatively lower cost. Therefore FMEA/FMECA is a tool that has been adapted in various ways for different applications and purposes, some of which are listed here ... 

Benefit of FMEA/FMECA The following are some of the immediate benefits... 

An increasingly accurate estimate of probability of failure will be developed, with the help of FMEA/FMECA. 

Since in FMEA/FMECA corrective action and risk ranking are done on the basis of RPN, which is a product of severity, occurrence, and detection, it is possible that risk with less severity may get more priority (through RPN) than risk with more severity. 

Timing is very important in FMEA/FMECA if it is too late then the purpose will be defeated. 

General terms in FMEA/FMECA The following terms are commonly used in FMEA/FMECA short discussions on these are also presented to understand the meaning associated with them. 

RPN This is expressed in FMEA/FMECA in terms of product of severity, occurrence, and detection ranking in the scale of 0—1000. 

Severity The measure of seriousness of the effect of failure mode (consequence of a failure classified by the degree of injury, property damage, system damage, and mission loss that could occur) it is described in FMEA/ FMECA in the scale of 1—10. 

Like HAZOP, here also a team is entrusted to carry out the study. Another important point to be remembered is that this is usually a live document so that any changes, etc. that can affect safety/risks can be reassessed with the help of the analysis. So it is not a case of once it is done the responsibility is over it needs to be updated. FMEA/FMECA methodology is presented in Figs. IV/2.1-1 and IV/2.1-2. It is worth noting that these two drawings should be viewed in conjunction, as one is related to the other. Fig. IV/2.1-3 is presented to depict PFMEA and its relation with DFMEA. 

Scope and boundary For defining the scope and boundaries of FMEA/FMECA the major questions are Is it for conceptual, design, process, or software and services Also the purpose of the study shall be questioned. The scope of analysis shall take into account the physical boundaries, operating phases (operational or startup/shutdown phase, etc.), and any other assumptions considered in the referendum. In brief the following points shall constitute the scope and boundary of analysis. It is worth noting that all interface points should be included in the scope even if these are beyond the physical boundaries defined. 

Information/documentation requirements For a successful FMEA/FMECA analysis a number of documents are necessary. The variety of information/ documentations necessary for DFMEA/DFMECA and PFMEA/PFMECA will be different. Also there wiU be variations in documentations for functional and hardware FMEA/FMECA approaches ... 

Table IV/2.1.2-1 Documentation for Hardware and Functional FMEA/FMECA (Typical Indicative Only) ...

As discussed earlier in Clause 2.0.4, there are two different approaches for FMEA/ FMECA with different purposes. One approach is top down, while the other is bottom up. Complexity of design, development stages, and amount of data availability actually determine which approach is better suited. In some complex laige systems it is not uncommon to use both in tandem. These approaches can he applied at any level. Functional analysis may be considered as an input for determining failure mode in both approaches. The top-down approach is better suited for functional analysis. 

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