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. 

FMECA - Failure Modes, Effects and Criticality Analysis. 

Answer You can treat them as one component in the component column and prepare the FMECA for ways they can fail together. For random failure of both valves in a mission time, estimate the failure probability as one-half the failure rate of one times the probability of ailing in the mission time of the other. 

All of these factors determine the stress experienced by the workers and the extent to which operational errors will be recovered before disastrous consequences have ensued. In this context, hazard identification techniques, such as hazard and operability studies (HAZOP), failure modes and effects and criticality analysis (FMECA), fault trees, and others are useful in making the process environment more forgiving. 

FMECA Failure Modes and Effects of Criticality Analysis... 

Failure modes effects criticality analysis (FMECA)... 

Perhaps the key to detcrnuiiiiig die consequences of an accident is die study of accident mininiization/prcvendon. This topic receives extensive treatment in Section 17.2. The estimation (not calculadon) of consequences is treated in Section 17.3, which is followed by evacuation procedures (Section 17.4). The next section e.xaniiiies failure modes, effects and critical analysis (FMECA). The cluipter concludes with vulnerability analysis (Section 17.6) and event tree analysis (Section 17.7). 

The first step in FMECA is to determine a level of resolution. If a system-level liazard is to be addressed, equipment in die system must be studied for a plant-level hazard, individual systems within die plant must be examined. Once the level of resolution luis been determined, a format must be developed-one to be used consistendy tlu oughout die study. A minimal format should include each item, its description, failure modes, effects, and criticality ranking. 

The FMECA table should be concise, complete, and well organized. This table should identify equipment and relate it to a system drawing or location. This is to prevent confusion when similar equipment is used in different locations. One of tlie limitations of FMECA is tliat the table must include ALL failure modes for each piece of equipment and effects of each failure along witli tlie associated criticality ranking. Table 17.5.3 shows a sample chart tliat can be completed for tlie FMECA table. 

The final step in conducting a FMECA is to report tlie results. If tlie prepared table (see Table 17.5.3) is complete, tliat may be sufficient. Often, however, a report of suggested design changes or alterations should also be included. 

It should be noted tliat FMECA identifies single failure modes tliat eitlier directly result in or contribute significantly to important accidents. Human/operator errors are generally not examined in a FMECA however, tlie effects of a misoperation are usually described by an equipment failure mode. It should also be noted that FMECA is not efficient for identifying combinations of equipment failures tliat lead to accidents. 

Failure Modes, Effects and Criticality Evaluation (FMECA) is a systematic qualitative metliod by which equipment and system failures and tlie resulting effects of these failures are detennined. FMECA studies possible events, but not tlie reasons for tlieir occurrences. 

Failure mode, effects, and criticality analysis (FMECA) This method tabulates a list of equipment in the process along with all the possible failure modes for each item. The effect of a particular failure is considered with respect to the process. 

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. 

From those techniques given in Table 1 my personal preference is for failure mode, effects, and criticality analysis (FMECA). This technique can be applied to both equipment and facilities and can be used to methodically break down the analysis of a complex process into a series of manageable steps. It is a powerful tool for summarizing the important modes of failure, the factors that may cause these failures, and their likely effects. It also incorporates the degree of severity of the consequences, their respective probabilities of occurrence, and their detectability. It must be stressed, however, that the outcome of the risk assessment process should be independent of the tool used and must be able to address all of the risks associated with the instrument that is being assessed. 

From this assessment Table 3 can then be used to determine the level and frequency of testing. From the example of FMECA in Table 4, the following actions should be performed ... 

Note Risk is the product of the severity and occurrence scores from the FMECA table. See Table 4. 

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.)... 

The next section e. aniines failure m es, effects and critical analysis (FMECA). The cliapter concludes with xaihierability analysis (Section 17.6) and event tree analysis (Section 17.7). 

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