Criticality ranking

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. 

TABLE 17.5.2 Suggested Scale for Criticality Ranking for a Qualitative ... 

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. 

Failure Mode Effect (and Criticality) Analysis [FME(C)A] a technique in which all known failure modes of components or features of a system are considered in turn and undesired outcomes are noted a criticality ranking of equipment may also be estimated. 

TABLE 17.5.1 Suggested Criticality Rankings Based on Aerospace Hazard Classification... 

C. Preventive/predictive maintenance Cost-effective plants have an equipment criticality ranking scheme and use rehability-centered maintenance (RCM) techniques to determine the equipment that requires preventive maintenance. They use a combination of equipment manufacturers reconunendations, experience, and available PM databases to validate or modify existing PMs and to create new preventive maintenance schedules where necessary then they faithfully execute their PM programs. 

Many hazard analysis and risk assessment techniques have been developed. These are just a few of the methodologies mentioned in the literature preliminary hazard analysis gross hazard analysis hazard criticality ranking catastrophe analysis change analysis energy flow/barrier analysis energy transfer analysis event tree analysis human factors review the hazard totem pole and double failure analysis. There are many other hazard analysis systems. 

When applied to models of local propagation of a fixed amount of load and of redistribution of the failure load, three of the proposed criticality indicators, namely/i, di and st, are consistent in their criticality ranking of the components. When applied to the model of cascading failures due to targeted intentional attacks fi and di are consistent in ranking the components according to their criticality. 

Figure 8. Criticality ranking of equipment contribution to unavailability.

For the base design configuration the overall production efficiency of the system is 96.05%. The criticality ranking is shown in Figure 8 and tells us that the compression systems contribute the majority of production losses. The HP compression system causes 64% of losses and the LP compression system causes 31% of losses. 

Description Failure modes relevant for the operational mode indicated Effects of failure mode on adjacent components and surrounding environment Effects on the functionality of the subsystem Effects on the functionality and availability of the entire plant Probability of failure occurrence (sometimes qualitative) Criticality rank of the failure mode on the basis of its effects and probability (qualitative estimation of risk) Methods of detection of the occurrence of the failure event Protections and measures to avoid the failure occurrence Remarks and suggestions on the need to consider the failure mode as accident initiator... 

Analyse and rank the vulnerability scenarios, resulting in a criticality ranking of the scenarios. 

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