FMECA, process and industrial maintenance analysis

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Failure Modes, Effects and Criticality Analysis (FMECA) is a reliable study methodology, widely used in areas such as process analysis and industrial maintenance.

FMECA was developed by NASA in 1949 and used during the Apollo space program project.

Procedure for Failure Mode, Effects, e Criticality Analysis (FMECA)

Procedure for Failure Mode, Effects, e Criticality Analysis (FMECA)

In Italy it was used for the first time by Fiat Auto group in 1985.

FMECA is based on two fundamental principles:

  • Hierarchical decomposition of the entity into subgroups with decreasing complexity, up to the desired level of detail (eventually up to the elementary components);
  • Performing the reliability analysis at each level, i.e. determining the manner, cause, mechanism and effect of the failure at any level, appropriately assessing the criticality of the entity under examination.

The entity is broken down from the overall entity (example: machine tool) to the desired level of detail. Potentially, you can reach the level of the single component.


Hierarchical decomposition of the entity under examination in subgroups with decreasing complexity

Hierarchical decomposition of the entity under examination in subgroups with decreasing complexity

The decomposition of the entity can be done applying two criteria:

  • The first is the decomposition based on the risk associated to the breakdowns of the critical entities;
  • The second is the decomposition guided by maintenance logistical support tasks.

After decomposition, a criticality analysis is carried out, which aims to quantify the functional risk of each failure mode. To do this, a "criticality index" is assigned, which can be calculated in different ways. One of the methods to define this index is RISK PRIORITY NUMBER, that is a number ranging from 1 to 1000 and is calculated with the following formula:

​​​​​​Critical Analysis

Hierarchical decomposition of the entity under examination in subgroups with decreasing complexity

  • O = Occurrence, i.e. the probability of occurrence estimated for the failure, if it is high, it is higher the failure is to occur. For example, O = 1 means that the failure occurs less frequently than once every 25000 hours, while O = 10 means that one failure occurs every hour.
  • S = Severity, i.e. the severity of the effects of the fault, if it is high, it is more severe the fault. For example, S = 1 indicates that the production process keeps its parameters within the limits and therefore the quality of the product is not affected, while S = 10 indicates that the failure makes the entity unusable creating a risk for operators and potentially also an environmental risk.
  • D = Detectability, i.e. the ease with which the fault can be detected in advance; if it is high, it is more difficult to notice the fault. For example, D = 1 indicates that the fault will definitely be detected and therefore machine checks are not necessary, while D = 10 indicates that it is impossible to detect the fault.

Once the scale is chosen for the 3 factors, it is used to calculate the RPN risk index for each failure of the entity. Once finished it, the different faults are sorted by decreasing RPN risk index. It is therefore important to identify which faults have the highest RPN. Various techniques can be used to do this, such as Pareto analysis. 

As soon as the most critical failure modes have been identified, we will proceed to identify and propose the interventions to be implemented in the maintenance plan.  The lowest-risk failure modes, on the other hand, will only be reconsidered after the identification of the maintenance required for the highest-risk failure modes has been completed, in order to evaluate the possibility of planning opportunistic maintenance.
The last step is the identification of corrective actions and maintenance planning to prevent or limit a cause of failure. Among the corrective actions that are typically under the responsibility of maintenance, are to be considered:

  • Non-periodic maintenance improvement measures (minor plant design changes).
  • Periodic revisions to the current maintenance plan, including changes to current maintenance procedures and operations.
  • Changes to the management of maintenance parts, such as a change in inventory management policies, or the choice to stock up with higher quality materials.

At the end of the FMECA analysis, through the rigorous procedure applied and the techniques of analysis mentioned above, a plan will be obtained that clearly indicates which are the most critical parts of the entity under examination, so as to be able to focus its efforts on improving the criticality related to these parts.

FMECA is a widely used technique for a quantitative definition of the criticality of your production plant.



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