Monday, 2 June 2014

RCM 4: Logic Trees


When the FMEA or COFA is finished, the maintenance tasks or actions must be chosen; to do it we will use a logic tree.

To choose the more appropriate task, the following factors must be considered:

1.    Shall be technically feasible and worth doing, in accordance with the section 5.6.2 of SAE JA 1011. A task is considered as worth doing if reduce (avoid, eliminate or minimize) the failure consequences. The task must be technically feasible, applicable and effective in the equipment work conditions.

2.    Cost – effective, in accordance with the section 5.6.3 of SAE JA 1011, if two or more tasks are technically feasible the most cost-effective task shall be selected.

3.    Probability of failure modes and age, the selection of tasks must take account the fact that the probability of some failure modes shall increase with age or that the probability not change with age, in accordance with section 5.6.1 of SAE JA 1011. To know if the failure mode ratio curve is random or age-related is essential for the process.  

4.    Selection of failure management policy, a failure management policy shall be selected if no specific task is currently being done to anticipate, prevent, or detect the failure, in accordance with the section 5.6.4 of SAE JA 1011.

The next step is to design a logic tree that categorize the failure consequences and provides a selection of failure management policies:

1.    Evident failure mode with safety or environmental consequences, the task shall assess the risk and implant tasks that reduce the probability of the failure mode to a level that is tolerable to the owner or user of the asset, in accordance with section 5.7.1.1 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time based task, if it is not feasible then proposes a combination of tasks and, finally, a redesign of equipment.

 

2.    Hidden failure mode with safety or environmental consequences, the task shall reduce the probability of the hidden failure mode to an extent which reduces the probability of the associated multiple failure to a tolerable level to the owner or user, in accordance with section 5.7.1.2 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time based task, if it is not feasible then proposes a failure-finding tasks and, finally, a redesign of equipment.

3.    Evident failure mode with economic consequences, the direct and indirect costs of doing the task shall be less than the costs of the failure mode, in accordance with section 5.7.1.3 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time based task, if it is not feasible then proposes no scheduled maintenance but a redesign of equipment is recommended.

4.    Hidden failure with economic consequences, the direct and indirect costs of doing the task shall be less than the costs of the multiple failure mode plus the cost of repairing the hidden failure, in accordance with section 5.7.1.4 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time based task, if it is not feasible then proposes failure-finding tasks and, finally, proposes no scheduled maintenance but a redesign of equipment is recommended.
 



COFA methodology takes under consideration the component classification. For the Critical, Commitment, or Economics components an on-condition task, if this task is not applicable or effective then proposes a time directed task, if this task also is not applicable or effective then proposes to initiate a design change or accept the risk.

For the Potentially Critical components an on-condition task, if this task is not applicable or effective then proposes a time directed task, if this task also is not applicable or effective then proposes failure finding task, if this task is not applicable or effective then to initiate a design change or accept the risk.

Indicate that the Run-to-Failure components not scheduled maintenance is done.

We have seen that the results of the logic trees can be:

·     On-Condition based tasks, or predictive tasks, they can be done without move equipment, without stop it or during a planned stop; they give information about the failure with time enough to plan reparations.

·    Time based tasks, or preventive tasks, they require to stop equipment and, usually, move it to a workshop; this task acts over all components, even those that don’t require maintenance.

·     Failure finding tasks, their mission is to find hidden failures, but they assume that find the failure is not immediate.

·     Tasks combination, when to implant only one task is not enough to reduce risk to reasonable values.

·    Run-to-Failure, it is the best solution when the risk is under acceptable values, because it use all components life.

·    Redesign, they can include as equipment redesign as process or operation redesign. Before to propose a redesign the complexity and time to implant them, the costs and the risk to no avoid the failure mode probability or consequence must be considered.