Thursday, 4 December 2014
If there are not any technically feasible tasks to predict or prevent the failure in the work conditions, or they are not cost effective, we must perform actions.
These tasks can be Run-to-Failure or Redesign.
- Run-to-Failure, only if the risk of the failure is in acceptable values and has not got safety or environmental consequences.
In these cases we must prepare an emergency response to act once the failure has been. To design this response the probability of the failure and the time between failures must be considered.
The emergency response must be based in a protocol that can include work orders to repair the failures, which allow acting immediately the failure is detected, the actions to take when the failure is detected to affect the failure affected, and the spare parts forecast to use to repair the failure.
- Redesign, if the risk of the failure is out acceptable values and has got safety or environmental consequences we must think in redesign, the redesign can be change in design of equipment, change in process, and change in operations. The redesign can be focused in prevention of failures, avoiding them; detection of failures, before the consequences of failure; control of the failures, reducing the probability of the failures modes; or mitigation, reducing the consequences of the failure.
These redesigns can affect both the equipment and the operation conditions, and the redesigns can include physical modifications of equipment or the environment, change in operation procedures, operation or maintenance staff training, etc.
Before start the redesign we must check that they are cost-effective, comparing the redesign and implantation cost and the estimated risk reduction, this risk never can be reducing to zero but it is right if the risk is reduced to reasonable values, or allows using effective condition based maintenance, time based maintenance or failure finding tasks.
Redesign must be considered as a project, with financial, human and material resources that must be assigned, and must be time limited. Finally, to check the redesign meets the targets should be checked.
Tuesday, 11 November 2014
Si no existe ninguna tarea que sea efectiva para predecir o prevenir el fallo en las condiciones de utilización del equipo, o existe pero no resulta rentable, debemos plantear acciones.
Estas acciones pueden ser Funcionar hasta fallar o Cambiar el diseño.
· Funcionar hasta fallar, sólo en los casos en los que el riesgo se encuentra en valores aceptables.
En estos casos se debe tener preparada una respuesta de emergencia para actuar una vez que este fallo se ha producido. Para esta respuesta hay que tener en cuenta la probabilidad de que este fallo se produzca y una estimación de cada cuanto tiempo se producirá el fallo.
Esta respuesta de emergencia debe estar basada en un protocolo de actuación que puede incluir las órdenes de trabajo de las reparaciones de los fallos, para actuar de forma inmediata a la detección del fallo, las actuaciones a tomar en el momento en que se detecta el fallo que afecten a los afectados por el fallo, y la previsión de repuestos a utilizar para solucionar el fallo.
· Cambiar de diseño, si el riesgo de fallo no se encuentra en valores aceptables se debe plantear un cambio de diseño, este puede tratarse tanto de cambios en el diseño del equipo como en el proceso o las operaciones, y pueden estar orientados a prevenir el fallo, evitando que se produzca, detectar el fallo, antes de que tenga consecuencias importantes, reducir la probabilidad de que se produzca el fallo, con lo que se reduce el riesgo, o mitigar las consecuencias del fallo.
Como hemos visto, estos cambios de diseño pueden afectar tanto al equipo como a sus condiciones de operación, y pueden incluir tanto modificaciones físicas del equipo o su entorno, cambios en los procedimientos de operación, formación del personal de operación o mantenimiento, etc.
Antes de implantarlos debemos comprobar que son rentables, comparando el coste de su diseño e implantación con la disminución del riesgo que supone su implantación, este riesgo nunca se puede reducir a cero pero la modificación del diseño será aceptable si lo reduce a un valor razonable o permite que en el equipo se utilicen tareas basadas en la condición, tareas basadas en tiempo o tareas de búsqueda de fallos que sean efectivas para prevenir o detectar el fallo.
Los cambios de diseño deben plantearse como un proyecto, al que se asignan unos recursos personales, económicos y materiales, y que debe estar limitado en el tiempo. A su vez se debe comprobar que, una vez aplicados, cumplen con sus objetivos.
Monday, 13 October 2014
Thursday, 25 September 2014
The first result of the Logic Tree is task, in this chapter we go to explain the different type of tasks that we can implant in our RCM program.
On Condition Tasks, they should be our first option. They are defined as periodic or continuous inspections that are designed to detect potential failure conditions and allow corrections prior to functional failures.
To develop an On Condition task, the following questions must be addressed:
· What is the Functional Failure? It can normally be obtained from the RCM analysis; however, additional definition of clarification related to the specific failure mode may be required during development of the On Condition task.
· What is the Potential Failure? It can be obtained from a study of the equipment work and environmental conditions.
· What is the Potential Failure to Functional Failure (PF) interval; is it consistent? It can be obtained from the P-F Curve.
P-F Curve, from NASA RCM Guide, Reliability-Centered Maintenance Guide, for facilities and collateral equipment. Page 4-2.
· Can a task interval be developed that ensures the probability of failure is at an acceptable level (considering the consequences of the failure mode)? We can calculate the interval given the chance to develop in Functional Failure, according to the formulas:
R(t) is the Reliability Function, it gives the probability of an item operating for a certain amount of time without failure. It is a function of time t and of failure rate l.
R(t) is the Reliability Function, it is a function of time t, of the scale parameter h, that indicate life of the equipment, and of the shape parameter b, that indicate the failure rate distribution.
· Is it feasible to perform predictive technique in that time slot? To choose the predictive task we can help further guidance provided by NASA.
We can calculate the costs to implant on condition tasks, considering the cost of inspection and the probability of not detect a potential failure, according to the formula:
Coc = Cf X Pf + Ctc + (Ctc + Cf) X Pfc X Pf
Coc is the cost of the on conditions maintenance, Cf is the cost of the failure, Pf is the probability of failure during the task interval, Ctc is the cost of the on condition task, and Pfc is the probability that the on condition task doesn’t avoid the failure.
Predictive techniques application, from NASA RCM Guide, Reliability-Centered Maintenance Guide, for facilities and collateral equipment. Page 6-2.
Time Based Tasks, they consist of regularly scheduled tasks, and Failure Finding Tasks that consist in techniques to find hidden failures. We go to study them together because they are performed without regard to equipment condition.
The elements of a Time Based Maintenance are:
1. Lubrication / Servicing: Cleaning, lubricating, charging, preservation, etc., of items/materials periodically to prevent the occurrence of incipient failures. It can be consider a failure finding task.
2. Testing (Operational, Visual, and Automatic): Periodically testing or checking out to determine serviceability and detect electrical/mechanical-related degradation. It can be consider a failure finding task.
3. Inspection / Functional Test: Periodically inspecting materials/items to determine their serviceability by comparing their physical, electrical, mechanical, etc., characteristics (as applicable) to expected standards. It can be consider a failure finding task.
4. Restoration: Periodically tasks to restore the design conditions of an item, it can include cleaning some elements a full overhaul.
5. Installation: Periodic replacement of limited-life items or the items experiencing time cycle or wear degradation, to maintain the specified system tolerance.
Also, we can calculate the optimum maintenance interval based in the failure rate that we need to keep, according the formula:
Exponential distribution, with failure rate l constant, the time based maintenance is not recommended because to define the maintenance interval is not definable.
Finally, we can calculate the time based maintenance costs, considering the cost of the task, the risk of a failure due the task and the probability of failure during the maintenance interval, according the formula:
Ctb = Cf X Pf + Ctp + Cf X Pfp + (Ctp + Cf) X Pft X Pf
Ctb is the cost of time based maintenance, Cf is the cost of failure, Pf the probability of failure during the maintenance interval, Cpt is the cost of the preventive task, Ppf is the probability of failure due the preventive task, and Pft is the probability of the preventive task doesn’t avoid the failure.
Tuesday, 8 July 2014
Como hemos visto, los primeros resultados del árbol de decisión son tareas, en este capítulo vamos a explicarlos:
Tareas basadas en la condición, esta será siempre nuestra primera opción, podemos definirlas como inspecciones periódicas o continuas diseñadas para detectar fallos potenciales y permitir su solución antes de que produzca un fallo funcional.
Para desarrollar el plan debemos realizar las siguientes preguntas:
1. ¿Cuál es el Fallo Funcional? Esto se obtiene directamente del análisis RCM, aunque puede hacer falta más información o una mayor clarificación relacionada con la especificidad del fallo.
2. ¿Cuál es el Fallo Potencial? Que averiguamos con un estudio de las condiciones de funcionamiento del equipo.
3. ¿Cuál es el intervalo entre el Fallo Potencial y el Fallo Funcional (intervalo P-F)? ¿Es un intervalo consistente? Para responder a estas preguntas nos ayudamos de la curva P-F.
Monday, 2 June 2014
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 18.104.22.168 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 22.214.171.124 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 126.96.36.199 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 188.8.131.52 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.