Thursday, 25 September 2014

RCM 5: Focused in Tasks.


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:
 
         Exponential distribution.
 
 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. 
 
     Weibull distribution.
 
 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:
 Weibull distribution.
 
 
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.