Reliability Centered Maintenance

 

 Reliability Centered Maintenance

 

 

An RCM process systematically identifies all of the asset’s functions and functional failures, and identifies all of its reasonably likely failure/causes. It then proceeds to identify the effects of these likely failure modes and to identify in what way those effects matter. Once it has gathered this information, the RCM process then selects the most appropriate asset management policy.  

RCM considers all asset management options: on-condition task, scheduled restoration task, scheduled discard task, failure-finding task, and one-time change (to hardware design, operating procedures, personnel training, or other aspects of the asset outside the strict world of maintenance). This consideration is unlike other "Maintenance Development Processes".

Funcitional Failures

2. In what ways can it Fail to fulfill its Functions (Functional Failures)?

This question has only one specific criterion: All the failed states associated with each function shall be identified. If functions are well defined, listing functional failures is relatively easy. For example, if a function is to “keep system temperature between 50 C and 70 C,” then functional failures might be:

 

Failure Modes

3. What Causes each Functional Failure (Failure Modes)? 

In Failure Modes, Effects and Criticality Analysis (FMECA), the term “failure mode” is used in the way that RCM uses the term “functional failure.” However, the RCM community uses the term “failure mode” to refer to the event that causes functional failure. The standard’s criteria for a process that identifies failure modes are:

 

Seven Questions Addressed By RCM

Fundamentally, the RCM process seeks to answer the following seven questions in sequential order:

1. What are the functions and associated desired standards of performance of the asset in its present operating context (functions)?

The Specific Criteria that the Process Must Satisfy are:

The "Operating Context" is the circumstance in which the asset is operated. The same hardware does not always require the same "Failure Management Policy in all installations.

For Example:

"A single pump in a system will usually need a different failure management policy from a pump that is one of several redundant units in a system. A pump moving corrosive fluids will usually need a different policy from a pump moving benign fluids." Protective devices are often overlooked; an RCM process shall ensure that their functions are identified. Finally, the owner/user shall dictate the level of performance that the maintenance program shall be designed to sustain. 

All Failure Modes Reasonably Probable to Cause each Functional failure shall be Identified.

The method used to decide what constitutes a “reasonably probable” failure mode shall be acceptable to the owner/user of the asset. Failure modes shall be identified at a level of causation that makes it possible to identify an appropriate failure management policy. 

RCM is the most thorough of the analytic processes that develop maintenance programs and manage physical assets. It is therefore appropriate for RCM to identify every reasonably likely failure mode.

Failure Effects

4. What Happens when Each of the Failures Occur (Failure Effects)? 

The criteria for identifying Failure Effects are: 

 

Failure Consequences

5. In what way does each Failure Matter (Failure Consequences)? 

The standard’s criteria for a process that identifies failure consequences are:

RCM assesses failure consequences as if nothing is being done about it. Some people are tempted to say, “Oh, that failure doesn’t matter because we always do (something), which protects us from it.” However, RCM is thorough it checks the assumption that this action that “we always do” actually does protect them from it, and it checks the assumption that this action is worth the effort.

RCM assesses failure consequences by formally assigning each failure mode into one of four categories: hidden, evident safety/environmental, evident operational, and evident non-operational.

The explicit distinction between "Hidden and Evident failures", performed at the outset of consequence assessment, is one of the characteristics that most clearly distinguishes RCM, as defined by Stan Nowlan and Howard Heap which was supported by the US Department of Defense and earlier U.S. Civil Aviation. 

Proactive Tasks

6. What should be done to Predict or Prevent each Failure (Proactive Tasks and Task Intervals)? 

This is a complex topic, and so its criteria are presented in two groups. The first group pertains to the overall topic of selecting Failure Management Policies. The Second Group of criteria pertains to scheduled tasks and intervals, which comprise Proactive Tasks as well as one default action - Failure Finding Tasks. 

This criteria is:

Scheduled Tasks are tasks that are “performed at fixed, predetermined intervals, including ‘continuous monitoring’ (where the interval is effectively zero).” Scheduled tasks should be identified that fit the following criteria:

> In the case of an evident failure mode that has safety or environmental consequences, the task shall reduce the probability of the failure mode to a level that is tolerable to the owner/user of the asset. 

> In the case of a hidden failure mode where the associated multiple failure has 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 level that is tolerable to the owner/user of the asset. 

> In the case of an evident failure mode that does not have safety or environmental consequences, the direct and indirect costs of doing the task shall be less than the direct and indirect costs of the failure mode when measured over comparable periods of time. 

> In the case of a hidden failure mode where the associated multiple failure does not have safety or environmental consequences, the direct and indirect costs of doing the task shall be less than the direct and indirect costs of the multiple failure plus the cost of repairing the hidden failure mode when measured over comparable periods of time.

Categories of Tasks

There are three general categories of tasks that are considered to be proactive in nature: 

> On-Condition Tasks

> Scheduled Discard Task 

 

The next kind of task is a scheduled discard task, defined as “a scheduled task that entails discarding an item at or before a specified age limit regardless of its condition at the time.” A scheduled discard task must be subjected to the following criteria before accepting the task:

> Scheduled Restoration Tasks 

The next kind of task is a scheduled restoration task, defined as “a scheduled task that restores the capability of an item at or before a specified interval (age limit), regardless of its condition at the time, to a level that provides a tolerable probability of survival to the end of another specified interval.” The following criteria must be applied to a scheduled restoration task before accepting the task: 

Default Actions

7. What should be done if a Suitable Proactive Task cannot be found (Default Actions?)

This question pertains to unscheduled failure management policies: the decision to let an asset run to failure, and the decision to change something about the asset’s operating context (such as its design or the way it is operated.) 

Failure-finding tasks usually apply to protective devices that fail without notice. This task represents a transition from the sixth question (proactive tasks) to the seventh question (default actions, or actions taken in the absence of proactive tasks.) Failure-finding tasks are scheduled tasks like the proactive tasks. However, failure-finding tasks are not proactive. They do not predict or prevent failures. They detect failures that already have happened, in order to reduce the chances of a multiple failure and the failure of a protected function while a protective device is already in a failed state.

Run to Failure (RTF)

If a process offers a decision to let an asset Run-to-failure (RTF), the following criteria should be applied before accepting the decision.

Please post your comments or share your knowledge/experience on RCM. 

The Author: Ricky Smith CMRP, CMRT, CRL

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