In this section we describe some RCM terms for those not familiar.
Functions are the reasons why an organization acquires physical asset systems and subsystems in the first place. Another way to view this is: assets perform a job or duty for us, which helps us accomplish our business goals. From the perspective of the maintenance function, we must preserve the physical assets’ operating state so that they continue to do whatever their users want them to do in their Operating Context. Note that RCM Functions are typically numbered sequentially: 1, 2, 3, …
As such, the RCM process begins by asking: “What are the functions and associated performance standards of our systems and subsystems physical assets in their present Operating Context?”
Note that every asset system has Primary Functions and Secondary Functions. The Primary Functions can usually be defined by asking: “Why does the system/asset exist?” Secondary Functions can be developed by Walking the Dog (stepping through and around the RCM analysis boundary) to determine all the relevant other functions that the system must provide beyond just the Primary Functions. These are usually in the following general categories:
Environmental
Safety / Structural
Control / Containment / Comfort
Appearance
Protection
Economy / Efficiency
Superfluous
Example of a duty and standby pump’s primary function:
To transfer water up to 300 GPM in the presence of a similar stand-by pump
To be capable of transferring water up to 300 GPM in the event of duty pump failure.
The following are examples which may be primary OR secondary functions:
To contain hydrogen in the fuel pump system.
To emit less than 50 dB measured 0.1 m behind exhaust outlet.
To regulate oven temperature from 100 C to 500 C.
To be capable of transmitting a signal to shutdown the steam turbine in the event the main oil pressure drops below 35 psi.
To be capable of stopping the conveyor in the event of an emergency at any point along its length. (Ex: A tripwire).
Functional Failures are defined to be any physical asset’s inability to do what its users want it to do. It follows then that Functional Failures are defined as any physical asset’s inability to meet a function’s performance standard in a way that is acceptable to those that use the assets.
The Functional Failure is the exact failed state or condition when an asset is unable to do what its users want it to do. They are lettered sequentially – A, B, C, … Y, Z, AA, AB, …
Example:
Unable to supply oil to the turbogenerator at all.
Unable to supply oil to the turbogenerator at a min. pressure of 35 PSI.
Example for hidden functions of a protective device:
Incapable of stopping the conveyor in the event of an emergency. (A tripwire)
Failure Modes are events that cause Functional Failures. In this context an event is any root cause failure that leaves the asset in a functionally failed state. RCM Failure Modes are numbered sequentially – 1, 2, 3, …
The RCM subject matter expert team decides what prevents the asset system from doing what its users want it to do by stepping through the system assets. This is done by exploring only the most reasonably likely causes of the failed states in any or all the 5 Mission Phases: Start, Run, Stop, Store & Maintain, which may lead to serious Safety, Environmental & Economic consequences that matter. Always consider reasonably likely Failure Modes that have happened in the past, any PM tasks being done now and whether any significant / catastrophic disaster could manifest in the future. And ask “Why?” enough times (5Ys is not always 5) to find a root cause failure reason that has a PM task that is technically feasible and worth doing.
Failure Effects describe what happens when a Failure Mode occurs. In other words, it is a short description (or story) of the event sequence that transpires when the asset fails according to the associated Failure Mode. The story begins with incipient asset failure, develops evidence of failure, until its final demise leads to the failed operational state. (Functional Failure)
However, the Failure Effect also includes all the failure consequences (safety, environmental, and economic impacts) that result along the way as asset deteriorates, degrades and failure progresses in an unimpeded manner. The Failure Effect is not complete without coverage of what cost and effort it requires (including manpower, OEM parts, tools, rentals, other resources, testing & validation, etc.) to correct and restore operations to normal conditions again.
Example:
Effect Indicator:
OAPOT, regular pump operation with poor quality water containing solid particles and other chemicals form abrasive matter that wears down the surface material of the seal. This eventually causes the seal to leak by when it gets too thin. Continued use makes the leak progress from shaft wetness, to water dripping out, followed by water streaming or gushing out of the pump seal area.Functional Failure:
Unable to transfer at least 1,000 GPM of water from the Reservoir 1 to by-pass A.Effect Consequence:
This will lead to no water being transferred to the reservoir by-pass spillway. The reservoir water levels will rise and, if no backup water pumping out of the reservoir can be arranged, the reservoir will spill over its banks onto the native reservation located below the reservoir valley. Should this occur, the result ranges from nuisance flooding of native lands to loss of wildlife or even human life, depending how much overflow occurs. If not acted upon before the pump shuts down, this may also result in a tricky and dangerous (read: unsafe) repair job partly in a confined space. It may also require contracting underwater divers, special rigging, and rental equipment at a significant and unexpected expense / cost.
Some other RCM terms:
A Proactive Task is worth doing if it reduces the consequences of the associated failure mode to an extent that justifies both the direct and indirect costs of doing the task.
A Hidden Function is one whose failure, if it happens on its own, will not become evident to the operating crew under normal operating circumstances.
A ‘Fail-Safe’ device is one whose failure on its own will become evident to the operating crew under normal circumstances.
A Multiple Failure only happens if a protected function fails while the protective device is in a failed state.
The task is Technically Feasible if it is physically possible for the task to reduce, or enable action to be taken to reduce, the consequences of the associated failure mode to an extent that is acceptable to the owner or user of the asset.
Scheduled Restoration tasks entail restoring initial capability of an existing item or component at, or before, a specific age limit, regardless of its apparent condition at that time.
Scheduled Discard tasks entails discarding an item or component at, or before, a specific age limit, regardless of its condition at that time.
On-Condition tasks entail checking for potential failures, so that action can be taken to prevent the functional failure or to avoid the consequences of the functional failure. On-condition tasks must be carried out at intervals less than the P-F interval (Potential Failure - Functional Failure).
A Potential Failure is an identifiable condition which indicates that a functional failure is either about to occur or in the process of occurring.
Failure-Finding entails testing a hidden function at specific, regular intervals to find out whether it has already failed. Failure-finding is worth doing if it reduces the probability of the associated Multiple Failure to a tolerably low level.