In the context of Reliability Centered Maintenance, understanding functions, functional failures, and failure modes is fundamental to the analysis process. Here's a brief explanation of each:
- Functions
Definition: Functions represent the desired capabilities of the system/asset within the operational context you will use it in. It does not represent the asset's requirements, design or specifications - RCM is concerned with how you (the owner or user) wish to use an asset in a specific operating context.
Functions should ideally contain a verb (the action), an object (the subject under action), and a performance standard (how well must the action perform).
They are often framed as: To [verb] [object] to [performance standard].
Example: For a diesel generator, a function could be "To generate(v) electrical power(o) to power lighting and heating(ps)".
- Functional Failures
Definition: Functional failures occur when a component or system fails to perform its intended function within the specified performance standards or criteria. Functional failures define all possible failed states for an asset/system.
Example: For the function above we could have "supplies no power" or "unable to provide sufficient voltage for heating."
- Failure Modes
Definition: Failure modes are the events that cause a functional failure. Each failure mode represents a unique failure scenario.
Example: For the function failure "supplies no power", failure modes could include "fuel system blockage" or "faulty fuel injector calibration."
We should also mention the Failure Mechanism. This is not part of the 7 Questions, and not core to the RCM process, but other processes use it. In essence, the failure mode is the event or scenario that leads to a functional failure, while the failure mechanism is the underlying process or cause that results in that specific failure mode.
- Failure Mechanism
Definition: A failure mechanism is the underlying process or physical mechanism that leads to the manifestation of a failure mode.
Example: If the failure mode is "corrosion," the failure mechanism could be the chemical process causing the material to degrade over time.
Quantitative Functions
When defining functions and functional failures, effort should be made to provide quantitative functions. If the function is measurable, we have an objective way to measure the functional failure.
In the examples above, we have a function: "To generate electrical power". This would be improved by adding a measurable factor to the function - how much power is required? By stating the function as: "To generate 8,000 kWe of power (+ 10%) @ 480V (+/- 4.0%) and 60Hz (+/- 4.5%)" we get objective limits (criteria).
We can now state objective functional failures:
- Engine generates less than 8,000 kWe of power
- Engine generates more than 8,800 kWe of power
- Power output does not remain with the voltage range of 480V (+/- 4.0%)
- Power output does not remain in the frequency range of 60Hz (+/- 4.5%)
Quantitative functions and functional failures not only help the analysis team identify functional failures, it provides the objective measure. This can then be used to calculate P-F intervals or be used in condition-based maintenance.
Goal
In the RCM analysis process, the goal is to identify and analyze these functions, functional failures, and failure modes systematically. This involves answering questions such as how things can go wrong (failure modes) and what happens if it goes wrong (effects). Subsequently, we can ask how each failure mode matters (the consequence), which provides the needed information to develop maintenance tasks or strategies to address the identified failure modes, aiming to optimize the reliability of the system while managing costs and resources effectively.